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Microwave spectrum of ethyl hypochlorite
JOURNAL
OF MOLECULAR
69,45&472
SPECTROSCOPY
Microwave
(1978)
Spectrum
of Ethyl Hypochlorite
R. D. SUENRAM,’ F. J. LOVAS, AND D. R. JOHNSON National Bureau of Standards, Washington, D. C. 20234 The microwave spectrum of ethyl hypochlorite has been analyzed in detail in the region of 20-60 GHz. Observed transitions for C~H~OWI in the ground state have been fit to a Hamiltonian model which includes y centrifugal distortion terms. The lowest vibrationally excited state of CzH&Wl and the ground state and lowest vibrationally excited state of CzH@Cl were analyzed with a rigid rotor model. This lowest vibrational mode lies at 125 f 23 cm-l and is most likely the torsional motion about the C-O bond. The dipole moment has been measured and found to have two nonzero components; p. = 1.623 f 0.010 D, pb = 1.097 f 0.005 D. No A-E torsional splittings were observed in either the ground state or the v = 1 state implying a lower limit for the barrier to internal rotation of ~3.0 kcal/mole. Ethyl hypochlorite was synthesized in the waveguide by the reaction of chlorine nitrate with ethanol. INTRODUCTION During
the recent
we observed ing that
work
chemical
experiments
ethyl
the
hypochlorite
moment,
investigation
and assigned
of the microwave
the spectrum
identity
of the
we were (C2H,OCl).
and nuclear
second
able
species
coupling
Instrumental. 80-kHz 1.1-m
square P-band
Teflon
The
microwave
spectrometer
wave
modulation.
Due
Stark
cell constructed
end windows
the high-voltage which
were sealed
Stark
feedthrough.
were lubricated
Initial
broad
backward
wave
free running
with
banded oscillator.
klystrons
Kel-F
scans
with
used
Teflon
observed
1 NRC-NBS
Postdoctoral
with
a spectrum
of stainless
a phase had been
in oil baths
in the chlorine
458
Copyright
Q 1978 by Academic
AU rights of reproduction
Press, Inc.
in any form
mend
additional species
spectrum,
as
dipole
hypochlorite.
of conventional nature steel
design
contained
with
of CH&H20Cl,
and
Teflon
O-rings as was the vacuum
Fellow, 19751977.
0022-2852/78/0693-045%$02.00/O
with
unknown
a
was used.
connection
two ground
for
glass joints
grease.
which were stabilized
as an impurity
was
reactive
the microwave transitions. Sample preparation and handling. As was mentioned was first
(I),
DETAILS
The inlet system
were made
Once
for ethyl
to the highly entirely
but
this
the microwave
constants
nitrate
For some time follow-
uncertain
identify
we report
I. EXPERIMENTAL
of chlorine
species.
was
to conclusively
In this paper
quadrupole
spectrum
of a new molecular
locked, observed
R-band
(26.5-40
GHz),
and roughly
assigned,
were used to accurately
measure
in the introduction,
nitrate
(ClNO,)
study.
CH&H20Cl It was later
4.59
MICROWAVE SPECTRUM OF CH&HZOCI
found that the easiest way to prepare it was to cool the absorption cell to - 78°C with dry ice, introduce several hundred millitorr pressure of ClNOs and then an approximately equal amount of ethanol. The cell pressure could then be reduced to the normal working pressure S-20 mTorr (1 mTorr = 0.133 Pa). The first sample introduced in this fashion usually lasted for only a few minutes but the second sample lasted considerably longer (1-2 hr). It shouId be emphasized that it was only when the absorption cell was cooled with dry ice could we observe the spectrum of CH,CH20Cl. Furthermore, the other reaction product, nitric acid, does not have sufficient vapor pressure at - 78°C to give rise to a microwave spectrum so any possibility of interference from nitric acid spectral lines is eliminated. II. SPECTRAL ASSIGNMENT AND MOLECULE IDENTIFICATION The microwave spectrum observed was that of a typical slightly asymmetric prolate rotor and the spectral assignment was far less of a problem than was the molecular identification. For slightly asymmetric prolate rotors, the strong u-type R-branch transitions occur in bands which are spaced at intervals of B + C. The transitions with high K-r values which form these bands have very fast Stark effects and are readily identified in wide frequency scans such as the one illustrated in Fig. 1. The transitions with low values of K-1 are split out from the main cluster of transitions and are easily identified. The K-1 = 0 transition will be found in the vicinity of the band center and is characteristically identified by its slow second-order Stark effect. Accordingly, the K-1 = 0 and 1 transitions for the J = 6-7 and J = 7-8 bands were identified and fit to a rigid rotor model. It was then possible to predict the remainder of the u-type spectrum, and the molecule was found to be quite prolate (K = - 0.94). The u-type R-branch transitions have very little dependence on the A rotational constant, hence, the A rotational constant was not well determined at this stage. The b-type transitions have a sizable dependence on the A rotational constant and as a result they were not well
ETHYL
HYPOGHLORITF
J=6-5 J=7-6 0
40
FREQ”ENCY3;Hd FIG. 1. The low-resolution microwave spectrum of ethyl hypochlorite
in
the 30 to 40 GHz range.
41,4
41.3
41.3
4 0.4
44 324.44(6)
F -,I,2 F - 9,2
-
J/2 9,2)
30.3 F - 5,2 - 3/2) F - J/2 - 5,2
9,2
F - 9,2
49 314.42(13)
49 311.87(13)
-0.18
0.14
-0.04
29 614.10(10)
- J/2
F = J/2
49 313.83
-0.08
29 616.52(E)
-0.04
29 609.92(S)
0.17
29 607.73&i)
29 612.32
-I,/2
21 268.60(12)
0.25
0.05 0.03
-0.05 O.D3
0.05
-
- J/2 -1312)
-
'I.7
‘I,6
-
F -1312
F -11/z
F ‘1512
61.5
61,6
60.6
50.5
-13,2
-II/2
-15/2
60.6 F = 912 - 9/2
F = 912 F -15/2
'0.7
61.6
61.5
51.4
51.5
52,3 F -,I,2 9,2 F -1312 -1112)
-
61.5
F =13/2 -II,2 F -15/Z -1312) 61.6
62,4
50.5
41.3
F '1112 - 9/2 F = 912 - J/2)
-
60.6
0.00 0.00
-
F -13/2 -,I,2
F 3 J/2 - 5,2 F -,I,2 - 912)
41.4 F - 9/2 J/2
F -1112 - 9,2 F -1312 -11/21
40.4 F - 9,2 - J/2 F = J/2 512’
51.4
51.5
50.5
-0.23
-0.04
-0.02
0.02
0.03
-0.05
0.02
-O.Zlb
0.00
-0.13b
F -1112
- 3,2> ¶I2
003.57
321 .27
21 267.62
2,
44
29 348.25
39 264.40b
29 151.43b
40.4 F = 512 - 512
-
F - 5,2 F -1112
31.2 F - l/2 - 5/2) F - 9/2 J/2
21 266.00(R)
21 004.32(8)
F - 9/2 F -,I,2
- J/2) - 9,2
21 001.52LS)
F - 512 312) F = J/2 - 5,2
30.3
- 5,2
F = J/2
29 354.01(5)
F - 712 - J/2
44 321.36(6)
29 349.97(5)
F = 512 - 512
20.2 F = 9/2 - J/2
29 345.37(5)
- 9,2
F - 9,2
31.3
29 34X.31(5)
39 263.88l9)
20.3 F - 3,2 - 3,2
31,2
29 147.27(lo)d
'0.1 F = J/2 - 5,2
-
- 712
20.2
21.2
F = J/2
21.1
I
36 302.32
36 302.32(14)
37 213.06
36 738.42
37 213.0617)
59 110.26 36 738,42(J)
30 346.54
31 525.07
59 110.26(16)
30 349.12(5)
30 348.35(5)
30 344.66(5)
30 343.69(5)
31 526.36(12)
3, 523.66(12)
31 898.Y(
-0.17
0.08
-0.49
0.58
-0.17
0.14
-0.05
0.07
-0.20
0.03 0.01
-0.02 -0.03
-0.05
-0.25
0.03
31 117.75
31 117.75(15) 31 898.38(10)
0.02 -0.02
0.01 -0.03
31 496.78(6)
31 495.70(12)
0.11
0.30 31 496.42
-0.04
-0.01 -0.06
0.07
-0.05
-0.04 -0.07
0.07 Cl.04
0.03
(MHZ)
ho(obs-talc)=
26 583.60(10)
26 583.60
25 932.45
26 251.06
"0' IMHZ)
25 933.35(5)
25 932.15(5)
25 931.05(5)
26 251.50(5)
26 250.W!5)d
"ohs (MHz)
The Measured Rotational Transitions for C~H~OWI in the Ground Vibrational State
TABLE
'1.7
71.6
-
-
%,8
I.8
‘00 10
9
927
F -1912 ,9,2 F =21/2 - 21/Z)
' F =17/2 : ,7,2 F -23/2 - 23/2'
'028
-
F -17/2 17/2 F =19,2 - 19/2'
-
go.9 F -,5,2 - ,5,2 F -21/2 21/2)
3 ‘019
9
- 80.8 F -13,2 - 13,2 F -,9,2 19121
81,7
F -15,2 - 15,2 F -17,2 - 1712)
72.5
8 2,6
81.7
70,7
-
F ‘1,,2 - 9,2 F -1712 1512'
- 62.4 F -1512 - 13,2 F '13,2 11/Z]
F -1112 9/2 F =,7,2 - 15121
- 62,5 F =15,2 - ,3,2 F -l3,2 ,112'
60.8
72.5
72,6
0.18
42 527.00
42 527.00(14)
32 7W.75(5)
32 698.18(10)
52 579.70(10)
31 996.85(10)
3, 993.20(10)
31 371.80(10)
31 367.94(10)
0.03 -0.07
52 579.70
32 699.99
0.05
0.29
31 369.9,
31 995.06
-0.2,
42 046.55
0.04
-0.09
41 485.86
42 046.55111)
-0.19
4, 978.20
-0.03
0.10 -0.11
-0.11 0.10
41 485.86(11)
36 784.67
36 760.32
41 978.20(7)
36 785.39(14)
36 783.85(14)
36 76,.10(14)
36 759.42(14)d
-
'lo,11
-
'01.10
-
'30.13
'31,13
F -27,2 -25,2> F -29,2 -27/Z
F -2512 -23,2> F =3,/2 -29/2
140.14
F -25/2 -25,2] F -2712 -27/Z
F -23,2 -23,2) F =29/Z -29,2
'31.12
F -2312 -23,2) F =25/2 -25/2
F -21/2 -2,,2> F -2712 -27,2
'20.12
49 837.63(g)
49 836.01(g)
35 327.36(35)
35 323.92(35)
34 363.75(30)
34 360.39(30)
33 653.31(10)
-
F -1912 -2,,2, F -21,2 -23,2 l21.11
33 648.08(14)
"1.11
32 ,3,.72(10)
32 130.30(10)
33 489.35(10)
33 486.13(10)d
F -1712 -,7,2> F -2312 -25,2
'O2,8
F -U/2 -,9,2) F -2312 -21/2
F -19,2 -,7,2' F -2512 -23/2
"0,ll
F -2112 -2,,2) F -2312 -23/2
F -1912 -,9/q F =25/Z -25/2
"l,,o
TABLE I-Continued
49 836.84
35 325.66
34 362.09
33 650.70
32 131.05
33 487.76
(continued)
-0.02
0.04
0.21
-0.05
0.25
-0.13
SUENRAM,
462
LOVAS, AND
TABLE
I-Continued
%bS (‘w '41.13
-
'40,14 36 381.00(14)d
F ‘27/Z -27,2, F -2912 -2912
36 384.36(14)
-
160,16
F '2912 -29/2) F =35,2 -3w
38 792.24(6)
F -3112 -3112 F -33/2 -3312'
38 795.78(6)
"1,16
-
'70.17
F -31/2 -3112, F '37/Z -37/Z
40 153.89(101
F ‘3312 -33/Z, F -3512 -35/Z
40 157.43(10)
'61.17
-
'80.18
F -3312 -3312% F =39/z -3912
41 625.71(11)
F -3512 -3512' F =37/2 -37/Z
41 629.3601)
201.19
-
'92.18
F =37/2 -3512, F =43/2 -4112
31 403.36(10)
F '3912 -37/Z> F -4'12 -3912
31 405.10(10)
"3,16
-
202.19
A$obs-talc)' VW
36 Z32.69
F '2512 -25/Z> F -3112 -3112
'61.15
JOHNSON
40 637.80(20)
-0.21
38 794.02
0.09
40 155.67
-0.16
41 627.54
0.11
31 404.24
0.W
40 637.80
0.0'
%ypothetical rotatfonal tranrltion freqwn~y employed in the centrifugal distortion ana,yr,s. b Not Included in fit since only one hyperfrnc component was measured. 'For the hyperflne c~n,,onentr Rnployed In the fit, the observed mlnur calculated values of the hyperfine splittlngn are give". dThe nu"berr in parentheses are the actual estimated uncertainties in the frequency measurenent and they refer to the 'list dlglts given.
