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Microwave spectra of ground and excited vibrational states of cis-methyl nitrite, CH3ONO
Chemical Physics 53 (1980) 39-50 0 North-Holland Publishing Company
MICROWAVE SPECTRA OF GROUND AND EXCITED VIBRATIONAL OF CIS-METHYL NITRITE, CHJONO
STATES
Pradip N. GHOSH *, A. BAUDER and Hs.H. GaNTHARD Laboratory forPllysical C~~emistry,Swiss Federal Institu te of Technology. CH-809-7 Zwich, Switzerland Received 13 !4ay 1980
Rotational transitions of the vibrational ground state of cis-methyl nitrite have been assigned up to J= 20 over the frequency range from 12 to 40 GHz. Rotational constants, centrifugal distortiqn constants and quadrupolc coupling constants have been fitted to the measured lransition frequencies. In addition, rotational transitions in singly and doubly exited states of the NO and CIIJ torsions and in the singly excited state of the CON deformation mode have been assigned. Many of the absorption lines exhibited splittings due to the internal rotation of the methyl goup. The internal rotation barrier has been determined to be I;, = 734(Z) cm -t from the ground state splittings. Relative intensity measurements have yielded a transition frequency of 210(31) cm -’ foi the NO torsion. Furthermore, the energy diffcrencc between the more stable cis conformer and the trans conformer was found to be 314(Z) cm-t front the temperature dependence of the intensities of rotational transitions.
1. Introduction
The microwave spectrum of methyl nitrite was investigated first by Gwinn et al. [ 11. They showed that methyl nitrite occurs as cis and tram conformers in the gas phase around room temperature. Recently Turner et al. [2] reported the complete r, structure of both conformers thus extending a partial structure determination of the cis conformer by Endo and Kamura [3]. Turner et al. [2] also determined the internal rotation barriers of the methyl group for both conformers. They confirmed the high barrier of 2090 cal/mol in the cis conformer. For the trans conformer, they reported a much lower value of 29 cal/mol for the threefold barrier than that of 188 cal/mol previously given by Gwinn et al. [l] - The unusually large difference between the internal rotation barriers,of the methyl group in the two conformers prompted us to compliment the experimental investigations with an
l
On study leave from the Department ofCalcutta, Calcutta-700009, India
of Physics, University
ab initio calculation of the potential energy surface for both internal rotations of the CHa and NO groups [4]. For the comparison with the theoretical calculations we needed more information on the cis conformer. In the present work we have extended the assignment of the rotational spectrum of the cis form in the ground vibrational state up to J = 20 based on measurements with higher resolution. The newly measured transition frequencies allowed us to determine more reliable rotational and quadrupole coupling constants as well as the centrifugal distortion constants which were not reported in the earlier works. With the help of microwave-microwave double resonance experiments we could assign several excited states of the CH, and NO torsional modes and the in-plane CON bending mode. From the splittings of rotational transitions in ground and excited torsional states the barrier to internal rotation of the methyl group was determined. The torsional frequency of the NO group was obtained from relative intensity measurements. An independent value of the energy separation between cis and tram conformer was found from the temperature dependence of intensities of absorption lines.
P.N. Gltosh et al. /Microwave
40
2. Eqerimental details 2.1. Chemicals MeQunol and sodium nitrite were reacted with dilute sulphuric acid to form methyl nitrite [51. The crude product was purified by low temperature distillation. 2.2. Spectrometers The microwave spectrum of methyl nitrite was recorded over the frequency range from 12 to 40 GHz with conventional 30 kHz Stark modulated spectrometers. The backward wave oscillator sources were phase-stabilized to the harmonics of a digitally controlled reference oscillator. The 4 to 6 m long Stark cells were mostly operated at room temperature, occasionally they were cooled to -4O’C. Methyl nitrite strongly adsorbed to the walls of the waveguide cells. Frequent refilling of the cell after every hour or a slow flow through the cell was necessary in order to keep the pressure within narrow limits around 1 to 2 mTorr. The microwave power was kept around 100 ;JWin order to avoid saturation broadening. Frequency measurements were averaged from sweeps in both directions. For signal enhancement and digital filtering the spectrometers could be connected to a small computer. This mode of operation was used mainly for the measurements of transitions in the vibrational ground stdte. The accuracy oi the frequency measurements under computer control was estimated to be better thrn 20 kNz. The frequencies for transitions in excited states were read directly from the strip.chart recordings and are accurate to within 0.1 Mti. Microwave-microwave double resonance was used exunsively for the assignments of satellite spectra with an instrument described previously [6]. The pump power of the order of several hundred milliwafts from phase-stabilized klystron sources was smpll~udtimoduluted a1 a rate of 30 kHz.
3. bdysis
OP microwave spectra and
results
3. I. GroutId vibrutior~al state lhc microwuve spectrum of methyl nitrite shows a number of absorption lines due to the presence of two
spectra of cis-methyl nitrite
conformers in a number of vibrational states. Excited states are easily observed for the low frequency torsional motions of the methyl and NO group. In the spectrum of cis-methyl nitrite under high resoiution each rotational transition is split into several components arising from interactions with the quadrupole moment of the nitrogen nucleus and with the intplnal rotation of the methyl group. Rotational constants and quadrupole coupling constants were derived from measured transition frequencies with J up to 6 in previous investigations [ 1,2]. These constants were used to predict rotational transitions for higher J values which could be assigned readily from their quadrupole splitting pattern. The newly measured tiansition frequencies were subjected to a least-squares analysis for improved rotational and quadrupole coupling constants. Centrifugal distortion was found to affect noticeably even low J transitions and had to be included in the analysis. The hamiltonian !?=A$ - A&
tBp,z +Ci), - A_@)’ - [(i; - &(S#
+ (S_r@t @;))(i;
- $)]
- AJK&‘: + 6&) )
(1)
according to Watson’s asymmetric reduction 171 was expressed in the prolate representation I’, where A, B, Care the rotational constants, AJ, AJK, AK, fj~, 6K the centrifugal distortion constants and icz, Pb, kc the principal axis rsomponents of the angular momentum operator i. The full set of improved constants yielded better predictions and allowed the assignment to be carried out to still higher J values. This stepwise procedure finally enabled us to assign rotational transitions up toJ = 20. The results are collected in table 1. The transitions with J < 6 were remeasured with higher resolution than previou$y reported [2]. The prominent quadrupole components were usually completely resolved and even some of the weaker components could be measured. For the transitions with J 2 10 the quadr!Jpole components were only partly resolved into a doublet. The internal rotation splittings were too small to be obti,ved for transitions with J Q 2. These splittings were easily resolved for transitions with hi&r J and are presented in table 1, Three rotational constants, five centrifugal distortion constants and two independem quadrupole coupling constants were determined simultaneously in
P.N. Ghosh et al. /Microwave spectra of c&methyl nitrite Table 1 Measured and calculated transition
JK_K+-J&;
F-F’
41
frequencies (in MHz) of cis-methyl nitrite in the vibrational ground state
vA(obs.) -.-.--__-
V~(ObS.) VA(cak.)
VA - VE u*,centar (ohs.) (ohs.)
l’A,ceatcr(obs*) J’¢er(calc*)
--.
loi-000
2-l 1-1 o-1
13068.389 13068.751 13067.750
-0.008 -0.079 0.003
IIt-000
2-1 l-l o-1
25903.272 25901.885 25905.464
-0.057 0.019 m-O.058
Ilo-lot
2-1 2-2 1-2
14641.430 14641.820 14642.718
0.053 0.010 -0.121
202-101
3-2 2-I 1-O
25958.997 25958.897 25959.528
212-111
3-2 2-l 1-o
211-110
3-2 2-1 1-O 2-2 1-I
212-101
u* -
“1;
(ohs.)
;I)
_ ..__
“A
v1(
(talc.)
0.045
13068.441
-0.028
0.096 0.093 0.180
25903.053
-0.032
0.122
0. I ‘1G
0.169 0.134
14641.890
-0.019
0.152
0
-0.058 -0.011 -0.017
25959.017
-0.029
o.ocr2
24329.568 24329.827 24328.100
-0.019 -0.119 0.094
24329.573
-0.014
ILO’.
27943.815 27941.425
27944.016
-0.055
o.I:~z
27945.382 27941.921
-0.079 -0.007 -0.143 -0.079 -0.074
3-2 2-1
37164.461 37162.833
-0.057 -0.149
37164.098
-0.103
0.175
202-111
3-2 2-l 1-o
13124.201 13125.955 13121.816
0.078 0.083 0.045
13124.499
0.069
Il.040
21 I -202
3-3 2-2 1-I
16626.500 16628.008 16625.890
-0.150 0.078 -0.049
16626.894
-11.041
It.226
3-3 2-2
38681.585 38679.743
0.049 0.060
38681.178
0.054
303-202
4-3 3-2 2-1
38505.056
-0.03 I 0.223 -0.178
38505.056
0.025
32 I -220
4-3 3-2 2-1
39904.632 39905.259 39904.277
O.OG7 -0.101 -0.032
39904.756
- 0.045
3i3-212
4--3 3-2 2-l
36387.884
-0. I48 -0.173 0.088
322-221
4-3 3-2
39205.011 39205.444
0,043 0.012
303-2.2
4--3 3-2 2-l
27299.623 27300.753 27298.990
0.000 - 0.005 -0.009
ho-21
I
0.357 0.383
0.171 0.149 0.252
36387.929
._
h,
- 0.078
39205.098
0.0?7
1
- 0.005
27299.87
0.370
I”0
0.360 IhI
O.lRH
0 IHI
It.1111 I
.. 42.
