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Molecular structure and nuclear quadrupole coupling constants of monochloroamine from microwave spectroscopy
JOURNAL OF MOLECULARSPECTROSCOPY4,
286-295 (1972)
Molecular Structure and Nuclear Quadrupole Coupling Constants of Monochloroamine from Microwave Spectroscopy G. CAZZOLI, D. G. LISTER ASD I’. G. FrlvE~o Laboratosio
di Spettroscopia Molecolare, Universitd di
C.N.R.
and Istituto
Chimico
“G. Ciamician”,
Bologna, Boloyna, Italy
The rotational constants B and C and the nuclear quadrupole coupling constants of six isotopic species of monochloroamine have been obtained from low J&? branch transitions. The ra structure has been determined as 7.X-R = 1.017 f 0.005 fi, HNCl
= 103”41’ f
22’,
1’N-Cl= 1.7480 f 0.0001 HNH
d,
= 107” Z!Z2’.
INTRODUCTION
Monochloroamine (NHzCl) is the last member of the isoelectronic series of molecules CH&l (I), NH&Z1 (2), HOC1 (3) and FCl (4) to be studied in detail by microwave spectroscopy. It is also of interest because of the question of the nonplanarity and the inversion of the amine group in NHzX molecules. Moore and Badger (5) have observed the infrared spectrum of this molecule and Lister and Millen (2) have measured the OoO--+ loI transitions of five isotopic species and the pG component of the dipole moment (6). In order to determine the molecular structure completely and to obtain thfl nitrogen and chlorine nuclear quadrupole coupling con&ants \ve have repeated and extended the microwave measurements. EXPERIMENTAL
PROCEI>URF,
Samples of monochloroamine were prepared by the method first used by Lister and Millen (2) to observe the microwave spectrum of this molecule but with the following modification. Dichlorine oxide vapor and the vapor from an aqueous ammonia solution were mixed at a low pressure in a trap at -40” and after .? min the trap was cooled to -SO” and the excess ammonia was removed by pumping. The solution used to prepare the deuterated monochloroamine was prepared by placing ci ml of DzO in a 2-1 bulb, pumping out the air and filling the bulb to a pressure of one atmosphere with ordinary ammonia. A solution of 151Vammonia was prepared by placing 0.5 g of 90 7% enriched 15?;$ammonium chloride and 0.4 g sodium hydroxide in a tube and distilling 0.5 ml of water into the tube. 286 Cupyripht
0
1972 by Academic
Press,
Inc.
MOLECULAR
STRUCTURE
OF NH&l
287
A 6-kHz Stark modulation spectrometer with a 3-m J band waveguide cell and phase stabilized Oki 24V10A and 30Vll klystrons was used for this work. Transitions in the region 48-58 GHz were observed using the second harmonic of these klystrons. Frequency measurements are accurate to ~0.02 MHz on the fundamental and the resolutions and sensitivity obtained may be judged from Fig. 1 which shows a recording of the 000+ 101, AFI = + 1 transition of 14NHi7C1. Measurements on the 1 -+ 2 transitions are somewhat less accurate than this because of the more complicated nuclear quadrupole coupling effects. In order to reduce the decomposition of monochloroamine, the cell was cooled to -30” and samples were flowed through it. TREATMENT
OF RESULTS
of six isotopic species of The 000 --) 101, Ilo -+ 211 and 111 -+ 2~ transitions monochloroamine have been observed to determine the rotational constants B and C and the nuclear quadrupole coupling constants. Centrifugal distortion effects have been neglected as work on hypoclorous acid (3) indicates that they should introduce errors of not greater than 0.5 MHz if the rotational constants are determined from the equations ~(0004 Y(Ilo --+ 211) -
101) = B + C,
Y(111-+ 212) = 2(B - C).
The quadrupole treated
coupling of the chlorine (II) and nitrogen (12) nuclei has been using the II, J, (F1), 12, F formulation of Bardeen and Townes (6) and
27d5.025 FIG.
1. Pen recording
27606 MHz.
Frequency
27d8.025
27d7.025
AF = +I transition of 14NH:‘C1 of 000 -) 101 _ AFI = fl, increases from left to right and markers are every 0 .l MHz.
288
CAZZOLI, LISTER,
AND FAVERO
the full secular equations have been solved. The nuclear quadrupole coupling of the lo1 level depends only on xW and the frequencies of the hyperfine components of each 0~ --+ lo1 transitions have been fitted by least squares to xaa(Cl), xaa(N) and the frequency (vo) of the transition in the absence of nuclear quadrupole coupling effects. The frequencies of the hyperfine components of each of the observed 1 -+ 2 transitions have been fitted by least squares to XW,(Cl) XW(N ) and y. using the values of xaa(C1) and xQa(N) determined from the appropriate Ooo-+ lo1 transitions. The measured line frequencies of the 000+ lo1 transitions, the differences between the observed and calculated line frequencies and the values of xaa and vo with their standard errors are shown in Table I. Results for the 110-+ 211and 111+ 212transitions are given in Table II and Table III. In the case of the 110--+ 211 transitions of 14NHD3%1 it was only possible to measure lines with little dependence on x&V) and the value for this quantity given in Table III is considered unreliable. The rotational constants B and C and the nuclear quadrupole coupling constants are collected in Table IV. Except for xti (N) of “NHD35C1 the values of xbbfor both nuclei are the mean of the values from the 110-+ 211and 111+ a12 transitions. The value of &b(N) for “NHD35C1 given in Table IV is that from the llo --, 211 transition. MOLECULAR
STRUCTURE
A r. structure determined by least-squares fitting of the observed moments of inertia Ib and I, to the four structural parameters of monochloroamine is shown in the first row of Table V. The quantity A - (B + C)/2 has beendetermined for the ground rotational state of 14NH?C1 as 8.56 f 0.01 cm-’ and for that of “NHD35C1 as 5.83 =k 0.01 cm-’ by Moore and Badger (5). Combination of these results with the present microwave data gives the following values of A: ‘*NHi5C1
270650 f
300 MHz
‘*NHD%l
157900 f
300 MHz
The structure in the second row of Table V was obtained by including the values of A in the least-squares calculation. The standard deviations on B and C are 1.8 MHz for both structures and the differences between the observed values of A and those calculated from the second structure is less than 150 MHz. The error on these values of A is considerably larger than the errors on rotational constants usually used to determine molecular structures by the isotopic substitution method (7). However, the uncertainty in the inertial difference AI, used in Kraitchman’s equation (8) leads to the following errors in the principal axis coordinat.es of the hydrogen atoms (&a = 0.001 A, 6b = 0.002 8, 6c = 0.007 A). The errors givr rise to uncertainties of 0.005 b in the N-H bond length and l”30’ in the HNH bond angle.