predicted. It should be noted at this point that the molecule remained unidentified although the basic spectrum had been assigned, and it was still not known whether the molecule might have other active dipole moment components. From the observed broadbanded scan, a series of strong transitions, which had similar Stark effects and were approximately equally spaced, were singled out. From the predicted spectrum it was possible to identify these transitions as members of the b-type Q-branch series J1,.r--l - Jo,r. All transitions of this Q-branch series have approximately the same dependence on the A rotational constant, thus if A is poorly determined these transitions will all exhibit a systematic shift from the predicted frequencies. These transitions were fit to the rigid rotor model and aside from some small centrifugal distortion corrections the fit appeared to be quite consistent. The rotational constants were now quite well determined and c-type transitions could be predicted with considerable precision. None of the predicted c-type transitions could be observed. The available spectral information provided the following clues as to the identity of the compound. First, a weaker set of u-type bands of about i the intensity of the most intense set had been observed. These were in accord with a chlorine isotope effect suggesting that the molecule contained chlorine. The magnitude of the nuclear quadrupole splittings were such that the chlorine atom would most likely be in an end position on the molecule. Second, the rotational constants indicated that the molecule was nearly a prolate symmetric top and the second moment in the c-axis direction,
MICROWAVE
SPECTRUM
4-63
OF CHaCHzOCl
le) clearly implied that the molecule was not planar, i.e., P,,#O. PC, = $(Ia+rbAlso from the magnitude of the rotational constants it was clear that there were at least three but no more than four heavy atoms arranged in a chainlike fashion beside the chlorine atom. Finally, from the relative intensities of the a- and b-type transitions it appeared that p, > pb. From these observations we concluded that the molecule must have the general structure Cl-X-X-X-(X) where the X’s could be oxygen, nitrogen, carbon, or some combination of the three. In view of the compound originally being studied (chlorine nitrate) one possibility would be Cl-O-N-O, the chlorine analog of nitrous acid (HONO), which conceivably might have been produced as a byproduct in the synthesis of chlorine nitrate. Although the observed rotational constants were in fair agreement with an estimated structure for ClONO, the observed second moment, 2P,, - + 6.4 u.A2, required that the dihedral angle between the Cl-O-N and O-N=0 planes be -60” which was inconsistent with other previously studied XONO molecules. In addition, the spectrum was observed only during the first several weeks of the study of chlorine nitrate and could not be obtained in later studies employing samples of chlorine nitrate of various purity obtained during the fractional distillation purification process. Thus, it appeared that the new molecule was a product of the reaction of chlorine nitrate with some impurity in the absorption cell or gas handling system. Since the inertial defect was the most difficult to explain, we first considered species which would have a nonplanar moment of - 4-6.4 u. AZ and still be consistent with the remaining characteristics. Noting that the ethyl group, C2H5, would provide the required nonplanar moment, the only ethane derivative which could be consistent with all the observations is C2H50Cl with all the heavy atoms lying in a plane, i.e., trans-ethyl
TABLE Rotational,
II
Centrifugal Distortion, and Quadrupole Coupling CZH&YI in the Ground Vibrational State
Constants
31582.01171” 2691.084(10) 2560.916(8) 0.0013(46) -0.00113(48) 0.45(8) -0.577(90) -0.00177(20) -O.OOl37(19)
-93.96(62) 35.76(39) 58.20(28)
"The uncerta,nries ,isted in parentheres are the calculated standard deviations. b
The paraneter '3 is fixed by setting R6 - 0 (see reference 5)
for
464
SUENRAM,
LOVAS, AND JOHNSON TABLE
III
The Measured Rotational Transitions for CoH&PC1 in the Vibrationally Excited State (u = 1) d
"obs (MHz) 41.3
-
F =
28 752.60
40.4 5,2 -
-
CL
5/2
28 748.35(16)'
yrr - "ca1c W)
-0.06
F = 11,2 - Ill2
28 750.2702)
7/z -
l/2
28 754.36(E)
0.02
F =
9/2 -
912
28 756.46(U)
-0.01
r/2
29 069.20(20)
0.06
F = 13,2 - 1312
29 070.37(20)
-0.16
F =
912
29 074.20(20)
0.11
F = 11/Z - 11/2
29 075.47(20)
-0.02
-
F =
9,2 -
61.5
-
51.4
'1.5
-
60,6
F =
9/Z -
-0.12
29 072.44
50.5 7/z -
31 923.65(15)
912
-0.02
-0.05
-0.33
29 459.51
-0.17
-0.13
29 456.78(20)
0.03 -0.03
F = 15/Z - 15/Z
29 457.75(20) 29 461.17(20)
0.09
F = 1312 - 1312
29 462.02(20)
-0.09
61.6 7
-
%.5
-
70.7
1.6 62.6 '01,9
81.7 -
72,5
37 242.35(7)
37 242.35
-0.20
-0.51
42 032.64(11)
42 032.64
-0.09
-0.74
41 558.20(11)
41 558.20
-0.23
-0.67
42 098.07(22)
42 098.07
0.09
-0.64
31 724.09
0.07
-0.26
32 481.96
0.11
-0.37
33 322.44
-0.03
-0.52
36 766.71
0.29
-2.98
34 249.40
0.11
-0.73
- 'Oo.10
F - 17/Z - 17/Z) F = 2312 - 23.12
31 722.35(10)
F = 19/Z - 19/Zj F = 2112 - 2112
31 725.78(10)
"1,lO
-0.12
31 923.65
F = 1,,2 - Ill2
*0.8
-0.12
0.09
F =
51.4
P4 tern
- "0,ll
F = 19/Z - 19/Z) F = 25/Z - 2512
32 480.32(10)
F = 21/Z - 2112, F = 23,2 - 2312
32 483.57(10)
12 1.11 - 120.12 F = 21/Z - 21/2, F = 27/Z - 2112
33 320.85(10)
F = 2312 - 23/2) F = 25/Z - 2512
33 324.00(10)
'20,lZ - "1.11 F = 2112 - 19/Z, F = 27/Z - 25,2
38 766.00(10)
F = 23,2 - 21/Z> F = 2512 - 23/Z
38 767.35(10)
'31,12 - '30,13 F = 23,2 - 2312) F = 29/Z - 29/Z
34 247.77(30)
; 1 ;;;; 1 ;;;;I
34 251.00(30)
hypochlorite? This hypothesis was tested by adding both chlorine nitrate and ethanol, which had been employed in cleaning portions of the sampling system, to the absorption cell. A strong spectrum of the assigned species was readily produced in this manner confirming the hypothesis. The reaction : CH&HzOH + ClNOa + CHzCHzOCl + HN03 2Two other isomeric forms which have an empirical formula of C&,ClO, 2-chloroethanol and chloromethyl methyl ether, were ruled out because they have been previously studied and the lowest energy forms were found to have gauche configurations (cf., R. G. Azrak and E. B. Wilson, J. Chem. Phys. 52, 5299 (1970), and T. Ikeda, R. F. Curl, and H. Karlsson, J. Mol. Speclrosc. 53, 101 (1974).
MICROWAVE
SPECTRUM
465
OF CHaCHoOCl
TABLE IIIG-Cmtiwed
- J;:,K;, K.~K;,
J’,
‘41,13
"ohs (MHZ)
- '40.14
F - 25/2 - 25,2 F _ 3,,2 _ 3,,$
35 264.46(251C
; : ;;;; : :;;;>
35 X7.70(25)
'71.16 - '70.17
-- 3712' 3112
38 891.30(10)
; : ;;;; : ::;;I
38 894.57(10)
F - 3,,2 37/2
F-
'%,I7
- '80,18
F - 33/2 - 33/2 F - 3g,2
- 39/z’
40 305.86(10)
F - 35,2 - 35/Z F - 37,2 - 37/Z)
40 309.15(10)
'91.18 - '90,19 ; : j;;; : :;;;I
41 829.6101)
; : :;;2' : :;;;I
41 832.95(11)
=1,24
a
- 250,25
F - 47,2 - 47/Z F _ 53,2 _ 53,2)
53 452.,,(,4)
F = 49/2 - 4912 F - 5,/2 - 51/Z]
53 455.81(14)
"Oa (MHz)
%
- %alcb (MHZ)
P4
ten
35 266.09
0.16
-1.0,
38 892.94
-0.03
-2.40
40 307.5,
0.13
-3.1,
41 831.29
0.34
-3.97
53 453.96
-0.34
-13.78
Hypothetical rotational transition frequency employed in the calculated fit.
bFor the hyperfine components employed in the fit, the observed minus calculated values of the hyperflne splittings we given. 'The numbers given in parentheses are the actual estimated uncertainties in the measurement and they refer to the last digits given.
is not unreasonable since both ethyl hypochlorite and nitric acid are readily observed and chlorine nitrate is completely consumed. In subsequent studies, methanol and chlorine nitrate were reacted in the absorption cell and the methyl hypochlorite rotational spectrum (2) was easily observed. III. SPECTRAL ANALYSIS
Many of the observed transitions listed in Tables I, III, IV, and V were split by the nuclear quadrupole moment of the chlorine nucleus. In order to adequately account for the observed transition frequencies, it was necessary to iterate between the fitting of the hypothetical centers of the rotational transitions and the fitting of the individual quadrupole splittings from the hypothetical center frequency. This procedure has been outlined by Kirchhoff and Johnson (3). For the ground state of the 35Cl species, a centrifugal distortion treatment of the Watson type (4) was employed using a computer program and fitting techniques developed by Kirchhoff (5). For the lowest torsional states of each species (v = 1) and the ground state of the “7Cl species rigid rotor fits were adequate. The data available was not sufkiently sensitive to centrifugal distortion to provide a more complete analysis. Since the primary interest in the ZJ= 1 states is for the determination of the torsional frequency via relative intensity measurements, it was deemed necessary only to measure a suflicient number of transitions to verify a correct rotational assignment. Similarly, for the 37Clform the objective was the determination of the nuclear quadrupole Coupling constants.