PN. Ghosk et aL /Microwave spectra of cis-methyl nitrite
Table 1 (continued) JK_K+-J&;
F-F’
vA(ObS.)
vA(ObS.) -
312-303
4-4 3-3 2-2
19919.708 19921.547 19919.245
-0.280 -0.051
321-312
4-4 3-3
36787.808 36786.550
0.071 0.048
0.358 0.371
36181.487
5-5
24826.231
3-3
24828.257 24825.760
0.130 0.197 0.161
0.471 0.372 0.447
24826.786
4-4 5-5
35 107.367
0.069
0.328
I 413-404
422-41:
3-3
35 106.651
0.085
6-5 5-4 4.-3
36411.061 36412.472
0.074 0.072 0.084
514-50s
6-6 5-5 4-4
31577.171 31579.346 31576.570
6-6 4-4 5-5
725-;2b
36410.790
35107.187
0.084
0.338
0.332
36411.470
0.089
0.126
0.343
34218.982
0.073 0.039
0.363
8-8
17399.667 17401.376 17399.427
b,
0.052
0.628 b,
0.624
34219.207
0.079
0.332
0.342
0.413
34640.745
0.106
0.413
0.400
0.462 0.516 0.484
17400.165
0.246
0.486
0.552
36774.849
0.488 b,
0.512
0.160 -0.027 0.191 0.235 0.276 0.235
36775.230
-0.289
0.577
835-744
9-8 7-6 8-7
14405.115
-0.l-ll7 0.082
-0.800
14405.990
0.022
-0.710
937-844
10-9 9-8 8-7
22523.107
0.046 -0.031 0.047
93b-845
LO-9 9-8 8-7
31199.227
0.002
31200.159
~14%3-1%5
0.444
b,
0.327
0.444
1037-103s
0.463
34219.325
0.307
-0.278 -0.183
1038-945
0.164
0.170
0.326
36774.653
._
0.348
0.359
8-8 6-6 7-7
725-716
0.312
0.364
31577.781
34640.745
6-6
b) -0.180
“A-YE (G-k.)
0.059
0.601 0.655 - c)
7-7 6-6 S-5 7-7
VA - VE ‘) (ohs.)
-0.119
4-4
624~-615
19920.220
vA,center(~lc-)
-0.165
514..423
523-514
VA,center(obs.) -
vA - UE “A,center (ohs.) (ohs.)
vA(dc.)
31199.168
-0.010 0.043
II-10 10-9 9-8
34521530
-0.174
11-11 IO-10
14194.897 14196.010
12-11 IO-9
24112.998
11-10
24113.307
b,
-0.254
14405.410
0.029
-0.770
-0.631
-0.460
22523.107
0.02 1
-0.460
-0.508
-0.361 _ c)
31199.541
b)
0.032
-0.401
b)
-0.274
-0.620
34521.530
b)
-0.620
b,
-0.547
0.547 0.566
1419.5271
-0.442 -0.144
-0.069 -0.192 0.147 0.197 -0.042 -0.022
0.008
24113.101
-
0.199 -0.019
0.584
0.572 -0.274
spectra of cis-methyl nitrite
P.N. Gltosh er OL /Microwave Table
43
1 (continued)
F-F’
1138-1139
1249-1156
1239-123.10
1359-1266
VA(obs.)
12-12
22050.882
IO-10 11-11
22052.237
“A-YE
(talc.)
-0.471 -0.355 -0.391
- c)
22051.337
-0.406
-0.437
37711.384”)
-0.092
31956.472b)
-0.339
24108.771
-0.061
37711.475 31955.915
-0.412
31957.455
14-13
24108.683 24108.945
-0.041 -0.083
0.575 b,
0.831
0.575 -0.473
-0.407
-0.545 -
c)
-0.353
-
c)
-0.059
0.528
1.121
-0.332
15 4,11-154.12
16-16 14-14
~~s,1s’m,l6
“A - % ‘)
Cobs.)
13-13 11-11 12-12
16714.091
19 5,14-195,15
VA.center(obs.) ~A,centrr(‘=IC.)
-0.080 -0.069 -0.125
15-15 13-13 14-14
185,13-185.14
“A,center (obs.)
37711.339
14 4,10-144.11
177,10-168,9
“A-% (obs.)
13-12 11-10 12-11
12-11 13-12
177,11--168,s
vA(ObS.) VA(CalC.)
0.652
0.688
0.790 0.779
16714.943
0.142 0.203 0.135
0.852
25325.075
0.248
1.096
16714.376
0.160
0.803
0.767
25325.440
0.286
1.095
1.077
3.884
4.095
0.319
1.5-15
25326.167
18-17
22695.995
-0.017
16-15 17-16
22696.202
-0.005 -0.048
18-17
22780.279
16-15 17-16 19-19
0.290
1.092
3.895
22696.064
-0.023
3.863 -5.6 10
22780.455
0.067 0.080 -0.001
18383.437
-0.272
0.896
17-17 18-18
18384.347
-0.232 -0.082
0.910
20-20
27504.700
18-18 19-19
0.049
22780.338
-5.643
-5.539
-5.709
27505.641
0.033 0.079 0.079
1.141
21-21 !9-19
39049.781
0.58k
1.676
20-20
39050.860
1.275
-0.196
18383.741
0.900
0.921
b,
0.064
1.230
1.271
39050.14 1 b)
0.612
1.670
1.643
27505.015
0.638
‘) Difference of hypothetical center frequencies.
0.6 16
1.659
b, Not included in the least-squares fit.
an iterative least-squares fit from the A state transition frequencies up to J = 14. Including higher J transitions did not improve the adjustment of the quadrupole coupling constants. The quadrupole coupling constants were then used to derive hypothetical unsptit centers for all measured rotational transitions. The center frequencies are included in table 1. Thirty one hypothetical center frequencies of the A state up to J= 20 were used to determine the fial rotational
‘) Obscured
by other rransitions.
and centrifugal distortion constants. A standard deviation of 110 !-Sk was estimated for a single measurement. This is somewhat larger than the accuracy of the frequency measurements of 20 kHz. This discrepancy may be attributed to the neglect. of higher order centrifugal distortion terms and the fact, that the A state frequencies were fitted. The rotational and centrifugal distortion constants from the last fit are shown in table 2 together with quadrupole coupling from the
P.N. Glrosh et al. /Microwave spectra of cis-methyl &rife
44 Table 2
Rotational corlstants (in MHz), centrifugal distortion constants (in kHz)
and quadrupolecoupling constants (in MHZ)of cis-
methyl nrtrite ir: the A state of the vibrational ground state -~
Present work ___________._____.___._.-_--._...--_~ A B c ,*J *JK *K
LJ &K xall Xbb xec
20272.540(36) 7437.914(4)
5630.582(9) 6.74(20)
Gwinn et al.
[ I]
20273.008 7438.113 5629.631 -
-12.90(130) 54.97(2 IO) 2.1(l) 5.4(20) 1.443(81) -4.874(76) 3.431(80)
.___ ,..__ 20272.38(20) 7437.81(5) 5633.58(S) -
_ _. 1.39(3) -4.86(3) 3.47(3)
Xab
I;“)
.--__r--. Turner et al. 121
3.120
3.101 _. -
_.
b) b) b) 2.8(6) ‘) 3.12
-
.--_______
b, Taken from ref. [ 11. ‘1 Obtained from the determination of xpa and Xbb in cis-CDBONO and cis_CH~ONO.
zy,
= /, -- A = Ia +I;
-
Jc (in uA2).
first fit. The errors represent one standard deviation. The distortion constant 6~ is not well determined from the assigned transitions. Complete neglect of ail centrifugal distortion constants, however, would raise the standard deviation of a transition frequency to an unacceptably high value of 16 MHz. Table 2 also gives a comparison of the molecular constants with those determined previously by Turner et al. [2] and Gwinn et al. [I]. The internal rotation splittings (A-E) due to methyl torsion were calcdated as iscribed in section 3.2. The calculated values are presented in table 1. 3.2. Vibrutiomlly excited states Rotational transitions in the first and second excited state of the NO torsion were assigned initially by microwave-microwave double resonance experiments. The A-E splittings due to the internal rotation of the methyl group were expected for these excited states to be of roughly the same magnitude as for the ground state. Since only low J transitions were assigned, the .A-E splitting: were not resolved. The quadrupole splitting could be resolved only for a few truw4ions in the first excited state of the NO torsion. In 111. t ~35~9 the frequency of the strongest componenr was measured accurately. Hypothetical unsplit centers were czalculated from the measured frequencies
taking over the quadrupole coupling constants of the ground state. In all other cases quadrupole splittings could not be resolved and the measured frequency was taken as the center frequency. These frequencies are collected in table 3. They are used to obtain the rotational constants in a least-squares fit. The centrifugal distortion constants were taken from the ground state and were not adjusted. The results are shown in table 4. The dependence of the rotational constants on torsional excitation is almost linear as shown by the results in tables 2 and 4. The rotational transitions in the first excited state of the CON angle deformation mode (V;,, = 365 cm-’ [S]) were assigned mainly from the similar Stark patterns and relative intensity of the transitions with respect to the first and second excited state of the NO torsion. The analysis of the rotational constants was made in a way similar to that for the NO torsional excited states. The measured transition frequencies and the rotational constants are included in tables 3 and 4, respectively. First and second excited states of the methyl torsion were assigned partially by double resonance experiments and partially by the similar Stark effect of corresponding rotational transitions. Again onI:/ low J transitions were assigned. The measured transition frequencies are collected in table 5. The internal
P.N. Cihoshet al./Microwave spectraofcis-trretl~~~l~itrite
Table 3 Measured
transition
state of CON
frequencies
deformation
.~
(in Mllz)
of cis-methyl
ni!rite
____________--._
___-_-_-_--.
"14 = 1
JK_K&K;
%bs. - “talc. “ohs. ___.__.._ .__ ~._-___-.--.----
la-000 111-000 202-101 212-111
13025.62 25899.30 25880.00b) 24280.50b)
-0.01 0.22 -0.65 0.43
211-110
27882.10 37153.60 a*b) 13006.60 '1 16587.00 a)
-0.11 0.08 -0.59 0.82
202-111 211-202
excited
“ohs. - “cult. lobs. --.._~__-~--“.