E
,
N-?S
w-w
72-?S
-0.02
28 088.45
TABLE
-0.02 f f f
27 621.78 27 602.07 -78.50 5.10
0.01 0.03 0.04
0.00 -0.01 0.00
0.01 0.01 0.01
Obs.Calc.
27 027.75 -99.65
27 052.66
27 007.81
27 032.73
Obs.
f f
‘eNH;‘Cl
I
0.01 0.0;
0.00
0.00
0.00
26 564.27 -78.51
26 583.88
26 548.56
26 568.21
-0.01
-0.01
0.02
Ohs.CELIC.
f 0.01 xk 0.06
‘“NH”CI 2
24 649.72 -99.07 5.10
24 630.09 24 628.99 24 630.93
24 654.42 24 655.48 24 653.96
Obs.
0.00 -0.02 0.02
0.00 -0.02 0.02
rt 0.01 I f 0.0: I f o.oi
“ND%1 2
STRUCTIJ~ZE (IN MHz)
Obs.
COUPLING
Obs.CdC.
WITH QUAURUPOLE
27 586.55 27 585.46 27 587.38
27 605.76 27 606.85 27 605.27
Obs.
‘“NH:‘Cl
lo, TRANSITION
0.01 0.04 0.05
0.00 -0.01 0.00
28 043.74 28 042.67 28 044.57
28 063.48 f -99.61 f 5.10 f
0.01 0.01 0.02
28 068.22 28 069.31 28 067.75
Obs.
I”NH;‘Cl
Ooo +
:26 213.02 -99.1 5.2
26 237.82
26 193.36 26 192.25
26 217.74 26 218.86 26 217.29
f f f
0.03 0.03 0.06
0.03 0.2 0.2
-0.07
-0.02 -0.04
“NHDWI
F,”
+
F*’
55 520.70 55 521.83 55 520.24
0.08 0.10
0.05 0.07 0.06
5G 455.42 56 456.53
56 430.92 56 429.73 56 431.82
0.03 0.02 0.01 0.1
56 474.69
56 u9.59 47.7 0.3
-0.06 55 508.14 -0.08 55 506.89 -O.OOt 1 55 508.94
56 439.32 56 438.12 56 440.18 0.02
-0.02 -0.07 -0.03
0.00 0.03 0.00
55 51G.18 & 0.02 37.6 f 0.1 0.2 f 0.1
55 536.031
55 511.27 55 512.43 55 510.77
-0,OG -0.02 -0.03
56 443.34 56 444.52 56 442.88
0.04 0.02 0.04
0.04 0.06 0.07
Ohs.CdC.
WITH
55 526.471 -0.15
-0.04
56 462.69
55 501.50 55 500.2B 55 502.37
Obs.
‘4NHS7C1 2
TRANSITION
Obs.
f f f
“NH3’CI 1
Ilo -+ 211
I
.I.
TABLE
0.05
0.00
54 352.49 48.1
54 377.411
54 342.16 /
1
f f
0.01 0.1
0.01
-0.01
53 415.34 37.7
53 434.84
53 407.28
53 410.67
53 425.62
53 400.51
-0.03
0.1
0.01
0.07
0.06
-0.12
zt 0.05 f 0.3
16NH:‘Cl
STRUCTURE
53 419.96
COUPLING
-0.03
II
54 346.471 -0.01 I
54 365.38
54 333.55
54 358.56
WH;%
QUADRUPOLE
--
(IN
49 836.25 48.1 0.2
49 861.201
49 849 l(
49 817.7’ 49 816.5: 49 818.5:
49 841.94 49 842.9! 49 841.4(
‘4ND”CI P
MHz)
zt 0.02 f 0.3 f 0.2
0.01
0.00
0.06 0.06 0.05
-0.04 -0.09 -0.05
Obs. Calc.
52 917.04 47.8 -0.1
52 941.91 3
52 930.0:
52 898.3: 52 897.3 52 899.1:
52 922.71 52 923.91 52 922.3,
Obs.
f f f
0.01 0.1 0.1
0.00
0.00
-0.03 -0.03 -0.03
0.02 0.02 0.05
“,k:
F’
m(N)
:bb(Cl)
0
w-%
%-?4 W-N
F” +
-
0.03 0.16
xix 0.02 I * 0.1 f 0.2
55 824.28
55 799.18 47.7 0.1
-0.03
55 789.53
55 789.53
-0.01 -0.06
0.04
55 811.10
55 792.14 55 792.5G
-0.07 0.00 0.00 -0.03 -0.01
804.84 806.33 780.21 780.67 779.85
55 55 55 55 55
_
54 886.98 37.7 0.3
54 906.741
54 879.26
f f f
0.02 0.1 0.1
0.09
0.08
-0.01 0.00
54 881.41 54 881.83
0.02 0.08 -0.03 -0.04 0.03 -0.04
891.42 892.94 871.94 872.46 871.57
Ohs: Calc.