SUENRAM, LOVAS, AND JOHNSON
466
TABLE IV The Measured Rotational Transitions for CzHsW7C1in the Ground Vibrational State
31.2
-
P/Z
29 374.12(16)'
3/2-
3f2
29 378.13(16)
112 -
7f2
29 380.88(16)
¶/2 -
F= F= 5.3
F =
29 376.56
%.3
F -
0.28 0.01 29 628.30
40.4
-0.02
3,2
29 624.83(15)
-0.22
F = 11/z - ,112
29 626.1l(Ja)
-0.05
F =
?I2 -
7f2
29 63D.lU(M~
-0.23
F -
912 _
9/2
29 631.38(10)
41.4
312 -
-
F F =
312 _
7f2 -
48 918.69(131
F = 912 F = 1112 -
II2 912)
48 920.56(131
-
60,6
'1,7
-
72.3
-
32.3
72.5
-
62.4
66.8
-
70.7
81.*
-
'1.1
61.7
-
71.6
91.8
- 90.9
61.6
-0.10
-0.26 0.05 33 923.73
-0.27
-0.30
33 305.51(14)
35 303.3,
0.07
-0.44
35 943.85(35)
35 943.85
0.40
-0.40
33 963.69(14)
35 965.69
-0.20
-0.40
41 047.63(111
41 047.63
-0.22
-0.74
40 373.45107)
40 373.43
-D.%
-0.67
41 571.41(11)
41 571.41
-0.09
-0.77
31 903.82
-0.02
-0.19
32 376.16
0.06
-0.26
34 139.12
0.01
-0.32
0.20
-2.98
31 902.40(10)
F - 17f2 _ 1712 F f 19/z. 1912'
31 w3.2a(lo~
- '"o,lo F - 1712 - I,,2 F - 2312 - 2312'
32 374.89(10)
F - 1912 - 19,2 F - 21/2 - 21d
32 377.40(10)
'21.11 - '20,lZ F = 21/Z - 2lf2 F = 27/Z - 21f2'
34 157.75(05)
F = 23f2 - 2312 F = 25/2 - 2312)
34 160.47(S) 36 396.23
'20.12 - "1.11 F = 21/2 - 19/q F = 2712 - 25/Z
36 395X,7(15)
0.42 0.03
c - 2312 - W2) F - 2312 - 2312
'J6 39&78(J51
-0.a3 -0.42
from transitions which had small unresolvable nuclear quadrupole splittings (lower J, K- 1 = 0, 1, u-type, R-branch transitions) and from transitions which were split into symmetric douMets by the nuclear quadrupole moment
spectral
-0.19
0.23 -0.06
35 923.75(TO1
F = 13/z - 1612 F = 21/2 . 2112'
'?A
-0.12
0.03 48 920.11
30.3 312, 312
7D,7
Initial
-0.28
fits were obtained
(principally
higher J, b-type, Q-branch
transitions).
The center frequency
of
these symmetric doublets was obtained by measuring the frequency of each component and then taking the average. Preliminary quadrupole fits were obtained by assigning quantum numbers to the components of these symmetric doublets. The resulting quadrupole coupling constants were of sticient accuracy to permit the assignment of individual quadrupole components of transitions which were split into asymmetric multiplets by the nuclear quadrupole moment. The frequencies of these components were then measured and the center frequency of the transition determined by iteration
MICROWAVE
SPECTRUM
TABLE
IV-Continued
35 076.44
'31,lZ - '30.13 F = 23,2 F - 29/z
23/Z) 29/z
14
1.13 - '40.14
42 160.11
0.08
-3.83
36 081.77
-0.02
-1.01
F = 25/z - 25/Z) F - 31,2 3112
36 080.38(10)
0.19 -0.11
F - 2912 - 29/z F = 27/Z 27/Z'
36 083.13(10)
-0.20 0.10
'61.15 - '60,16 F - 29/2 F - 35/Z
29/Z] 3512
3% 371.33(10)
F - X,2 - 3112 F - 33,2 - 33/2'
38 373.90(10)
"1.16
- "0.17
F = 31,2 31/2) F - 37,2 - 3712
36 372.62
0.02
-1.83
39 665.64
0.17
-2.40
41 061.40
-0.33
-3.1,
49 720.946
-0.58
-9.46
39 664.28(11)
F - 33,2 - 33/2) F = 35/Z - 3512
39 666.98(111
'61,17 - '60,18 F - 33,2 - 3312) F = 3912 3912
41 060.02(11)
F - 35/2 3512) F - 37,2 - 37/Z
41 062.77(11)
231.22 - 230.23 F - 4x,2 43/q F = 49,2 - 4912
49 719.46(09)
F - 45,2 - 45/Z> F - 47,2 - 4712
49 722.41(09)
%ypothetical
-0.73
0.05 -0.27
35 077.73(25) 42 160.11(22)
'30.13 - '21.12
0.17 0.27 -0.05
35 075.12(2S)c
F - 25/z - 2512) F = 27,2 27/2
467
OF CHaCHzOCl
mtationsl
transition frequency used in the calculated fit.
bFor the hyperfine components employed in the fit. the observed minus calculated values of the hyperfine splittings are given. 'The numbers in parentheses are the actual estimated uncertainties in the frequency measurements and refer to the la*t digits given. dNat included in rigid rotor fit.
between the quadrupole fit and the centrifugal distortion fit. The hypothetical transition frequencies, Q, presented in Tables I, III, IV, and V represent the final iterative stages of the fit. For the ground state of CzH5035C1 a complete P4 centrifugal distortion analysis provided the rotational and distortion constants shown in Table II. With the exception of rI, the taus are fairly well determined and the standard deviation of the fit, 0.202 MHz, agrees well with the average measurement uncertainty. The nuclear quadrupole coupling constants for 35C1listed in Table II are also well determined. Since ~~~~~ could not be determined from the measurements on the ZJ= 1 state of CZH~O~~CI and both the ground and the vibrational state of C2H60”Cl, a rigid rotor analysis was employed in the analysis of their spectra. Since the magnitude of the centrifugal distortion contribution, P&,, is rather small for all the observed transitions, the rigid rotor frequencies were calculated according to Eq. (1) : vrr
where the P&,
represents
=
the calculated
vu
-
4 p,mn,
centrifugal
(1)
distortion
for the ground state of
468
SUENRAM,
LOVAS, TABLE
The Measured
Rotational
- 5+,
Ji’ K’ -1 t, 70,'
-
60.6
71,'
-
61.6
7
1.6
61.5
60.8
-
70.7
81.8
-
71.7
91.7
-
'1.6
"1.10
Transitions
for C2H0’Cl
"obs (MHz)
(ML?;
Excited
State
yrr -Yca1c. (MHZ)
P4 term
-0.50
-0.18
35 567.40
0.28
-0.44
36 406.27(15)
36 406.27
0.09
-0.51
41 100.93(11)
41 100.93
-0.10
-0.74
40 646.10(10)
40 646.10
0.00
-0.67
41 604.85(11)
40 604.85
-0.07
-0.77
32 326.09
0.10
-0.37
33 126.42
-0.11
-0.52
34 008.49
-0.09
-0.73
34 975.34
-0.10
-1.01
36 421.13
0.4,
-2.40
39 762.80
0.04
-3.11
41 207.46
-0.24
-3.97
32 324.79(10) 32 327.35(10)
'21.11 - '20,12 F - 2112 - 21,2 F = 2712 - 2712'
33 125.10(10)
F = 2312 - 2312 F = 2512 - 2512'
33 127.72(10)
'31,12 - '30.13 F - 2312 - 23,2) F = 2912 - 29,2
34 00'.2O(lO)
F s 2512 - 2512 F = 2712 - 2712'
34 009.75(10)
'41.13 - '40.14 F - 25/2 - 25/2, F = 3112 - 31/Z
34 974.15(5)
F = 2712 - 2712, F - 29/Z - 29/2
34 976.75(5)
- '70.17
F - 31/2 - 31,2I F - 37/2 - 37/2
38 419.95(20)
F = 3312 - 3312, F = 35/2 - 35/2
38 422.30(20)
‘Oo,la
- 3312) F = 3912 - 3912
39 761.48('1)
F = 3512 - 3512) F = 37/2 - 3712
39 764.11(11)
'91.18 - '90.19 F - 3512 - 3512) F = 4112 - 4'12
41 206.20(22)
F - 37/2 - 37/Z) F = 39,2 - 3912
41 208.70(22)
Frmthetical
in the Vibrationally
35 970.05
F = 21/Z - 21,2 F = 23/Z - 2312'
F = 3312
V
35 567.40(10)
- 'lo,11
'91.17 -
JOHNSON
35 970.05(lO)b
F = 1912 - 19,2 F = 2512 - 25121
"I.16
AND
(v = 1)
rotational transition frequency used in the calculated fit.
The numbers in parentheses ape the actual estimated uncertainties in the frequency marurements and they refer to the last digits given.
CzHr,035C1and y. is the hypothetical rotational line frequency, for the species being analyzed. The rigid rotor frequencies, vrr, were then least-squares fit to A, B, and C with the results shown in Table VI. The standard deviations of these fits were more than five times smaller than similar fits to u. which ignored centrifugal distortion contributions. The nuclear quadrupole coupling constants, X’S, were obtained in the same manner as described for ground state CZH~,O~~C~. IV. DIPOLE
MOMENT
The electric dipole moment of ethyl hypochlorite was determined by measuring the second-order Stark effect of several of the rotational transitions. The spectrometer was calibrated using OCS for which the reported dipole moment is 0.71519(3) (6).
