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Microwave spectrum, quadrupole coupling constants, and structure of N-chlorodifluoromethanimine
Journal of Molecular Structure, 160 (1987) 37-56 Elsevier Science Publishers B.V., Amsterdam -Printed
in The Netherlands
MICROWAVE SPECTRUM, QUADRUPOLE COUPLING CONSTANTS, AND STRUCTURE OF N-CHLORODIFLUOROMETHANIMINE
P. GRONER, H. NANAIE and J. R. DURIG Department
of Chemistry,
University of South Carolina, Columbia, SC 29208
(U.S.A.)
(Received 13 January 1987)
ABSTRACT The microwave spectra of N-chlorodifluoromethanimine (chlorocarbonimidic difluoride), CF,N35C1 and CF,N37C1, have been investigated from 18 to 39.5 GHz. The hyperfine structure due to the quadrupole coupling with the chlorine and 14N nuclei has been analyzed for a- and b-type transitions of both isotopic species. From experimental diagonal and off-diagonal quadrupole coupling constants, principal coupling constants of -94.7(l) and -74(l) MHz were calculated for the 35C1and 3’C1 isotopes. A molecular structure compatible with rotational and quadrupole coupling constants has been determined with r(N-Cl) and r(C=N) fixed at 1.700 and 1.254 A, respectively, and L(CNC1) = 116.4(14)0 with reasonable assumptions for the other structural parameters. The strong correlations among all structural parameters are discussed and the determined parameters are compared to the corresponding parameters of similar molecules. INTRODUCTION
The chlorine-nitrogen bond distance has been determined only for a small number of molecules in the gas phase. The reported values display rather large variations. The N-Cl bond distances of 1.9734 + 0.0017 a, 1.840 f 0.002 A, and 1.705 + 0.005 A for nitrosyl chloride [l] (ClNO), nitryl chloride [2] (ClNO*), and chlorine isocyanate [ 31 (ClNCO), respectively, provide excellent examples. Although the first two values are larger than the sum of the covalent radii of 1.73 _i$for nitrogen and chlorine [4], the third value is smaller than this sum. Nevertheless, several chlorine-nitrogen bond distances have been determined which are only slightly larger than the sum of these radii, i.e., 1.7480 + 0.0001 A for chloroamine, C1NH2 [5], 1.750 + 0.003 A for N-chloromethanamine, C1(CH3)NH [6], and 1.759 f 0.002 A for trichloroamine, NC13 [7]. Since the substitution of a methyl group in N-chloromethanamine for a hydrogen atom in chloroamine had essentially no effect on the N-Cl bond distance, we recently determined the N-Cl bond distance in N-chloro-N-methyl methanamine, (CH&NCl, from an analysis of its microwave spectrum [B] . The resulting value of 1.749 + 0.001 a shows rather conclusively that this distance does not change if a hydrogen atom in chloroamine is replaced by a methyl group. This result is somewhat surprising since the comparison of the carbon-chlorine 0022-2860/87/$03.50
o 1987 Elsevier Science Publishers B.V.
38
bond distances in the corresponding carbon analogues shows that such a bond is elongated by 0.007 8, with the substitution of a hydrogen atom by a methyl group [9-111. Because of the limited amount of structural data available on chlorine-nitrogen bonds, it seems it is not possible to predict N-Cl distances confidently at this time and further investigations are necessaryAs might be expected from the variations in the N-Cl bond lengths, large differences are also reported for the chlorine quadrupole coupling constants in these compounds. In nitrosyl chloride [12] and nitryl chloride [2], the 35C1 coupling constants have values of -57.20 and -94.28 MHz, respectively, whereas for N-chloromethanamine a value of approximately -100 MHz is estimated from the microwave data [6, 131. For N-chloro-N-methyl methanamine we recently reported quadrupole coupling constants of -91.50 MHz for 35C1and -72.12 MHz for 37C1[8]. In order to provide additional information about NC1 bonds, we have investigated the microwave spectrum of iV-chlorodifluoromethanimine (chloro-carbonimidic difluoride), CF*NCl. The results of this investigation and the conclusions to be drawn about the molecular structure and the quadrupole coupling constants of the 3sC1, 37C1and 14Nnuclei are reported herein. EXPERIMENTAL
The CFzNCl sample was prepared by the method of Zheng and DesMarteau [14] in which ClCN was vacuum transferred to a stainless steel reaction vessel maintained at -196°C followed by the addition of ClF. The reaction vessel was slowly warmed to 22°C and maintained at this temperature for 20 h. The contents of the reaction vessel were separated through a -78°C trap where the CICFzNClz sample was collected. The sample of CICFzNClz was placed in a Pyrex vessel and heated for 5 h at 135°C. The contents were then transferred on to Hg and allowed to stand for 20 mm at 22°C; the volatiles were then separated through three traps at -95, -125, and -196°C. The CFINCl was collected in the -125°C trap and then further purified using a low temperature vacuum fractionating column. The purified sample was stored at Dry Ice temperature. The microwave spectrum of N-chlorodifluoromethanimine was recorded in the region between 18 and 39.5 GHz using a Hewlett-Packard model 8460A MRR spectrometer with 33.3 kHz Stark modulation. In order to reduce thermal decomposition of the sample and increase the population in the low rotational and vibrational states, the Stark cell was cooled with a packing of Dry Ice. Because CFzNCl decomposed quickly in the unconditioned cell, resampling was necessary initially every 10 to 15 min. Later on, this period could be extended to approximately 2 h. The pressure of the sample in the Stark cell was maintained between 5 and 20 mTorr. Accurate frequencies were measured for transitions in the R-band region (26.539.5 MHz). The accuracy of the measured frequencies is estimated to be
39
between 0.1 and 0.05 MHz, depending on the degree of resolution achievable for incompletely resolved bands. RESULTS
Initial assignments of the microwave spectrum The microwave spectrum of CF2=NC1 was predicted with rotational constants from a structure very similar to the one reported [15] for CF2=NF, but using an N-Cl distance of 1.75 A. Both a- and b-type R-transitions were predicted in addition to several b-type Q-transition series. The survey spectrum in the region between 18 and 40 GHz showed many very strong bands. They were soon identified to be the predicted b-type Q-transitions of CF2=N3’C1. This was followed by the assignment of the same series for the molecule containing 3’Cl . It took a little longer to identify the first b-type R- and later also a-type R-transitions successfully for both isotopic modifications. Under increased resolution, most bands showed a complicated hyperfine splitting pattern due to the interaction of the rotating molecular electric field gradients with the nuclear quadrupole moment of the Cl and 14Nnuclei. Using average frequencies of the prominent components, approximate rotational and centrifugal distortion constants were obtained from a least squares fit. The very small inertial defects, A = 0.184 amu A2 for both isotopes, confirmed that the molecule is planar. Theory of the hyperfine structure The operator for the energy of a rotating molecule with two nuclei possessing a nuclear quadrupole moment is set up as I I I . H=H, +H,, +HQ2. fiR is the operator of an asymmetric rotor with centrifugal distortion. The operator goj containing the coupling of the rotational angular momentum J with the spins, Ii, of the quadrupole nuclei are written in terms of the scalar product of two irreducible tensors of rank 2, fioj = 4 ;
(-1)/” I$,? Q(,2?_fi.
Q!2’ represents the tensor of the nuclear quadrupole moment of nucleus j multiplied by the nuclear charge, and q2) is the negative tensor of the electric field gradients, produced by the other nuclei and all electrons, at the location of nucleus j. For a rotating molecule, the components of q2) depend on the orientation of the molecule and therefore on the angular momentum J, whereas Qj(2) depends only on the spin lj of nucleus j. Because of the coupling, J is no longer a good quantum number. The following coupling scheme is used to generate the total angular momentum quantum number F
40 s+zi
IJ--III
=i&,
“F*f i; = i?,
IFI ---I2 I < F < F1 + 12.
The matrix elements of fi were calculated in a basis corresponding to this coupling scheme. The basis functions are defined according to Wigner [16]
W(FI,12)F3 F1 (J,I,)I,Kj
=
F
1 I (-l)F1 13’ 23
- ‘1 +F, (2F+
1)1’2 (;I
; 13
_; 23
) 3
The normalized rotation group representation coefficients [16] 19 $& and ggI are used as basis functions (JKMI and (113 I, respectively. F3, F13, I13, 123, ‘and A4 are the components of F, F1, II, 12, and J, respectively, along the space fixed g3 axis. The matrix elements of fro, + &02 in this basis are derived by using repeatedly the definition of the reduced matrix element (double bar element) of a tensor operator T [ 161 --
(JM@”
1J M) = (-l)J-w
+M (27+
The result is (F(F,,I,)F,
+ (-l)F+l~
The 3 -j (
(-1)’
(-1)‘1
[t&F,
Jo
-I,
and 6 -j
jI
j2
j3
ml
m2
m3 >
ICI&I [(ZJ+
+
&42
1)(2Y+
IW’,
7
(
M p -iGi >
-F1 (J,I,)I,
= ~FF~F,F~
x
1)“2
(JII+”
IIF>.
--,12)F3
1)/24]1’2
F,
(i
(JAI2z) i_,
_g)
+Fl
-J-J[(2F,
+ 1)(2F,
+ 1)]1’2
symbols and
{ ;:
;I
f)
,
respectively, are used as defined by Edmonds [17] . The elements irreducible quadrupole coupling tensors, xpi, are abbreviations for x;$, = (2/3)“”
I$; CIjljIQ/,? IriIj)
of the
41
They are related to the corresponding
Cartesian tensors by
xi’“; = (3/2)1’* Xj,zz Xj;Z,‘, = T Xj,x.z + iXj,yz Yjz,), = (1/2)(Xj,,,
-
under the assumption Xj,zz + Xj.xx
+ Xj,yy
Xj,yy
)T
iXj, xy
that = O*
As a consequence, the general matrix element derived above is a complex quantity. It becomes, except for a numerical factor involving the definitions of the x’s, identical to the expression given by Benz et al. [18] for the case of a single quadrupole nucleus, if either I1 and all xi’: or 1, and all x$?L are set equal to zero.
Analysis of hyperfine
structure
The molecule CF2=NC1 has already been shown to be planar, the molecular plane being a symmetry plane. In this case, the axes may be chosen such that the x(2) tensors and all matrix elements become real. For the I’ representation Af the principal inertial axes, the components of XI*)become xi’*; = (3/2)“*
xaa
XV) I,+_1= T Xab xj:!_*
=
W2)(Xbb
-xc,)
The centrifugal distortion constants are defined according asymmetric reduction in the r representation [ 191. The energy levels are obtained from the matrix elements ization. The dimension, d, of the energy matrix increases with increasing quantum number F, as
to
Watson’s
by diagonalvery rapidly
d = (2F + 1)(21, + 1)(21* + 1). In the case of CF,=NCl,
we have I1 = 3/2, I2 = 1 and
d = 12(2F + 1). For this reason, all matrix elements with F1 # F1 are from the expression for the matrix element, that only ing I2 and x$*) are neglected, whereas the contributions are still completely included in this approximation. diagonalized now have the dimensions
neglected. It is seen contributions involvoriginating from I1 The matrices to be
d = (211 + 1)(2F1 + 1) = 4(2F1 + 1). The effects of the quadrupole coupling become smaller and smaller as J increases [20]. Therefore, all matrix elements off diagonal in J are neglec-
42
ted if J exceeds 9. Transition frequencies approximate selection rules [ 201
were calculated
according
to the
AF=AF,-AJ where AJ= 0,l. The first step in the analysis of the hyperfine structure of the observed transitions was the calculation of a spectrum using the approximate rotational and centrifugal distortion constants. The quadrupole coupling constants of the 14N nucleus reported for CF2=NF [21] were used directly as initial values. Approximations to the corresponding constants of the 35C1 nucleus were found by trial and error by comparing the magnitude of calculated and observed splittings. Better values were then obtained by a least squares fit of the frequencies and splittings of the J3,J_ 3 + J2, J_-l transitions. During this procedure, A, B, (2 and xaa and Xbb - xcc of 35C1 were allowed to vary. Gradually, more and more transitions and individual components were included in the fitting process. The additional assignments were based on successively updated predictions of the spectrum. An increasing number of the quartic centrifugal distortion constants and finally the quadrupole coupling constants of the 14N nucleus were allowed to vary. As the calculated transition frequencies became more precise, some reassignments of individual hyperfine components became necessary. After a satisfactory fit of all transitions and hyperfine components was obtained, the coupling constant Xab of the 35C1nucleus was included in the list of variables with an initial value of 10 MHz. The iterative least squares process ended with a standard deviation of 0.052 MHz and a value of 40.8 MHz for &b . All other parameters were similar to the final values. The analysis of the hyperfine structure of the transitions of CFz=14N3’Cl followed the same procedure. The initial quadrupole coupling constants for 37Cl, except Xab, were obtained by dividing those for 35C1 by 1.26878, the ratio of the nuclear quadrupole moments [2O]. Xab was kept fixed at the zero value throughout the calculations. It was included among the variable parameters only after a reasonable fit had been obtained. A value of 32.4 MHz for &‘b and a standard deviation of 0.069 MHz resulted from the least squares fitting process, the other parameters being close to the final values obtained later. The transformation to the principal axes of the quadrupole coupling tensor yielded a value of 20.4“ for the angle 0 between the a principal inertial axis and the z principal axis of the quadrupole coupling tensor for both isotopes. Assuming that the z axis should coincide with the direction of the NC1 bond, 0 is also the angle between that bond and the a principal axis. The initial attempts to determine a molecular structure from the observed rotational constants yielded a value of approximately 30” for the angle 8 (see section on “molecular structure”). Because of the huge discrepancy in the values for 0 from both methods, we tried to improve on the values for x&. Upon the analysis of the rotational energy levels without quadrupole inter-
43
action, it was observed that the energy levels J2,J_ 1 and J + 1 l,J+ 1 for J = 5, 6, 7, 8 are the most likely ones to be perturbed by xo6. Therefore, the quadin a least squares fit of rupole coupling constants for 35C1 were determined only the b-type transitions with J< 9. All other constants were kept fixed at the previously determined values. Avalue of 49.0 MHz resulted for xab. After a few reassignments of hyperfine components were made, xob rose even further to over 53 MHz. Starting with this new value, all assigned transitions and hyperfine components were included in a simultaneous adjustment of all parameters. The standard deviation dropped to 0.047 MHz and a final value of 52.42 MHz was obtained for x,“;. Subsequently, this value was divided by the ratio of the nuclear quadrupole components for the chlorine isotopes to obtain a new value for ~2;. The spectrum of CF,‘4N37C1 was predicted again using the new Xab value and the previous values of the other parameters. After making a few reassignments, all parameters, except AK which was fixed at the value of the other isotope, were adjusted in a fit of all transitions and hyperfine components. During this process, the standard deviation for the 37C1 isotope wa s reduced to 0.046 MHz. A value of 40.8 MHz was obtained for xi;. The complete lists of assigned transitions and hyperfine components and the differences to the calculated frequencies and splittings are shown in Tables 1 and 2 for CF,N35C1 and CF,N37C1, respectively. The final parameters for both isotopic molecules are listed in Table 3. With the new values for X&, the angle 0 was now calculated to be 24.0” and 23.9” for the two molecules.