54 896.27
54 54 54 54 54
Obs.
53 753.56 47.9
53 778.66
53 743.88
53 746.69
53 765.48
53 734.76
53 759.59
Obs.
f f
0.05 0.3
0.19
-0.09
-0.02
-0.05
0.02
-0.05
Ohs.Cak.
_.
-
52 836.89 37.7
52 856.61)
52 829.21
52 831.39
f f
0.05 0.3
0.08
-0.13
-0.11
0.00
0.09
52 822.16
52 846.33
0.06
I-
Ohs: ChlC.
52 841.75
Obs.
‘GNHarC1 2
-
-
48 759.70 47.9 0.1
48 784.44
48 750.01
48 750.01
48 771.62
48 7G5.36 48 766.93
Obs.
i 0.04 i 0.3 z!z 0.4
-0.OG
0.03
-0.05
0.00
-0.03 0.1
‘4ND3%J 2
TABLE III lx1 -+ 21e TKANSITION WITH QUADRUPOLE COUPLING STRUCTURE (IN MHz)
51 931.21 47.9 -0.7
51 956.03
51 921.60
51 924.28 51 924.88 51 923.98
51 943.15
f f f
0.01 0.1 0.2
0.02
-0.01
-0.03 0.01 0.01
0.00
CAZZOLI, LISTER, AND FAVERO
292
“NK~5C1 was chosen as the parent molecule and the atoms were located in its principal inertial axis system using Kraitchman’s equations and the appropriate inertial differences. Figure 2 shows the orientation of the principal axe3 relative to the molecular framework and it can be seen that the c coordinates of the nitrogen and chlorine atoms are very small. These coordinates were determined from the first moment (I;mici = 0) and product of inertia conditions (Zmiaici = 0). The coordinates are given in Table VI and the structure calculat,ed from them is given in the third row of Table V. TABLE
IV
ROTATION-AL CONSTANTS AND QUADRUPOLE COUPLING CONSTANTS (IN MHz) Molecules
&I
CO
x&l)
xmz
Xbb&%’
6% -__-
_______
‘4NH:kl 14NH;‘C1 lSNH:sC1 1SNH;‘Cl 14NHDasC1 ‘“ND:?1
14 13 13 13 13 12
194.3413 958.3413 663.6113 426.7513 352.9712 594.0012
-
xbmb
869.14-99.61 643.73 -78.50 364.14 -99.65 137.52 -78.51 860.05-99.1 055.67 -99.07
f f f f zk ziz
0.0447.7 0.0337.6 0.0248.08 0.0737.7 0.2 47.8 0.0848.0
f f f f f f
0.1 0.1 0.09 0.3 0.1 0.3
* Mean of values from the 1x0+ 211and 111---f 212 transitions. h Mean of values from the 110 + 211 and 111---) 212 transitions for 14NHD3W which is the result from the llo + 2,* transition.
TABLE
5.1 5.1
f f -
0.2 + 0.1 0.2 & 0.1
0.05 0.04
5.2 f 0.2 -0.1 f 5.10 zk 0.071 0.2 f
with the exception
0.1 0.1
of t.hat
V
MOLECULAR STRUCTURES
ra(1) ro(2)
-
r,(l) rs(2)
rN-CL&
rN -II@)
HrjH
HGCI
V
1.7523 1.7522 1.7480 1.7481
1.0218 1.0166 1.0161 1.0185
107” 107”13’ 108”23’ lO6”25’
103”29’ 103”42’ 103”52’ 103”30’
66”55’ 66”29’ 65”48’ 66”38’
&+ is the angle between angle.
the extension
of the NC1 bond and the bisector
TABLE
of the HNH
VI
PRINCIPAL AXIS COORDINATES8 a(&
N
-1.4546 - 1.4546 -1.1865
Cl
-1.1865 0.5597
H
b(A)
____
-
f0.8241 hO.8156
0.5597 a Calculated,
using the conversion factor 5.05376 X 105a.m.u. Aa.
c(A) 0.4551 0 4699 -0.0756 -0.0780 0.0040 a.0042
MOLECULAR
STRUCTURE
OF NH&l
293
Cl
___--
-a’
FIG. amine.
2. Orientation
of different
The axes are labeled as principal inertial principal inertial principal axes of 2, 2
a, c a’, 8
axis
systems
follows: axes of NH&I, axes of ND&l, the chlorine nuclear
in the symmetry
quadrupole
coupling
plane
of monochloro-
tensor.