MICROWAVE
SPECTRUM
OF CHKHzOCl
469
From preliminary observations it was evident that both p, and pb were sizableexhibits unresolvwhile from symmetry consideration pc = 0. The 61~-505 transition able quadrupole splitting so the strong field approximation can be used. The MJ = 1, 3, and 4 Stark lobes of this transition were used in the fit. The 413-312 transition is slightly split into a doublet by the 35C1 nuclear quadrupole interaction. The frequency of the MJ = 1 lobe was measured as a function of the applied field. Each component of the doublet was measured at every field and an average frequency taken. The 211-202 transition has a large nuclear quadrupole splitting. The g-3 component is the strongest component and is well separated from the other components. Measurements on the MJ = 2 Stark transition of this quadrupole component were used in the fit. Also included in the analysis were measurements on the MJ = 1 Stark transition for the 2r2l& = g-5 line. These measurements were least-squares fit to Eq. (2) (7) : AV = (A + BMJ~)/&~~ + (A’ + B’MJ2)pb2p,
(2)
where the coefficients A, B, A’, and B’ were calculated by a rigid rotor computer program described by Beaudet (8). The results of the fit are pa= 1.632(10) D, pb= 1.097(5) D, and pr = 1.97(l) D. The only other alkyl hypochlorite for which accurate dipole moments have been determined is methyl hypochlorite. For CH30WI Suzuki and Guarnieri (9) obtained p, = 1.373(4) D, j& = 1.46(7) D, and br = 1.788 D which are comparable to the present results on ethyl hypochlorite. V. RELATIVE
INTENSITIES
In order to determine the energy difference between the ground and excited state the relative intensities of transitions of the 35C1species were compared for the 514-413, 616-515, 615-514, 7r,-6re, and 716-615 tra.nsitions. The average of these measurements resulted in an energy of 125 cm-‘. Measurements of this type are generally carried out at room temperature in order to alleviate any error which might arise due to uncertainties in the temperature. However, this was not possible for ethyl hypochlorite since it is not stable in our absorption cell at room temperature. All the measurements reported here were carried out with the cell cooled with dry ice. Under such circumstances, it is difficult to accurately measure the average temperature of the gas molecules in the cell. It was assumed that the average temperature was - 70 f 10°C. This introduces an TABLE Rotational
VI
Constants and Hypertine Coupling Constants for C2H&Y5C1 v = 1, and CzH&YCl in the Vibrational Ground and Excited States in MHz 37 35Cl 37 m1ecu1.w C2H50 Cl cp5ll Cl C*H+l COnstant
v-1
gVX"d 31568.64(8)
v-1 30734.25(19)
A
30750.34(7)a
B
X91.943(5)
2629.909(4)
2630.813(S)
c
2566.633(5)
2505.372(4)
2510.923(6)
-91.2(60)
-67.4(25)
35.1(37)
24.8(13)
24.8
56.1(24)
42.6(12)
42.gb
-%a %b xcc
-67.4b
'The nunkrs in parentheses are the calculated standard deviations of the fit and they refer to the last digits given. bThe nuclear quadrupole coupling constants for the ground state were used to obtain the hypothetical rotatianal transition frequencies elnployed in the analysis.
470
SUENRAM,
LOVAS, AND JOHNSON TABLE VII
Comparison of Observed Rotational Constants with Those Calculated from an Assumed Structure B0”d
Assumed Value
Assumed Value (degrees)
Angle
ci, CH
1.09
HCC
109.5"
CC
1.51
CC0
107.8"
CO
1.43
LOCI
113"
DC1
1.674
Rotational Constants C2H5036Cl Calc.
C2H5037C1 Calc.
ohs.
A
32996.6
31568.64
Obr.
A
33009.6
31582.01
8
2662.4
2691.084
6
2601.9
2629.909
C
2541.3
2560.916
C
2486.0
2505.372
uncertainty of about f 10 cm-‘. The uncertainty resulting from the intensity measurement is f13 cm-’ so the sum provides a total uncertainty of f23 cm-‘. The lowest vibrational state is therefore 125 f 23 cm+ and is most likely the lowest level of the torsional motion about the C-O bond. Some evidence for this conclusion can be extracted from the second moments perpendicular to the heavy atom plane, 2P,, = I, + Ib - I,. For both chlorine isotopic forms we obtain 2P,, = + 6.46 u.A for the ground state and 2P,, = + 7.27 u.A for the excited state. The change in this out-of-plane moment, 0.81 u.A2, is quite large and indicates the motion of chlorine out of the C-C-O-Cl plane. By way of comparison, the difference in inertial defects between the ground state and lowest torsional state of chlorine nitrate, ClON02, is 0.76 u.Az (I). VI. MOLECULAR
STRUCTURE
The present results do not provide a means for determining the molecular structure of tram- ethyl hypochlorite. However, as further evidence for the molecular identification, the observed rotational constants can be compared to those derived from a structure estimated from analogous molecules whose structures are well determined. The assumed structural parameters are shown in Table VII. The values for the CzHbO geometry were taken from the structure for trans- ethanol (IO), ignoring slight asymmetry in the CH bond lengths and HCC angles. The OCl bond length and COCl angle were assumed to be identical to those reported for methyl hypochlorite (2). In the lower portion of Table VII the calculated and observed rotational constants are listed for both 35Cl and 3’Cl forms of tram- C2H60Cl. Note that the heavy atoms lie in the a-b plane as required by the fact that 2P,, is identical for both chlorine isotopic species. No spectral evidence was obtained for any other conformers. VII. HYPERFINE
STRUCTURE
AND INTERNAL
ROTATION
Ethyl hypochlorite exhibits hyperfine structure typical of a prolate rotor with a terminal chlorine atom. The nuclear quadrupole coupling tensors for GH603’Cl are listed in Table II. In order to compare these results with related molecules these tensors
MICROWAVE
SPECTRUM
OF CHsCHzOCl
471
must be converted from the principal axis frame to a bond axis system. For the transformation of a + z and c + y with z along the O-Cl bond, the angle, 8, between the u-axis and O-Cl bond was calculated from the assumed structure given in Table VII. The quadrupole hyperfine tensors obtained with 0 = 20.1 f 1” are shown in Table VIII. For comparison the hypertine constants reported for HOC1 and CH,OCl are also listed in Table VIII. Within the uncertainty from the ethyl hypochlorite structure, these results agree excellently with those for HOC1 and CHaOCl. Ethyl hypochlorite is expected to exhibit internal rotation splittings similar to other ethane derivatives such as ethanol or ethyl cyanide. For both ethanol and ethyl cyanide the barrier to internal rotation is very high, -3.2 kcal/mole and splittings in the ground state spectrum were typically less than 0.1 MHz and unresolvable while the first torsional state has resolvable splittings on the order of 1 MHz. For ethyl hypochlorite no internal rotation splittings could be resolved in the ground state or in the excited state. Splittings less than 0.5 MHz would be unresolvable in this case due to the rather broad linewidths obtained. Since no internal rotation splitting could be observed, we conclude that the barrier to internal rotation is >3 kcal/mole and is consistent with other ethane derivatives. VII. ALKYL
HYPOCHLORITE
SYNTHESES
Few spectroscopic studies on alkyl hypochlorites have been reported. This may be due, in part, to difficulties in synthesizing the desired molecular species. In order to determine whether the reaction of chlorine nitrate with alcohols would be generally applicable for producing the hypochlorite, CINOl was reacted with methyl, i-propyl, and n-propyl alcohols in the septum absorption cell cooled to - 78’C. After warming the absorption cell to -4O”C, spectral scans over the range 30 to 40 GHz were then carried out in order to detect the a-type bands. For i-propyl hypochlorite three isomers are possible, trans, cis, and gauche (defined as methylene C-H bond with respect to the O-Cl bond orientation). Structural estimates of each were used to predict B + C values shown in Table IX which will be characteristic of the u-type band spectra. Medium strength bands were observed at 30.73 GHz (J = 7-6), 35.19 GHz (J = 8-7), and 39.48 GHz (J = 9-8) which correspond to a value of B + C = 4.392 GHz. Although the observed value is closest to the predicted B -l- C for guuclze-isopropyl hypochlorite, it is intermediate between the cis and gauche values. More detailed studies are needed to determine which conformation is correct. TABLE
VIII
Nuclear Quadrupole Coupling Constants in the Bond Axis System (MHz)
-
C2H50
CH 035Cla 3
H035Clb
-114.2(20)
-117.1(20)
-121.86(28)
x (35Cl) xx
56.0(23)
58.0(20)
59.56(25)
x,(35c1)
58.20(28)
59.1
62.30
20.1(10)"
25.6(3)'
xzz(35cl)
%a %f.
9.
bRef. 11.
Cl
_-_
472
SUENRAM,
LOVAS, TABLE
AND JOHNSON Ix
Calculated and Observed Values of B + C for n-Propyl and Isopropyl Hypochlorites Alkyl Hypochlorite ('W)
Calculated BtC (MHz)
=-isopropyl
5618
c&-isopropyl
4259
Observed B+C (MHZ)
gauche-isopropyl
4442
4392(5)
~-normal
2842
2829(Z)
propy1
The spectrum observed for n-propyl hypochlorite was considerably sparser than that found for isopropyl hypochlorite. A group of closely spaced bands were observed at intervals of -2830 MHz. Within each group the lowest frequency band was strongest and each successive band in the group got progressively weaker. Assigning the strongest band to the ground state and the remaining bands to vibrationally excited states, the observations fit the relation (B + C), = 2829 + 4.5
MHz,
where v is an integer indicating the vibrational state. As can be seen in Table IX, these observations agree with the trans-n-propyl hypochlorite (extended form). Calculations of all the conformers produced by stepwise rotation about the C&a and C-0 bonds (C~--G-C&O-Cl) give progressively higher values of B + C as the rotations deviate from the totally extended conformation. VII.
SUMMARY
The spectroscopic and chemical evidence presented in the previous sections firmly establish ethyl hypochlorite as the source of the rotational spectrum observed and assigned while studying chlorine nitrate (1). The rotational constants, hyperfine structure, and electric dipole moment are all consistent with this identification. In addition the reaction of ClNO, with ethanol was employed in synthesizing GHsOCl. The chlorine nitrate-alcohol reaction also was found to be generally applicable in the synthesis of alkyl hypochlorites. RECEIVED: August
15, 1977 REFERENCES
1. R. D. SUENRAY, D. R. JOHNSON,L. C. GLASGOW,AND P. Z. MEAKIN, Geophys. Res. Lett. 3, 611 (1976) ; Erratum, 3, 758 (1976). 2. J. S. RIGDEN AND S. S. BUTCHER,J. Chem. Phys. 40, 2109 (1964). 3. W. H. K~CHEOFF AND D. R. JOHNSON,J. Mol. Spectrosc. 45, 159 (1973). 4. J. K. G. WATSON,J. Chem. Phys. 46, 1935 (1967). 5. W. H. KIRCHHOFP, J. Mol. Spectrosc. 41, 333 (1972). 6. J. M. L. J. RFXNARTZAND A. DWANUS, Chem. Phys. Lett. 24, 346 (1974). 7. S. GOLDENAND E. B. WILSON, JR., J. Chem. Phys. 16, 669 (1948). 8. R. A. BEAUDET,Ph.D. Thesis, Harvard University, (1963). 9. M. SUZUKIAND A. GUARNIERI,Z. Neturforsch. A 31, 1242 (1976). 10. J.-P. CULOT, University of Louvain, Belgium, private communication. 11. A. M. MIRRI,F. S~APPINI,AND A. CAZZOLI,/. &foZ. Spectrosc. 38,218 (1971).