Molecular structure Because the rotational constants A a.re essentially identical in both isotopic molecules and because the molecules are planar, only two of seven structural parameters are well determined. Diagnostic least squares calculations [22, 231 showed that all parameters contribute to one of the determinable linear combinations with the CNCl angle contributing about 50%. The other determinable parameter is essentially a linear combination of the CF, distance and the NCFt angle where F, and Ft refer to the F atoms cis and tram to Cl, respectively. The observed rotational constants may be reproduced reasonably well with almost any set of starting parameters by the diagnostic least squares method if the CF, distance and the CNCl and NCF, angles are allowed to vary. The resulting values depend very much on the initial assumptions, particularly on those about the NC1 and CN distances. The sensitivity of the results is demonstrated by the fact that a change in any of the fixed CF distances by 0.01 A results in changes of up to 0.03 a in the CN and NC1 distances, if those are allowed to vary. Initially, the r-i data reported for CFzNF [15] (see Table 4) were used as a starting point together with a NC1 distance of 1.74 A. Once a reasonable agreement between observed and calculated rotational constants was obtained, the angle f3 between the NC1 bond and the a IYncipal axis was calculated to be -30” with the CNCl angle below 110”. Assuming that one
9/Z 11/2 1312 1512
vorAv
b
dC
Transition F;
912 11/2 13/2 1512
7/2 912 11/2 13/2
F’
d --
Transition
F;
625+ 52,
F’
vorAv
d
2312 2712 2512 2912 2712 2912 3112
29520.64 0.01 0.00-0.03 2.83-0.05 2.83-0.08 3.20 0.02 3.73 0.12 3.20-0.01
29233.614.01 12,,,, + 12,,,, 21/2 19/2 35598.09-0.01 11.36 0.05 21/2 21/2 0.87-0.01 15.32 0.03 2112 2312 0.00-0.08 2312 2112 3.98 0.01 5.34 0.04 2312 2312 6.18 0.02 2312 2512 5.34-0.03 30445.29 0.00 8.71 0.04 2512 2312 5.97 0.02 2512 2512 6.79-0.01 11.15 0.03 2.45 0.01 5.97-0.04 2512 2712 0.62-0.03 2712 2512 31908.04 0.02 2712 2712 1.51-0.01 0.29-0.02 2712 2912 0.62-0.09 0.00-0.06 7.25 0.03 13,,,, + 13,,,,2312 2112 27027.36 0.00 1.53 0.03 2312 2512 0.00-0.03 2.60-0.02 7.25-0.01 2512 2312 2512 2712 2.60-0.05 8.88 0.00 9.16 0.00 2112 2512 2.95 0.04 2712 2912 2.95 0.01 8.88-0.04 1.72-0.01 2912 2712 0.30 0.01 2.03 0.00 2912 2912 0.65-0.01 1.72-0.04 2912 3112 0.30-0.02
vorAv
912 l/2 27605.29 0.05 0.00 0.01 912 11/2 -2.09 -0.01 6,,+ 6,, 912 712 11/2 912 -2.09 -0.01 912 912 11/2 13/2 -2.09 0.04 912 11/2 13/2 11/2 11/2 9/2 -2.09 0.03 1312 1512 11/2 11/2 0.00-0.02 1512 1312 11/2 1312 0.00-0.02 15/2 1'7/2 13/2 11/2 1312 1312 624+ 52, 1312 1312 28220.40-0.05 1312 1512 -0.20 0.03 1312 1512 1512 1312 1.70-0.01 1512 1512 1512 1512 1.53 0.03 1512 1712 1512 1712 Io,+ 60, 1312 1312 31349.00 0.00 +-14,,,,2512 1512 1512 0.52-0.02 7,,+-7,, 11/2 11/2 33624.98 0.01 14,,,, 11/2 1312 -0.34 -0.01 2512 1712 1712 1.00 0.06 13/2 11/2 5.75 0.01 2712 1312 1312 6.08-0.03 2112 7 1,+ 61, 1312 1312 30442.00-0.03 1312 1512 5.75-0.04 2912 0.52-0.03 15/2 15/2 1512 1312 7.16 0.04 2912 1712 1112 1.01 0.02 15/2 15/2 7.48 0.00 2912
712 912 11/2 1312
5/2 912 27070.00 0.10 4,,+ 4,, -0.44 -0.06 l/2 11/2 0.29-0.09 912 1312 11/2 0.67-0.08 1512
Fla
6 LS+ 51, 11/Z 912 28935.30 0.04 5,,+ 5,, 1.00-0.09 1512 1312
606+ 50,
Transition F;
Microwavespectrum of CF,=14N35C1. Observedfrequencies and splittings of hyperfinecomponents of rotational transitions and deviations of calculated values (MHz)
TABLE1
2
1512 1512 1712 2112
921+ 91,
1712 1512 1712 19/2
17/2 19/2 21/2 2112 2312 2312 2512
1512 1312 1512 1512 1512 1712 1712 1512 1712 1712 1712 1912 1912 1712 1912 19/2 19/2 21/2 2112 1912 21/2 21/2 2112 2312
13/2 11/2 1312 1312 1312 1512 15/2 13/2 1512 1512 1512 1712 1712 1512 1712 1712 17/2 19/2 19/2 17/2 19/2 19/2 19/2 21/2
1512 17/2 1712 1712
38367.34-0.09 -0.30 0.08 0.00 0.11 0.37 0.08 lo,,+9,, 17/2 17/2 19/2 8?.1+72, 1712 1512 36691.02-0.03 1712 1912 2112 0.00 0.01 19/2 1712 1.06 0.07 2112 1912 2112 1.06 0.08 2312 2312
8 1'1 + 71, 1312 1512 1712 1912
34721.04-0.03 0.59 0.06 0.79 0.03
1312 35558.25 0.04 1512 -0.44 -0.03 1712 0.00 0.00 19/2 0.43 0.02
8 1s+ 71, 1512 1512 1712 1712 1912 1912
808+-70, 1312 1512 1712 19/2
33110.14 0.02 -0.98 -0.05 -0.98 4.05 -0.98 -0.05 0.00-0.02 0.00-0.02
0.38 0.04 0.83 0.00
72s+ 62, 11/2 11/2 1312 1312 1512 1312 1512 1712 17/2 15/2 1712 1912
1512 1312 1712 1712
615 1312 11/2 33673.77-0.02
32159.02-0.05 -1.52 0.05 8,,+-8,, -1.52 0.05 -1.21 0.10 -1.21 0.10 0.00 0.03 0.00 0.03
+
726+ 6,s 11/2 11/2 13/2 11/2 1312 1512 1512 1312 1512 1712 1712 1512 1712 1912
7 16
3112 2912 31/2 31/2 3112 3312
-0.01 0.74 0.00 0.29-0.04 0.29
15,,,, + 15,,,,2712 2512 32588.37 0.00 0.00-0.04 35594.87-0.03 2712 2912 3.09-0.02 0.42-0.02 2912 2712 3.09-0.05 0.00-0.06 2912 3112 3.45 0.04 5.73 0.06 3112 2912 4.01 0.10 6.15 0.07 3112 3112 3.45 0.01 5.73 0.01 3112 3312 0.30-0.01 6.68 0.04 3312 3112 0.81 0.00 7.07 0.01 3312 3312 0.30-0.03 6.68-0.01 3312 3512 1.10 0.03 1.52 0.02 16,,,,+16,,,, 29/2 27/2 36207.53 0.01 0.00-0.04 1.10-0.01 2912 3112 3.27-0.03 31j2 29/2 3.27-0.06 3112 3312 37816.324.02 3.61 0.01 0.48 0.01 3312 3112 4.22 0.07 0.00-0.06 3312 3312 3.61-0.02 5.25 0.05 3312 3512 0.29-0.01 3512 3312 5.72 0.06 5.25 0.00 0.85-0.01 3512 3512 0.29-0.04 6.04 0.03 3512 3712 6.49 0.02 6.04-0.02 ll,,+ ll,, 19/2 19/2 38746.41-0.05 0.88 0.02 0.55 0.07 2112 2112 1.35 0.02 2112 2312 1.02 0.13 0.88-0.02 2312 2312 0.55 0.01 2312 2512 1.02 0.07 30485.79 0.01 2512 2512 0.00-0.06 2512 2712 -0.56 -0.05 0.55 0.07 0.00-0.06 0.84-0.05 12,,+-12,,,, 2112 2312 37386.48-0.01 2312 2512 0.26-0.03 0.32-0.07 0.32-0.01 2512 2312 0.26-0.10 -0.26-0.07 2512 2712 0.26-0.07
7.16 0.00 0.79 0.00 1.19 0.02 0.79-0.04
%
Fta
vorAv
b dC
Transition F;
F’
220+ 11,
512 512 512 712
+-13,,,, 2712 2712 312 36352.28 0.01 14,,,, 512 2912 2712 -1.72--O.O2 2912 3112 712 -O.63 0.05 512 3112 2912 -12.99 0.12 3112 3112 515+-40, 912 912 28653.90 0.02 3112 3312 0.33-O.ll 912 712 912 1112 0.33 0.02 lo,,'lo,,,, 1712 1512 1312 1112 1712 1712 2.38-O.O7 1312 1312 1712 19/2 1.80-O.O5 1312 1512 1912 1712 2.38 0.04 1912 1912 616+ 50, 1112 912 32085.36 0.02 191.22112 1112 11/2 21/2 1912 -O.48 -O.O5 1112 1312 2112 2112 0.00 0.10 1312 1112 21/2 23/2 0.49-O.o7 1312 1512 2312 2112 0.49 0.02 1512 1312 2312 2312 1.77-+I.04
t lo,, 21/2 1912 826+-72, 1312 1512 38046.52 0.01 ll,,,, 1512 1712 2112 2112 -0.51 0.01 2112 2312 1712 19/Z -0.34-O.03 1912 2112 2312 2312 0.22-0.01 2312 2512 2512 2312 909+ 80, 1512 1712 39712.37-0.06 1712 1912 2512 2512 -0.34 0.06 1912 2112 2512 2712 0.00 0.07 0.35 0.02 2112 2312 13z,LL + 12,,,, 2512 2512 2712 2512 9 19+ 8,s 1512 1512 38979.59 0.04 1712 1712 2712 2912 -0.26-0.04 2912 2712 1912 1912 0.00-0.11 2112 2112 2912 3112 0.35-O.o5
Transition F;
TABLE 1 (continued) or Av d
Transition
F;
F’
27416.99 0.01 14,,,, + 14,,,,2512 2712 -O.39 0.03 2712 2712 -O.39 --O.Ol 2712 2912 -O.89 -O.O4 2912 2912 -O.89 -O.O7 2912 3112 3112 3312 34623.54 0.00 + 15,,,,2712 2512 -O.30 0.08 15,,,, -O.30 0.04 2712 2912 -1.10 0.00 2912 2712 -O.54 -O.Ol 2912 3112 -1.10 -O.o3 3112 2912 3112 3312 27010.25-0.01 3312 3112 0.82--O.OB 3312 3512 0.00-O.lO + 16,,,, 2912 2712 5.56 0.04 16,,,, 6.34-0.05 2912 3112 5.56-O.O4 3112 2912 6.34 0.02 3112 3312 7.18 0.00 3312 3312 6.34-0.05 3512 3512 0.82 0.02 + 17,,,, 3112 3112 1.75 0.06 17,,,,
36871.41 0.01 27/Z 2512 0.45-0.03 2712 2712 0.00-0.03 2712 2912 0.90 0.11 0.45 0.11 13mJ + 13,,,, 2312 2112 -0.50 -O.70 2312 2512 0.00-0.06 2912 2712 +I.50 -O.lO 2912 3112
v d
34246.46 0.00
34047.85-O.01 0.00 0.01 0.69-O.01 0.69 0.00 0.69 0.00 0.00 0.01
34351.95-O.02 0.00 0.01 0.48 0.00 0.48 0.01 0.48-O.O5 0.48-0.04 0.00-O.o5 0.00-O.o4
35076.95 0.05 0.00-O.lO 0.27-0.06 0.00-0.14 0.27-0.09 0.00-O.o4
36125.08 0.07 0.00 0.03 o.oo-O.03 0.00-O.Ol
0.00-0.07 -0.38 -0.05 0.00-0.04
uorAv
%
1312 1312 1512 1512 1512 1712 1712 1912 1912 1912
1312 1312 1712 1712 1712 19/2 1912
8 1* + 70,
8 08 +- 71,
11/2 1512 1512 1712 1912 17/2 2112
1512 11/2 1312 1512 1712 1512 1712 1712 1912 2112
11/2 11/2 1312 1312 1512 1712 1512 1712
1512 1712
with the upper
31448.77 0.00 0.00 -0.02 0.96 -0.01 1.24 0.02 0.96 -0.03 0.51 -0.01 0.51-0.03
38830.97 0.00 0.00 -0.07 -1.14 0.01 -1.45 0.02 -1.14 0.06 -0.79 0.03 -1.14 0.01 0.29 0.01 0.00 0.07 0.29 0.06
35458.57 0.00 -0.83 -0.06 -1.16 0.00 -0.48 -0.04 -0.83 0.00 -0.48 0.02 0.86 0.00 0.42 -0.01
1.28 -0.02 1.77 0.04 + ll,,,,
19/2 19/2 21;2 2112 2112 2312 2312 2312 2512 2512 2512
level in the rotational
ll,,,,
2312
transition
17/2 21/2 19;2 2112 2312 2112 2312 2512 2312 2512 2712
2512
Au = V’ -
+ 19,,,,
+ 18,,,,
3912 4312 4112 4512
4112 4112 4312 4312
0.95 0.95 0.95 0.00 0.00
0.00 -0.06 -0.06 -0.06 -0.07
0.00
v’ = frequency
rule
0.00 -0.01 0.00 -0.02
the selection
38430.35 0.00 -1.77 -1.77
-1.51-0.02 -1.51-0.02
0.00 -0.01
36397.84
35012.98 -0.03 1.22 0.00 1.22 0.01 1.22 0.00 0.00 0.00 0.00 0.00
of first component,
from
3712 4112 3912 4312
3312 3512 3512 3912 3712 4112
3312 3512 3712 3712 3912 3912 3912 3912 4112 4112
3312 3312 3712 3512 3912
3312 3512 3512 3712 3712
Fy and F” are obtained
203,,, + 202,,*
19,,,,
18,,,
v, v = frequency
J’dadc + J?;&;.
