Microwave spectra and structures of cis-and trans-methyl nitrite

The Journal

cis- and trans-Methyl Nitrite

of Physical Chemistry, Vol. 83, No. 11, 7979

1473

Microwave Spectra and Structures of cis- and trans-Methyl Nitrite. Methyl Barrier in trans -Methy I Nitrite Paul H. Turner,$ Michael J. Corklll, and A. Peter Cox*+ Department of Physical Chemistry, University of Brisfol, Cantock's Close, Bristol BSB ITS, United Kingdom (Received October 25, 1978) Publication costs assisted by the University of Bristol

Accurate structures for both cis- and trans-methyl nitrite have been derived from isotopic measurements for each atomic position. For the cis isomer the following structure has been obtained from nine isotopic species: O=N = 1.182 A, N-0 = 1.398 A, 0-C = 1.437 A, C-Hi (in-plane) = 1.09 A, C-H, (out-of-plane) = 1.102 A, LONO = 114.8', LNOC = 114.7", LOCH, = 1 0 1 . 8 O , LOCH, = 109.9', and LH,CH, = 108.1". The methyl group was found to be tilted by -5' with the expected conformation. Nitrogen quadrupole coupling constants determined for cis-CD30N0 have allowed the complete field gradient tensor to be obtained. Analysis of isotopic spectra for the trans molecule was complicated by nonrigidity due to the very low methyl barrier. Splittings of the = 0, m = f3, pa R-branch transitions gave V3values of 29 and 46 cal mol-' for CH30N0 and CD,ONO, respectively (61 and 49 cal mol-' when V, is taken into account), considerably lower than that previously reported for CH30N0. Analysis of spectra for six isotopic species gave the following structural parameters for the trans isomer: O=N = 1.164 A, N-0 = 1.415 A, 0-C = 1.436 A, LON0 = 111.8"and LNOC = 109.9'. The structural properties of cis- and trans-methyl nitrite are discussed.

Introduction The microwave spectrum of the normal species of cisand trans-methyl nitrite has previously been studied by Gwinn, Anderson, and Stelman in some detai1.l For the cis isomer, they found a high V3barrier value (1912 cal mol-l) for the methyl group, whereas the trans molecule exhibited a very nonrigid spectrum and a much lower V3 value (188 cal mol-') was obtained. The cis rotamer was found to be more stable by 275 cm-l in good agreement with the liquid-phase NMR value.2 Recently a microwave structure for cis-CH30NO has been published but no isotopic oxygen data were reported in that s t ~ d y In . ~the present work, we have determined accurate structures for both isomers from complete isotopic data. Comparison of structural paramleters will be of particular interest in connection with cis/trans isomer stability and the unusually large difference in the barriers to internal rotation of the methyl groups. Few detailed structural studies have been made for different rotational isomers of the same molecule; notable exceptions are nitrous acid,4 monothioformic acid,5 3-fl~oropropene,~ ethanethi01,~fluoroacetic acid, and fluoroacetyl fluoride.8 The results obtained for methyl nitrite should stimulate theoretical calculations on the system. Several ab initio molecular orbital calculations have been reported for the isoelectronic molecule methyl f ~ r m a t ebut , ~ in that case experimental data for the cis isomer only have been obtained.1° For the det,erminntion of V3 for trans-methyl nitrite, Gwinn et al.' fitted the separation of the m = 0 and m = fl states a t the low barrier limit for some a-type, Rbranch, K-, = 1 and 2 transitions. In this study, in order to determine rotational constants for the isotopic molecules, a fairly complete assignment over several low-lying torsional states was necessary. Splittings of the K-, = 0, m = f 3 , a-type, R-branch transitions in trans-CH30N0 and -CD,ONO have been analyzed to give an improved value for the low barrier. Few asymmetric top molecules have been found to hlave a low V3barrier comparable with trans-methyl nitrite; some other reported examples are *Department of Chemistry, Harvard University, Cambridge, Mass. 02138. 0022-3654/79/2083-1473$0 1,OO/O

1-chloro-2-butyne (V3 < 100 cal mol-l),ll methyl isothiocyanate (V3= 304 cal mol-'),12 methyl isocyanate (V3 = 83 cal m ~ l - ' ) , ' ~ Jand ~ rn-fluorotoluene (V3 = 46 cal m ~ l - ~The ) . ~spectrum ~ for trans-methyl nitrite is difficult to fit well and improvements obtained with refinements in the model will be discussed. Experimental Section The microwave spectra of all methyl nitrite species were studied using a conventional klystron-operated 100-kHz Stark modulation spectrometer with a 3-m X-band cell cooled with dry ice. Initial broad band searches were obtained using the backward wave oscillator sweep unit from a Camspek microwave spectrometer in conjunction with the above spectrometer. Carbon-13 spectra were measured with the isotope in natural abundance. The normal and enriched methyl nitrite species were prepared on approximately M scale according to the following reaction sequence:I5 4 N 0 + 02 2N203 (1)

Nz03 + 2CH30H

-

2CH3ONO

+ H20

(2)