12983.20
24231.40 27700.25 37141.20
322.-221 313-212 312-321 312-303 -
_ ._-..
a) Hypothetical for 2x2 -lo, b)Observed
38405.20 39748.50 39078.30
b, ‘)
36317.50 36918.00 19804.00
b, c) ‘1
0 48 0.4 1
1.74 -0.48 0.24 1.87
0.46 0.73 -0.29
16542.50
0.0.1
38303.70 39590.25 38948.00
-0.13 -0.96 0.32
37044.25 _
-0.08
.,_._ -_-_
25945.50 26090.00 24419.50 28141.00 37234.00
-0.76 -1.08 1.13 0.40
16726.00 38636.00
-0.12
39418.00 36708.00 20134.00
0.14
I ‘70 1.07 -0.09 -0.28
. _._
_
in MW-MW
double
constants (in Mllz:, ..___. .._~_
__^_.
.~_~
resonance.
c) Not included
in the least-squares
ofcis-methyl
VI4 = I
._ .~~
20271.878(262) 7398.417194) S&27.243(72) 3.430 distortion
nitrite
component
(I;=
3
I;’
:
2
fit.
whereas the moment of inertia of the methyl top, Jr, and the angle 4 between the internal rotation axi!; and the principal II axis were kept constant at the values reported from the structure determination 1.21.The latter two parameters were found to have little effect on the small splittings of the ground state. For the excited torsional states, the parameters Ap) = E,,,(u) - L?E(u), the A-E energy separation of the torsional state u, p = I7 [(x,/l,)’ + @xl,/lb)21”’ and 0 = arc tan[XbIa/Xalb] were varied, where IO, I,,, I,, h, and I,,, are the three principal moments of inertia and the direction cosines of the internal rotation
in the excited
states of NO torsion
(~~4) and CON dut’c*rlnarion
(vlo)
:I)
__.... ~. .~_
-_.__.___.___.___.~__. n)Centrifugal
- ._----.-._.-_-
_
Table 4 Rotational __-_
A
an6 first cvrhed
“ohs. .. “talc. ___~__.___
center frequency obtained from the measured frequency of the etroneest quadrupule and F= 3-F’ = 3 for 211-2o2) and the shift to the center as shown in table I,
rotation splittings increase by an order of magnitude upon each step of torsional excitation of the methyl group. The internal rotation parameters of the methyl group were adjusted from the measured transition frequencies and A-E splittings in separate leastsquares fits for the ground and each excited state of the methyl torsion. The internal axis method in the high barrier approximation given by Woods [91 was applied for the simultaneous adjustment of rotational constants and the internal rotation parameters. For the ground state only the barrier, V3, was varied
B C I; (uA2)
forr~on
0.55
220-211
303 -202 321-220
state of/IO
.._. ..- .-.- .~-. u10= 1
2
VI4 =
“obs. -___
212-101
in the first and ..econd
45
constants
II,4 =
_
2
v,o=
20315.989(205) 7500.5081853 5639.314(127) 2.638
20269.094(218) 7358.538(116) 5624.161(175) 3.754 _ _.-I-..~.~ were taken from
the ground
I
^_
----.state (teble
2) and
were kept
constant.
PN. Gfxxh er at. / Microware spectra of chnetlzyl nitrile
46
-
TabIe 5 hleasuredtransit&
JK_K++K;
frequencies ----
(in MHz) of c&methyl ---
nitrite in the excited states of methyl torsion (vtS) ---
“1s = 1
v,;=2
vA(obr)
“A - YE (ohs.)
“A - UE (talc.)
L’A(ObS.)
VA - % (obs.)
VA - VE (talc.)
-2.83
12957.6
36.0
43.05
-7.0 -16.8
-6.66 -16.14
24881.5
813.5
819.05 __
25726.5 a) 27648.5
-5.0 -5.5
-5.26 -5.26
25741.0 27701.0
79.5 499.5
79.30 498.29
24135.9 3) 37148.0
-5.6 -9.5
-5.95 -9.71
24147.23) 36065.0
-334.8 448.0
-328.40 447.58
15660.5 b, -
-53.0
-60.14
38165.2 39616.0 b, 36111.2”) 38886.4 18928.0
103.0 507.4 -74.7 -230.1 157.0
102.63 500.43 -76.46 -229.12 159.5’
lot -%a
12946.4
IIt-000 ~lO--~OI
25958.0 14769.6
202-101 211-110 712-111 212-101
1 -11-202 220-h
16692.4 b, 39 205.0 b)
I
303-202 32t-120
38182.5 a) 39491.7 36102.6 5) 38838.0
:l3-2,2 322-221
-2.8
-14.8 -141.5
-16.15 -139.88
-6.9 68.9 -4.4 -87.0
-6.86 68.97 -4.73 -86.85
312-303 a) Observed in MW-BIW double resonance.
b, Not included in the last-squares fit.
axis with respect to the principal axes, respectively [IO]. The centrifugal distortion constants were not adjusted but kept at the values of the ground state given in table 2. The resuhs of the Ieast-squares adjustments for the three states are shown in table 6. TabIe 6 Rotational
constants
and internal rotation parameters
This treatment of the measured data for the second excited stateghes only approximate results because the lower limit of the high barrier approximation was
surpassed. A better insight into the internal rotation
of cis-methyl nitrite in the pound
and exited
of the
states of methyl torsion e)
(w) uls=o
u,5 = 1 .--.--~
20272.461(19) b, 7437.888(6) b, 5630.585(5) b, 3.18 c) 0589 c, 0.808 53.9 14 0.08411 - 14.239 0.4663 56.930 734.1(21) 2.099(6) b,
20371.541(395)b)
______-.
A (MHz) B (MHz) c (WLZ) Ir(uA2)
ho hb
0 (W P A0 (MHz) P (md) S
V3 (cm-t) V3 (kcal/mol)
7353.297(152) b) 5594.568(98) b, 2.90 0586 0.810 54.118 0.07636(350) b, 1116.3(471) b, O-4633(152) b, 51.387 720.8(35) 2.061(10)
Lq5 = 2 19176.615(1412) b, 7341.923(386) b, 5592.613(259) b, 3.27 0.605 0.796 52.745 0.08411 -13763.7(270) b, 0.4663 58.100 728.9(17) 2.084(5)
a) Centrift& distortion constants were used from the ground state (table 2) and were kept constant. b, These p&meters were varied to fit the observed A-E frequencies. =) Taken from ref. 121.
Cl14= 1.u15 = 1 20360.014(1100) 7325.444(380)
b, b,
5588.846(190)b)
3.206 0.582 0.813 54.438 0.08411 955.5(60) b) 0.4663 51.912 663.1(49) 1.896(14)
Table 7 Measured transition frequencies (in MHz) of c&methyl
nitrite in a mixed excited state of both the NO and CH3 torsion (~14 = 1,
UlS = 0
JK_K+-J&i IlO--101 202-101 2 1z-lr1 211-110 2,,-202 303-202 313-212
;
u_+y(ObS.)
VA
14763.5 25661.8 24090.1 27558.2 16660.1 38098.2 36037.1
-16.1 -5.6 -5.8 -6.0 -16.5 -7.2 -4.8
“A - q(obs.) ---.--- 14.3 a) -5.8 -5.7 -7.2 -15.9 -6.7 a) -5.4
-
v~(calc.) --___
---a) Not included in the least-squares fit.
methyl group was attempted by simultaneously considering the A-E splittings of ground and both excited states of the methyl torsion. The variation of the V, potential for the different states points to the contribution of higher terms in the potential expansion of the methyl barrier. Transition frequencies and their variation with respect to internal rotation parameters were calculated from eigenvalues obtained by direct diagonalization of the hamiltonian matrix of a semirigid model [lo] _But a simultaneous adjustment of the A-E splittings in the ground and two excited states of the methyl torsion could not determine a single set of internal rotation parameters Z,, 9, V3 and V, _This failure of the semirigid model could not be corrected by inclusion of non-rigidity terms as used for a similar analysis of the spectrum of acetaldehyde [lo] _It is apparently due to a specific interaction of the methyl torsion with the NO torsion. A typical example of a strong interaction between two torsional modes in a molecule was recently studied in pyruvic acid (111. Finally we were able to assign a mixed excited state where both the NO and the CHJ torsion are singly excited. The A-E splittings are similar to the first excited state of the CH3 torsion alone. From the measured transition frequencies in table 7 the rotational constants and the internal rotation parameters were adjusted as explained above. The results are included in table 6. 3.3. Relative intensity measurement Additional information on the energy separations of excited states of the NO torsion could be gained
from relative intensity,measurements. The method of Esbitt and Wilson [12] was applied in order to measure the peak intensities of transitions with corresponding assignments in ground and first excited state. Measurements were carried out in a few system and the pressure was adjusted such that the A-E splittings were not resolved in both states. The strongest quadrupole components of the transitions 2O2-1o1 and 2:12-1I L were measured over the temperature range X0-290 K. Furthermore we studied the temperature dependence of the peak intensities of the transitions 322-221 and l&,- 11s6 in the ground
state of the cis conformer and of the transitions 4,3-3 12and 414-3 13 of the ground state of the trans conformer [2] over the same temperature range. Measurements were made with a constant flow as well as at constant pressure v&hout a flow in the absorption cell. The energy separationsof the states involved were deduced from the temperature dependence of the peak intensities according to the procedure discussed below. The peak absorption coefficient of a rotational transition as a function of temperature T is given by t131
x (1 - IlLJ~/X~ I(mIPlrr)12,
(3-j where u,, = (I?,, - E,,)/Iz is the resonance t’requency, Av(T) the line width parameter of the assumed lorentzian line shape, N the number of molecules per unit volume,f,,(7J the fraction of molecules in the lower state for a transition between states m and II with a connecting dipole element (nzlpin), c the velocity of light, k the Boltzmann constant and II the
p.N, Ghmh ct al. /Microwave
48
yields the normal frequency Cl, The ratio of the total number of molecules of two conformers in thermal equilibrium is given by
Planck constant. The factor (! - hv,/ZkT) can be neglected in the frequency range considered. The fraction of molecules in the lower state m is calculated from the Boltzmann distribution
= f;.(T)g, exp(-Er/k7’)(h3.4BC/lnk3 T’)“’
WN, (3)
for an asymmetric rotor with rotational constants A, B and C and a rotational energy E, in the lower state m with rotational degeneracy g,, where f,(T) = gVexp(--&/kn/Q,(’ represents the fraction of molecules in the vibrational state with energy E, and degeneracy g,. Q, and Q, denote the rotational and total vibrational partition function, respectively. For internal rotation sta!es with intermediate or high barriers, the partition function may be approximated by a vibrational partition ftinction and will be included in Qv. It is very difficult to determine the absorption coefficients in microwave spectroscopy absolutely because of the problems of accurate power measurements in the microwave region. Therefore only the ratio of absorption coefficients is usually measured between different vibrational states of the same molecule or between the ground states of two conformers. Higher accuracy can be obtained if the ratio of the products of peak absorption coefficient and Ime width parameter is formed instead of the ratio of the peak absorption coefflfirients alone according lo the exprtssion
wbrre the indices I and 2 refer to the two vibrational stales compared. Transition frequencies, rotational constants and line strengtha are known from the nnalysis of the corresponding rotational spectra, The ro!atio~!! energy difference ErI - .Fr2 is wually very small compared to kT and can safely be neglected. if 1”etrleasure the relative intensities of rotational tranions in the ground and first excited state of a nondc8enerate normal vibration i, then the expression Ink
(7’J/fv2(T)]
- -/6,/ckT
spectra of cis-methyl nitrite
(5)
= exp HE1 - GW”l
et/Q, ,
(6)
where Et - EC refers to the energy difference between the ground states of the two conformers considcced. Qt and Q, are the partition functions of the two conformers. They can be expanded into a product of contributions from translation, rotation, internal rotation and vibrations. Since the masses of the two conformers are equal, the translational partition functions cancel. The ratio of the number of molecules in a giver. lower state mt or m, is expressed as
= [gnrtw(-E~t/kW~m, ev(--Ek/kT)l X exp[-(4 - &W’l~
(7)
where EL, or EL, is the energy of rotation, internal rotation and vibrations with respect to the ground state of the corresponding conformer. The logarithmic ratio of the products of peak absorption coefficient and hne width parameter then reads [cf. eq. (211
Ad
ln[~t(~ot)A~tloc(~oc)
= 2 W0tlu0d
+ lt&Ad
+ ~n[l~mtl~tlnt~12/l~m,l~cl~t,~121 -
(Et -
E,\/kT - (Eh, -- EA&kT.