As a check the hydrogen b and c coordinates were calculated from AIb and AI, for 14NHi5C1 and “ND?C1 using the a coordinate of the hydrogen atom determined from the single deuterium substitution. The coordinates determined in this way are given beneath the first set of coordinates in Table VI and the structure derived from them is given in the bottom row of Table V. Finally the nitrogen and chlorine atoms were located in the principal axis system of 14NHi7C1assuming that A for 14NHi7C1is the same as that of 14NHi5C1 and the geometry of the amine group given above. The nitrogen chlorine bond length of 1.7479 A calculated in this way is in excellent agreement with the t‘#values in Table VI. DISCUSSION
The nitrogen chlorine bond length may be compared with that in nitrogen trichloride (TV = 1.759 f 0.002 8) as determined by electron diffraction (9).
a 2
294
CAZZOLI, LISTER,
AND FAVERO
The nitrogen-hydrogen bond lengthoand the NHN angle are very similar to those in ammonia (T, E-H = 1.012 A, HNH = 106”40’) (10) and the degree of nonplanarity 4 (Table V) is slightly larger than that in ammonia (4 = 62”). The observed transitions of monochloroamine have not shown any splitting that may be attributed to the inversion of the amine group. Moore and Badger (5) have measured the fundamental and first overtone of the NH2 wagging vibraO-l and l-3 tion as 1032 cm-’ and 2020 cm-‘. The nearly equal separations show that the v = 2 levels must lie below the top of the barrier to the inversion of the amine group. This places a lower limit of E (v = 2) N 2500 cm-’ on the barrier, and it is therefore considerably higher than that in ammonia (2020 cm-‘) (11). This barrier height may be used with the frequency ~3, the molecular structure, and a Dennison-Uhlenbeck potential function (12) to show that the splitting of the Of and O- states is unlikely to be more than a few hundred megacycle/ second. The p. component of the dipole moment is symmetric with respect’ to the inversion of the amine group and the observed transitions are purely rotational, Any inversion doubling, therefore, depends on differences between the rotational constants of the O+ and O- states and will be extremely small because those states are almost degenerate and their rotational constants nearly equal. The chlorine nuclear quadrupole coupling constants (xua) of ‘*NHi’Cl and ‘“ND”,“Cl may be used with the molecular structure to show that the principal axes of the chlorine nuclear quadrupole coupling tensor are oriented at an angle of Y”5’ & 45’ to the principal inertial axes of ‘*NHpCl as shown in Fig. 2. As the nitrogen chlorine bond is inclined at an angle of 2”36’ f 5’ to the axes of ‘*KHi5C1 one of the principal axes of the chlorine quadrupole coupling tensor nearly (hointides with this bond. The principal elements of the tensor are: xc = -99.81
f
0.05 MHz,
xy = 47.10 f
0.10 MHZ,
x+ = 52.71 & 0.15 MHZ, where the y direction is perpendicular to the molecular symmetry plane. If the usual method of interpreting the asymmetry in the nuclear quadrupole coupling tensor is adopt’ed (IS), there is 2.6% double-bond character in the nitrogen chlorine bond and it is such that a loss of electrons from the chlorine 3p, orbital takes place. A similar effect has been observed in hypochlorous acid (4) indicating 1.6 % double-bond character and a loss of electrons from the chlorine 3p, orbital. The ionic character of the nitrogen chlorine bond calculated from xz assuming no s hybridization of the chlorine is 0.09. This may be compared with the value of 0.12 predicted using Gordy’s relationship between ionic character and electronegativity difference (14) taking the electroncgativit8y of the amine group as 2.76 (15).
MOLECULAR
STRUCTURE
OF NH&l
295
The nuclear quadrupole coupling constants of the nitrogen atom may be accounted for approximately using the approach of Townes and Dailey as applied to NXY, molecules by Pierce et al. (16). The nitrogen atom is assumed to form four orthonormal sp3 hybrid orbitals and these are determined by the molecular geometry. If the ionic character of the N-H bonds is assumed to be the same as that in ammonia and that of the N-Cl bond as given above, the quadrupole coupling constants are calculated to be xacl= 3.6 MHz and XM,= 0.6 MHz. ACKNOWLEDGMENTS We wish to thank Professor D. J. Millen for the sample of 16NH&1 and for many helpful discussions. D. G. L. thanks the Royal Society and Accademia die Lincei for a fellowship.
RECEIVED:
September 20, 1971 REFERENCES
1. J. M. MAYS AND B. P. DAILEY, J. &em. Phys. 20, 1695 (1952). 2. D. G. LISTER _&NDD. J. MILLEN, C&m. Commun. 1606(1970). A. M. Mirri, F. Scappini and F. Cazzoli, J. Mol. Spectrosc. 38, 218 (1971). :: D. A. GILBERT, A. ROBERTS AND P. A. GRISWOLD, Phys. Rev. 76,1723 L (1949). 6. G. E. MOORE AND R. M. BADGER, J. Amer. Chem. Sot. 74, 6076 (1952). 6. J. BARDEEN AND C. H. TONNES, Phys. Rev. 73, 97 (1948). Y. C. C. COST~IN, J. Chem. Phys. 29, 864 (1958). 8. J. KRAITCHMAN, Amer. J. Phys. 21, 17 (1953). 9. H. B. BURGI, D. STEDMAN AND L. S. BARTELL, J. Mol. Struct., in press. 10. W. S. BENEDICT AND E. K. PLYLER, Can. J. Phys. 36, 1235 (1957). il. J. D. SWALEN AND J. A. IBERS, J. Chem. Phys. 36, 1914 (1962). id. D. M. DENNISON BND G. E. UHLENBECK, Phys. Rev. 41, 313 (1932). 1s. J. H. GOLDSTEIN, J. Chem. Phys. 24,106 (1955). 1-L W. GORDY AND R. L. COOK, “Microwave Molecular Spectra,” Chap. 14, Wiley, New York, 1970. lb. J. HINGE, M. A. WHITEHEAD AND H. H. JAFFE, J. Amer. Chem.Sot. 86,148(1963). 16. L. PIERCE, R. G. HAYES AND J. E. BEECHER, J. Chem.Phys. 46,4352 (1966).