0.02 -0.09 0.02 -0.12 -0.06 0.03 -0.04 -0.04 0.00 -0.05 -0.07
0.82 -0.06 31147.89 0.00 5.46 6.19 5.46 6.19 6.99 6.19 0.72 1.56 0.72
AF, = AF = AJ. bFrequency v given for first component in each multiplet. of any other component. Cd = v&S - vcalc or Avobs -AU&.
aF; and F’ are associated
11/2 1312 1312 1512 1512 1512 1712 1712
7 1, +- 60,
15/2 1512
2
7 2s
*
624
7 26 +- 62,
+
11/2 1312
11/2 1312
1912
1712
1512 13/2 1712 19/2
1312 1712
1312 11/2 1512 1712
615
7 16
1512 1312 1712 1912
1912 13/2 17/2
1112 13/2 1512 1712
1312 11/2 15/2 1712
F’a
11/2 1512
1312 11/2 1512 1712
+-
61,
7 07
7 ,, +
912 11/2 1312 1512
6 24 + 52,
1712 11/2 1512
11/2 912 13/2 15/2
6 15 + 51,
60,
F;
Transition
b
dC
-0.04 0.08 -0.03
32200.93 -0.73
-0.06 0.00
31341.48 0.02 -0.94 0.01 0.00 -0.03
32788.60 0.01 0.29 -0.07 0.29 0.01 0.64 -0.02
29713:75 0.00 0.40 0.08 0.40 -0.04 0.79 0.00
30609.54 -0.36 -0.36
27455.54 -0.01 -1.34 0.00 -1.34 -0.02 0.00 -0.05
frequencies
1712
1712
9,, + 8,,
8,, + 8,,
7,, + 7,,
1512 1712 1712 1712 19/2 19/2
1312 1312 1312 1512 1712 1512 1712 1912 17/2 1912
11/2 13/2 1512 1312
11/2 912 1312 1512 1912
2112 1712 19/2
1912 1512 1712
8,, + 7,,
11/2 11/2 1112 1712 1712
11/2 912 1312 1312 15/2 1512 17/2
912 912 1112 1312 1312 1512 1512
+- 5,,
6,,
F’
-0.03
4.62 0.05 5.28 0.05 5.60 -0.02 5.28 0.01 0.93 0.04 1.31 0.01
35323.22 0.02 0.40 -0.01 0.00 -0.06 4.62 0.09
33455.00 -0.06 -0.39 -0.03 -6.39 -0.09 0.63 0.08 0.63 0.05
35311.35
30404.59 -0.05 0.00 0.09 0.32 -0.07
31641.70 0.05 -0.49 -0.12 -0.93 0.04 -0.93 -0.04 -0.49 0.03 0.00 -0.06 0.51 0.01
d
of hyperfine
vorAv
and splittings
F;
Transition
Observed
28168.46 0.01 0.49 -0.06 0.49 0.10 0.81 0.00
vorAv
Microwave spectrum of CF,=‘4N37Cl. ations of calculated values (MHz)
TABLE
12,,
ll,,
17,,,,
16,,,,
+- 12,,,,
+ ll,,
+ 17,,,,
1912 2112 2112 2312 2312 2512
1912 1912 2112 2112 2312 2312
2312 2512
2512 2712
2512 2112
2912 3112 3312 3512 3912
3112 3112 3112 3712 3712
2512 2512
2712 2912 3112 2912 3112 3312 3112 3512 3312 3512 3712
F’
0.02 -0.01 -0.04 -0.03 -0.06
38120.85 -0.08 0.00 -0.02
0.00 -0.04 0.45 -0.03
39457.89 0.07 0.45 0.02 0.45 0.09 0.79 0.01 0.45 0.05 0.79 -0.04
38464.06 0.56 0.00 0.20 0.20
-0.03 0.01 -0.03 -0.01 0.06 -0.04 0.00 -0.03 0.00 0.02 -0.03
d
and devi-
34666.98 0.54 0.00 2.52 3.11 2.52 2.76 2.76 0.23 0.78 0.23
vorAv
transitions
2912 2912 2912 3112 3112 3112 3312 3312 3512 3512 3512
F;
of rotational
+ 16,,,,
Transition
components
?z
37368.81 0.05 0.23-0.08 0.23 0.01 0.49-0.05
36993.82 0.02 -0.41 0.00 -0.41 -0.02 0.00-0.03
38801.89 0.04 0.26-0.05
28280.59 0.00 0.35-0.09 0.35 0.04 1.11-0.10 0.69-0.04 1.11 0.01 1.97-0.07 1.34-0.09 1.97 0.04
1712 1512 19/2 2112
8I,+ 71, 1512 1312 17/2 1912
826+ 72, 1312 1312 1512 1512 1712 1912 19/2 21/2
909+ 80, 1912 2112 2112 2312
515+ 40,
a*bvCForexplanation offootnotesseeTable 1.
912 912 912 712 912 11/2 11/2 9/2 11/2 11/2 11/2 13/2 1312 11/2 1312 1312 1312 1512
0.01
33894.97
8 ,Ic 71, 15/2 17/2
1912 2112
0.93 0.00 13,,,, + 13,,,, 2512 2712 2712 2912 1512 1312 37431.16 0.08 1512 1712 0.00-0.05 14,,,, + 14,,,, 2712 2912 2912 3112 4.11-0.02 1712 19/2 4.73 0.00 1912 1712 5.16 0.00 16,<,, + 16,,,,2912 3112 19/2 19/2 3112 3312 4.73-0.04 1912 2112 2112 1912 3312 3512 0.67 0.00 3512 3712 1.13 0.01 2112 2112 0.67-0.04 2112 2312 17%1,+ 17,,,,3112 3312 3312 3512 +14,,,,2512 2312 28611.44-0.02 14,,,, 3512 3712 0.00-0.03 2512 2712 3712 3912 2.21 0.04 2712 2512 2.21 0.01 2712 2912 + 18,,,,3312 3512 2.39-0.01 18,,,, 2912 2712 3512 3712 2.39-0.03 2912 3112 3712 3912 0.22-0.01 3112 2912 3912 4112 0.64 0.01 3112 3112 0.22-0.03 3112 3312 + 19,,,, 3512 3712 19,,,, 3712 3912 + 15,,,, 2712 2512 31370.94-0.03 15,,,, 3912 4112 0.48 0.01 2712 2712 4112 4312 0.00-0.03 2712 2912 2.37 0.01 2912 2712 + 20,,,,3712 3912 2.86 0.03 20,,,, 2912 2912 2.37-0.02 2912 3112 3912 4112 4112 4312 2.59 0.00 3112 2912 4312 4512 3.14 0.08 3112 3112 2.59-0.03 3112 3312 0.23 0.00 3312 3112 0.70 0.00 3312 3312 0.23-0.03 3312 3512
-0.73 0.05 0.64-0.02 34732.62*.02 9,,+ 9,, -0.27 0.05 0.00 0.00 0.36 0.03
1512 1712 17/2 19/2 801+ 70, 1312 1512 1512 1712 17/2 1912 19/2 2112
37414.80-0.01 1.26 0.01 1.26-a.09 0.00-0.09
35792.62 0.01 1.05 0.01 1.05-0.07 0.00-9.08
34771.80 0.02 0.83 0.01 0.83-0.06 0.00-0.07
34314.14 0.02 0.61-0.01 0.61-0.06 0.00~a.05
34366.83 0.02 0.44 0.01 0.44-0.03 0.00-0.04
35718.33-0.04 0.00-0.02
36839.11-0.03 o.oo- 0.02
50 TABLE 3 Rotational (MHz), centrifugal distortion constants in N-chlorodifluoromethanimine
A B c
(kHz),
CF,=14N35C1
CF,=‘4N3’C1
11260.86058(l) 2542.205431(l)
11260.8548(14) 2470.93487(33)
2072.428539(l)
2024.81508(32)
*J
0.377273(2)
O-4206( 27)
and nuclear quadrupole
CF,==‘4N3SCl X2 X2; -Xc&
-71.3445(62) -25.7144(18)
lX;jl
52.42(13) 22.8150(35)’
coupling
(MHz)
CF,=‘4NJ7Cl -56.06(13) -20.237(31) 40.8( 16) 17.913(71)a
*JK
3.96432( 2)
3.932(11)
*K
5.5716(4)
5.5716f
XOb Cl _ Cl - Xcc XEI’ X22
6J
0.0694046(4)
0.06715(18)
Cl XXX
46.193(95)a
2.57222(3)
2.160(16)
eb
24.04(3)a
23.9(6)a
3.84(28)
0.029(17)a
-2.920(57)
nc ad
0.0247( 10)a
XZb - Xz
3.552(90) -2.958(31)
0.0467
0.0458
";b
-3.255(48)a
-3.38(
ne
323
157
X cc
-0.297(47y
-0.46(13)a
“N” XM
15y
48.5295(29)= -94.723(95)a
38.150(65)a -74.2(12)a 36.0( 12)a
aDependent parameter. b 0 - angle between the z and the a axis in degrees. ‘11 = (Xxx -X,,)/ X **. do = Standard deviation of least squares fit in MHz. en = Number of data points (frequencies and splittings). fFixed at value of other isotope.
principal axis of the quadrupole coupling tensor of 35C1should be along the NC1 bond, the angle 8 should be only 20.4”) as obtained from the initial value for xab (see previous section). Therefore, we sought to reduce the discrepancy between the values for f3 by changing the initial parameters in the molecular structure calculations. It was found that 0 is reduced upon increasing the CNCI angle significantly from the initial value of 107.9”. The increase in the CNCl angle requires a significant decrease in the NC1 and CN distances from the original values of 1.74 and 1.274 A, respectively. However, it was very difficult to determine a satisfactory structure, where the NC1 distance is at least 1.70 8, by the trial and error method. For this reason, we determined the fully optimized structure by ab initio SCF calculations at the 3-21G* level with the program GAUSSIAN 82 [26]. The results are listed in Table 4. The angle 0 for this structure is 26.5”. In order to assess the reliability of the ab initio results for CF,=NCl, we performed the same 3-21G* calculations on ClN=C=O for which the complete r, structure has been determined by Hocking et al. [3]. Except for the CO distance and the NC0 angle, the fully optimized structural parameters were not even close to the r, values. Therefore, the ab initio results were not used as initial values for the diagnostic least squares refinement, in contrast to our earlier intentions. After numerous calculations, we decided to vary only the well determined parameters (CF, distance and CNCl and NCF, angles) while keeping the other parameters constant at assumed values. A value of 1.700 8, was chosen for the NC1 distance. For comparison, the corresponding r, distance [3] in
2470.935 2024.815
c
11260.855
A B
C
11260.861 2542.206 2072.429
Obs.
fixed fixed fixed 1.299(34) 116.4(14) 119.5(34) fixed 27.15
2025.580
2469.927
11259.317
11259.561 2541.377 2073.395
Calc.
1.274 1.300 1.300 (107.9) 119.8 127.2
rk ’
CF =NFC
-0.765
1.008
1.538
1.300 0.829 -0.966
Obs.-Calc.
(1.690) (1.310) 1.323 1.326 (123.3) 126.0 124.2 25.8(11p
CF,==CHCld r.