(8)
For the comparison of rotational transitions of the vibration and internal rotation ground state the energy difference Eht - EL, is of the order of a rotational energy. It will be neglected considering the experimental uncertainties of the relative intensity measurements. Measurements of relative peak intensities are usually made at a single temperature. They are Table 8 NO torsional from r&he __.~_.___..~ ‘kw.ition
frequency
~~4 (in cm-‘) of cirmethyl measurements
.___--
Torsional frequency
---
202-101 212-111
nitrite
peak intensity
207(75) 213(57)
---
Mean :orsiond
210(31)
frequency
P.N. Gl~oslr et al. /Microwave Table 9 Energy difference Transitions
_-__-__
322-h -P-A-
between
cis and trans conformers
compared
cis
1249-1156
(in cm-‘)
Energy difference trans
--..
__
413-3t2 414-313
-_-_-.-_
..__~
closed cell ~~_ 36 I(W) 286U49) ._ _
spectra of cis-met/l vl nitrite
in methyl
Etrans
nitrite
- &is
Mean energy
differcnrc
flow system 267(43) 423f159) ~. -~~-.-_--.-
analyzed according to eqs. (4)-(5) or (8). As an alternative procedure to determine Pi or E, - E, thy temperature dependence of ln(a, CV,/a2Au2) or Irn(a,Av,/a,Av,) was studied. Thus, the torsional frequency c,, or Etrans - &is could be determined easily from plots of In@, Av, /o12Avl) or In(a,Au,/ acAu,) versus E/T. Thereby no prior hncwledge of rotational constants, line strengths or statistical weight factors according to ea. (4) or (8) was necessary. The results of the analysis for the frequency of the NO torsion are given in table 8 and for the energy difference between cis and trans methyl nitrite in table 9. The trans conformer was found to be higher in energy by 314(22) cm-’ . The errors are considerable due to the large scatter of the individual measurements at a given temperature. Measurements at several temperatures should therefore provide more reliable data than a measurement at a single temperature.
4. Discussion-and conclusions In the work of Turner et al. [2] on the abundant isotopic form of cis-methyl nitrite, twelve transitions up to J = 6 were assigned in the frequency range from 12 to 40 GHz. Only five of the transiticns were resolved into nuclear hyperfine components and none showed resolution into internal rotation doublets. Most of the strong absorption lines remained unassigned in this region. The barrier to internai rot+ tion of the methyl group was determined f:om the splittings of six transitions of the 15Nspeclcs alone. This work provides a much extended assignment for the vibrational ground state including some thirty high J transitions up to J = 20. Ail of them >howed at least a partial resolution into hyperfine ccmp~
314(22) _._.
..--
__
__
nents. The newiy determined quadrupole coupling constants exhibit only small changes within the un. certainty limits compared to the previous results ( I 1. Turner et al. 121 were able to estimate the out-ofdiagonal constant hfi = 2.8 + 0.6 MHz from changes of the diagonal constants in CD30N0. Combined with the diagonal part the quadrupole coupling tensor was transformed to principal axes with the following results: hX = --5.95(50) MHz, xjaY= 3.43(G) MHz and xrz = 2.55(50) MHz, where the?) axis is perpendicular to the molecular symmetry plane and the z axis forms an angle of 20.9(30)* with the principal a axis. Internal rotation doublet splittings were measured Fpr half of the asisigned transitions in the ground stale. In addition, transitions in singly or ioubly excited b;ates of the NO and methyl torsions were assignstl. Combined with the measurements on tbc ground state the measured A-E splittings in excited methyl torsional states allowed a much more reliable determination of the barrier hindering the internal rota1.io.lof the methyl group. Comparing the new value for V, = 2098.7(63) cal/mol from the analysis of th: ground state alone with previous results shows good agreement with the value of V3 = 2090 cal/mol reported by Tu:ner et al. [2] but is higher than tile vafareof V3 = 1912 cal/mol of Gwinn et al. 111,Frori: the ab initio calculations on methyl nitriic [4J :I barrier of 1930 cal/mol was obtained for lhc cis 1’01m in fair agreement with the experimental obsurvatir)n. On the other hand a 10s barrier of 270 C~/IIKJ! vm calculated for the trans conformer. This result c011i~ pares well within the expected accuracy of the ;III initio calculation for the almost free internal rot;lIi(ltl with a small barrier of30 :o 60 cnl/mol dcducc~l from the microwave spectrum. Our own checks WI the assignments and our calculations confirm the
50
P_N. Gitosh et al. /Micrmvnw
results of Turner et al. [Z] for the tram conformer. The energy difference of 210(3 1) cm-’ between ground and first excited state of the NO torsion of the cis conformer is similar to the same difference in the tram form of 230(30) cm-’ [2] _The ab initio calculation of the force constants for the NO torsional vibrations yielded 0.181 mdyn/rad and 0.109 mdyn/rad which correspond to torsional frequencies of336 cm-’ and 187 cm-’ for the cis and tram conformers, respectively, assuming the torsional states to be pure. But the NO torsion in the cis conformer will have considerable mktiing with the methyl torsion and will give rise to a large shift in the frequency. In the case of the tram conformer, however, there is practically no such mixing. Ali known measurements of the relative stability of the cis and trans conformer showed invariably the cis form to be more stable_ A ground state energy difference of 275 cm-’ [l] and 296 cm-’ [ 141 from relative intensity measurements in the microwave spectrum is in good agreement with our vahre of 314(22) cm-r from the temperature dependence of absorption lines in the microwave spectrum. Similar measurements from relative intensities in the infrared spectrum of trapped effusive or supersonic molecular beams yielded 625 cal/mol(2 18 cm-’ ) [15]. investigation of the NMR spectrum in the liquid phase gave a difference of 275 cm-’ [16]_ A considerably larger difference of 2940 cal/mol (1028 cm-‘) was obtamed from the ab initio calculation. This value refers actual!y to the minimn of the potential surface. The difference of the zero point energies of the two conformers which is mainly due to different internal rotation barriers of the methyl group reduces this value by approximately 100 cm-’ _
spectra of cis-methyl nitrite
Acknowledgement Financial support by the Swiss National Foundation (Project No. 2.033-0.78 and 2.X19-0.79) and a research grant from the administration of the ETHZ (No. 13223/41-0430.5 and 14651/41-0430.5) are gratefully acknowledged. Furthermore we wish to thank Mr. N. Schwizgebel for preparing methyl nitrite. Finally, the ETHZ Computer Center generously granted free computer time.
References
[I J W.D. Gwinn, R.J. Anderson and D. Stelman, Bull. Am. Phys. Sot. 13 (1968) 831. [2] P.H. Turner, MJ. Corkill and A.P. Cos, J. Phys. Chem. 83 (1979) 1473. [3] K:. Endo and Y. Kamura, J. Chem. SW. Japan 5 (1977) 729. [4] T.-K. Ha, unpublished results. [S] A.H. Blatt, ed.. Orgmic syntheses (Wiley, New York, 1963) p. 363. [6] J. Ekkers, A. Bnuder and Hs.H. Giinthard, 5. Phys. E: Sci. Instrum. 8 (1975) 819. [7] J.K.G. Watson, J. Chem. Phys. 46 (1967) 1935. [8] P. KJaboe. D. Jonesand E.R. Lippincotr, Spectrochim. Actn 23A (1967) 2957. [V] R.C. Woods, J. Mol. Spectry. 21 (1966) 4. [ 101 A. Bnuder and Hs.H. GBnthnrd, J. Mol. Spectry. 60 (1976) 290. [ 111 A. Bauder and R. Meyer, unpublished results. [ 121 AS. Esbitt and E.B. Wilson. Rev. Sci. Instrum. 34 (1963) 901. [ 131 W. Gordy and R.L. Cook. Microwwe molecular spectra (Interscience, New York, 1970) p. 41. [14] A.P. Cos, private communication. [ 151 P. ITelder end Hs.H. Giinthard, Chrm. Phys.Letters 66 (1979) 283. [16] H.W. Brown and D.P. Hollis, J. Mol. Spectry. 13 (1964) 30.5.