OF MOLECULAR
69,45&472
SPECTROSCOPY
Microwave
(1978)
Spectrum
of Ethyl Hypochlorite
R. D. SUENRAM,’ F. J. LOVAS, AND D. R. JOHNSON National Bureau of Standards, Washington, D. C. 20234 The microwave spectrum of ethyl hypochlorite has been analyzed in detail in the region of 20-60 GHz. Observed transitions for C~H~OWI in the ground state have been fit to a Hamiltonian model which includes y centrifugal distortion terms. The lowest vibrationally excited state of CzH&Wl and the ground state and lowest vibrationally excited state of CzH@Cl were analyzed with a rigid rotor model. This lowest vibrational mode lies at 125 f 23 cm-l and is most likely the torsional motion about the C-O bond. The dipole moment has been measured and found to have two nonzero components; p. = 1.623 f 0.010 D, pb = 1.097 f 0.005 D. No A-E torsional splittings were observed in either the ground state or the v = 1 state implying a lower limit for the barrier to internal rotation of ~3.0 kcal/mole. Ethyl hypochlorite was synthesized in the waveguide by the reaction of chlorine nitrate with ethanol. INTRODUCTION During
the recent
we observed ing that
work
chemical
experiments
ethyl
the
hypochlorite
moment,
investigation
and assigned
of the microwave
the spectrum
identity
of the
we were (C2H,OCl).
and nuclear
second
able
species
coupling
Instrumental. 80-kHz 1.1-m
square P-band
Teflon
The
microwave
spectrometer
wave
modulation.
Due
Stark
cell constructed
end windows
the high-voltage which
were sealed
Stark
feedthrough.
were lubricated
Initial
broad
backward
wave
free running
with
banded oscillator.
klystrons
Kel-F
scans
with
used
Teflon
observed
1 NRC-NBS
Postdoctoral
with
a spectrum
of stainless
a phase had been
in oil baths
in the chlorine
458
Copyright
Q 1978 by Academic
AU rights of reproduction
Press, Inc.
in any form
mend
additional species
spectrum,
as
dipole
hypochlorite.
of conventional nature steel
design
contained
with
of CH&H20Cl,
and
Teflon
O-rings as was the vacuum
Fellow, 19751977.
0022-2852/78/0693-045%$02.00/O
with
unknown
a
was used.
connection
two ground
for
glass joints
grease.
which were stabilized
as an impurity
was
reactive
the microwave transitions. Sample preparation and handling. As was mentioned was first
(I),
DETAILS
The inlet system
were made
Once
for ethyl
to the highly entirely
but
this
the microwave
constants
nitrate
For some time follow-
uncertain
identify
we report
I. EXPERIMENTAL
of chlorine
species.
was
to conclusively
In this paper
quadrupole
spectrum
of a new molecular
locked, observed
R-band
(26.5-40
GHz),
and roughly
assigned,
were used to accurately
measure
in the introduction,
nitrate
(ClNO,)
study.
CH&H20Cl It was later
4.59
MICROWAVE SPECTRUM OF CH&HZOCI
found that the easiest way to prepare it was to cool the absorption cell to - 78°C with dry ice, introduce several hundred millitorr pressure of ClNOs and then an approximately equal amount of ethanol. The cell pressure could then be reduced to the normal working pressure S-20 mTorr (1 mTorr = 0.133 Pa). The first sample introduced in this fashion usually lasted for only a few minutes but the second sample lasted considerably longer (1-2 hr). It shouId be emphasized that it was only when the absorption cell was cooled with dry ice could we observe the spectrum of CH,CH20Cl. Furthermore, the other reaction product, nitric acid, does not have sufficient vapor pressure at - 78°C to give rise to a microwave spectrum so any possibility of interference from nitric acid spectral lines is eliminated. II. SPECTRAL ASSIGNMENT AND MOLECULE IDENTIFICATION The microwave spectrum observed was that of a typical slightly asymmetric prolate rotor and the spectral assignment was far less of a problem than was the molecular identification. For slightly asymmetric prolate rotors, the strong u-type R-branch transitions occur in bands which are spaced at intervals of B + C. The transitions with high K-r values which form these bands have very fast Stark effects and are readily identified in wide frequency scans such as the one illustrated in Fig. 1. The transitions with low values of K-1 are split out from the main cluster of transitions and are easily identified. The K-1 = 0 transition will be found in the vicinity of the band center and is characteristically identified by its slow second-order Stark effect. Accordingly, the K-1 = 0 and 1 transitions for the J = 6-7 and J = 7-8 bands were identified and fit to a rigid rotor model. It was then possible to predict the remainder of the u-type spectrum, and the molecule was found to be quite prolate (K = - 0.94). The u-type R-branch transitions have very little dependence on the A rotational constant, hence, the A rotational constant was not well determined at this stage. The b-type transitions have a sizable dependence on the A rotational constant and as a result they were not well
ETHYL
HYPOGHLORITF
J=6-5 J=7-6 0
40
FREQ”ENCY3;Hd FIG. 1. The low-resolution microwave spectrum of ethyl hypochlorite
in
the 30 to 40 GHz range.
41,4
41.3
41.3
4 0.4
44 324.44(6)
F -,I,2 F - 9,2
-
J/2 9,2)
30.3 F - 5,2 - 3/2) F - J/2 - 5,2
9,2
F - 9,2
49 314.42(13)
49 311.87(13)
-0.18
0.14
-0.04
29 614.10(10)
- J/2
F = J/2
49 313.83
-0.08
29 616.52(E)
-0.04
29 609.92(S)
0.17
29 607.73&i)
29 612.32
-I,/2
21 268.60(12)
0.25
0.05 0.03
-0.05 O.D3
0.05
-
- J/2 -1312)
-
'I.7
‘I,6
-
F -1312
F -11/z
F ‘1512
61.5
61,6
60.6
50.5
-13,2
-II/2
-15/2
60.6 F = 912 - 9/2
F = 912 F -15/2
'0.7
61.6
61.5
51.4
51.5
52,3 F -,I,2 9,2 F -1312 -1112)
-
61.5
F =13/2 -II,2 F -15/Z -1312) 61.6
62,4
50.5
41.3
F '1112 - 9/2 F = 912 - J/2)
-
60.6
0.00 0.00
-
F -13/2 -,I,2
F 3 J/2 - 5,2 F -,I,2 - 912)
41.4 F - 9/2 J/2
F -1112 - 9,2 F -1312 -11/21
40.4 F - 9,2 - J/2 F = J/2 512’
51.4
51.5
50.5
-0.23
-0.04
-0.02
0.02
0.03
-0.05
0.02
-O.Zlb
0.00
-0.13b
F -1112
- 3,2> ¶I2
003.57
321 .27
21 267.62
2,
44
29 348.25
39 264.40b
29 151.43b
40.4 F = 512 - 512
-
F - 5,2 F -1112
31.2 F - l/2 - 5/2) F - 9/2 J/2
21 266.00(R)
21 004.32(8)
F - 9/2 F -,I,2
- J/2) - 9,2
21 001.52LS)
F - 512 312) F = J/2 - 5,2
30.3
- 5,2
F = J/2
29 354.01(5)
F - 712 - J/2
44 321.36(6)
29 349.97(5)
F = 512 - 512
20.2 F = 9/2 - J/2
29 345.37(5)
- 9,2
F - 9,2
31.3
29 34X.31(5)
39 263.88l9)
20.3 F - 3,2 - 3,2
31,2
29 147.27(lo)d
'0.1 F = J/2 - 5,2
-
- 712
20.2
21.2
F = J/2
21.1
I
36 302.32
36 302.32(14)
37 213.06
36 738.42
37 213.0617)
59 110.26 36 738,42(J)
30 346.54
31 525.07
59 110.26(16)
30 349.12(5)
30 348.35(5)
30 344.66(5)
30 343.69(5)
31 526.36(12)
3, 523.66(12)
31 898.Y(
-0.17
0.08
-0.49
0.58
-0.17
0.14
-0.05
0.07
-0.20
0.03 0.01
-0.02 -0.03
-0.05
-0.25
0.03
31 117.75
31 117.75(15) 31 898.38(10)
0.02 -0.02
0.01 -0.03
31 496.78(6)
31 495.70(12)
0.11
0.30 31 496.42
-0.04
-0.01 -0.06
0.07
-0.05
-0.04 -0.07
0.07 Cl.04
0.03
(MHZ)
ho(obs-talc)=
26 583.60(10)
26 583.60
25 932.45
26 251.06
"0' IMHZ)
25 933.35(5)
25 932.15(5)
25 931.05(5)
26 251.50(5)
26 250.W!5)d
"ohs (MHz)
The Measured Rotational Transitions for C~H~OWI in the Ground Vibrational State
TABLE
'1.7
71.6
-
-
%,8
I.8
‘00 10
9
927
F -1912 ,9,2 F =21/2 - 21/Z)
' F =17/2 : ,7,2 F -23/2 - 23/2'
'028
-
F -17/2 17/2 F =19,2 - 19/2'
-
go.9 F -,5,2 - ,5,2 F -21/2 21/2)
3 ‘019
9
- 80.8 F -13,2 - 13,2 F -,9,2 19121
81,7
F -15,2 - 15,2 F -17,2 - 1712)
72.5
8 2,6
81.7
70,7
-
F ‘1,,2 - 9,2 F -1712 1512'
- 62.4 F -1512 - 13,2 F '13,2 11/Z]
F -1112 9/2 F =,7,2 - 15121
- 62,5 F =15,2 - ,3,2 F -l3,2 ,112'
60.8
72.5
72,6
0.18
42 527.00
42 527.00(14)
32 7W.75(5)
32 698.18(10)
52 579.70(10)
31 996.85(10)
3, 993.20(10)
31 371.80(10)
31 367.94(10)
0.03 -0.07
52 579.70
32 699.99
0.05
0.29
31 369.9,
31 995.06
-0.2,
42 046.55
0.04
-0.09
41 485.86
42 046.55111)
-0.19
4, 978.20
-0.03
0.10 -0.11
-0.11 0.10
41 485.86(11)
36 784.67
36 760.32
41 978.20(7)
36 785.39(14)
36 783.85(14)
36 76,.10(14)
36 759.42(14)d
-
'lo,11
-
'01.10
-
'30.13
'31,13
F -27,2 -25,2> F -29,2 -27/Z
F -2512 -23,2> F =3,/2 -29/2
140.14
F -25/2 -25,2] F -2712 -27/Z
F -23,2 -23,2) F =29/Z -29,2
'31.12
F -2312 -23,2) F =25/2 -25/2
F -21/2 -2,,2> F -2712 -27,2
'20.12
49 837.63(g)
49 836.01(g)
35 327.36(35)
35 323.92(35)
34 363.75(30)
34 360.39(30)
33 653.31(10)
-
F -1912 -2,,2, F -21,2 -23,2 l21.11
33 648.08(14)
"1.11
32 ,3,.72(10)
32 130.30(10)
33 489.35(10)
33 486.13(10)d
F -1712 -,7,2> F -2312 -25,2
'O2,8
F -U/2 -,9,2) F -2312 -21/2
F -19,2 -,7,2' F -2512 -23/2
"0,ll
F -2112 -2,,2) F -2312 -23/2
F -1912 -,9/q F =25/Z -25/2
"l,,o
TABLE I-Continued
49 836.84
35 325.66
34 362.09
33 650.70
32 131.05
33 487.76
(continued)
-0.02
0.04
0.21
-0.05
0.25
-0.13
SUENRAM,
462
LOVAS, AND
TABLE
I-Continued
%bS (‘w '41.13
-
'40,14 36 381.00(14)d
F ‘27/Z -27,2, F -2912 -2912
36 384.36(14)
-
160,16
F '2912 -29/2) F =35,2 -3w
38 792.24(6)
F -3112 -3112 F -33/2 -3312'
38 795.78(6)
"1,16
-
'70.17
F -31/2 -3112, F '37/Z -37/Z
40 153.89(101
F ‘3312 -33/Z, F -3512 -35/Z
40 157.43(10)
'61.17
-
'80.18
F -3312 -3312% F =39/z -3912
41 625.71(11)
F -3512 -3512' F =37/2 -37/Z
41 629.3601)
201.19
-
'92.18
F =37/2 -3512, F =43/2 -4112
31 403.36(10)
F '3912 -37/Z> F -4'12 -3912
31 405.10(10)
"3,16
-
202.19
A$obs-talc)' VW
36 Z32.69
F '2512 -25/Z> F -3112 -3112
'61.15
JOHNSON
40 637.80(20)
-0.21
38 794.02
0.09
40 155.67
-0.16
41 627.54
0.11
31 404.24
0.W
40 637.80
0.0'
%ypothetical rotatfonal tranrltion freqwn~y employed in the centrifugal distortion ana,yr,s. b Not Included in fit since only one hyperfrnc component was measured. 'For the hyperflne c~n,,onentr Rnployed In the fit, the observed mlnur calculated values of the hyperfine splittlngn are give". dThe nu"berr in parentheses are the actual estimated uncertainties in the frequency measurenent and they refer to the 'list dlglts given.