286-295 (1972)
Molecular Structure and Nuclear Quadrupole Coupling Constants of Monochloroamine from Microwave Spectroscopy G. CAZZOLI, D. G. LISTER ASD I’. G. FrlvE~o Laboratosio
di Spettroscopia Molecolare, Universitd di
C.N.R.
and Istituto
Chimico
“G. Ciamician”,
Bologna, Boloyna, Italy
The rotational constants B and C and the nuclear quadrupole coupling constants of six isotopic species of monochloroamine have been obtained from low J&? branch transitions. The ra structure has been determined as 7.X-R = 1.017 f 0.005 fi, HNCl
= 103”41’ f
22’,
1’N-Cl= 1.7480 f 0.0001 HNH
d,
= 107” Z!Z2’.
INTRODUCTION
Monochloroamine (NHzCl) is the last member of the isoelectronic series of molecules CH&l (I), NH&Z1 (2), HOC1 (3) and FCl (4) to be studied in detail by microwave spectroscopy. It is also of interest because of the question of the nonplanarity and the inversion of the amine group in NHzX molecules. Moore and Badger (5) have observed the infrared spectrum of this molecule and Lister and Millen (2) have measured the OoO--+ loI transitions of five isotopic species and the pG component of the dipole moment (6). In order to determine the molecular structure completely and to obtain thfl nitrogen and chlorine nuclear quadrupole coupling con&ants \ve have repeated and extended the microwave measurements. EXPERIMENTAL
PROCEI>URF,
Samples of monochloroamine were prepared by the method first used by Lister and Millen (2) to observe the microwave spectrum of this molecule but with the following modification. Dichlorine oxide vapor and the vapor from an aqueous ammonia solution were mixed at a low pressure in a trap at -40” and after .? min the trap was cooled to -SO” and the excess ammonia was removed by pumping. The solution used to prepare the deuterated monochloroamine was prepared by placing ci ml of DzO in a 2-1 bulb, pumping out the air and filling the bulb to a pressure of one atmosphere with ordinary ammonia. A solution of 151Vammonia was prepared by placing 0.5 g of 90 7% enriched 15?;$ammonium chloride and 0.4 g sodium hydroxide in a tube and distilling 0.5 ml of water into the tube. 286 Cupyripht
0
1972 by Academic
Press,
Inc.
MOLECULAR
STRUCTURE
OF NH&l
287
A 6-kHz Stark modulation spectrometer with a 3-m J band waveguide cell and phase stabilized Oki 24V10A and 30Vll klystrons was used for this work. Transitions in the region 48-58 GHz were observed using the second harmonic of these klystrons. Frequency measurements are accurate to ~0.02 MHz on the fundamental and the resolutions and sensitivity obtained may be judged from Fig. 1 which shows a recording of the 000+ 101, AFI = + 1 transition of 14NHi7C1. Measurements on the 1 -+ 2 transitions are somewhat less accurate than this because of the more complicated nuclear quadrupole coupling effects. In order to reduce the decomposition of monochloroamine, the cell was cooled to -30” and samples were flowed through it. TREATMENT
OF RESULTS
of six isotopic species of The 000 --) 101, Ilo -+ 211 and 111 -+ 2~ transitions monochloroamine have been observed to determine the rotational constants B and C and the nuclear quadrupole coupling constants. Centrifugal distortion effects have been neglected as work on hypoclorous acid (3) indicates that they should introduce errors of not greater than 0.5 MHz if the rotational constants are determined from the equations ~(0004 Y(Ilo --+ 211) -
101) = B + C,
Y(111-+ 212) = 2(B - C).
The quadrupole treated
coupling of the chlorine (II) and nitrogen (12) nuclei has been using the II, J, (F1), 12, F formulation of Bardeen and Townes (6) and
27d5.025 FIG.
1. Pen recording
27606 MHz.
Frequency
27d8.025
27d7.025
AF = +I transition of 14NH:‘C1 of 000 -) 101 _ AFI = fl, increases from left to right and markers are every 0 .l MHz.
288
CAZZOLI, LISTER,
AND FAVERO
the full secular equations have been solved. The nuclear quadrupole coupling of the lo1 level depends only on xW and the frequencies of the hyperfine components of each 0~ --+ lo1 transitions have been fitted by least squares to xaa(Cl), xaa(N) and the frequency (vo) of the transition in the absence of nuclear quadrupole coupling effects. The frequencies of the hyperfine components of each of the observed 1 -+ 2 transitions have been fitted by least squares to XW,(Cl) XW(N ) and y. using the values of xaa(C1) and xQa(N) determined from the appropriate Ooo-+ lo1 transitions. The measured line frequencies of the 000+ lo1 transitions, the differences between the observed and calculated line frequencies and the values of xaa and vo with their standard errors are shown in Table I. Results for the 110-+ 211and 111+ 212transitions are given in Table II and Table III. In the case of the 110--+ 211 transitions of 14NHD3%1 it was only possible to measure lines with little dependence on x&V) and the value for this quantity given in Table III is considered unreliable. The rotational constants B and C and the nuclear quadrupole coupling constants are collected in Table IV. Except for xti (N) of “NHD35C1 the values of xbbfor both nuclei are the mean of the values from the 110-+ 211and 111+ a12 transitions. The value of &b(N) for “NHD35C1 given in Table IV is that from the llo --, 211 transition. MOLECULAR
STRUCTURE
A r. structure determined by least-squares fitting of the observed moments of inertia Ib and I, to the four structural parameters of monochloroamine is shown in the first row of Table V. The quantity A - (B + C)/2 has beendetermined for the ground rotational state of 14NH?C1 as 8.56 f 0.01 cm-’ and for that of “NHD35C1 as 5.83 =k 0.01 cm-’ by Moore and Badger (5). Combination of these results with the present microwave data gives the following values of A: ‘*NHi5C1
270650 f
300 MHz
‘*NHD%l
157900 f
300 MHz
The structure in the second row of Table V was obtained by including the values of A in the least-squares calculation. The standard deviations on B and C are 1.8 MHz for both structures and the differences between the observed values of A and those calculated from the second structure is less than 150 MHz. The error on these values of A is considerably larger than the errors on rotational constants usually used to determine molecular structures by the isotopic substitution method (7). However, the uncertainty in the inertial difference AI, used in Kraitchman’s equation (8) leads to the following errors in the principal axis coordinat.es of the hydrogen atoms (&a = 0.001 A, 6b = 0.002 8, 6c = 0.007 A). The errors givr rise to uncertainties of 0.005 b in the N-H bond length and l”30’ in the HNH bond angle.