118.83(5)
1.705(5) 1.226(5)
ClN=C=@ rS
1.683(10) 1.266(5) 1.718(6)$ 1.718(6)? 117.11(40) 118.68(50)? 127.52(40)$
CC1,=NClf rk
aDistances r in A, angles in degrees. bTwo linear combinations of three variable parameters were adjusted; the calculated rotational constants were obtained from the final structural parameters. ‘Ref. 15. The value in parentheses is for the C=NF angle. dRef. 24. The values in parentheses are for the C-Cl and C=C distances and for the C=CCl angle. eRef. 3. fRef. 25. The values indicated with a dagger (t) are for carbon--chlorine parameters rather than carbon-fluorine parameters. % = Angle between the “a” principal inertial axis and the NC1 bond hAngle between the Q inertial axis and z principal quadrupole axis.
CF zN”CI
A B
CF,N=Cl
1.700 1.254 1.300 1.300(50) 115.0(50) 118.8(50) 126.2
(MHz)
1.704 1.231 1.318 1.326 119.39 122.45 127.51 26.5
Diagnostic least squaresb (initial) (final)
Rotational constantsb
r(C-F,) r(C-F,) L( CNCl) L(NCFt 1 UNCF,) es
r( C=N)
r(N-Cl)
Ab initio 3-21G*
CF,=NCl
Molecular structures of CF,NCl and related molecules?
TABLE 4
52
ClNCO (1.705 A) and the rt Cl-N distance [25] in Cl&NC1 are at the lower end of the range of values reported in the literature. A value of 1.254 A was assumed for the CN distance. It is about halfway between the value obtained by electron diffraction in CFzNF [15] (1.274 a) and the ab initio value (1.2312 A). The values for both CF distances (1.300 a) were transferred from the structure of CFzNF [15]. The NCF angles reported in ref. 15 were reduced by 1” each. With these assumptions and modifications, a diagnostic least squares refinement with an initial value of 115” for the CNCl angle yielded a good fit of the observed rotational constants and a set of acceptable parameters. The results are shown in Table 4. The final values for the varied structural parameters are 1.299 A for the CF, distance, and 116.4” and 119.5” for the CNCl and NCFt angles, respectively. For this structure, 0 is 27.15”. For comparison, the structures for ClNCO (rs) [3], CF2NF (rz) [15], and CF,=CHCI (ro) [24] are also given in Table 4. For the last of these molecules, which is very similar to CF,=NCl, Stone and Flygare [24] obtained an angle 0 of 25.8” from the off-diagonal element of the quadrupole coupling tensor, xab, of the 35C1isotope. DISCUSSION
The hyperfine structure due to the presence of the Cl nucleus could be resolved fully for most b-type transitions. This was not possible for the atype transitions. The splitting due to the interaction with the 14N quadrupole nucleus was resolvable only partially in some b-type transitions. All three components due to this nucleus were only resolved for the F1 = 5/2 component of the 220 +- 111 transition in the spectrum of CF2=14 N3%1. The other components of this weak transition were unfortunately blended with other absorptions. In many cases, two or more components of the hyperfine pattern had to be assigned to the same absorption band. Many times, nearby components were barely resolved, rendering a precise determination of the exact line positions difficult or even impossible. As a consequence, the precision of the line positions is estimated to be between 0.1 and 0.05 MHz. Whenever lines with a calculated separation of up to 0.15 MHz had to be assigned to the same frequency, a deviation larger than the estimated precision had to be expected for at least one of these lines. In view of these facts, the standard deviations of 0.047 and 0.046 MHz achieved for CF,=N35C1 and CF,=N37C1, respectively, are quite acceptable. The standard errors of the rotational and centrifugal distortion constants, particularly in the case of CF2=14N35C1,are surprisingly low. In the original version of our computer program, the absorption frequencies of all assigned hyperfine components were fitted directly. In this case, the standard errors were even lower. In order to change the weights of the data points, the presently used scheme was introduced whereby only the frequency of one arbitrarily chosen hyperfine component was fitted directly. For the other components, the difference to the directly fitted frequency were taken as
53
data points to be fitted. This resulted in an increase of up to 30% in the standard errors of rotational and centrifugal distortion constants accompanied by a decrease of similar magnitude in the standard errors of the quadrupole coupling constants. Still, the standard errors look very good. Strong dependences of splittings on rotational and centrifugal distortion constants and/or accidental near degeneracies of hyperfine levels with different rotational quantum numbers seem to be the only possible reasons. Jemson et al. [27] have recently exploited these opportunities in the case of BrNCO and INCO to determine the constants A and Ax accurately from only u-type transitions. The agreement of the centrifugal distortion constants between the isotopes is not very good. This was particularly true for Ax. Therefore, for CFzN3’C1, this parameter was fixed at the value obtained for the 35C1isotope. Since the approximate planarity relations between the centrifugal distortion constants in the I’ representation A reduction [19] does not involve Ax, we suspect that the much smaller data set for CFz=N37C1 might not be sufficient for an accurate determination of Ax in this isotope. The planarity relation is definitely not fulfilled for the data of the 35C1 isotopic modification. The diagonal quadrupole coupling constants with respect to the principal inertial axes are reasonably well determined. The constants for 14N for both isotopic species are equal within the error limits. The ratios of corresponding constants for the “‘Cl and 37C1nuclei vary between 1.272 and 1.274, close to the ratio [20] of the nuclear quadrupole moments, 1.26878. From the practical identity of the A rotational constants in both isotopes, it follows that the a principal axis passes through the chlorine nucleus in both isotopes. Therefore, the principal inertial axes systems in both molecules are parallel to each other. It is for this reason, that the ratios of all quadrupole coupling constants with respect to the principal inertial axes agree so well with the ratio of the nuclear quadrupole moments. The main point of discussion concerns the off-diagonal element j&b for the chlorine nuclei. This constant has not been reported very often in the literature because its effects are so small. It is not accessible through first order perturbation calculations. Only higher order treatments give access to it and these are usually not carried out for molecules containing chlorine. The CF&lz [28] and CF2= CHCl [24] molecules are examples where Xab has been determined for a “Cl nucleus. The initial results for $2 in both molecules were in good agreement with each other, the ratio xii /xi; = 1.258 being close to the ratio of the nuclear quadrupole moments. This result is expected because the principal inertial axes are parallel in the two molecules. When the discrepancy between the values for the angle 0 as obtained from initial values of Xab and from the initial molecular structure calculations was noted, three possible reasons came to mind. (1) The quadrupole coupling constants x:2 are significantly larger than the values obtained from the initial analysis of the hyperfine structure in the spectrum. (2) The molecular structure is compatible with the observed rotational constants but otherwise
54
inappropriate. (3) The model which assumes that one principal axis of the quadrupole coupling tensor coincides with the straight line connecting the chlorine and nitrogen nuclei (direction of NC1 u-bond) is not adequate. The first point was addressed in the re-analysis of the hyperfine patterns in the section “analysis of hyperfine structure”. As described there, it was possible to find values for $5 and xii which were not only in the correct ratio (1.284) but also led to an angle 13 of about 24”. The reassignments made were of two types. The first type comprises about six cases of u-type transitions in the spectrum of the “Cl isotope where the assignment of the F quantum number was changed while maintaining F1. The second type involved a redefinition of the “arbitrarily chosen” hyperfine component which was fitted directly and served as a reference point in the calculation of the splittings. This was done in a few cases for both isotopic molecules where it was observed that a relatively large deviation for the directly fit component obtained a large weight because it contributed again to the residuals of the splittings. This “redefinition” is merely a question of different weights. It was found that this weighting scheme influences not only the resulting value for &b but also some of the centrifugal distortion constants such as AK and 6K. We have not yet developed a weighting scheme yielding believable standard errors on rotational and centrifugal distortion constants which puts equal weights on all hyperfine components of a rotational transition. The second possible reason for the discrepancy in the values for 0, an inappropriate molecular structure, has also been addressed (see section on “molecular structure”). A further increase of the CNCl angle in the structure calculations would easily reduce the angle 8 to values near 25”. However, the most likely necessary reduction of the NC1 and CN distances to values below those assumed, seems to be unacceptable at this point. The possibility cannot be ruled out that further modifications in the structural parameters of the CF2 group allow an opening of the CNCl angle without a considerable shortening of the CN and NC1 distances. However, reliable data about the CF2 moiety are scarce and difficult to obtain. For these reasons, we propose for CF2=NC1 the structure obtained by the diagnostic least squares method and listed in Table 4 since this structure reproduces the rotational constants. Additionally, the value for the NC1 distance and the CNCl angle are in good agreement with the r, values [3] for ClNCO (Table 4) as well as the rz values [25] for Cl,CNCl. Also the angle 8 of 27.2’is in acceptable agreement with the value obtained from the diagonalization of the quadrupole coupling tensors, 24.0”, considering that X&, might be even somewhat larger than the value obtained by our analysis. We consider it unlikely that it is as large as 65 MHz which would bring 0 up to 27”. Stone and Flygare [24] reported an angle of 25.8 rt 1.1” for 0 in the very similar molecule CF,=CHCl, also derived from ~2;. The third possibility for the discrepancy in the values for 0, i.e., a small deviation of the z axis of the quadrupole coupling tensor from the straight line connecting the nitrogen and chlorine nuclei, cannot be ruled out.
55
Conclusions about ionic and double bond character of the NC1 bond are premature as long as the structural problems remain unsolved. We expect that the analysis of the microwave spectrum of both chlorine isotopic modifications of CF2--lSNC1 would provide some definite answers for two reasons. Substitution of the nitrogen nucleus would be extremely valuable for the determination of the NC1 distance. The lack of a nuclear quadrupole moment in the 15N nucleus simplifies the hyperfine structure considerably so that a more accurate determination of &$b for both chlorine atoms becomes feasible. Therefore, a microwave investigation of this isotopic species would probably lead to definitive structural parameters for this molecule. ACKNOWLEDGEMENT
The authors gratefully acknowledge the financial support of this study from the National Science Foundation by Grant CHE-83-11279. REFERENCES 1 G. Cazzoli, C. Degli Esposti, P. Palmieri and S. Simeone, J. Mol. Spectrosc., 97 (1983) 165. 2 D. J. Millen and K. M. Sinnott, J. Chem. Sot., (1958) 350. 3 W. H. Hocking, M. L. Williams and M. C. L. Gerry, J. Mol. Spectrosc., 58 (1975) 250. 4 L. Pauling, The Nature of the Chemical Bond, 3rd edn., Cornell University Press, Ithaca, New York, 1960, p. 229. 5 G. Cazzoli, D. G. Lister and P. G. Favero, J. Mol. Spectrosc., 42 (1972) 286. 6 W. Caminati, R. Cervellati and A. M. Mirri, J. Mol. Spectrosc., 51 (1974) 288. 7 H. B. Biirgi, D. Stedman and L. S. Bartell, J. Mol. Struct., 10 (1971) 31. 8 J. R. Durig, K. K. Chatterjee, N. E. Lindsay and P. Groner, J. Am. Chem. Sot., 108 (1986) 6903. 9 S. L. Miller, L. C. Aamodt, G. Dousmanis, C. H. Townes and J. Kraitchman, J. Chem. Phys., 20 (1952) 1112. 10 R. H. Schwendeman and J. D. Kelly, J. Chem. Phys., 42 (1965) 1132. 11 R. H. Schwendeman and G. D. Jacobs, J. Chem. Phys., 36 (1962) 1245. 12 D. J. Millen and J. Pannell, J. Chem. Sot., (1961) 1322 13 A. M. Mirri and W. Caminati, J. Mol. Spectrosc., 47 (1973) 204. 14 Y. Y. Zheng and D. D. DesMarteau, J. Org. Chem., 48 (1983) 4844. 15 D. Christen, H. Oberhammer, R. M. Hammaker, S. C. Chang and D. D. DesMarteau, J. Am. Chem. Sot., 104 (1982) 6186. 16 E. P. Wigner, Group Theory and Its Application to the Quantum Mechanics of Atomic Spectra, Academic Press, New York, 1959. 17 A. R. Edmonds, Angular Momentum in Quantum Mechanics, Princeton University Press, Princeton, NJ, 1957. 18 H. P. Benz, A. Bauder and Hs. H. Giinthard, J. Mol. Spectrosc., 21 (1966) 156. 19 J. K. G. Watson, in J. R. Durig (Ed.), Vibrational Spectra and Structure, Vol. 6, Elsevier, Amsterdam, 197 7. 20 W. Gordy and R. L. Cook, Microwave Molecular Spectra, Vol. 9, Part II of A. Weissberger, Technique of Organic Chemistry, Interscience, New York, 1970. 21 D. Christen, J. Mol. Struct., 79 (1982) 221. 22 R. F. Curl, J. Comp. Phys., 6 (1970) 367.
56 23 24 25 26
P. Groner, J. S. Church, Y. S. Li and J. R. Durig, J. Chem. Phys., 82 (1985) 3894. R. G. Stone and W. H. Flygare, J. Chem. Phys., 49 (1968) 1943. D. Christen and K. Kalcher, J. Mol. Struct., 97 (1983) 143. J. S. Binkley, M. J, Frisch, D. J. DeFrees, K. Raghavachari, R. A. Whiteside, H. B. Schelgel, E. M. Fluder and J. A. Pople, Gaussian 82, Carnegie Mellon University, Pittsburgh, PA, 1984. 27 H. M. Jemson, W. Lewis-Bevan, N. P. C. Westwood and M. C. L. Gerry, J. Mol. Spectrosc., 118 (1986) 481; 119 (1986) 22. 28 C. F. Su and E. L. Beeson, J. Chem. Phys., 66 (1977) 330.
in The Netherlands
MICROWAVE SPECTRUM, QUADRUPOLE COUPLING CONSTANTS, AND STRUCTURE OF N-CHLORODIFLUOROMETHANIMINE
P. GRONER, H. NANAIE and J. R. DURIG Department
of Chemistry,
University of South Carolina, Columbia, SC 29208
(U.S.A.)