predicted. It should be noted at this point that the molecule remained unidentified although the basic spectrum had been assigned, and it was still not known whether the molecule might have other active dipole moment components. From the observed broadbanded scan, a series of strong transitions, which had similar Stark effects and were approximately equally spaced, were singled out. From the predicted spectrum it was possible to identify these transitions as members of the b-type Q-branch series J1,.r--l - Jo,r. All transitions of this Q-branch series have approximately the same dependence on the A rotational constant, thus if A is poorly determined these transitions will all exhibit a systematic shift from the predicted frequencies. These transitions were fit to the rigid rotor model and aside from some small centrifugal distortion corrections the fit appeared to be quite consistent. The rotational constants were now quite well determined and c-type transitions could be predicted with considerable precision. None of the predicted c-type transitions could be observed. The available spectral information provided the following clues as to the identity of the compound. First, a weaker set of u-type bands of about i the intensity of the most intense set had been observed. These were in accord with a chlorine isotope effect suggesting that the molecule contained chlorine. The magnitude of the nuclear quadrupole splittings were such that the chlorine atom would most likely be in an end position on the molecule. Second, the rotational constants indicated that the molecule was nearly a prolate symmetric top and the second moment in the c-axis direction,
MICROWAVE
SPECTRUM
4-63
OF CHaCHzOCl
le) clearly implied that the molecule was not planar, i.e., P,,#O. PC, = $(Ia+rbAlso from the magnitude of the rotational constants it was clear that there were at least three but no more than four heavy atoms arranged in a chainlike fashion beside the chlorine atom. Finally, from the relative intensities of the a- and b-type transitions it appeared that p, > pb. From these observations we concluded that the molecule must have the general structure Cl-X-X-X-(X) where the X’s could be oxygen, nitrogen, carbon, or some combination of the three. In view of the compound originally being studied (chlorine nitrate) one possibility would be Cl-O-N-O, the chlorine analog of nitrous acid (HONO), which conceivably might have been produced as a byproduct in the synthesis of chlorine nitrate. Although the observed rotational constants were in fair agreement with an estimated structure for ClONO, the observed second moment, 2P,, - + 6.4 u.A2, required that the dihedral angle between the Cl-O-N and O-N=0 planes be -60” which was inconsistent with other previously studied XONO molecules. In addition, the spectrum was observed only during the first several weeks of the study of chlorine nitrate and could not be obtained in later studies employing samples of chlorine nitrate of various purity obtained during the fractional distillation purification process. Thus, it appeared that the new molecule was a product of the reaction of chlorine nitrate with some impurity in the absorption cell or gas handling system. Since the inertial defect was the most difficult to explain, we first considered species which would have a nonplanar moment of - 4-6.4 u. AZ and still be consistent with the remaining characteristics. Noting that the ethyl group, C2H5, would provide the required nonplanar moment, the only ethane derivative which could be consistent with all the observations is C2H50Cl with all the heavy atoms lying in a plane, i.e., trans-ethyl
TABLE Rotational,
II
Centrifugal Distortion, and Quadrupole Coupling CZH&YI in the Ground Vibrational State
Constants
31582.01171” 2691.084(10) 2560.916(8) 0.0013(46) -0.00113(48) 0.45(8) -0.577(90) -0.00177(20) -O.OOl37(19)
-93.96(62) 35.76(39) 58.20(28)
"The uncerta,nries ,isted in parentheres are the calculated standard deviations. b
The paraneter '3 is fixed by setting R6 - 0 (see reference 5)
for
464
SUENRAM,
LOVAS, AND JOHNSON TABLE
III
The Measured Rotational Transitions for CoH&PC1 in the Vibrationally Excited State (u = 1) d
"obs (MHz) 41.3
-
F =
28 752.60
40.4 5,2 -
-
CL
5/2
28 748.35(16)'
yrr - "ca1c W)
-0.06
F = 11,2 - Ill2
28 750.2702)
7/z -
l/2
28 754.36(E)
0.02
F =
9/2 -
912
28 756.46(U)
-0.01
r/2
29 069.20(20)
0.06
F = 13,2 - 1312
29 070.37(20)
-0.16
F =
912
29 074.20(20)
0.11
F = 11/Z - 11/2
29 075.47(20)
-0.02
-
F =
9,2 -
61.5
-
51.4
'1.5
-
60,6
F =
9/Z -
-0.12
29 072.44
50.5 7/z -
31 923.65(15)
912
-0.02
-0.05
-0.33
29 459.51
-0.17
-0.13
29 456.78(20)
0.03 -0.03
F = 15/Z - 15/Z
29 457.75(20) 29 461.17(20)
0.09
F = 1312 - 1312
29 462.02(20)
-0.09
61.6 7
-
%.5
-
70.7
1.6 62.6 '01,9
81.7 -
72,5
37 242.35(7)
37 242.35
-0.20
-0.51
42 032.64(11)
42 032.64
-0.09
-0.74
41 558.20(11)
41 558.20
-0.23
-0.67
42 098.07(22)
42 098.07
0.09
-0.64
31 724.09
0.07
-0.26
32 481.96
0.11
-0.37
33 322.44
-0.03
-0.52
36 766.71
0.29
-2.98
34 249.40
0.11
-0.73
- 'Oo.10
F - 17/Z - 17/Z) F = 2312 - 23.12
31 722.35(10)
F = 19/Z - 19/Zj F = 2112 - 2112
31 725.78(10)
"1,lO
-0.12
31 923.65
F = 1,,2 - Ill2
*0.8
-0.12
0.09
F =
51.4
P4 tern
- "0,ll
F = 19/Z - 19/Z) F = 25/Z - 2512
32 480.32(10)
F = 21/Z - 2112, F = 23,2 - 2312
32 483.57(10)
12 1.11 - 120.12 F = 21/Z - 21/2, F = 27/Z - 2112
33 320.85(10)
F = 2312 - 23/2) F = 25/Z - 2512
33 324.00(10)
'20,lZ - "1.11 F = 2112 - 19/Z, F = 27/Z - 25,2
38 766.00(10)
F = 23,2 - 21/Z> F = 2512 - 23/Z
38 767.35(10)
'31,12 - '30,13 F = 23,2 - 2312) F = 29/Z - 29/Z
34 247.77(30)
; 1 ;;;; 1 ;;;;I
34 251.00(30)
hypochlorite? This hypothesis was tested by adding both chlorine nitrate and ethanol, which had been employed in cleaning portions of the sampling system, to the absorption cell. A strong spectrum of the assigned species was readily produced in this manner confirming the hypothesis. The reaction : CH&HzOH + ClNOa + CHzCHzOCl + HN03 2Two other isomeric forms which have an empirical formula of C&,ClO, 2-chloroethanol and chloromethyl methyl ether, were ruled out because they have been previously studied and the lowest energy forms were found to have gauche configurations (cf., R. G. Azrak and E. B. Wilson, J. Chem. Phys. 52, 5299 (1970), and T. Ikeda, R. F. Curl, and H. Karlsson, J. Mol. Speclrosc. 53, 101 (1974).
MICROWAVE
SPECTRUM
465
OF CHaCHoOCl
TABLE IIIG-Cmtiwed
- J;:,K;, K.~K;,
J’,
‘41,13
"ohs (MHZ)
- '40.14
F - 25/2 - 25,2 F _ 3,,2 _ 3,,$
35 264.46(251C
; : ;;;; : :;;;>
35 X7.70(25)
'71.16 - '70.17
-- 3712' 3112
38 891.30(10)
; : ;;;; : ::;;I
38 894.57(10)
F - 3,,2 37/2
F-
'%,I7
- '80,18
F - 33/2 - 33/2 F - 3g,2
- 39/z’
40 305.86(10)
F - 35,2 - 35/Z F - 37,2 - 37/Z)
40 309.15(10)
'91.18 - '90,19 ; : j;;; : :;;;I
41 829.6101)
; : :;;2' : :;;;I
41 832.95(11)
=1,24
a
- 250,25
F - 47,2 - 47/Z F _ 53,2 _ 53,2)
53 452.,,(,4)
F = 49/2 - 4912 F - 5,/2 - 51/Z]
53 455.81(14)
"Oa (MHz)
%
- %alcb (MHZ)
P4
ten
35 266.09
0.16
-1.0,
38 892.94
-0.03
-2.40
40 307.5,
0.13
-3.1,
41 831.29
0.34
-3.97
53 453.96
-0.34
-13.78
Hypothetical rotational transition frequency employed in the calculated fit.
bFor the hyperfine components employed in the fit, the observed minus calculated values of the hyperflne splittings we given. 'The numbers given in parentheses are the actual estimated uncertainties in the measurement and they refer to the last digits given.
is not unreasonable since both ethyl hypochlorite and nitric acid are readily observed and chlorine nitrate is completely consumed. In subsequent studies, methanol and chlorine nitrate were reacted in the absorption cell and the methyl hypochlorite rotational spectrum (2) was easily observed. III. SPECTRAL ANALYSIS
Many of the observed transitions listed in Tables I, III, IV, and V were split by the nuclear quadrupole moment of the chlorine nucleus. In order to adequately account for the observed transition frequencies, it was necessary to iterate between the fitting of the hypothetical centers of the rotational transitions and the fitting of the individual quadrupole splittings from the hypothetical center frequency. This procedure has been outlined by Kirchhoff and Johnson (3). For the ground state of the 35Cl species, a centrifugal distortion treatment of the Watson type (4) was employed using a computer program and fitting techniques developed by Kirchhoff (5). For the lowest torsional states of each species (v = 1) and the ground state of the “7Cl species rigid rotor fits were adequate. The data available was not sufkiently sensitive to centrifugal distortion to provide a more complete analysis. Since the primary interest in the ZJ= 1 states is for the determination of the torsional frequency via relative intensity measurements, it was deemed necessary only to measure a suflicient number of transitions to verify a correct rotational assignment. Similarly, for the 37Clform the objective was the determination of the nuclear quadrupole Coupling constants.