MICROWAVE SPECTRA OF GROUND AND EXCITED VIBRATIONAL OF CIS-METHYL NITRITE, CHJONO
STATES
Pradip N. GHOSH *, A. BAUDER and Hs.H. GaNTHARD Laboratory forPllysical C~~emistry,Swiss Federal Institu te of Technology. CH-809-7 Zwich, Switzerland Received 13 !4ay 1980
Rotational transitions of the vibrational ground state of cis-methyl nitrite have been assigned up to J= 20 over the frequency range from 12 to 40 GHz. Rotational constants, centrifugal distortiqn constants and quadrupolc coupling constants have been fitted to the measured lransition frequencies. In addition, rotational transitions in singly and doubly exited states of the NO and CIIJ torsions and in the singly excited state of the CON deformation mode have been assigned. Many of the absorption lines exhibited splittings due to the internal rotation of the methyl goup. The internal rotation barrier has been determined to be I;, = 734(Z) cm -t from the ground state splittings. Relative intensity measurements have yielded a transition frequency of 210(31) cm -’ foi the NO torsion. Furthermore, the energy diffcrencc between the more stable cis conformer and the trans conformer was found to be 314(Z) cm-t front the temperature dependence of the intensities of rotational transitions.
1. Introduction
The microwave spectrum of methyl nitrite was investigated first by Gwinn et al. [ 11. They showed that methyl nitrite occurs as cis and tram conformers in the gas phase around room temperature. Recently Turner et al. [2] reported the complete r, structure of both conformers thus extending a partial structure determination of the cis conformer by Endo and Kamura [3]. Turner et al. [2] also determined the internal rotation barriers of the methyl group for both conformers. They confirmed the high barrier of 2090 cal/mol in the cis conformer. For the trans conformer, they reported a much lower value of 29 cal/mol for the threefold barrier than that of 188 cal/mol previously given by Gwinn et al. [l] - The unusually large difference between the internal rotation barriers,of the methyl group in the two conformers prompted us to compliment the experimental investigations with an
l
On study leave from the Department ofCalcutta, Calcutta-700009, India
of Physics, University
ab initio calculation of the potential energy surface for both internal rotations of the CHa and NO groups [4]. For the comparison with the theoretical calculations we needed more information on the cis conformer. In the present work we have extended the assignment of the rotational spectrum of the cis form in the ground vibrational state up to J = 20 based on measurements with higher resolution. The newly measured transition frequencies allowed us to determine more reliable rotational and quadrupole coupling constants as well as the centrifugal distortion constants which were not reported in the earlier works. With the help of microwave-microwave double resonance experiments we could assign several excited states of the CH, and NO torsional modes and the in-plane CON bending mode. From the splittings of rotational transitions in ground and excited torsional states the barrier to internal rotation of the methyl group was determined. The torsional frequency of the NO group was obtained from relative intensity measurements. An independent value of the energy separation between cis and tram conformer was found from the temperature dependence of intensities of absorption lines.
P.N. Gltosh et al. /Microwave
40
2. Eqerimental details 2.1. Chemicals MeQunol and sodium nitrite were reacted with dilute sulphuric acid to form methyl nitrite [51. The crude product was purified by low temperature distillation. 2.2. Spectrometers The microwave spectrum of methyl nitrite was recorded over the frequency range from 12 to 40 GHz with conventional 30 kHz Stark modulated spectrometers. The backward wave oscillator sources were phase-stabilized to the harmonics of a digitally controlled reference oscillator. The 4 to 6 m long Stark cells were mostly operated at room temperature, occasionally they were cooled to -4O’C. Methyl nitrite strongly adsorbed to the walls of the waveguide cells. Frequent refilling of the cell after every hour or a slow flow through the cell was necessary in order to keep the pressure within narrow limits around 1 to 2 mTorr. The microwave power was kept around 100 ;JWin order to avoid saturation broadening. Frequency measurements were averaged from sweeps in both directions. For signal enhancement and digital filtering the spectrometers could be connected to a small computer. This mode of operation was used mainly for the measurements of transitions in the vibrational ground stdte. The accuracy oi the frequency measurements under computer control was estimated to be better thrn 20 kNz. The frequencies for transitions in excited states were read directly from the strip.chart recordings and are accurate to within 0.1 Mti. Microwave-microwave double resonance was used exunsively for the assignments of satellite spectra with an instrument described previously [6]. The pump power of the order of several hundred milliwafts from phase-stabilized klystron sources was smpll~udtimoduluted a1 a rate of 30 kHz.
3. bdysis
OP microwave spectra and
results
3. I. GroutId vibrutior~al state lhc microwuve spectrum of methyl nitrite shows a number of absorption lines due to the presence of two
spectra of cis-methyl nitrite
conformers in a number of vibrational states. Excited states are easily observed for the low frequency torsional motions of the methyl and NO group. In the spectrum of cis-methyl nitrite under high resoiution each rotational transition is split into several components arising from interactions with the quadrupole moment of the nitrogen nucleus and with the intplnal rotation of the methyl group. Rotational constants and quadrupole coupling constants were derived from measured transition frequencies with J up to 6 in previous investigations [ 1,2]. These constants were used to predict rotational transitions for higher J values which could be assigned readily from their quadrupole splitting pattern. The newly measured tiansition frequencies were subjected to a least-squares analysis for improved rotational and quadrupole coupling constants. Centrifugal distortion was found to affect noticeably even low J transitions and had to be included in the analysis. The hamiltonian !?=A$ - A&
tBp,z +Ci), - A_@)’ - [(i; - &(S#
+ (S_r@t @;))(i;
- $)]
- AJK&‘: + 6&) )
(1)
according to Watson’s asymmetric reduction 171 was expressed in the prolate representation I’, where A, B, Care the rotational constants, AJ, AJK, AK, fj~, 6K the centrifugal distortion constants and icz, Pb, kc the principal axis rsomponents of the angular momentum operator i. The full set of improved constants yielded better predictions and allowed the assignment to be carried out to still higher J values. This stepwise procedure finally enabled us to assign rotational transitions up toJ = 20. The results are collected in table 1. The transitions with J < 6 were remeasured with higher resolution than previou$y reported [2]. The prominent quadrupole components were usually completely resolved and even some of the weaker components could be measured. For the transitions with J 2 10 the quadr!Jpole components were only partly resolved into a doublet. The internal rotation splittings were too small to be obti,ved for transitions with J Q 2. These splittings were easily resolved for transitions with hi&r J and are presented in table 1, Three rotational constants, five centrifugal distortion constants and two independem quadrupole coupling constants were determined simultaneously in
P.N. Ghosh et al. /Microwave spectra of c&methyl nitrite Table 1 Measured and calculated transition
JK_K+-J&;
F-F’
41
frequencies (in MHz) of cis-methyl nitrite in the vibrational ground state
vA(obs.) -.-.--__-
V~(ObS.) VA(cak.)
VA - VE u*,centar (ohs.) (ohs.)
l’A,ceatcr(obs*) J’¢er(calc*)
--.
loi-000
2-l 1-1 o-1
13068.389 13068.751 13067.750
-0.008 -0.079 0.003
IIt-000
2-1 l-l o-1
25903.272 25901.885 25905.464
-0.057 0.019 m-O.058
Ilo-lot
2-1 2-2 1-2
14641.430 14641.820 14642.718
0.053 0.010 -0.121
202-101
3-2 2-I 1-O
25958.997 25958.897 25959.528
212-111
3-2 2-l 1-o
211-110
3-2 2-1 1-O 2-2 1-I
212-101
u* -
“1;
(ohs.)
;I)
_ ..__
“A
v1(
(talc.)
0.045
13068.441
-0.028
0.096 0.093 0.180
25903.053
-0.032
0.122
0. I ‘1G
0.169 0.134
14641.890
-0.019
0.152
0
-0.058 -0.011 -0.017
25959.017
-0.029
o.ocr2
24329.568 24329.827 24328.100
-0.019 -0.119 0.094
24329.573
-0.014
ILO’.
27943.815 27941.425
27944.016
-0.055
o.I:~z
27945.382 27941.921
-0.079 -0.007 -0.143 -0.079 -0.074
3-2 2-1
37164.461 37162.833
-0.057 -0.149
37164.098
-0.103
0.175
202-111
3-2 2-l 1-o
13124.201 13125.955 13121.816
0.078 0.083 0.045
13124.499
0.069
Il.040
21 I -202
3-3 2-2 1-I
16626.500 16628.008 16625.890
-0.150 0.078 -0.049
16626.894
-11.041
It.226
3-3 2-2
38681.585 38679.743
0.049 0.060
38681.178
0.054
303-202
4-3 3-2 2-1
38505.056
-0.03 I 0.223 -0.178
38505.056
0.025
32 I -220
4-3 3-2 2-1
39904.632 39905.259 39904.277
O.OG7 -0.101 -0.032
39904.756
- 0.045
3i3-212
4--3 3-2 2-l
36387.884
-0. I48 -0.173 0.088
322-221
4-3 3-2
39205.011 39205.444
0,043 0.012
303-2.2
4--3 3-2 2-l
27299.623 27300.753 27298.990
0.000 - 0.005 -0.009
ho-21
I
0.357 0.383
0.171 0.149 0.252
36387.929
._
h,
- 0.078
39205.098
0.0?7
1
- 0.005
27299.87
0.370
I”0
0.360 IhI
O.lRH
0 IHI
It.1111 I
.. 42.