E
,
N-?S
w-w
72-?S
-0.02
28 088.45
TABLE
-0.02 f f f
27 621.78 27 602.07 -78.50 5.10
0.01 0.03 0.04
0.00 -0.01 0.00
0.01 0.01 0.01
Obs.Calc.
27 027.75 -99.65
27 052.66
27 007.81
27 032.73
Obs.
f f
‘eNH;‘Cl
I
0.01 0.0;
0.00
0.00
0.00
26 564.27 -78.51
26 583.88
26 548.56
26 568.21
-0.01
-0.01
0.02
Ohs.CELIC.
f 0.01 xk 0.06
‘“NH”CI 2
24 649.72 -99.07 5.10
24 630.09 24 628.99 24 630.93
24 654.42 24 655.48 24 653.96
Obs.
0.00 -0.02 0.02
0.00 -0.02 0.02
rt 0.01 I f 0.0: I f o.oi
“ND%1 2
STRUCTIJ~ZE (IN MHz)
Obs.
COUPLING
Obs.CdC.
WITH QUAURUPOLE
27 586.55 27 585.46 27 587.38
27 605.76 27 606.85 27 605.27
Obs.
‘“NH:‘Cl
lo, TRANSITION
0.01 0.04 0.05
0.00 -0.01 0.00
28 043.74 28 042.67 28 044.57
28 063.48 f -99.61 f 5.10 f
0.01 0.01 0.02
28 068.22 28 069.31 28 067.75
Obs.
I”NH;‘Cl
Ooo +
:26 213.02 -99.1 5.2
26 237.82
26 193.36 26 192.25
26 217.74 26 218.86 26 217.29
f f f
0.03 0.03 0.06
0.03 0.2 0.2
-0.07
-0.02 -0.04
“NHDWI
F,”
+
F*’
55 520.70 55 521.83 55 520.24
0.08 0.10
0.05 0.07 0.06
5G 455.42 56 456.53
56 430.92 56 429.73 56 431.82
0.03 0.02 0.01 0.1
56 474.69
56 u9.59 47.7 0.3
-0.06 55 508.14 -0.08 55 506.89 -O.OOt 1 55 508.94
56 439.32 56 438.12 56 440.18 0.02
-0.02 -0.07 -0.03
0.00 0.03 0.00
55 51G.18 & 0.02 37.6 f 0.1 0.2 f 0.1
55 536.031
55 511.27 55 512.43 55 510.77
-0,OG -0.02 -0.03
56 443.34 56 444.52 56 442.88
0.04 0.02 0.04
0.04 0.06 0.07
Ohs.CdC.
WITH
55 526.471 -0.15
-0.04
56 462.69
55 501.50 55 500.2B 55 502.37
Obs.
‘4NHS7C1 2
TRANSITION
Obs.
f f f
“NH3’CI 1
Ilo -+ 211
I
.I.
TABLE
0.05
0.00
54 352.49 48.1
54 377.411
54 342.16 /
1
f f
0.01 0.1
0.01
-0.01
53 415.34 37.7
53 434.84
53 407.28
53 410.67
53 425.62
53 400.51
-0.03
0.1
0.01
0.07
0.06
-0.12
zt 0.05 f 0.3
16NH:‘Cl
STRUCTURE
53 419.96
COUPLING
-0.03
II
54 346.471 -0.01 I
54 365.38
54 333.55
54 358.56
WH;%
QUADRUPOLE
--
(IN
49 836.25 48.1 0.2
49 861.201
49 849 l(
49 817.7’ 49 816.5: 49 818.5:
49 841.94 49 842.9! 49 841.4(
‘4ND”CI P
MHz)
zt 0.02 f 0.3 f 0.2
0.01
0.00
0.06 0.06 0.05
-0.04 -0.09 -0.05
Obs. Calc.
52 917.04 47.8 -0.1
52 941.91 3
52 930.0:
52 898.3: 52 897.3 52 899.1:
52 922.71 52 923.91 52 922.3,
Obs.
f f f
0.01 0.1 0.1
0.00
0.00
-0.03 -0.03 -0.03
0.02 0.02 0.05
“,k:
F’
m(N)
:bb(Cl)
0
w-%
%-?4 W-N
F” +
-
0.03 0.16
xix 0.02 I * 0.1 f 0.2
55 824.28
55 799.18 47.7 0.1
-0.03
55 789.53
55 789.53
-0.01 -0.06
0.04
55 811.10
55 792.14 55 792.5G
-0.07 0.00 0.00 -0.03 -0.01
804.84 806.33 780.21 780.67 779.85
55 55 55 55 55
_
54 886.98 37.7 0.3
54 906.741
54 879.26
f f f
0.02 0.1 0.1
0.09
0.08
-0.01 0.00
54 881.41 54 881.83
0.02 0.08 -0.03 -0.04 0.03 -0.04
891.42 892.94 871.94 872.46 871.57
Ohs: Calc.