SUENRAM, LOVAS, AND JOHNSON
466
TABLE IV The Measured Rotational Transitions for CzHsW7C1in the Ground Vibrational State
31.2
-
P/Z
29 374.12(16)'
3/2-
3f2
29 378.13(16)
112 -
7f2
29 380.88(16)
¶/2 -
F= F= 5.3
F =
29 376.56
%.3
F -
0.28 0.01 29 628.30
40.4
-0.02
3,2
29 624.83(15)
-0.22
F = 11/z - ,112
29 626.1l(Ja)
-0.05
F =
?I2 -
7f2
29 63D.lU(M~
-0.23
F -
912 _
9/2
29 631.38(10)
41.4
312 -
-
F F =
312 _
7f2 -
48 918.69(131
F = 912 F = 1112 -
II2 912)
48 920.56(131
-
60,6
'1,7
-
72.3
-
32.3
72.5
-
62.4
66.8
-
70.7
81.*
-
'1.1
61.7
-
71.6
91.8
- 90.9
61.6
-0.10
-0.26 0.05 33 923.73
-0.27
-0.30
33 305.51(14)
35 303.3,
0.07
-0.44
35 943.85(35)
35 943.85
0.40
-0.40
33 963.69(14)
35 965.69
-0.20
-0.40
41 047.63(111
41 047.63
-0.22
-0.74
40 373.45107)
40 373.43
-D.%
-0.67
41 571.41(11)
41 571.41
-0.09
-0.77
31 903.82
-0.02
-0.19
32 376.16
0.06
-0.26
34 139.12
0.01
-0.32
0.20
-2.98
31 902.40(10)
F - 17f2 _ 1712 F f 19/z. 1912'
31 w3.2a(lo~
- '"o,lo F - 1712 - I,,2 F - 2312 - 2312'
32 374.89(10)
F - 1912 - 19,2 F - 21/2 - 21d
32 377.40(10)
'21.11 - '20,lZ F = 21/Z - 2lf2 F = 27/Z - 21f2'
34 157.75(05)
F = 23f2 - 2312 F = 25/2 - 2312)
34 160.47(S) 36 396.23
'20.12 - "1.11 F = 21/2 - 19/q F = 2712 - 25/Z
36 395X,7(15)
0.42 0.03
c - 2312 - W2) F - 2312 - 2312
'J6 39&78(J51
-0.a3 -0.42
from transitions which had small unresolvable nuclear quadrupole splittings (lower J, K- 1 = 0, 1, u-type, R-branch transitions) and from transitions which were split into symmetric douMets by the nuclear quadrupole moment
spectral
-0.19
0.23 -0.06
35 923.75(TO1
F = 13/z - 1612 F = 21/2 . 2112'
'?A
-0.12
0.03 48 920.11
30.3 312, 312
7D,7
Initial
-0.28
fits were obtained
(principally
higher J, b-type, Q-branch
transitions).
The center frequency
of
these symmetric doublets was obtained by measuring the frequency of each component and then taking the average. Preliminary quadrupole fits were obtained by assigning quantum numbers to the components of these symmetric doublets. The resulting quadrupole coupling constants were of sticient accuracy to permit the assignment of individual quadrupole components of transitions which were split into asymmetric multiplets by the nuclear quadrupole moment. The frequencies of these components were then measured and the center frequency of the transition determined by iteration
MICROWAVE
SPECTRUM
TABLE
IV-Continued
35 076.44
'31,lZ - '30.13 F = 23,2 F - 29/z
23/Z) 29/z
14
1.13 - '40.14
42 160.11
0.08
-3.83
36 081.77
-0.02
-1.01
F = 25/z - 25/Z) F - 31,2 3112
36 080.38(10)
0.19 -0.11
F - 2912 - 29/z F = 27/Z 27/Z'
36 083.13(10)
-0.20 0.10
'61.15 - '60,16 F - 29/2 F - 35/Z
29/Z] 3512
3% 371.33(10)
F - X,2 - 3112 F - 33,2 - 33/2'
38 373.90(10)
"1.16
- "0.17
F = 31,2 31/2) F - 37,2 - 3712
36 372.62
0.02
-1.83
39 665.64
0.17
-2.40
41 061.40
-0.33
-3.1,
49 720.946
-0.58
-9.46
39 664.28(11)
F - 33,2 - 33/2) F = 35/Z - 3512
39 666.98(111
'61,17 - '60,18 F - 33,2 - 3312) F = 3912 3912
41 060.02(11)
F - 35/2 3512) F - 37,2 - 37/Z
41 062.77(11)
231.22 - 230.23 F - 4x,2 43/q F = 49,2 - 4912
49 719.46(09)
F - 45,2 - 45/Z> F - 47,2 - 4712
49 722.41(09)
%ypothetical
-0.73
0.05 -0.27
35 077.73(25) 42 160.11(22)
'30.13 - '21.12
0.17 0.27 -0.05
35 075.12(2S)c
F - 25/z - 2512) F = 27,2 27/2
467
OF CHaCHzOCl
mtationsl
transition frequency used in the calculated fit.
bFor the hyperfine components employed in the fit. the observed minus calculated values of the hyperfine splittings are given. 'The numbers in parentheses are the actual estimated uncertainties in the frequency measurements and refer to the la*t digits given. dNat included in rigid rotor fit.
between the quadrupole fit and the centrifugal distortion fit. The hypothetical transition frequencies, Q, presented in Tables I, III, IV, and V represent the final iterative stages of the fit. For the ground state of CzH5035C1 a complete P4 centrifugal distortion analysis provided the rotational and distortion constants shown in Table II. With the exception of rI, the taus are fairly well determined and the standard deviation of the fit, 0.202 MHz, agrees well with the average measurement uncertainty. The nuclear quadrupole coupling constants for 35C1listed in Table II are also well determined. Since ~~~~~ could not be determined from the measurements on the ZJ= 1 state of CZH~O~~CI and both the ground and the vibrational state of C2H60”Cl, a rigid rotor analysis was employed in the analysis of their spectra. Since the magnitude of the centrifugal distortion contribution, P&,, is rather small for all the observed transitions, the rigid rotor frequencies were calculated according to Eq. (1) : vrr
where the P&,
represents
=
the calculated
vu
-
4 p,mn,
centrifugal
(1)
distortion
for the ground state of
468
SUENRAM,
LOVAS, TABLE
The Measured
Rotational
- 5+,
Ji’ K’ -1 t, 70,'
-
60.6
71,'
-
61.6
7
1.6
61.5
60.8
-
70.7
81.8
-
71.7
91.7
-
'1.6
"1.10
Transitions
for C2H0’Cl
"obs (MHz)
(ML?;
Excited
State
yrr -Yca1c. (MHZ)
P4 term
-0.50
-0.18
35 567.40
0.28
-0.44
36 406.27(15)
36 406.27
0.09
-0.51
41 100.93(11)
41 100.93
-0.10
-0.74
40 646.10(10)
40 646.10
0.00
-0.67
41 604.85(11)
40 604.85
-0.07
-0.77
32 326.09
0.10
-0.37
33 126.42
-0.11
-0.52
34 008.49
-0.09
-0.73
34 975.34
-0.10
-1.01
36 421.13
0.4,
-2.40
39 762.80
0.04
-3.11
41 207.46
-0.24
-3.97
32 324.79(10) 32 327.35(10)
'21.11 - '20,12 F - 2112 - 21,2 F = 2712 - 2712'
33 125.10(10)
F = 2312 - 2312 F = 2512 - 2512'
33 127.72(10)
'31,12 - '30.13 F - 2312 - 23,2) F = 2912 - 29,2
34 00'.2O(lO)
F s 2512 - 2512 F = 2712 - 2712'
34 009.75(10)
'41.13 - '40.14 F - 25/2 - 25/2, F = 3112 - 31/Z
34 974.15(5)
F = 2712 - 2712, F - 29/Z - 29/2
34 976.75(5)
- '70.17
F - 31/2 - 31,2I F - 37/2 - 37/2
38 419.95(20)
F = 3312 - 3312, F = 35/2 - 35/2
38 422.30(20)
‘Oo,la
- 3312) F = 3912 - 3912
39 761.48('1)
F = 3512 - 3512) F = 37/2 - 3712
39 764.11(11)
'91.18 - '90.19 F - 3512 - 3512) F = 4112 - 4'12
41 206.20(22)
F - 37/2 - 37/Z) F = 39,2 - 3912
41 208.70(22)
Frmthetical
in the Vibrationally
35 970.05
F = 21/Z - 21,2 F = 23/Z - 2312'
F = 3312
V
35 567.40(10)
- 'lo,11
'91.17 -
JOHNSON
35 970.05(lO)b
F = 1912 - 19,2 F = 2512 - 25121
"I.16
AND
(v = 1)
rotational transition frequency used in the calculated fit.
The numbers in parentheses ape the actual estimated uncertainties in the frequency marurements and they refer to the last digits given.
CzHr,035C1and y. is the hypothetical rotational line frequency, for the species being analyzed. The rigid rotor frequencies, vrr, were then least-squares fit to A, B, and C with the results shown in Table VI. The standard deviations of these fits were more than five times smaller than similar fits to u. which ignored centrifugal distortion contributions. The nuclear quadrupole coupling constants, X’S, were obtained in the same manner as described for ground state CZH~,O~~C~. IV. DIPOLE
MOMENT
The electric dipole moment of ethyl hypochlorite was determined by measuring the second-order Stark effect of several of the rotational transitions. The spectrometer was calibrated using OCS for which the reported dipole moment is 0.71519(3) (6).