PN. Ghosk et aL /Microwave spectra of cis-methyl nitrite
Table 1 (continued) JK_K+-J&;
F-F’
vA(ObS.)
vA(ObS.) -
312-303
4-4 3-3 2-2
19919.708 19921.547 19919.245
-0.280 -0.051
321-312
4-4 3-3
36787.808 36786.550
0.071 0.048
0.358 0.371
36181.487
5-5
24826.231
3-3
24828.257 24825.760
0.130 0.197 0.161
0.471 0.372 0.447
24826.786
4-4 5-5
35 107.367
0.069
0.328
I 413-404
422-41:
3-3
35 106.651
0.085
6-5 5-4 4.-3
36411.061 36412.472
0.074 0.072 0.084
514-50s
6-6 5-5 4-4
31577.171 31579.346 31576.570
6-6 4-4 5-5
725-;2b
36410.790
35107.187
0.084
0.338
0.332
36411.470
0.089
0.126
0.343
34218.982
0.073 0.039
0.363
8-8
17399.667 17401.376 17399.427
b,
0.052
0.628 b,
0.624
34219.207
0.079
0.332
0.342
0.413
34640.745
0.106
0.413
0.400
0.462 0.516 0.484
17400.165
0.246
0.486
0.552
36774.849
0.488 b,
0.512
0.160 -0.027 0.191 0.235 0.276 0.235
36775.230
-0.289
0.577
835-744
9-8 7-6 8-7
14405.115
-0.l-ll7 0.082
-0.800
14405.990
0.022
-0.710
937-844
10-9 9-8 8-7
22523.107
0.046 -0.031 0.047
93b-845
LO-9 9-8 8-7
31199.227
0.002
31200.159
~14%3-1%5
0.444
b,
0.327
0.444
1037-103s
0.463
34219.325
0.307
-0.278 -0.183
1038-945
0.164
0.170
0.326
36774.653
._
0.348
0.359
8-8 6-6 7-7
725-716
0.312
0.364
31577.781
34640.745
6-6
b) -0.180
“A-YE (G-k.)
0.059
0.601 0.655 - c)
7-7 6-6 S-5 7-7
VA - VE ‘) (ohs.)
-0.119
4-4
624~-615
19920.220
vA,center(~lc-)
-0.165
514..423
523-514
VA,center(obs.) -
vA - UE “A,center (ohs.) (ohs.)
vA(dc.)
31199.168
-0.010 0.043
II-10 10-9 9-8
34521530
-0.174
11-11 IO-10
14194.897 14196.010
12-11 IO-9
24112.998
11-10
24113.307
b,
-0.254
14405.410
0.029
-0.770
-0.631
-0.460
22523.107
0.02 1
-0.460
-0.508
-0.361 _ c)
31199.541
b)
0.032
-0.401
b)
-0.274
-0.620
34521.530
b)
-0.620
b,
-0.547
0.547 0.566
1419.5271
-0.442 -0.144
-0.069 -0.192 0.147 0.197 -0.042 -0.022
0.008
24113.101
-
0.199 -0.019
0.584
0.572 -0.274
spectra of cis-methyl nitrite
P.N. Gltosh er OL /Microwave Table
43
1 (continued)
F-F’
1138-1139
1249-1156
1239-123.10
1359-1266
VA(obs.)
12-12
22050.882
IO-10 11-11
22052.237
“A-YE
(talc.)
-0.471 -0.355 -0.391
- c)
22051.337
-0.406
-0.437
37711.384”)
-0.092
31956.472b)
-0.339
24108.771
-0.061
37711.475 31955.915
-0.412
31957.455
14-13
24108.683 24108.945
-0.041 -0.083
0.575 b,
0.831
0.575 -0.473
-0.407
-0.545 -
c)
-0.353
-
c)
-0.059
0.528
1.121
-0.332
15 4,11-154.12
16-16 14-14
~~s,1s’m,l6
“A - % ‘)
Cobs.)
13-13 11-11 12-12
16714.091
19 5,14-195,15
VA.center(obs.) ~A,centrr(‘=IC.)
-0.080 -0.069 -0.125
15-15 13-13 14-14
185,13-185.14
“A,center (obs.)
37711.339
14 4,10-144.11
177,10-168,9
“A-% (obs.)
13-12 11-10 12-11
12-11 13-12
177,11--168,s
vA(ObS.) VA(CalC.)
0.652
0.688
0.790 0.779
16714.943
0.142 0.203 0.135
0.852
25325.075
0.248
1.096
16714.376
0.160
0.803
0.767
25325.440
0.286
1.095
1.077
3.884
4.095
0.319
1.5-15
25326.167
18-17
22695.995
-0.017
16-15 17-16
22696.202
-0.005 -0.048
18-17
22780.279
16-15 17-16 19-19
0.290
1.092
3.895
22696.064
-0.023
3.863 -5.6 10
22780.455
0.067 0.080 -0.001
18383.437
-0.272
0.896
17-17 18-18
18384.347
-0.232 -0.082
0.910
20-20
27504.700
18-18 19-19
0.049
22780.338
-5.643
-5.539
-5.709
27505.641
0.033 0.079 0.079
1.141
21-21 !9-19
39049.781
0.58k
1.676
20-20
39050.860
1.275
-0.196
18383.741
0.900
0.921
b,
0.064
1.230
1.271
39050.14 1 b)
0.612
1.670
1.643
27505.015
0.638
‘) Difference of hypothetical center frequencies.
0.6 16
1.659
b, Not included in the least-squares fit.
an iterative least-squares fit from the A state transition frequencies up to J = 14. Including higher J transitions did not improve the adjustment of the quadrupole coupling constants. The quadrupole coupling constants were then used to derive hypothetical unsptit centers for all measured rotational transitions. The center frequencies are included in table 1. Thirty one hypothetical center frequencies of the A state up to J= 20 were used to determine the fial rotational
‘) Obscured
by other rransitions.
and centrifugal distortion constants. A standard deviation of 110 !-Sk was estimated for a single measurement. This is somewhat larger than the accuracy of the frequency measurements of 20 kHz. This discrepancy may be attributed to the neglect. of higher order centrifugal distortion terms and the fact, that the A state frequencies were fitted. The rotational and centrifugal distortion constants from the last fit are shown in table 2 together with quadrupole coupling from the
P.N. Glrosh et al. /Microwave spectra of cis-methyl &rife
44 Table 2
Rotational corlstants (in MHz), centrifugal distortion constants (in kHz)
and quadrupolecoupling constants (in MHZ)of cis-
methyl nrtrite ir: the A state of the vibrational ground state -~
Present work ___________._____.___._.-_--._...--_~ A B c ,*J *JK *K
LJ &K xall Xbb xec
20272.540(36) 7437.914(4)
5630.582(9) 6.74(20)
Gwinn et al.
[ I]
20273.008 7438.113 5629.631 -
-12.90(130) 54.97(2 IO) 2.1(l) 5.4(20) 1.443(81) -4.874(76) 3.431(80)
.___ ,..__ 20272.38(20) 7437.81(5) 5633.58(S) -
_ _. 1.39(3) -4.86(3) 3.47(3)
Xab
I;“)
.--__r--. Turner et al. 121
3.120
3.101 _. -
_.
b) b) b) 2.8(6) ‘) 3.12
-
.--_______
b, Taken from ref. [ 11. ‘1 Obtained from the determination of xpa and Xbb in cis-CDBONO and cis_CH~ONO.
zy,
= /, -- A = Ia +I;
-
Jc (in uA2).
first fit. The errors represent one standard deviation. The distortion constant 6~ is not well determined from the assigned transitions. Complete neglect of ail centrifugal distortion constants, however, would raise the standard deviation of a transition frequency to an unacceptably high value of 16 MHz. Table 2 also gives a comparison of the molecular constants with those determined previously by Turner et al. [2] and Gwinn et al. [I]. The internal rotation splittings (A-E) due to methyl torsion were calcdated as iscribed in section 3.2. The calculated values are presented in table 1. 3.2. Vibrutiomlly excited states Rotational transitions in the first and second excited state of the NO torsion were assigned initially by microwave-microwave double resonance experiments. The A-E splittings due to the internal rotation of the methyl group were expected for these excited states to be of roughly the same magnitude as for the ground state. Since only low J transitions were assigned, the .A-E splitting: were not resolved. The quadrupole splitting could be resolved only for a few truw4ions in the first excited state of the NO torsion. In 111. t ~35~9 the frequency of the strongest componenr was measured accurately. Hypothetical unsplit centers were czalculated from the measured frequencies
taking over the quadrupole coupling constants of the ground state. In all other cases quadrupole splittings could not be resolved and the measured frequency was taken as the center frequency. These frequencies are collected in table 3. They are used to obtain the rotational constants in a least-squares fit. The centrifugal distortion constants were taken from the ground state and were not adjusted. The results are shown in table 4. The dependence of the rotational constants on torsional excitation is almost linear as shown by the results in tables 2 and 4. The rotational transitions in the first excited state of the CON angle deformation mode (V;,, = 365 cm-’ [S]) were assigned mainly from the similar Stark patterns and relative intensity of the transitions with respect to the first and second excited state of the NO torsion. The analysis of the rotational constants was made in a way similar to that for the NO torsional excited states. The measured transition frequencies and the rotational constants are included in tables 3 and 4, respectively. First and second excited states of the methyl torsion were assigned partially by double resonance experiments and partially by the similar Stark effect of corresponding rotational transitions. Again onI:/ low J transitions were assigned. The measured transition frequencies are collected in table 5. The internal
P.N. Cihoshet al./Microwave spectraofcis-trretl~~~l~itrite
Table 3 Measured
transition
state of CON
frequencies
deformation
.~
(in Mllz)
of cis-methyl
ni!rite
____________--._
___-_-_-_--.
"14 = 1
JK_K&K;
%bs. - “talc. “ohs. ___.__.._ .__ ~._-___-.--.----
la-000 111-000 202-101 212-111
13025.62 25899.30 25880.00b) 24280.50b)
-0.01 0.22 -0.65 0.43
211-110
27882.10 37153.60 a*b) 13006.60 '1 16587.00 a)
-0.11 0.08 -0.59 0.82
202-111 211-202
excited
“ohs. - “cult. lobs. --.._~__-~--“.