54 896.27
54 54 54 54 54
Obs.
53 753.56 47.9
53 778.66
53 743.88
53 746.69
53 765.48
53 734.76
53 759.59
Obs.
f f
0.05 0.3
0.19
-0.09
-0.02
-0.05
0.02
-0.05
Ohs.Cak.
_.
-
52 836.89 37.7
52 856.61)
52 829.21
52 831.39
f f
0.05 0.3
0.08
-0.13
-0.11
0.00
0.09
52 822.16
52 846.33
0.06
I-
Ohs: ChlC.
52 841.75
Obs.
‘GNHarC1 2
-
-
48 759.70 47.9 0.1
48 784.44
48 750.01
48 750.01
48 771.62
48 7G5.36 48 766.93
Obs.
i 0.04 i 0.3 z!z 0.4
-0.OG
0.03
-0.05
0.00
-0.03 0.1
‘4ND3%J 2
TABLE III lx1 -+ 21e TKANSITION WITH QUADRUPOLE COUPLING STRUCTURE (IN MHz)
51 931.21 47.9 -0.7
51 956.03
51 921.60
51 924.28 51 924.88 51 923.98
51 943.15
f f f
0.01 0.1 0.2
0.02
-0.01
-0.03 0.01 0.01
0.00
CAZZOLI, LISTER, AND FAVERO
292
“NK~5C1 was chosen as the parent molecule and the atoms were located in its principal inertial axis system using Kraitchman’s equations and the appropriate inertial differences. Figure 2 shows the orientation of the principal axe3 relative to the molecular framework and it can be seen that the c coordinates of the nitrogen and chlorine atoms are very small. These coordinates were determined from the first moment (I;mici = 0) and product of inertia conditions (Zmiaici = 0). The coordinates are given in Table VI and the structure calculat,ed from them is given in the third row of Table V. TABLE
IV
ROTATION-AL CONSTANTS AND QUADRUPOLE COUPLING CONSTANTS (IN MHz) Molecules
&I
CO
x&l)
xmz
Xbb&%’
6% -__-
_______
‘4NH:kl 14NH;‘C1 lSNH:sC1 1SNH;‘Cl 14NHDasC1 ‘“ND:?1
14 13 13 13 13 12
194.3413 958.3413 663.6113 426.7513 352.9712 594.0012
-
xbmb
869.14-99.61 643.73 -78.50 364.14 -99.65 137.52 -78.51 860.05-99.1 055.67 -99.07
f f f f zk ziz
0.0447.7 0.0337.6 0.0248.08 0.0737.7 0.2 47.8 0.0848.0
f f f f f f
0.1 0.1 0.09 0.3 0.1 0.3
* Mean of values from the 1x0+ 211and 111---f 212 transitions. h Mean of values from the 110 + 211 and 111---) 212 transitions for 14NHD3W which is the result from the llo + 2,* transition.
TABLE
5.1 5.1
f f -
0.2 + 0.1 0.2 & 0.1
0.05 0.04
5.2 f 0.2 -0.1 f 5.10 zk 0.071 0.2 f
with the exception
0.1 0.1
of t.hat
V
MOLECULAR STRUCTURES
ra(1) ro(2)
-
r,(l) rs(2)
rN-CL&
rN -II@)
HrjH
HGCI
V
1.7523 1.7522 1.7480 1.7481
1.0218 1.0166 1.0161 1.0185
107” 107”13’ 108”23’ lO6”25’
103”29’ 103”42’ 103”52’ 103”30’
66”55’ 66”29’ 65”48’ 66”38’
&+ is the angle between angle.
the extension
of the NC1 bond and the bisector
TABLE
of the HNH
VI
PRINCIPAL AXIS COORDINATES8 a(&
N
-1.4546 - 1.4546 -1.1865
Cl
-1.1865 0.5597
H
b(A)
____
-
f0.8241 hO.8156
0.5597 a Calculated,
using the conversion factor 5.05376 X 105a.m.u. Aa.
c(A) 0.4551 0 4699 -0.0756 -0.0780 0.0040 a.0042
MOLECULAR
STRUCTURE
OF NH&l
293
Cl
___--
-a’
FIG. amine.
2. Orientation
of different
The axes are labeled as principal inertial principal inertial principal axes of 2, 2
a, c a’, 8
axis
systems
follows: axes of NH&I, axes of ND&l, the chlorine nuclear
in the symmetry
quadrupole
coupling
plane
of monochloro-
tensor.