(Received 13 January 1987)
ABSTRACT The microwave spectra of N-chlorodifluoromethanimine (chlorocarbonimidic difluoride), CF,N35C1 and CF,N37C1, have been investigated from 18 to 39.5 GHz. The hyperfine structure due to the quadrupole coupling with the chlorine and 14N nuclei has been analyzed for a- and b-type transitions of both isotopic species. From experimental diagonal and off-diagonal quadrupole coupling constants, principal coupling constants of -94.7(l) and -74(l) MHz were calculated for the 35C1and 3’C1 isotopes. A molecular structure compatible with rotational and quadrupole coupling constants has been determined with r(N-Cl) and r(C=N) fixed at 1.700 and 1.254 A, respectively, and L(CNC1) = 116.4(14)0 with reasonable assumptions for the other structural parameters. The strong correlations among all structural parameters are discussed and the determined parameters are compared to the corresponding parameters of similar molecules. INTRODUCTION
The chlorine-nitrogen bond distance has been determined only for a small number of molecules in the gas phase. The reported values display rather large variations. The N-Cl bond distances of 1.9734 + 0.0017 a, 1.840 f 0.002 A, and 1.705 + 0.005 A for nitrosyl chloride [l] (ClNO), nitryl chloride [2] (ClNO*), and chlorine isocyanate [ 31 (ClNCO), respectively, provide excellent examples. Although the first two values are larger than the sum of the covalent radii of 1.73 _i$for nitrogen and chlorine [4], the third value is smaller than this sum. Nevertheless, several chlorine-nitrogen bond distances have been determined which are only slightly larger than the sum of these radii, i.e., 1.7480 + 0.0001 A for chloroamine, C1NH2 [5], 1.750 + 0.003 A for N-chloromethanamine, C1(CH3)NH [6], and 1.759 f 0.002 A for trichloroamine, NC13 [7]. Since the substitution of a methyl group in N-chloromethanamine for a hydrogen atom in chloroamine had essentially no effect on the N-Cl bond distance, we recently determined the N-Cl bond distance in N-chloro-N-methyl methanamine, (CH&NCl, from an analysis of its microwave spectrum [B] . The resulting value of 1.749 + 0.001 a shows rather conclusively that this distance does not change if a hydrogen atom in chloroamine is replaced by a methyl group. This result is somewhat surprising since the comparison of the carbon-chlorine 0022-2860/87/$03.50
o 1987 Elsevier Science Publishers B.V.
38
bond distances in the corresponding carbon analogues shows that such a bond is elongated by 0.007 8, with the substitution of a hydrogen atom by a methyl group [9-111. Because of the limited amount of structural data available on chlorine-nitrogen bonds, it seems it is not possible to predict N-Cl distances confidently at this time and further investigations are necessaryAs might be expected from the variations in the N-Cl bond lengths, large differences are also reported for the chlorine quadrupole coupling constants in these compounds. In nitrosyl chloride [12] and nitryl chloride [2], the 35C1 coupling constants have values of -57.20 and -94.28 MHz, respectively, whereas for N-chloromethanamine a value of approximately -100 MHz is estimated from the microwave data [6, 131. For N-chloro-N-methyl methanamine we recently reported quadrupole coupling constants of -91.50 MHz for 35C1and -72.12 MHz for 37C1[8]. In order to provide additional information about NC1 bonds, we have investigated the microwave spectrum of iV-chlorodifluoromethanimine (chloro-carbonimidic difluoride), CF*NCl. The results of this investigation and the conclusions to be drawn about the molecular structure and the quadrupole coupling constants of the 3sC1, 37C1and 14Nnuclei are reported herein. EXPERIMENTAL
The CFzNCl sample was prepared by the method of Zheng and DesMarteau [14] in which ClCN was vacuum transferred to a stainless steel reaction vessel maintained at -196°C followed by the addition of ClF. The reaction vessel was slowly warmed to 22°C and maintained at this temperature for 20 h. The contents of the reaction vessel were separated through a -78°C trap where the CICFzNClz sample was collected. The sample of CICFzNClz was placed in a Pyrex vessel and heated for 5 h at 135°C. The contents were then transferred on to Hg and allowed to stand for 20 mm at 22°C; the volatiles were then separated through three traps at -95, -125, and -196°C. The CFINCl was collected in the -125°C trap and then further purified using a low temperature vacuum fractionating column. The purified sample was stored at Dry Ice temperature. The microwave spectrum of N-chlorodifluoromethanimine was recorded in the region between 18 and 39.5 GHz using a Hewlett-Packard model 8460A MRR spectrometer with 33.3 kHz Stark modulation. In order to reduce thermal decomposition of the sample and increase the population in the low rotational and vibrational states, the Stark cell was cooled with a packing of Dry Ice. Because CFzNCl decomposed quickly in the unconditioned cell, resampling was necessary initially every 10 to 15 min. Later on, this period could be extended to approximately 2 h. The pressure of the sample in the Stark cell was maintained between 5 and 20 mTorr. Accurate frequencies were measured for transitions in the R-band region (26.539.5 MHz). The accuracy of the measured frequencies is estimated to be
39
between 0.1 and 0.05 MHz, depending on the degree of resolution achievable for incompletely resolved bands. RESULTS
Initial assignments of the microwave spectrum The microwave spectrum of CF2=NC1 was predicted with rotational constants from a structure very similar to the one reported [15] for CF2=NF, but using an N-Cl distance of 1.75 A. Both a- and b-type R-transitions were predicted in addition to several b-type Q-transition series. The survey spectrum in the region between 18 and 40 GHz showed many very strong bands. They were soon identified to be the predicted b-type Q-transitions of CF2=N3’C1. This was followed by the assignment of the same series for the molecule containing 3’Cl . It took a little longer to identify the first b-type R- and later also a-type R-transitions successfully for both isotopic modifications. Under increased resolution, most bands showed a complicated hyperfine splitting pattern due to the interaction of the rotating molecular electric field gradients with the nuclear quadrupole moment of the Cl and 14Nnuclei. Using average frequencies of the prominent components, approximate rotational and centrifugal distortion constants were obtained from a least squares fit. The very small inertial defects, A = 0.184 amu A2 for both isotopes, confirmed that the molecule is planar. Theory of the hyperfine structure The operator for the energy of a rotating molecule with two nuclei possessing a nuclear quadrupole moment is set up as I I I . H=H, +H,, +HQ2. fiR is the operator of an asymmetric rotor with centrifugal distortion. The operator goj containing the coupling of the rotational angular momentum J with the spins, Ii, of the quadrupole nuclei are written in terms of the scalar product of two irreducible tensors of rank 2, fioj = 4 ;
(-1)/” I$,? Q(,2?_fi.
Q!2’ represents the tensor of the nuclear quadrupole moment of nucleus j multiplied by the nuclear charge, and q2) is the negative tensor of the electric field gradients, produced by the other nuclei and all electrons, at the location of nucleus j. For a rotating molecule, the components of q2) depend on the orientation of the molecule and therefore on the angular momentum J, whereas Qj(2) depends only on the spin lj of nucleus j. Because of the coupling, J is no longer a good quantum number. The following coupling scheme is used to generate the total angular momentum quantum number F
40 s+zi
IJ--III
=i&,
“F*f i; = i?,
IFI ---I2 I < F < F1 + 12.
The matrix elements of fi were calculated in a basis corresponding to this coupling scheme. The basis functions are defined according to Wigner [16]
W(FI,12)F3 F1 (J,I,)I,Kj
=
F
1 I (-l)F1 13’ 23
- ‘1 +F, (2F+
1)1’2 (;I
; 13
_; 23
) 3
The normalized rotation group representation coefficients [16] 19 $& and ggI are used as basis functions (JKMI and (113 I, respectively. F3, F13, I13, 123, ‘and A4 are the components of F, F1, II, 12, and J, respectively, along the space fixed g3 axis. The matrix elements of fro, + &02 in this basis are derived by using repeatedly the definition of the reduced matrix element (double bar element) of a tensor operator T [ 161 --
(JM@”
1J M) = (-l)J-w
+M (27+
The result is (F(F,,I,)F,
+ (-l)F+l~
The 3 -j (
(-1)’
(-1)‘1
[t&F,
Jo
-I,
and 6 -j
jI
j2
j3
ml
m2
m3 >
ICI&I [(ZJ+
+
&42
1)(2Y+
IW’,
7
(
M p -iGi >
-F1 (J,I,)I,
= ~FF~F,F~
x
1)“2
(JII+”
IIF>.
--,12)F3
1)/24]1’2
F,
(i
(JAI2z) i_,
_g)
+Fl
-J-J[(2F,
+ 1)(2F,
+ 1)]1’2
symbols and
{ ;:
;I
f)
,
respectively, are used as defined by Edmonds [17] . The elements irreducible quadrupole coupling tensors, xpi, are abbreviations for x;$, = (2/3)“”
I$; CIjljIQ/,? IriIj)
of the
41
They are related to the corresponding
Cartesian tensors by
xi’“; = (3/2)1’* Xj,zz Xj;Z,‘, = T Xj,x.z + iXj,yz Yjz,), = (1/2)(Xj,,,
-
under the assumption Xj,zz + Xj.xx
+ Xj,yy
Xj,yy
)T
iXj, xy
that = O*
As a consequence, the general matrix element derived above is a complex quantity. It becomes, except for a numerical factor involving the definitions of the x’s, identical to the expression given by Benz et al. [18] for the case of a single quadrupole nucleus, if either I1 and all xi’: or 1, and all x$?L are set equal to zero.
Analysis of hyperfine
structure
The molecule CF2=NC1 has already been shown to be planar, the molecular plane being a symmetry plane. In this case, the axes may be chosen such that the x(2) tensors and all matrix elements become real. For the I’ representation Af the principal inertial axes, the components of XI*)become xi’*; = (3/2)“*
xaa
XV) I,+_1= T Xab xj:!_*
=
W2)(Xbb
-xc,)
The centrifugal distortion constants are defined according asymmetric reduction in the r representation [ 191. The energy levels are obtained from the matrix elements ization. The dimension, d, of the energy matrix increases with increasing quantum number F, as
to
Watson’s
by diagonalvery rapidly
d = (2F + 1)(21, + 1)(21* + 1). In the case of CF,=NCl,
we have I1 = 3/2, I2 = 1 and
d = 12(2F + 1). For this reason, all matrix elements with F1 # F1 are from the expression for the matrix element, that only ing I2 and x$*) are neglected, whereas the contributions are still completely included in this approximation. diagonalized now have the dimensions
neglected. It is seen contributions involvoriginating from I1 The matrices to be
d = (211 + 1)(2F1 + 1) = 4(2F1 + 1). The effects of the quadrupole coupling become smaller and smaller as J increases [20]. Therefore, all matrix elements off diagonal in J are neglec-
42
ted if J exceeds 9. Transition frequencies approximate selection rules [ 201
were calculated
according
to the
AF=AF,-AJ where AJ= 0,l. The first step in the analysis of the hyperfine structure of the observed transitions was the calculation of a spectrum using the approximate rotational and centrifugal distortion constants. The quadrupole coupling constants of the 14N nucleus reported for CF2=NF [21] were used directly as initial values. Approximations to the corresponding constants of the 35C1 nucleus were found by trial and error by comparing the magnitude of calculated and observed splittings. Better values were then obtained by a least squares fit of the frequencies and splittings of the J3,J_ 3 + J2, J_-l transitions. During this procedure, A, B, (2 and xaa and Xbb - xcc of 35C1 were allowed to vary. Gradually, more and more transitions and individual components were included in the fitting process. The additional assignments were based on successively updated predictions of the spectrum. An increasing number of the quartic centrifugal distortion constants and finally the quadrupole coupling constants of the 14N nucleus were allowed to vary. As the calculated transition frequencies became more precise, some reassignments of individual hyperfine components became necessary. After a satisfactory fit of all transitions and hyperfine components was obtained, the coupling constant Xab of the 35C1nucleus was included in the list of variables with an initial value of 10 MHz. The iterative least squares process ended with a standard deviation of 0.052 MHz and a value of 40.8 MHz for &b . All other parameters were similar to the final values. The analysis of the hyperfine structure of the transitions of CFz=14N3’Cl followed the same procedure. The initial quadrupole coupling constants for 37Cl, except Xab, were obtained by dividing those for 35C1 by 1.26878, the ratio of the nuclear quadrupole moments [2O]. Xab was kept fixed at the zero value throughout the calculations. It was included among the variable parameters only after a reasonable fit had been obtained. A value of 32.4 MHz for &‘b and a standard deviation of 0.069 MHz resulted from the least squares fitting process, the other parameters being close to the final values obtained later. The transformation to the principal axes of the quadrupole coupling tensor yielded a value of 20.4“ for the angle 0 between the a principal inertial axis and the z principal axis of the quadrupole coupling tensor for both isotopes. Assuming that the z axis should coincide with the direction of the NC1 bond, 0 is also the angle between that bond and the a principal axis. The initial attempts to determine a molecular structure from the observed rotational constants yielded a value of approximately 30” for the angle 8 (see section on “molecular structure”). Because of the huge discrepancy in the values for 0 from both methods, we tried to improve on the values for x&. Upon the analysis of the rotational energy levels without quadrupole inter-
43
action, it was observed that the energy levels J2,J_ 1 and J + 1 l,J+ 1 for J = 5, 6, 7, 8 are the most likely ones to be perturbed by xo6. Therefore, the quadin a least squares fit of rupole coupling constants for 35C1 were determined only the b-type transitions with J< 9. All other constants were kept fixed at the previously determined values. Avalue of 49.0 MHz resulted for xab. After a few reassignments of hyperfine components were made, xob rose even further to over 53 MHz. Starting with this new value, all assigned transitions and hyperfine components were included in a simultaneous adjustment of all parameters. The standard deviation dropped to 0.047 MHz and a final value of 52.42 MHz was obtained for x,“;. Subsequently, this value was divided by the ratio of the nuclear quadrupole components for the chlorine isotopes to obtain a new value for ~2;. The spectrum of CF,‘4N37C1 was predicted again using the new Xab value and the previous values of the other parameters. After making a few reassignments, all parameters, except AK which was fixed at the value of the other isotope, were adjusted in a fit of all transitions and hyperfine components. During this process, the standard deviation for the 37C1 isotope wa s reduced to 0.046 MHz. A value of 40.8 MHz was obtained for xi;. The complete lists of assigned transitions and hyperfine components and the differences to the calculated frequencies and splittings are shown in Tables 1 and 2 for CF,N35C1 and CF,N37C1, respectively. The final parameters for both isotopic molecules are listed in Table 3. With the new values for X&, the angle 0 was now calculated to be 24.0” and 23.9” for the two molecules.