12983.20
24231.40 27700.25 37141.20
322.-221 313-212 312-321 312-303 -
_ ._-..
a) Hypothetical for 2x2 -lo, b)Observed
38405.20 39748.50 39078.30
b, ‘)
36317.50 36918.00 19804.00
b, c) ‘1
0 48 0.4 1
1.74 -0.48 0.24 1.87
0.46 0.73 -0.29
16542.50
0.0.1
38303.70 39590.25 38948.00
-0.13 -0.96 0.32
37044.25 _
-0.08
.,_._ -_-_
25945.50 26090.00 24419.50 28141.00 37234.00
-0.76 -1.08 1.13 0.40
16726.00 38636.00
-0.12
39418.00 36708.00 20134.00
0.14
I ‘70 1.07 -0.09 -0.28
. _._
_
in MW-MW
double
constants (in Mllz:, ..___. .._~_
__^_.
.~_~
resonance.
c) Not included
in the least-squares
ofcis-methyl
VI4 = I
._ .~~
20271.878(262) 7398.417194) S&27.243(72) 3.430 distortion
nitrite
component
(I;=
3
I;’
:
2
fit.
whereas the moment of inertia of the methyl top, Jr, and the angle 4 between the internal rotation axi!; and the principal II axis were kept constant at the values reported from the structure determination 1.21.The latter two parameters were found to have little effect on the small splittings of the ground state. For the excited torsional states, the parameters Ap) = E,,,(u) - L?E(u), the A-E energy separation of the torsional state u, p = I7 [(x,/l,)’ + @xl,/lb)21”’ and 0 = arc tan[XbIa/Xalb] were varied, where IO, I,,, I,, h, and I,,, are the three principal moments of inertia and the direction cosines of the internal rotation
in the excited
states of NO torsion
(~~4) and CON dut’c*rlnarion
(vlo)
:I)
__.... ~. .~_
-_.__.___.___.___.~__. n)Centrifugal
- ._----.-._.-_-
_
Table 4 Rotational __-_
A
an6 first cvrhed
“ohs. .. “talc. ___~__.___
center frequency obtained from the measured frequency of the etroneest quadrupule and F= 3-F’ = 3 for 211-2o2) and the shift to the center as shown in table I,
rotation splittings increase by an order of magnitude upon each step of torsional excitation of the methyl group. The internal rotation parameters of the methyl group were adjusted from the measured transition frequencies and A-E splittings in separate leastsquares fits for the ground and each excited state of the methyl torsion. The internal axis method in the high barrier approximation given by Woods [91 was applied for the simultaneous adjustment of rotational constants and the internal rotation parameters. For the ground state only the barrier, V3, was varied
B C I; (uA2)
forr~on
0.55
220-211
303 -202 321-220
state of/IO
.._. ..- .-.- .~-. u10= 1
2
VI4 =
“obs. -___
212-101
in the first and ..econd
45
constants
II,4 =
_
2
v,o=
20315.989(205) 7500.5081853 5639.314(127) 2.638
20269.094(218) 7358.538(116) 5624.161(175) 3.754 _ _.-I-..~.~ were taken from
the ground
I
^_
----.state (teble
2) and
were kept
constant.
PN. Gfxxh er at. / Microware spectra of chnetlzyl nitrile
46
-
TabIe 5 hleasuredtransit&
JK_K++K;
frequencies ----
(in MHz) of c&methyl ---
nitrite in the excited states of methyl torsion (vtS) ---
“1s = 1
v,;=2
vA(obr)
“A - YE (ohs.)
“A - UE (talc.)
L’A(ObS.)
VA - % (obs.)
VA - VE (talc.)
-2.83
12957.6
36.0
43.05
-7.0 -16.8
-6.66 -16.14
24881.5
813.5
819.05 __
25726.5 a) 27648.5
-5.0 -5.5
-5.26 -5.26
25741.0 27701.0
79.5 499.5
79.30 498.29
24135.9 3) 37148.0
-5.6 -9.5
-5.95 -9.71
24147.23) 36065.0
-334.8 448.0
-328.40 447.58
15660.5 b, -
-53.0
-60.14
38165.2 39616.0 b, 36111.2”) 38886.4 18928.0
103.0 507.4 -74.7 -230.1 157.0
102.63 500.43 -76.46 -229.12 159.5’
lot -%a
12946.4
IIt-000 ~lO--~OI
25958.0 14769.6
202-101 211-110 712-111 212-101
1 -11-202 220-h
16692.4 b, 39 205.0 b)
I
303-202 32t-120
38182.5 a) 39491.7 36102.6 5) 38838.0
:l3-2,2 322-221
-2.8
-14.8 -141.5
-16.15 -139.88
-6.9 68.9 -4.4 -87.0
-6.86 68.97 -4.73 -86.85
312-303 a) Observed in MW-BIW double resonance.
b, Not included in the last-squares fit.
axis with respect to the principal axes, respectively [IO]. The centrifugal distortion constants were not adjusted but kept at the values of the ground state given in table 2. The resuhs of the Ieast-squares adjustments for the three states are shown in table 6. TabIe 6 Rotational
constants
and internal rotation parameters
This treatment of the measured data for the second excited stateghes only approximate results because the lower limit of the high barrier approximation was
surpassed. A better insight into the internal rotation
of cis-methyl nitrite in the pound
and exited
of the
states of methyl torsion e)
(w) uls=o
u,5 = 1 .--.--~
20272.461(19) b, 7437.888(6) b, 5630.585(5) b, 3.18 c) 0589 c, 0.808 53.9 14 0.08411 - 14.239 0.4663 56.930 734.1(21) 2.099(6) b,
20371.541(395)b)
______-.
A (MHz) B (MHz) c (WLZ) Ir(uA2)
ho hb
0 (W P A0 (MHz) P (md) S
V3 (cm-t) V3 (kcal/mol)
7353.297(152) b) 5594.568(98) b, 2.90 0586 0.810 54.118 0.07636(350) b, 1116.3(471) b, O-4633(152) b, 51.387 720.8(35) 2.061(10)
Lq5 = 2 19176.615(1412) b, 7341.923(386) b, 5592.613(259) b, 3.27 0.605 0.796 52.745 0.08411 -13763.7(270) b, 0.4663 58.100 728.9(17) 2.084(5)
a) Centrift& distortion constants were used from the ground state (table 2) and were kept constant. b, These p&meters were varied to fit the observed A-E frequencies. =) Taken from ref. 121.
Cl14= 1.u15 = 1 20360.014(1100) 7325.444(380)
b, b,
5588.846(190)b)
3.206 0.582 0.813 54.438 0.08411 955.5(60) b) 0.4663 51.912 663.1(49) 1.896(14)
Table 7 Measured transition frequencies (in MHz) of c&methyl
nitrite in a mixed excited state of both the NO and CH3 torsion (~14 = 1,
UlS = 0
JK_K+-J&i IlO--101 202-101 2 1z-lr1 211-110 2,,-202 303-202 313-212
;
u_+y(ObS.)
VA
14763.5 25661.8 24090.1 27558.2 16660.1 38098.2 36037.1
-16.1 -5.6 -5.8 -6.0 -16.5 -7.2 -4.8
“A - q(obs.) ---.--- 14.3 a) -5.8 -5.7 -7.2 -15.9 -6.7 a) -5.4
-
v~(calc.) --___
---a) Not included in the least-squares fit.
methyl group was attempted by simultaneously considering the A-E splittings of ground and both excited states of the methyl torsion. The variation of the V, potential for the different states points to the contribution of higher terms in the potential expansion of the methyl barrier. Transition frequencies and their variation with respect to internal rotation parameters were calculated from eigenvalues obtained by direct diagonalization of the hamiltonian matrix of a semirigid model [lo] _But a simultaneous adjustment of the A-E splittings in the ground and two excited states of the methyl torsion could not determine a single set of internal rotation parameters Z,, 9, V3 and V, _This failure of the semirigid model could not be corrected by inclusion of non-rigidity terms as used for a similar analysis of the spectrum of acetaldehyde [lo] _It is apparently due to a specific interaction of the methyl torsion with the NO torsion. A typical example of a strong interaction between two torsional modes in a molecule was recently studied in pyruvic acid (111. Finally we were able to assign a mixed excited state where both the NO and the CHJ torsion are singly excited. The A-E splittings are similar to the first excited state of the CH3 torsion alone. From the measured transition frequencies in table 7 the rotational constants and the internal rotation parameters were adjusted as explained above. The results are included in table 6. 3.3. Relative intensity measurement Additional information on the energy separations of excited states of the NO torsion could be gained
from relative intensity,measurements. The method of Esbitt and Wilson [12] was applied in order to measure the peak intensities of transitions with corresponding assignments in ground and first excited state. Measurements were carried out in a few system and the pressure was adjusted such that the A-E splittings were not resolved in both states. The strongest quadrupole components of the transitions 2O2-1o1 and 2:12-1I L were measured over the temperature range X0-290 K. Furthermore we studied the temperature dependence of the peak intensities of the transitions 322-221 and l&,- 11s6 in the ground
state of the cis conformer and of the transitions 4,3-3 12and 414-3 13 of the ground state of the trans conformer [2] over the same temperature range. Measurements were made with a constant flow as well as at constant pressure v&hout a flow in the absorption cell. The energy separationsof the states involved were deduced from the temperature dependence of the peak intensities according to the procedure discussed below. The peak absorption coefficient of a rotational transition as a function of temperature T is given by t131
x (1 - IlLJ~/X~ I(mIPlrr)12,
(3-j where u,, = (I?,, - E,,)/Iz is the resonance t’requency, Av(T) the line width parameter of the assumed lorentzian line shape, N the number of molecules per unit volume,f,,(7J the fraction of molecules in the lower state for a transition between states m and II with a connecting dipole element (nzlpin), c the velocity of light, k the Boltzmann constant and II the
p.N, Ghmh ct al. /Microwave
48
yields the normal frequency Cl, The ratio of the total number of molecules of two conformers in thermal equilibrium is given by
Planck constant. The factor (! - hv,/ZkT) can be neglected in the frequency range considered. The fraction of molecules in the lower state m is calculated from the Boltzmann distribution
= f;.(T)g, exp(-Er/k7’)(h3.4BC/lnk3 T’)“’
WN, (3)
for an asymmetric rotor with rotational constants A, B and C and a rotational energy E, in the lower state m with rotational degeneracy g,, where f,(T) = gVexp(--&/kn/Q,(’ represents the fraction of molecules in the vibrational state with energy E, and degeneracy g,. Q, and Q, denote the rotational and total vibrational partition function, respectively. For internal rotation sta!es with intermediate or high barriers, the partition function may be approximated by a vibrational partition ftinction and will be included in Qv. It is very difficult to determine the absorption coefficients in microwave spectroscopy absolutely because of the problems of accurate power measurements in the microwave region. Therefore only the ratio of absorption coefficients is usually measured between different vibrational states of the same molecule or between the ground states of two conformers. Higher accuracy can be obtained if the ratio of the products of peak absorption coefficient and Ime width parameter is formed instead of the ratio of the peak absorption coefflfirients alone according lo the exprtssion
wbrre the indices I and 2 refer to the two vibrational stales compared. Transition frequencies, rotational constants and line strengtha are known from the nnalysis of the corresponding rotational spectra, The ro!atio~!! energy difference ErI - .Fr2 is wually very small compared to kT and can safely be neglected. if 1”etrleasure the relative intensities of rotational tranions in the ground and first excited state of a nondc8enerate normal vibration i, then the expression Ink
(7’J/fv2(T)]
- -/6,/ckT
spectra of cis-methyl nitrite
(5)
= exp HE1 - GW”l
et/Q, ,
(6)
where Et - EC refers to the energy difference between the ground states of the two conformers considcced. Qt and Q, are the partition functions of the two conformers. They can be expanded into a product of contributions from translation, rotation, internal rotation and vibrations. Since the masses of the two conformers are equal, the translational partition functions cancel. The ratio of the number of molecules in a giver. lower state mt or m, is expressed as
= [gnrtw(-E~t/kW~m, ev(--Ek/kT)l X exp[-(4 - &W’l~
(7)
where EL, or EL, is the energy of rotation, internal rotation and vibrations with respect to the ground state of the corresponding conformer. The logarithmic ratio of the products of peak absorption coefficient and hne width parameter then reads [cf. eq. (211
Ad
ln[~t(~ot)A~tloc(~oc)
= 2 W0tlu0d
+ lt&Ad
+ ~n[l~mtl~tlnt~12/l~m,l~cl~t,~121 -
(Et -
E,\/kT - (Eh, -- EA&kT.