As a check the hydrogen b and c coordinates were calculated from AIb and AI, for 14NHi5C1 and “ND?C1 using the a coordinate of the hydrogen atom determined from the single deuterium substitution. The coordinates determined in this way are given beneath the first set of coordinates in Table VI and the structure derived from them is given in the bottom row of Table V. Finally the nitrogen and chlorine atoms were located in the principal axis system of 14NHi7C1assuming that A for 14NHi7C1is the same as that of 14NHi5C1 and the geometry of the amine group given above. The nitrogen chlorine bond length of 1.7479 A calculated in this way is in excellent agreement with the t‘#values in Table VI. DISCUSSION
The nitrogen chlorine bond length may be compared with that in nitrogen trichloride (TV = 1.759 f 0.002 8) as determined by electron diffraction (9).
a 2
294
CAZZOLI, LISTER,
AND FAVERO
The nitrogen-hydrogen bond lengthoand the NHN angle are very similar to those in ammonia (T, E-H = 1.012 A, HNH = 106”40’) (10) and the degree of nonplanarity 4 (Table V) is slightly larger than that in ammonia (4 = 62”). The observed transitions of monochloroamine have not shown any splitting that may be attributed to the inversion of the amine group. Moore and Badger (5) have measured the fundamental and first overtone of the NH2 wagging vibraO-l and l-3 tion as 1032 cm-’ and 2020 cm-‘. The nearly equal separations show that the v = 2 levels must lie below the top of the barrier to the inversion of the amine group. This places a lower limit of E (v = 2) N 2500 cm-’ on the barrier, and it is therefore considerably higher than that in ammonia (2020 cm-‘) (11). This barrier height may be used with the frequency ~3, the molecular structure, and a Dennison-Uhlenbeck potential function (12) to show that the splitting of the Of and O- states is unlikely to be more than a few hundred megacycle/ second. The p. component of the dipole moment is symmetric with respect’ to the inversion of the amine group and the observed transitions are purely rotational, Any inversion doubling, therefore, depends on differences between the rotational constants of the O+ and O- states and will be extremely small because those states are almost degenerate and their rotational constants nearly equal. The chlorine nuclear quadrupole coupling constants (xua) of ‘*NHi’Cl and ‘“ND”,“Cl may be used with the molecular structure to show that the principal axes of the chlorine nuclear quadrupole coupling tensor are oriented at an angle of Y”5’ & 45’ to the principal inertial axes of ‘*NHpCl as shown in Fig. 2. As the nitrogen chlorine bond is inclined at an angle of 2”36’ f 5’ to the axes of ‘*KHi5C1 one of the principal axes of the chlorine quadrupole coupling tensor nearly (hointides with this bond. The principal elements of the tensor are: xc = -99.81
f
0.05 MHz,
xy = 47.10 f
0.10 MHZ,
x+ = 52.71 & 0.15 MHZ, where the y direction is perpendicular to the molecular symmetry plane. If the usual method of interpreting the asymmetry in the nuclear quadrupole coupling tensor is adopt’ed (IS), there is 2.6% double-bond character in the nitrogen chlorine bond and it is such that a loss of electrons from the chlorine 3p, orbital takes place. A similar effect has been observed in hypochlorous acid (4) indicating 1.6 % double-bond character and a loss of electrons from the chlorine 3p, orbital. The ionic character of the nitrogen chlorine bond calculated from xz assuming no s hybridization of the chlorine is 0.09. This may be compared with the value of 0.12 predicted using Gordy’s relationship between ionic character and electronegativity difference (14) taking the electroncgativit8y of the amine group as 2.76 (15).
MOLECULAR
STRUCTURE
OF NH&l
295
The nuclear quadrupole coupling constants of the nitrogen atom may be accounted for approximately using the approach of Townes and Dailey as applied to NXY, molecules by Pierce et al. (16). The nitrogen atom is assumed to form four orthonormal sp3 hybrid orbitals and these are determined by the molecular geometry. If the ionic character of the N-H bonds is assumed to be the same as that in ammonia and that of the N-Cl bond as given above, the quadrupole coupling constants are calculated to be xacl= 3.6 MHz and XM,= 0.6 MHz. ACKNOWLEDGMENTS We wish to thank Professor D. J. Millen for the sample of 16NH&1 and for many helpful discussions. D. G. L. thanks the Royal Society and Accademia die Lincei for a fellowship.
RECEIVED:
September 20, 1971 REFERENCES
1. J. M. MAYS AND B. P. DAILEY, J. &em. Phys. 20, 1695 (1952). 2. D. G. LISTER _&NDD. J. MILLEN, C&m. Commun. 1606(1970). A. M. Mirri, F. Scappini and F. Cazzoli, J. Mol. Spectrosc. 38, 218 (1971). :: D. A. GILBERT, A. ROBERTS AND P. A. GRISWOLD, Phys. Rev. 76,1723 L (1949). 6. G. E. MOORE AND R. M. BADGER, J. Amer. Chem. Sot. 74, 6076 (1952). 6. J. BARDEEN AND C. H. TONNES, Phys. Rev. 73, 97 (1948). Y. C. C. COST~IN, J. Chem. Phys. 29, 864 (1958). 8. J. KRAITCHMAN, Amer. J. Phys. 21, 17 (1953). 9. H. B. BURGI, D. STEDMAN AND L. S. BARTELL, J. Mol. Struct., in press. 10. W. S. BENEDICT AND E. K. PLYLER, Can. J. Phys. 36, 1235 (1957). il. J. D. SWALEN AND J. A. IBERS, J. Chem. Phys. 36, 1914 (1962). id. D. M. DENNISON BND G. E. UHLENBECK, Phys. Rev. 41, 313 (1932). 1s. J. H. GOLDSTEIN, J. Chem. Phys. 24,106 (1955). 1-L W. GORDY AND R. L. COOK, “Microwave Molecular Spectra,” Chap. 14, Wiley, New York, 1970. lb. J. HINGE, M. A. WHITEHEAD AND H. H. JAFFE, J. Amer. Chem.Sot. 86,148(1963). 16. L. PIERCE, R. G. HAYES AND J. E. BEECHER, J. Chem.Phys. 46,4352 (1966).