Molecular structure Because the rotational constants A a.re essentially identical in both isotopic molecules and because the molecules are planar, only two of seven structural parameters are well determined. Diagnostic least squares calculations [22, 231 showed that all parameters contribute to one of the determinable linear combinations with the CNCl angle contributing about 50%. The other determinable parameter is essentially a linear combination of the CF, distance and the NCFt angle where F, and Ft refer to the F atoms cis and tram to Cl, respectively. The observed rotational constants may be reproduced reasonably well with almost any set of starting parameters by the diagnostic least squares method if the CF, distance and the CNCl and NCF, angles are allowed to vary. The resulting values depend very much on the initial assumptions, particularly on those about the NC1 and CN distances. The sensitivity of the results is demonstrated by the fact that a change in any of the fixed CF distances by 0.01 A results in changes of up to 0.03 a in the CN and NC1 distances, if those are allowed to vary. Initially, the r-i data reported for CFzNF [15] (see Table 4) were used as a starting point together with a NC1 distance of 1.74 A. Once a reasonable agreement between observed and calculated rotational constants was obtained, the angle f3 between the NC1 bond and the a IYncipal axis was calculated to be -30” with the CNCl angle below 110”. Assuming that one
9/Z 11/2 1312 1512
vorAv
b
dC
Transition F;
912 11/2 13/2 1512
7/2 912 11/2 13/2
F’
d --
Transition
F;
625+ 52,
F’
vorAv
d
2312 2712 2512 2912 2712 2912 3112
29520.64 0.01 0.00-0.03 2.83-0.05 2.83-0.08 3.20 0.02 3.73 0.12 3.20-0.01
29233.614.01 12,,,, + 12,,,, 21/2 19/2 35598.09-0.01 11.36 0.05 21/2 21/2 0.87-0.01 15.32 0.03 2112 2312 0.00-0.08 2312 2112 3.98 0.01 5.34 0.04 2312 2312 6.18 0.02 2312 2512 5.34-0.03 30445.29 0.00 8.71 0.04 2512 2312 5.97 0.02 2512 2512 6.79-0.01 11.15 0.03 2.45 0.01 5.97-0.04 2512 2712 0.62-0.03 2712 2512 31908.04 0.02 2712 2712 1.51-0.01 0.29-0.02 2712 2912 0.62-0.09 0.00-0.06 7.25 0.03 13,,,, + 13,,,,2312 2112 27027.36 0.00 1.53 0.03 2312 2512 0.00-0.03 2.60-0.02 7.25-0.01 2512 2312 2512 2712 2.60-0.05 8.88 0.00 9.16 0.00 2112 2512 2.95 0.04 2712 2912 2.95 0.01 8.88-0.04 1.72-0.01 2912 2712 0.30 0.01 2.03 0.00 2912 2912 0.65-0.01 1.72-0.04 2912 3112 0.30-0.02
vorAv
912 l/2 27605.29 0.05 0.00 0.01 912 11/2 -2.09 -0.01 6,,+ 6,, 912 712 11/2 912 -2.09 -0.01 912 912 11/2 13/2 -2.09 0.04 912 11/2 13/2 11/2 11/2 9/2 -2.09 0.03 1312 1512 11/2 11/2 0.00-0.02 1512 1312 11/2 1312 0.00-0.02 15/2 1'7/2 13/2 11/2 1312 1312 624+ 52, 1312 1312 28220.40-0.05 1312 1512 -0.20 0.03 1312 1512 1512 1312 1.70-0.01 1512 1512 1512 1512 1.53 0.03 1512 1712 1512 1712 Io,+ 60, 1312 1312 31349.00 0.00 +-14,,,,2512 1512 1512 0.52-0.02 7,,+-7,, 11/2 11/2 33624.98 0.01 14,,,, 11/2 1312 -0.34 -0.01 2512 1712 1712 1.00 0.06 13/2 11/2 5.75 0.01 2712 1312 1312 6.08-0.03 2112 7 1,+ 61, 1312 1312 30442.00-0.03 1312 1512 5.75-0.04 2912 0.52-0.03 15/2 15/2 1512 1312 7.16 0.04 2912 1712 1112 1.01 0.02 15/2 15/2 7.48 0.00 2912
712 912 11/2 1312
5/2 912 27070.00 0.10 4,,+ 4,, -0.44 -0.06 l/2 11/2 0.29-0.09 912 1312 11/2 0.67-0.08 1512
Fla
6 LS+ 51, 11/Z 912 28935.30 0.04 5,,+ 5,, 1.00-0.09 1512 1312
606+ 50,
Transition F;
Microwavespectrum of CF,=14N35C1. Observedfrequencies and splittings of hyperfinecomponents of rotational transitions and deviations of calculated values (MHz)
TABLE1
2
1512 1512 1712 2112
921+ 91,
1712 1512 1712 19/2
17/2 19/2 21/2 2112 2312 2312 2512
1512 1312 1512 1512 1512 1712 1712 1512 1712 1712 1712 1912 1912 1712 1912 19/2 19/2 21/2 2112 1912 21/2 21/2 2112 2312
13/2 11/2 1312 1312 1312 1512 15/2 13/2 1512 1512 1512 1712 1712 1512 1712 1712 17/2 19/2 19/2 17/2 19/2 19/2 19/2 21/2
1512 17/2 1712 1712
38367.34-0.09 -0.30 0.08 0.00 0.11 0.37 0.08 lo,,+9,, 17/2 17/2 19/2 8?.1+72, 1712 1512 36691.02-0.03 1712 1912 2112 0.00 0.01 19/2 1712 1.06 0.07 2112 1912 2112 1.06 0.08 2312 2312
8 1'1 + 71, 1312 1512 1712 1912
34721.04-0.03 0.59 0.06 0.79 0.03
1312 35558.25 0.04 1512 -0.44 -0.03 1712 0.00 0.00 19/2 0.43 0.02
8 1s+ 71, 1512 1512 1712 1712 1912 1912
808+-70, 1312 1512 1712 19/2
33110.14 0.02 -0.98 -0.05 -0.98 4.05 -0.98 -0.05 0.00-0.02 0.00-0.02
0.38 0.04 0.83 0.00
72s+ 62, 11/2 11/2 1312 1312 1512 1312 1512 1712 17/2 15/2 1712 1912
1512 1312 1712 1712
615 1312 11/2 33673.77-0.02
32159.02-0.05 -1.52 0.05 8,,+-8,, -1.52 0.05 -1.21 0.10 -1.21 0.10 0.00 0.03 0.00 0.03
+
726+ 6,s 11/2 11/2 13/2 11/2 1312 1512 1512 1312 1512 1712 1712 1512 1712 1912
7 16
3112 2912 31/2 31/2 3112 3312
-0.01 0.74 0.00 0.29-0.04 0.29
15,,,, + 15,,,,2712 2512 32588.37 0.00 0.00-0.04 35594.87-0.03 2712 2912 3.09-0.02 0.42-0.02 2912 2712 3.09-0.05 0.00-0.06 2912 3112 3.45 0.04 5.73 0.06 3112 2912 4.01 0.10 6.15 0.07 3112 3112 3.45 0.01 5.73 0.01 3112 3312 0.30-0.01 6.68 0.04 3312 3112 0.81 0.00 7.07 0.01 3312 3312 0.30-0.03 6.68-0.01 3312 3512 1.10 0.03 1.52 0.02 16,,,,+16,,,, 29/2 27/2 36207.53 0.01 0.00-0.04 1.10-0.01 2912 3112 3.27-0.03 31j2 29/2 3.27-0.06 3112 3312 37816.324.02 3.61 0.01 0.48 0.01 3312 3112 4.22 0.07 0.00-0.06 3312 3312 3.61-0.02 5.25 0.05 3312 3512 0.29-0.01 3512 3312 5.72 0.06 5.25 0.00 0.85-0.01 3512 3512 0.29-0.04 6.04 0.03 3512 3712 6.49 0.02 6.04-0.02 ll,,+ ll,, 19/2 19/2 38746.41-0.05 0.88 0.02 0.55 0.07 2112 2112 1.35 0.02 2112 2312 1.02 0.13 0.88-0.02 2312 2312 0.55 0.01 2312 2512 1.02 0.07 30485.79 0.01 2512 2512 0.00-0.06 2512 2712 -0.56 -0.05 0.55 0.07 0.00-0.06 0.84-0.05 12,,+-12,,,, 2112 2312 37386.48-0.01 2312 2512 0.26-0.03 0.32-0.07 0.32-0.01 2512 2312 0.26-0.10 -0.26-0.07 2512 2712 0.26-0.07
7.16 0.00 0.79 0.00 1.19 0.02 0.79-0.04
%
Fta
vorAv
b dC
Transition F;
F’
220+ 11,
512 512 512 712
+-13,,,, 2712 2712 312 36352.28 0.01 14,,,, 512 2912 2712 -1.72--O.O2 2912 3112 712 -O.63 0.05 512 3112 2912 -12.99 0.12 3112 3112 515+-40, 912 912 28653.90 0.02 3112 3312 0.33-O.ll 912 712 912 1112 0.33 0.02 lo,,'lo,,,, 1712 1512 1312 1112 1712 1712 2.38-O.O7 1312 1312 1712 19/2 1.80-O.O5 1312 1512 1912 1712 2.38 0.04 1912 1912 616+ 50, 1112 912 32085.36 0.02 191.22112 1112 11/2 21/2 1912 -O.48 -O.O5 1112 1312 2112 2112 0.00 0.10 1312 1112 21/2 23/2 0.49-O.o7 1312 1512 2312 2112 0.49 0.02 1512 1312 2312 2312 1.77-+I.04
t lo,, 21/2 1912 826+-72, 1312 1512 38046.52 0.01 ll,,,, 1512 1712 2112 2112 -0.51 0.01 2112 2312 1712 19/Z -0.34-O.03 1912 2112 2312 2312 0.22-0.01 2312 2512 2512 2312 909+ 80, 1512 1712 39712.37-0.06 1712 1912 2512 2512 -0.34 0.06 1912 2112 2512 2712 0.00 0.07 0.35 0.02 2112 2312 13z,LL + 12,,,, 2512 2512 2712 2512 9 19+ 8,s 1512 1512 38979.59 0.04 1712 1712 2712 2912 -0.26-0.04 2912 2712 1912 1912 0.00-0.11 2112 2112 2912 3112 0.35-O.o5
Transition F;
TABLE 1 (continued) or Av d
Transition
F;
F’
27416.99 0.01 14,,,, + 14,,,,2512 2712 -O.39 0.03 2712 2712 -O.39 --O.Ol 2712 2912 -O.89 -O.O4 2912 2912 -O.89 -O.O7 2912 3112 3112 3312 34623.54 0.00 + 15,,,,2712 2512 -O.30 0.08 15,,,, -O.30 0.04 2712 2912 -1.10 0.00 2912 2712 -O.54 -O.Ol 2912 3112 -1.10 -O.o3 3112 2912 3112 3312 27010.25-0.01 3312 3112 0.82--O.OB 3312 3512 0.00-O.lO + 16,,,, 2912 2712 5.56 0.04 16,,,, 6.34-0.05 2912 3112 5.56-O.O4 3112 2912 6.34 0.02 3112 3312 7.18 0.00 3312 3312 6.34-0.05 3512 3512 0.82 0.02 + 17,,,, 3112 3112 1.75 0.06 17,,,,
36871.41 0.01 27/Z 2512 0.45-0.03 2712 2712 0.00-0.03 2712 2912 0.90 0.11 0.45 0.11 13mJ + 13,,,, 2312 2112 -0.50 -O.70 2312 2512 0.00-0.06 2912 2712 +I.50 -O.lO 2912 3112
v d
34246.46 0.00
34047.85-O.01 0.00 0.01 0.69-O.01 0.69 0.00 0.69 0.00 0.00 0.01
34351.95-O.02 0.00 0.01 0.48 0.00 0.48 0.01 0.48-O.O5 0.48-0.04 0.00-O.o5 0.00-O.o4
35076.95 0.05 0.00-O.lO 0.27-0.06 0.00-0.14 0.27-0.09 0.00-O.o4
36125.08 0.07 0.00 0.03 o.oo-O.03 0.00-O.Ol
0.00-0.07 -0.38 -0.05 0.00-0.04
uorAv
%
1312 1312 1512 1512 1512 1712 1712 1912 1912 1912
1312 1312 1712 1712 1712 19/2 1912
8 1* + 70,
8 08 +- 71,
11/2 1512 1512 1712 1912 17/2 2112
1512 11/2 1312 1512 1712 1512 1712 1712 1912 2112
11/2 11/2 1312 1312 1512 1712 1512 1712
1512 1712
with the upper
31448.77 0.00 0.00 -0.02 0.96 -0.01 1.24 0.02 0.96 -0.03 0.51 -0.01 0.51-0.03
38830.97 0.00 0.00 -0.07 -1.14 0.01 -1.45 0.02 -1.14 0.06 -0.79 0.03 -1.14 0.01 0.29 0.01 0.00 0.07 0.29 0.06
35458.57 0.00 -0.83 -0.06 -1.16 0.00 -0.48 -0.04 -0.83 0.00 -0.48 0.02 0.86 0.00 0.42 -0.01
1.28 -0.02 1.77 0.04 + ll,,,,
19/2 19/2 21;2 2112 2112 2312 2312 2312 2512 2512 2512
level in the rotational
ll,,,,
2312
transition
17/2 21/2 19;2 2112 2312 2112 2312 2512 2312 2512 2712
2512
Au = V’ -
+ 19,,,,
+ 18,,,,
3912 4312 4112 4512
4112 4112 4312 4312
0.95 0.95 0.95 0.00 0.00
0.00 -0.06 -0.06 -0.06 -0.07
0.00
v’ = frequency
rule
0.00 -0.01 0.00 -0.02
the selection
38430.35 0.00 -1.77 -1.77
-1.51-0.02 -1.51-0.02
0.00 -0.01
36397.84
35012.98 -0.03 1.22 0.00 1.22 0.01 1.22 0.00 0.00 0.00 0.00 0.00
of first component,
from
3712 4112 3912 4312
3312 3512 3512 3912 3712 4112
3312 3512 3712 3712 3912 3912 3912 3912 4112 4112
3312 3312 3712 3512 3912
3312 3512 3512 3712 3712
Fy and F” are obtained
203,,, + 202,,*
19,,,,
18,,,
v, v = frequency
J’dadc + J?;&;.