(8)
For the comparison of rotational transitions of the vibration and internal rotation ground state the energy difference Eht - EL, is of the order of a rotational energy. It will be neglected considering the experimental uncertainties of the relative intensity measurements. Measurements of relative peak intensities are usually made at a single temperature. They are Table 8 NO torsional from r&he __.~_.___..~ ‘kw.ition
frequency
~~4 (in cm-‘) of cirmethyl measurements
.___--
Torsional frequency
---
202-101 212-111
nitrite
peak intensity
207(75) 213(57)
---
Mean :orsiond
210(31)
frequency
P.N. Gl~oslr et al. /Microwave Table 9 Energy difference Transitions
_-__-__
322-h -P-A-
between
cis and trans conformers
compared
cis
1249-1156
(in cm-‘)
Energy difference trans
--..
__
413-3t2 414-313
-_-_-.-_
..__~
closed cell ~~_ 36 I(W) 286U49) ._ _
spectra of cis-met/l vl nitrite
in methyl
Etrans
nitrite
- &is
Mean energy
differcnrc
flow system 267(43) 423f159) ~. -~~-.-_--.-
analyzed according to eqs. (4)-(5) or (8). As an alternative procedure to determine Pi or E, - E, thy temperature dependence of ln(a, CV,/a2Au2) or Irn(a,Av,/a,Av,) was studied. Thus, the torsional frequency c,, or Etrans - &is could be determined easily from plots of In@, Av, /o12Avl) or In(a,Au,/ acAu,) versus E/T. Thereby no prior hncwledge of rotational constants, line strengths or statistical weight factors according to ea. (4) or (8) was necessary. The results of the analysis for the frequency of the NO torsion are given in table 8 and for the energy difference between cis and trans methyl nitrite in table 9. The trans conformer was found to be higher in energy by 314(22) cm-’ . The errors are considerable due to the large scatter of the individual measurements at a given temperature. Measurements at several temperatures should therefore provide more reliable data than a measurement at a single temperature.
4. Discussion-and conclusions In the work of Turner et al. [2] on the abundant isotopic form of cis-methyl nitrite, twelve transitions up to J = 6 were assigned in the frequency range from 12 to 40 GHz. Only five of the transiticns were resolved into nuclear hyperfine components and none showed resolution into internal rotation doublets. Most of the strong absorption lines remained unassigned in this region. The barrier to internai rot+ tion of the methyl group was determined f:om the splittings of six transitions of the 15Nspeclcs alone. This work provides a much extended assignment for the vibrational ground state including some thirty high J transitions up to J = 20. Ail of them >howed at least a partial resolution into hyperfine ccmp~
314(22) _._.
..--
__
__
nents. The newiy determined quadrupole coupling constants exhibit only small changes within the un. certainty limits compared to the previous results ( I 1. Turner et al. 121 were able to estimate the out-ofdiagonal constant hfi = 2.8 + 0.6 MHz from changes of the diagonal constants in CD30N0. Combined with the diagonal part the quadrupole coupling tensor was transformed to principal axes with the following results: hX = --5.95(50) MHz, xjaY= 3.43(G) MHz and xrz = 2.55(50) MHz, where the?) axis is perpendicular to the molecular symmetry plane and the z axis forms an angle of 20.9(30)* with the principal a axis. Internal rotation doublet splittings were measured Fpr half of the asisigned transitions in the ground stale. In addition, transitions in singly or ioubly excited b;ates of the NO and methyl torsions were assignstl. Combined with the measurements on tbc ground state the measured A-E splittings in excited methyl torsional states allowed a much more reliable determination of the barrier hindering the internal rota1.io.lof the methyl group. Comparing the new value for V, = 2098.7(63) cal/mol from the analysis of th: ground state alone with previous results shows good agreement with the value of V3 = 2090 cal/mol reported by Tu:ner et al. [2] but is higher than tile vafareof V3 = 1912 cal/mol of Gwinn et al. 111,Frori: the ab initio calculations on methyl nitriic [4J :I barrier of 1930 cal/mol was obtained for lhc cis 1’01m in fair agreement with the experimental obsurvatir)n. On the other hand a 10s barrier of 270 C~/IIKJ! vm calculated for the trans conformer. This result c011i~ pares well within the expected accuracy of the ;III initio calculation for the almost free internal rot;lIi(ltl with a small barrier of30 :o 60 cnl/mol dcducc~l from the microwave spectrum. Our own checks WI the assignments and our calculations confirm the
50
P_N. Gitosh et al. /Micrmvnw
results of Turner et al. [Z] for the tram conformer. The energy difference of 210(3 1) cm-’ between ground and first excited state of the NO torsion of the cis conformer is similar to the same difference in the tram form of 230(30) cm-’ [2] _The ab initio calculation of the force constants for the NO torsional vibrations yielded 0.181 mdyn/rad and 0.109 mdyn/rad which correspond to torsional frequencies of336 cm-’ and 187 cm-’ for the cis and tram conformers, respectively, assuming the torsional states to be pure. But the NO torsion in the cis conformer will have considerable mktiing with the methyl torsion and will give rise to a large shift in the frequency. In the case of the tram conformer, however, there is practically no such mixing. Ali known measurements of the relative stability of the cis and trans conformer showed invariably the cis form to be more stable_ A ground state energy difference of 275 cm-’ [l] and 296 cm-’ [ 141 from relative intensity measurements in the microwave spectrum is in good agreement with our vahre of 314(22) cm-r from the temperature dependence of absorption lines in the microwave spectrum. Similar measurements from relative intensities in the infrared spectrum of trapped effusive or supersonic molecular beams yielded 625 cal/mol(2 18 cm-’ ) [15]. investigation of the NMR spectrum in the liquid phase gave a difference of 275 cm-’ [16]_ A considerably larger difference of 2940 cal/mol (1028 cm-‘) was obtamed from the ab initio calculation. This value refers actual!y to the minimn of the potential surface. The difference of the zero point energies of the two conformers which is mainly due to different internal rotation barriers of the methyl group reduces this value by approximately 100 cm-’ _
spectra of cis-methyl nitrite
Acknowledgement Financial support by the Swiss National Foundation (Project No. 2.033-0.78 and 2.X19-0.79) and a research grant from the administration of the ETHZ (No. 13223/41-0430.5 and 14651/41-0430.5) are gratefully acknowledged. Furthermore we wish to thank Mr. N. Schwizgebel for preparing methyl nitrite. Finally, the ETHZ Computer Center generously granted free computer time.
References
[I J W.D. Gwinn, R.J. Anderson and D. Stelman, Bull. Am. Phys. Sot. 13 (1968) 831. [2] P.H. Turner, MJ. Corkill and A.P. Cos, J. Phys. Chem. 83 (1979) 1473. [3] K:. Endo and Y. Kamura, J. Chem. SW. Japan 5 (1977) 729. [4] T.-K. Ha, unpublished results. [S] A.H. Blatt, ed.. Orgmic syntheses (Wiley, New York, 1963) p. 363. [6] J. Ekkers, A. Bnuder and Hs.H. Giinthard, 5. Phys. E: Sci. Instrum. 8 (1975) 819. [7] J.K.G. Watson, J. Chem. Phys. 46 (1967) 1935. [8] P. KJaboe. D. Jonesand E.R. Lippincotr, Spectrochim. Actn 23A (1967) 2957. [V] R.C. Woods, J. Mol. Spectry. 21 (1966) 4. [ 101 A. Bnuder and Hs.H. GBnthnrd, J. Mol. Spectry. 60 (1976) 290. [ 111 A. Bauder and R. Meyer, unpublished results. [ 121 AS. Esbitt and E.B. Wilson. Rev. Sci. Instrum. 34 (1963) 901. [ 131 W. Gordy and R.L. Cook. Microwwe molecular spectra (Interscience, New York, 1970) p. 41. [14] A.P. Cos, private communication. [ 151 P. ITelder end Hs.H. Giinthard, Chrm. Phys.Letters 66 (1979) 283. [16] H.W. Brown and D.P. Hollis, J. Mol. Spectry. 13 (1964) 30.5.