MICROWAVE
SPECTRUM
OF CHKHzOCl
469
From preliminary observations it was evident that both p, and pb were sizableexhibits unresolvwhile from symmetry consideration pc = 0. The 61~-505 transition able quadrupole splitting so the strong field approximation can be used. The MJ = 1, 3, and 4 Stark lobes of this transition were used in the fit. The 413-312 transition is slightly split into a doublet by the 35C1 nuclear quadrupole interaction. The frequency of the MJ = 1 lobe was measured as a function of the applied field. Each component of the doublet was measured at every field and an average frequency taken. The 211-202 transition has a large nuclear quadrupole splitting. The g-3 component is the strongest component and is well separated from the other components. Measurements on the MJ = 2 Stark transition of this quadrupole component were used in the fit. Also included in the analysis were measurements on the MJ = 1 Stark transition for the 2r2l& = g-5 line. These measurements were least-squares fit to Eq. (2) (7) : AV = (A + BMJ~)/&~~ + (A’ + B’MJ2)pb2p,
(2)
where the coefficients A, B, A’, and B’ were calculated by a rigid rotor computer program described by Beaudet (8). The results of the fit are pa= 1.632(10) D, pb= 1.097(5) D, and pr = 1.97(l) D. The only other alkyl hypochlorite for which accurate dipole moments have been determined is methyl hypochlorite. For CH30WI Suzuki and Guarnieri (9) obtained p, = 1.373(4) D, j& = 1.46(7) D, and br = 1.788 D which are comparable to the present results on ethyl hypochlorite. V. RELATIVE
INTENSITIES
In order to determine the energy difference between the ground and excited state the relative intensities of transitions of the 35C1species were compared for the 514-413, 616-515, 615-514, 7r,-6re, and 716-615 tra.nsitions. The average of these measurements resulted in an energy of 125 cm-‘. Measurements of this type are generally carried out at room temperature in order to alleviate any error which might arise due to uncertainties in the temperature. However, this was not possible for ethyl hypochlorite since it is not stable in our absorption cell at room temperature. All the measurements reported here were carried out with the cell cooled with dry ice. Under such circumstances, it is difficult to accurately measure the average temperature of the gas molecules in the cell. It was assumed that the average temperature was - 70 f 10°C. This introduces an TABLE Rotational
VI
Constants and Hypertine Coupling Constants for C2H&Y5C1 v = 1, and CzH&YCl in the Vibrational Ground and Excited States in MHz 37 35Cl 37 m1ecu1.w C2H50 Cl cp5ll Cl C*H+l COnstant
v-1
gVX"d 31568.64(8)
v-1 30734.25(19)
A
30750.34(7)a
B
X91.943(5)
2629.909(4)
2630.813(S)
c
2566.633(5)
2505.372(4)
2510.923(6)
-91.2(60)
-67.4(25)
35.1(37)
24.8(13)
24.8
56.1(24)
42.6(12)
42.gb
-%a %b xcc
-67.4b
'The nunkrs in parentheses are the calculated standard deviations of the fit and they refer to the last digits given. bThe nuclear quadrupole coupling constants for the ground state were used to obtain the hypothetical rotatianal transition frequencies elnployed in the analysis.
470
SUENRAM,
LOVAS, AND JOHNSON TABLE VII
Comparison of Observed Rotational Constants with Those Calculated from an Assumed Structure B0”d
Assumed Value
Assumed Value (degrees)
Angle
ci, CH
1.09
HCC
109.5"
CC
1.51
CC0
107.8"
CO
1.43
LOCI
113"
DC1
1.674
Rotational Constants C2H5036Cl Calc.
C2H5037C1 Calc.
ohs.
A
32996.6
31568.64
Obr.
A
33009.6
31582.01
8
2662.4
2691.084
6
2601.9
2629.909
C
2541.3
2560.916
C
2486.0
2505.372
uncertainty of about f 10 cm-‘. The uncertainty resulting from the intensity measurement is f13 cm-’ so the sum provides a total uncertainty of f23 cm-‘. The lowest vibrational state is therefore 125 f 23 cm+ and is most likely the lowest level of the torsional motion about the C-O bond. Some evidence for this conclusion can be extracted from the second moments perpendicular to the heavy atom plane, 2P,, = I, + Ib - I,. For both chlorine isotopic forms we obtain 2P,, = + 6.46 u.A for the ground state and 2P,, = + 7.27 u.A for the excited state. The change in this out-of-plane moment, 0.81 u.A2, is quite large and indicates the motion of chlorine out of the C-C-O-Cl plane. By way of comparison, the difference in inertial defects between the ground state and lowest torsional state of chlorine nitrate, ClON02, is 0.76 u.Az (I). VI. MOLECULAR
STRUCTURE
The present results do not provide a means for determining the molecular structure of tram- ethyl hypochlorite. However, as further evidence for the molecular identification, the observed rotational constants can be compared to those derived from a structure estimated from analogous molecules whose structures are well determined. The assumed structural parameters are shown in Table VII. The values for the CzHbO geometry were taken from the structure for trans- ethanol (IO), ignoring slight asymmetry in the CH bond lengths and HCC angles. The OCl bond length and COCl angle were assumed to be identical to those reported for methyl hypochlorite (2). In the lower portion of Table VII the calculated and observed rotational constants are listed for both 35Cl and 3’Cl forms of tram- C2H60Cl. Note that the heavy atoms lie in the a-b plane as required by the fact that 2P,, is identical for both chlorine isotopic species. No spectral evidence was obtained for any other conformers. VII. HYPERFINE
STRUCTURE
AND INTERNAL
ROTATION
Ethyl hypochlorite exhibits hyperfine structure typical of a prolate rotor with a terminal chlorine atom. The nuclear quadrupole coupling tensors for GH603’Cl are listed in Table II. In order to compare these results with related molecules these tensors
MICROWAVE
SPECTRUM
OF CHsCHzOCl
471
must be converted from the principal axis frame to a bond axis system. For the transformation of a + z and c + y with z along the O-Cl bond, the angle, 8, between the u-axis and O-Cl bond was calculated from the assumed structure given in Table VII. The quadrupole hyperfine tensors obtained with 0 = 20.1 f 1” are shown in Table VIII. For comparison the hypertine constants reported for HOC1 and CH,OCl are also listed in Table VIII. Within the uncertainty from the ethyl hypochlorite structure, these results agree excellently with those for HOC1 and CHaOCl. Ethyl hypochlorite is expected to exhibit internal rotation splittings similar to other ethane derivatives such as ethanol or ethyl cyanide. For both ethanol and ethyl cyanide the barrier to internal rotation is very high, -3.2 kcal/mole and splittings in the ground state spectrum were typically less than 0.1 MHz and unresolvable while the first torsional state has resolvable splittings on the order of 1 MHz. For ethyl hypochlorite no internal rotation splittings could be resolved in the ground state or in the excited state. Splittings less than 0.5 MHz would be unresolvable in this case due to the rather broad linewidths obtained. Since no internal rotation splitting could be observed, we conclude that the barrier to internal rotation is >3 kcal/mole and is consistent with other ethane derivatives. VII. ALKYL
HYPOCHLORITE
SYNTHESES
Few spectroscopic studies on alkyl hypochlorites have been reported. This may be due, in part, to difficulties in synthesizing the desired molecular species. In order to determine whether the reaction of chlorine nitrate with alcohols would be generally applicable for producing the hypochlorite, CINOl was reacted with methyl, i-propyl, and n-propyl alcohols in the septum absorption cell cooled to - 78’C. After warming the absorption cell to -4O”C, spectral scans over the range 30 to 40 GHz were then carried out in order to detect the a-type bands. For i-propyl hypochlorite three isomers are possible, trans, cis, and gauche (defined as methylene C-H bond with respect to the O-Cl bond orientation). Structural estimates of each were used to predict B + C values shown in Table IX which will be characteristic of the u-type band spectra. Medium strength bands were observed at 30.73 GHz (J = 7-6), 35.19 GHz (J = 8-7), and 39.48 GHz (J = 9-8) which correspond to a value of B + C = 4.392 GHz. Although the observed value is closest to the predicted B -l- C for guuclze-isopropyl hypochlorite, it is intermediate between the cis and gauche values. More detailed studies are needed to determine which conformation is correct. TABLE
VIII
Nuclear Quadrupole Coupling Constants in the Bond Axis System (MHz)
-
C2H50
CH 035Cla 3
H035Clb
-114.2(20)
-117.1(20)
-121.86(28)
x (35Cl) xx
56.0(23)
58.0(20)
59.56(25)
x,(35c1)
58.20(28)
59.1
62.30
20.1(10)"
25.6(3)'
xzz(35cl)
%a %f.
9.
bRef. 11.
Cl
_-_
472
SUENRAM,
LOVAS, TABLE
AND JOHNSON Ix
Calculated and Observed Values of B + C for n-Propyl and Isopropyl Hypochlorites Alkyl Hypochlorite ('W)
Calculated BtC (MHz)
=-isopropyl
5618
c&-isopropyl
4259
Observed B+C (MHZ)
gauche-isopropyl
4442
4392(5)
~-normal
2842
2829(Z)
propy1
The spectrum observed for n-propyl hypochlorite was considerably sparser than that found for isopropyl hypochlorite. A group of closely spaced bands were observed at intervals of -2830 MHz. Within each group the lowest frequency band was strongest and each successive band in the group got progressively weaker. Assigning the strongest band to the ground state and the remaining bands to vibrationally excited states, the observations fit the relation (B + C), = 2829 + 4.5
MHz,
where v is an integer indicating the vibrational state. As can be seen in Table IX, these observations agree with the trans-n-propyl hypochlorite (extended form). Calculations of all the conformers produced by stepwise rotation about the C&a and C-0 bonds (C~--G-C&O-Cl) give progressively higher values of B + C as the rotations deviate from the totally extended conformation. VII.
SUMMARY
The spectroscopic and chemical evidence presented in the previous sections firmly establish ethyl hypochlorite as the source of the rotational spectrum observed and assigned while studying chlorine nitrate (1). The rotational constants, hyperfine structure, and electric dipole moment are all consistent with this identification. In addition the reaction of ClNO, with ethanol was employed in synthesizing GHsOCl. The chlorine nitrate-alcohol reaction also was found to be generally applicable in the synthesis of alkyl hypochlorites. RECEIVED: August
15, 1977 REFERENCES
1. R. D. SUENRAY, D. R. JOHNSON,L. C. GLASGOW,AND P. Z. MEAKIN, Geophys. Res. Lett. 3, 611 (1976) ; Erratum, 3, 758 (1976). 2. J. S. RIGDEN AND S. S. BUTCHER,J. Chem. Phys. 40, 2109 (1964). 3. W. H. K~CHEOFF AND D. R. JOHNSON,J. Mol. Spectrosc. 45, 159 (1973). 4. J. K. G. WATSON,J. Chem. Phys. 46, 1935 (1967). 5. W. H. KIRCHHOFP, J. Mol. Spectrosc. 41, 333 (1972). 6. J. M. L. J. RFXNARTZAND A. DWANUS, Chem. Phys. Lett. 24, 346 (1974). 7. S. GOLDENAND E. B. WILSON, JR., J. Chem. Phys. 16, 669 (1948). 8. R. A. BEAUDET,Ph.D. Thesis, Harvard University, (1963). 9. M. SUZUKIAND A. GUARNIERI,Z. Neturforsch. A 31, 1242 (1976). 10. J.-P. CULOT, University of Louvain, Belgium, private communication. 11. A. M. MIRRI,F. S~APPINI,AND A. CAZZOLI,/. &foZ. Spectrosc. 38,218 (1971).