0.02 -0.09 0.02 -0.12 -0.06 0.03 -0.04 -0.04 0.00 -0.05 -0.07
0.82 -0.06 31147.89 0.00 5.46 6.19 5.46 6.19 6.99 6.19 0.72 1.56 0.72
AF, = AF = AJ. bFrequency v given for first component in each multiplet. of any other component. Cd = v&S - vcalc or Avobs -AU&.
aF; and F’ are associated
11/2 1312 1312 1512 1512 1512 1712 1712
7 1, +- 60,
15/2 1512
2
7 2s
*
624
7 26 +- 62,
+
11/2 1312
11/2 1312
1912
1712
1512 13/2 1712 19/2
1312 1712
1312 11/2 1512 1712
615
7 16
1512 1312 1712 1912
1912 13/2 17/2
1112 13/2 1512 1712
1312 11/2 15/2 1712
F’a
11/2 1512
1312 11/2 1512 1712
+-
61,
7 07
7 ,, +
912 11/2 1312 1512
6 24 + 52,
1712 11/2 1512
11/2 912 13/2 15/2
6 15 + 51,
60,
F;
Transition
b
dC
-0.04 0.08 -0.03
32200.93 -0.73
-0.06 0.00
31341.48 0.02 -0.94 0.01 0.00 -0.03
32788.60 0.01 0.29 -0.07 0.29 0.01 0.64 -0.02
29713:75 0.00 0.40 0.08 0.40 -0.04 0.79 0.00
30609.54 -0.36 -0.36
27455.54 -0.01 -1.34 0.00 -1.34 -0.02 0.00 -0.05
frequencies
1712
1712
9,, + 8,,
8,, + 8,,
7,, + 7,,
1512 1712 1712 1712 19/2 19/2
1312 1312 1312 1512 1712 1512 1712 1912 17/2 1912
11/2 13/2 1512 1312
11/2 912 1312 1512 1912
2112 1712 19/2
1912 1512 1712
8,, + 7,,
11/2 11/2 1112 1712 1712
11/2 912 1312 1312 15/2 1512 17/2
912 912 1112 1312 1312 1512 1512
+- 5,,
6,,
F’
-0.03
4.62 0.05 5.28 0.05 5.60 -0.02 5.28 0.01 0.93 0.04 1.31 0.01
35323.22 0.02 0.40 -0.01 0.00 -0.06 4.62 0.09
33455.00 -0.06 -0.39 -0.03 -6.39 -0.09 0.63 0.08 0.63 0.05
35311.35
30404.59 -0.05 0.00 0.09 0.32 -0.07
31641.70 0.05 -0.49 -0.12 -0.93 0.04 -0.93 -0.04 -0.49 0.03 0.00 -0.06 0.51 0.01
d
of hyperfine
vorAv
and splittings
F;
Transition
Observed
28168.46 0.01 0.49 -0.06 0.49 0.10 0.81 0.00
vorAv
Microwave spectrum of CF,=‘4N37Cl. ations of calculated values (MHz)
TABLE
12,,
ll,,
17,,,,
16,,,,
+- 12,,,,
+ ll,,
+ 17,,,,
1912 2112 2112 2312 2312 2512
1912 1912 2112 2112 2312 2312
2312 2512
2512 2712
2512 2112
2912 3112 3312 3512 3912
3112 3112 3112 3712 3712
2512 2512
2712 2912 3112 2912 3112 3312 3112 3512 3312 3512 3712
F’
0.02 -0.01 -0.04 -0.03 -0.06
38120.85 -0.08 0.00 -0.02
0.00 -0.04 0.45 -0.03
39457.89 0.07 0.45 0.02 0.45 0.09 0.79 0.01 0.45 0.05 0.79 -0.04
38464.06 0.56 0.00 0.20 0.20
-0.03 0.01 -0.03 -0.01 0.06 -0.04 0.00 -0.03 0.00 0.02 -0.03
d
and devi-
34666.98 0.54 0.00 2.52 3.11 2.52 2.76 2.76 0.23 0.78 0.23
vorAv
transitions
2912 2912 2912 3112 3112 3112 3312 3312 3512 3512 3512
F;
of rotational
+ 16,,,,
Transition
components
?z
37368.81 0.05 0.23-0.08 0.23 0.01 0.49-0.05
36993.82 0.02 -0.41 0.00 -0.41 -0.02 0.00-0.03
38801.89 0.04 0.26-0.05
28280.59 0.00 0.35-0.09 0.35 0.04 1.11-0.10 0.69-0.04 1.11 0.01 1.97-0.07 1.34-0.09 1.97 0.04
1712 1512 19/2 2112
8I,+ 71, 1512 1312 17/2 1912
826+ 72, 1312 1312 1512 1512 1712 1912 19/2 21/2
909+ 80, 1912 2112 2112 2312
515+ 40,
a*bvCForexplanation offootnotesseeTable 1.
912 912 912 712 912 11/2 11/2 9/2 11/2 11/2 11/2 13/2 1312 11/2 1312 1312 1312 1512
0.01
33894.97
8 ,Ic 71, 15/2 17/2
1912 2112
0.93 0.00 13,,,, + 13,,,, 2512 2712 2712 2912 1512 1312 37431.16 0.08 1512 1712 0.00-0.05 14,,,, + 14,,,, 2712 2912 2912 3112 4.11-0.02 1712 19/2 4.73 0.00 1912 1712 5.16 0.00 16,<,, + 16,,,,2912 3112 19/2 19/2 3112 3312 4.73-0.04 1912 2112 2112 1912 3312 3512 0.67 0.00 3512 3712 1.13 0.01 2112 2112 0.67-0.04 2112 2312 17%1,+ 17,,,,3112 3312 3312 3512 +14,,,,2512 2312 28611.44-0.02 14,,,, 3512 3712 0.00-0.03 2512 2712 3712 3912 2.21 0.04 2712 2512 2.21 0.01 2712 2912 + 18,,,,3312 3512 2.39-0.01 18,,,, 2912 2712 3512 3712 2.39-0.03 2912 3112 3712 3912 0.22-0.01 3112 2912 3912 4112 0.64 0.01 3112 3112 0.22-0.03 3112 3312 + 19,,,, 3512 3712 19,,,, 3712 3912 + 15,,,, 2712 2512 31370.94-0.03 15,,,, 3912 4112 0.48 0.01 2712 2712 4112 4312 0.00-0.03 2712 2912 2.37 0.01 2912 2712 + 20,,,,3712 3912 2.86 0.03 20,,,, 2912 2912 2.37-0.02 2912 3112 3912 4112 4112 4312 2.59 0.00 3112 2912 4312 4512 3.14 0.08 3112 3112 2.59-0.03 3112 3312 0.23 0.00 3312 3112 0.70 0.00 3312 3312 0.23-0.03 3312 3512
-0.73 0.05 0.64-0.02 34732.62*.02 9,,+ 9,, -0.27 0.05 0.00 0.00 0.36 0.03
1512 1712 17/2 19/2 801+ 70, 1312 1512 1512 1712 17/2 1912 19/2 2112
37414.80-0.01 1.26 0.01 1.26-a.09 0.00-0.09
35792.62 0.01 1.05 0.01 1.05-0.07 0.00-9.08
34771.80 0.02 0.83 0.01 0.83-0.06 0.00-0.07
34314.14 0.02 0.61-0.01 0.61-0.06 0.00~a.05
34366.83 0.02 0.44 0.01 0.44-0.03 0.00-0.04
35718.33-0.04 0.00-0.02
36839.11-0.03 o.oo- 0.02
50 TABLE 3 Rotational (MHz), centrifugal distortion constants in N-chlorodifluoromethanimine
A B c
(kHz),
CF,=14N35C1
CF,=‘4N3’C1
11260.86058(l) 2542.205431(l)
11260.8548(14) 2470.93487(33)
2072.428539(l)
2024.81508(32)
*J
0.377273(2)
O-4206( 27)
and nuclear quadrupole
CF,==‘4N3SCl X2 X2; -Xc&
-71.3445(62) -25.7144(18)
lX;jl
52.42(13) 22.8150(35)’
coupling
(MHz)
CF,=‘4NJ7Cl -56.06(13) -20.237(31) 40.8( 16) 17.913(71)a
*JK
3.96432( 2)
3.932(11)
*K
5.5716(4)
5.5716f
XOb Cl _ Cl - Xcc XEI’ X22
6J
0.0694046(4)
0.06715(18)
Cl XXX
46.193(95)a
2.57222(3)
2.160(16)
eb
24.04(3)a
23.9(6)a
3.84(28)
0.029(17)a
-2.920(57)
nc ad
0.0247( 10)a
XZb - Xz
3.552(90) -2.958(31)
0.0467
0.0458
";b
-3.255(48)a
-3.38(
ne
323
157
X cc
-0.297(47y
-0.46(13)a
“N” XM
15y
48.5295(29)= -94.723(95)a
38.150(65)a -74.2(12)a 36.0( 12)a
aDependent parameter. b 0 - angle between the z and the a axis in degrees. ‘11 = (Xxx -X,,)/ X **. do = Standard deviation of least squares fit in MHz. en = Number of data points (frequencies and splittings). fFixed at value of other isotope.
principal axis of the quadrupole coupling tensor of 35C1should be along the NC1 bond, the angle 8 should be only 20.4”) as obtained from the initial value for xab (see previous section). Therefore, we sought to reduce the discrepancy between the values for f3 by changing the initial parameters in the molecular structure calculations. It was found that 0 is reduced upon increasing the CNCI angle significantly from the initial value of 107.9”. The increase in the CNCl angle requires a significant decrease in the NC1 and CN distances from the original values of 1.74 and 1.274 A, respectively. However, it was very difficult to determine a satisfactory structure, where the NC1 distance is at least 1.70 8, by the trial and error method. For this reason, we determined the fully optimized structure by ab initio SCF calculations at the 3-21G* level with the program GAUSSIAN 82 [26]. The results are listed in Table 4. The angle 0 for this structure is 26.5”. In order to assess the reliability of the ab initio results for CF,=NCl, we performed the same 3-21G* calculations on ClN=C=O for which the complete r, structure has been determined by Hocking et al. [3]. Except for the CO distance and the NC0 angle, the fully optimized structural parameters were not even close to the r, values. Therefore, the ab initio results were not used as initial values for the diagnostic least squares refinement, in contrast to our earlier intentions. After numerous calculations, we decided to vary only the well determined parameters (CF, distance and CNCl and NCF, angles) while keeping the other parameters constant at assumed values. A value of 1.700 8, was chosen for the NC1 distance. For comparison, the corresponding r, distance [3] in
2470.935 2024.815
c
11260.855
A B
C
11260.861 2542.206 2072.429
Obs.
fixed fixed fixed 1.299(34) 116.4(14) 119.5(34) fixed 27.15
2025.580
2469.927
11259.317
11259.561 2541.377 2073.395
Calc.
1.274 1.300 1.300 (107.9) 119.8 127.2
rk ’
CF =NFC
-0.765
1.008
1.538
1.300 0.829 -0.966
Obs.-Calc.
(1.690) (1.310) 1.323 1.326 (123.3) 126.0 124.2 25.8(11p
CF,==CHCld r.
118.83(5)
1.705(5) 1.226(5)
ClN=C=@ rS
1.683(10) 1.266(5) 1.718(6)$ 1.718(6)? 117.11(40) 118.68(50)? 127.52(40)$
CC1,=NClf rk
aDistances r in A, angles in degrees. bTwo linear combinations of three variable parameters were adjusted; the calculated rotational constants were obtained from the final structural parameters. ‘Ref. 15. The value in parentheses is for the C=NF angle. dRef. 24. The values in parentheses are for the C-Cl and C=C distances and for the C=CCl angle. eRef. 3. fRef. 25. The values indicated with a dagger (t) are for carbon--chlorine parameters rather than carbon-fluorine parameters. % = Angle between the “a” principal inertial axis and the NC1 bond hAngle between the Q inertial axis and z principal quadrupole axis.
CF zN”CI
A B
CF,N=Cl
1.700 1.254 1.300 1.300(50) 115.0(50) 118.8(50) 126.2
(MHz)
1.704 1.231 1.318 1.326 119.39 122.45 127.51 26.5
Diagnostic least squaresb (initial) (final)
Rotational constantsb
r(C-F,) r(C-F,) L( CNCl) L(NCFt 1 UNCF,) es
r( C=N)
r(N-Cl)
Ab initio 3-21G*
CF,=NCl
Molecular structures of CF,NCl and related molecules?
TABLE 4
52
ClNCO (1.705 A) and the rt Cl-N distance [25] in Cl&NC1 are at the lower end of the range of values reported in the literature. A value of 1.254 A was assumed for the CN distance. It is about halfway between the value obtained by electron diffraction in CFzNF [15] (1.274 a) and the ab initio value (1.2312 A). The values for both CF distances (1.300 a) were transferred from the structure of CFzNF [15]. The NCF angles reported in ref. 15 were reduced by 1” each. With these assumptions and modifications, a diagnostic least squares refinement with an initial value of 115” for the CNCl angle yielded a good fit of the observed rotational constants and a set of acceptable parameters. The results are shown in Table 4. The final values for the varied structural parameters are 1.299 A for the CF, distance, and 116.4” and 119.5” for the CNCl and NCFt angles, respectively. For this structure, 0 is 27.15”. For comparison, the structures for ClNCO (rs) [3], CF2NF (rz) [15], and CF,=CHCI (ro) [24] are also given in Table 4. For the last of these molecules, which is very similar to CF,=NCl, Stone and Flygare [24] obtained an angle 0 of 25.8” from the off-diagonal element of the quadrupole coupling tensor, xab, of the 35C1isotope. DISCUSSION
The hyperfine structure due to the presence of the Cl nucleus could be resolved fully for most b-type transitions. This was not possible for the atype transitions. The splitting due to the interaction with the 14N quadrupole nucleus was resolvable only partially in some b-type transitions. All three components due to this nucleus were only resolved for the F1 = 5/2 component of the 220 +- 111 transition in the spectrum of CF2=14 N3%1. The other components of this weak transition were unfortunately blended with other absorptions. In many cases, two or more components of the hyperfine pattern had to be assigned to the same absorption band. Many times, nearby components were barely resolved, rendering a precise determination of the exact line positions difficult or even impossible. As a consequence, the precision of the line positions is estimated to be between 0.1 and 0.05 MHz. Whenever lines with a calculated separation of up to 0.15 MHz had to be assigned to the same frequency, a deviation larger than the estimated precision had to be expected for at least one of these lines. In view of these facts, the standard deviations of 0.047 and 0.046 MHz achieved for CF,=N35C1 and CF,=N37C1, respectively, are quite acceptable. The standard errors of the rotational and centrifugal distortion constants, particularly in the case of CF2=14N35C1,are surprisingly low. In the original version of our computer program, the absorption frequencies of all assigned hyperfine components were fitted directly. In this case, the standard errors were even lower. In order to change the weights of the data points, the presently used scheme was introduced whereby only the frequency of one arbitrarily chosen hyperfine component was fitted directly. For the other components, the difference to the directly fitted frequency were taken as
53
data points to be fitted. This resulted in an increase of up to 30% in the standard errors of rotational and centrifugal distortion constants accompanied by a decrease of similar magnitude in the standard errors of the quadrupole coupling constants. Still, the standard errors look very good. Strong dependences of splittings on rotational and centrifugal distortion constants and/or accidental near degeneracies of hyperfine levels with different rotational quantum numbers seem to be the only possible reasons. Jemson et al. [27] have recently exploited these opportunities in the case of BrNCO and INCO to determine the constants A and Ax accurately from only u-type transitions. The agreement of the centrifugal distortion constants between the isotopes is not very good. This was particularly true for Ax. Therefore, for CFzN3’C1, this parameter was fixed at the value obtained for the 35C1isotope. Since the approximate planarity relations between the centrifugal distortion constants in the I’ representation A reduction [19] does not involve Ax, we suspect that the much smaller data set for CFz=N37C1 might not be sufficient for an accurate determination of Ax in this isotope. The planarity relation is definitely not fulfilled for the data of the 35C1 isotopic modification. The diagonal quadrupole coupling constants with respect to the principal inertial axes are reasonably well determined. The constants for 14N for both isotopic species are equal within the error limits. The ratios of corresponding constants for the “‘Cl and 37C1nuclei vary between 1.272 and 1.274, close to the ratio [20] of the nuclear quadrupole moments, 1.26878. From the practical identity of the A rotational constants in both isotopes, it follows that the a principal axis passes through the chlorine nucleus in both isotopes. Therefore, the principal inertial axes systems in both molecules are parallel to each other. It is for this reason, that the ratios of all quadrupole coupling constants with respect to the principal inertial axes agree so well with the ratio of the nuclear quadrupole moments. The main point of discussion concerns the off-diagonal element j&b for the chlorine nuclei. This constant has not been reported very often in the literature because its effects are so small. It is not accessible through first order perturbation calculations. Only higher order treatments give access to it and these are usually not carried out for molecules containing chlorine. The CF&lz [28] and CF2= CHCl [24] molecules are examples where Xab has been determined for a “Cl nucleus. The initial results for $2 in both molecules were in good agreement with each other, the ratio xii /xi; = 1.258 being close to the ratio of the nuclear quadrupole moments. This result is expected because the principal inertial axes are parallel in the two molecules. When the discrepancy between the values for the angle 0 as obtained from initial values of Xab and from the initial molecular structure calculations was noted, three possible reasons came to mind. (1) The quadrupole coupling constants x:2 are significantly larger than the values obtained from the initial analysis of the hyperfine structure in the spectrum. (2) The molecular structure is compatible with the observed rotational constants but otherwise
54
inappropriate. (3) The model which assumes that one principal axis of the quadrupole coupling tensor coincides with the straight line connecting the chlorine and nitrogen nuclei (direction of NC1 u-bond) is not adequate. The first point was addressed in the re-analysis of the hyperfine patterns in the section “analysis of hyperfine structure”. As described there, it was possible to find values for $5 and xii which were not only in the correct ratio (1.284) but also led to an angle 13 of about 24”. The reassignments made were of two types. The first type comprises about six cases of u-type transitions in the spectrum of the “Cl isotope where the assignment of the F quantum number was changed while maintaining F1. The second type involved a redefinition of the “arbitrarily chosen” hyperfine component which was fitted directly and served as a reference point in the calculation of the splittings. This was done in a few cases for both isotopic molecules where it was observed that a relatively large deviation for the directly fit component obtained a large weight because it contributed again to the residuals of the splittings. This “redefinition” is merely a question of different weights. It was found that this weighting scheme influences not only the resulting value for &b but also some of the centrifugal distortion constants such as AK and 6K. We have not yet developed a weighting scheme yielding believable standard errors on rotational and centrifugal distortion constants which puts equal weights on all hyperfine components of a rotational transition. The second possible reason for the discrepancy in the values for 0, an inappropriate molecular structure, has also been addressed (see section on “molecular structure”). A further increase of the CNCl angle in the structure calculations would easily reduce the angle 8 to values near 25”. However, the most likely necessary reduction of the NC1 and CN distances to values below those assumed, seems to be unacceptable at this point. The possibility cannot be ruled out that further modifications in the structural parameters of the CF2 group allow an opening of the CNCl angle without a considerable shortening of the CN and NC1 distances. However, reliable data about the CF2 moiety are scarce and difficult to obtain. For these reasons, we propose for CF2=NC1 the structure obtained by the diagnostic least squares method and listed in Table 4 since this structure reproduces the rotational constants. Additionally, the value for the NC1 distance and the CNCl angle are in good agreement with the r, values [3] for ClNCO (Table 4) as well as the rz values [25] for Cl,CNCl. Also the angle 8 of 27.2’is in acceptable agreement with the value obtained from the diagonalization of the quadrupole coupling tensors, 24.0”, considering that X&, might be even somewhat larger than the value obtained by our analysis. We consider it unlikely that it is as large as 65 MHz which would bring 0 up to 27”. Stone and Flygare [24] reported an angle of 25.8 rt 1.1” for 0 in the very similar molecule CF,=CHCl, also derived from ~2;. The third possibility for the discrepancy in the values for 0, i.e., a small deviation of the z axis of the quadrupole coupling tensor from the straight line connecting the nitrogen and chlorine nuclei, cannot be ruled out.
55
Conclusions about ionic and double bond character of the NC1 bond are premature as long as the structural problems remain unsolved. We expect that the analysis of the microwave spectrum of both chlorine isotopic modifications of CF2--lSNC1 would provide some definite answers for two reasons. Substitution of the nitrogen nucleus would be extremely valuable for the determination of the NC1 distance. The lack of a nuclear quadrupole moment in the 15N nucleus simplifies the hyperfine structure considerably so that a more accurate determination of &$b for both chlorine atoms becomes feasible. Therefore, a microwave investigation of this isotopic species would probably lead to definitive structural parameters for this molecule. ACKNOWLEDGEMENT
The authors gratefully acknowledge the financial support of this study from the National Science Foundation by Grant CHE-83-11279. REFERENCES 1 G. Cazzoli, C. Degli Esposti, P. Palmieri and S. Simeone, J. Mol. Spectrosc., 97 (1983) 165. 2 D. J. Millen and K. M. Sinnott, J. Chem. Sot., (1958) 350. 3 W. H. Hocking, M. L. Williams and M. C. L. Gerry, J. Mol. Spectrosc., 58 (1975) 250. 4 L. Pauling, The Nature of the Chemical Bond, 3rd edn., Cornell University Press, Ithaca, New York, 1960, p. 229. 5 G. Cazzoli, D. G. Lister and P. G. Favero, J. Mol. Spectrosc., 42 (1972) 286. 6 W. Caminati, R. Cervellati and A. M. Mirri, J. Mol. Spectrosc., 51 (1974) 288. 7 H. B. Biirgi, D. Stedman and L. S. Bartell, J. Mol. Struct., 10 (1971) 31. 8 J. R. Durig, K. K. Chatterjee, N. E. Lindsay and P. Groner, J. Am. Chem. Sot., 108 (1986) 6903. 9 S. L. Miller, L. C. Aamodt, G. Dousmanis, C. H. Townes and J. Kraitchman, J. Chem. Phys., 20 (1952) 1112. 10 R. H. Schwendeman and J. D. Kelly, J. Chem. Phys., 42 (1965) 1132. 11 R. H. Schwendeman and G. D. Jacobs, J. Chem. Phys., 36 (1962) 1245. 12 D. J. Millen and J. Pannell, J. Chem. Sot., (1961) 1322 13 A. M. Mirri and W. Caminati, J. Mol. Spectrosc., 47 (1973) 204. 14 Y. Y. Zheng and D. D. DesMarteau, J. Org. Chem., 48 (1983) 4844. 15 D. Christen, H. Oberhammer, R. M. Hammaker, S. C. Chang and D. D. DesMarteau, J. Am. Chem. Sot., 104 (1982) 6186. 16 E. P. Wigner, Group Theory and Its Application to the Quantum Mechanics of Atomic Spectra, Academic Press, New York, 1959. 17 A. R. Edmonds, Angular Momentum in Quantum Mechanics, Princeton University Press, Princeton, NJ, 1957. 18 H. P. Benz, A. Bauder and Hs. H. Giinthard, J. Mol. Spectrosc., 21 (1966) 156. 19 J. K. G. Watson, in J. R. Durig (Ed.), Vibrational Spectra and Structure, Vol. 6, Elsevier, Amsterdam, 197 7. 20 W. Gordy and R. L. Cook, Microwave Molecular Spectra, Vol. 9, Part II of A. Weissberger, Technique of Organic Chemistry, Interscience, New York, 1970. 21 D. Christen, J. Mol. Struct., 79 (1982) 221. 22 R. F. Curl, J. Comp. Phys., 6 (1970) 367.
56 23 24 25 26
P. Groner, J. S. Church, Y. S. Li and J. R. Durig, J. Chem. Phys., 82 (1985) 3894. R. G. Stone and W. H. Flygare, J. Chem. Phys., 49 (1968) 1943. D. Christen and K. Kalcher, J. Mol. Struct., 97 (1983) 143. J. S. Binkley, M. J, Frisch, D. J. DeFrees, K. Raghavachari, R. A. Whiteside, H. B. Schelgel, E. M. Fluder and J. A. Pople, Gaussian 82, Carnegie Mellon University, Pittsburgh, PA, 1984. 27 H. M. Jemson, W. Lewis-Bevan, N. P. C. Westwood and M. C. L. Gerry, J. Mol. Spectrosc., 118 (1986) 481; 119 (1986) 22. 28 C. F. Su and E. L. Beeson, J. Chem. Phys., 66 (1977) 330.