Nitric oxide was mixed with oxygen in the molar ratio 4:l. The dinitrogen trioxide formed was then allowed to react with methanol below 0 "C. The methyl nitrite was separated from the aqueous solution by low temperature vacuum distillation. A 30% enriched sample of CH30N180 gas (93 atom 70). For CH3015N0 was prepared using 1s02 the nitrogen-15 source was 7 mol dm-3 Hl5NO3 (95% enriched). This was converted into 15N0 by the reaction of nitric acid on mercury in excess sulfuric acid. CD30N0 was prepared using [2H,]methanol. The above synthesis of CH30N180 failed to yield any microwave spectral features attributable to CH3"ON0, and this is consistent with the mechanism for esterification of methanol involving retention of the methanolic oxygen atom. The synthesis of CH3180N0 therefore required a sample of oxygen-18 enriched methanol as did the preparation of bridging oxygen-18 methyl nitrate.16J7 The details of the CH3180Hpreparation are given in ref 17. A 10% enriched CH3180N0 sample was prepared. For CH,DONO, CH2DOH was prepared by reacting formaldehyde with G 1979 American Chemical Society

1474

P. H. Turner, M. J. Corkill, and A. P. Cox

The Journal of Physical Chemistry, Vol. 83, No. 11, 1979

TABLE I : Observed and Calculated Frequencies (MHz) of cis-CH30N0

TABLE 111: Observed and Calculated Frequencies (MHz) of C ~ ~ - C R , O N ’ ~ O

transition J-J

Vobsd calcd

”

b

;

F-F

vobsda

Vhc

2-1

1 3 068.71

1 3 068.61

0.22

25 905.41 25 903.27

25903.01

0.05

la~l-oo*o

1-1 0-1 2-1

ll,l-Oo,o

2o,z-~0,1 21,z-11,,

1-1 2-1 3-2 21,1-11,0 2-2 2-1 1-0 3-2

1-1 219z-10,1 1-0 3-2

1-1

25 959.02 24 331.60 24 329,71

-0.03 24 329.55

1-1

27 27 27 37 37

21,2-10,1 944’35 943.87 941.91 165.63 164.46

27 944.00

-0.01

37 164.13

0.01

;

11,0-10,1

20,i-10,1 21,i-I 1,l 21,1-11,0 21,2-10,1 22,0-21,1 i,i

31,3-21,2 30,3-2a,z

z - ~ I ?3

5 i . 4 - 4 z, 3 44.0-5333 44, ,-53,3 44.0-5 3, z 44.143,z

1-0

27 945.36

transition J-J 10,1-00,0 ~1,1-00,0

4z,

-3-2

TABLE 11: Observed and Calculated Frequencies (MHz) of c~s-CH,O”NO

%,2-22, 3z, 1-31, z

F’-F 2-1 0-1 2-1 1-1 21q2-1,,1 1-1 3-2 2-2 21,1-11,a 2-2

0.00

37 162.94 3 0 q 3-2 0,z 38 505.06 -0.34 31,3-21,2 36 387.98 -0.24 32,2-22,1 39 204.98 -0.19 5z, 3-5 I , 4 34 219.00 0.51 62,4-6 1,s 34 640.70 0.09 a Accurate to i 0 . 0 5 MHz. Hypothetical centers for resolved splittings obtained using coupling constants from ref 1. Rigid-rotor frequencies from constants of Table X.

3-2

S-J 10,l-Oa,o ll,l-Oo~o

1-1

2-1

3a,

transition

A E A E

E A E E A E

a Accurate to -0.05 MHz. from constants of Table XI.

Vobsda

Vobsd Vcalcd

1 2 942.77 25 628.97 1 4 480.72 25 708.26 24 091.02 27 679.99 36 777.23 38 235.47 27 061.23 36 030.49 38 130.19 38 828.12 36 368.57 36 358.22 34 697.67 34 697.33 36 154.87 29 575.83 29 572.30 29 566.16 29 272.85 29 268.64 29 263.20

0.02 0.01 0.02 - 0.02 0.01 0.00 0.01 - 0.38 -0.39 -0.25 -0.37 -0.13 -0.26 -0.27 0.15 0.14 -3.76 - 6.05 -5.74 - 6.00 - 5.89 - 5.77 - 5.82

Calculated frequencies

lithium triethylborodeuteride, LiB(C2HJ,D, in tetrahydrofuran, followed by low temperature vacuum distillation. Results cis-Methyl Nitrite. Microwave Spectrum. The observed microwave frequencies for cis-methyl nitrite are listed in Table I. The spectrum is that of an almost rigid asymmetric rotor ( K = -0.75) and both a- and b-type transitions are easily observed (pa= 1.66, p b = 1-20D).’ Hypothetical centers of the quadrupole structure for the

b

;

*obsd Vcalcd

Vobsda

hc

12 580.69 25 192.11 25 189.92 25 188.38 23 438.33 23 436.31 23435.16 26 887.87 26 886.83

1 2 580.76

0.08

25 189.65

0.05

23 436.27

0.01

26 886.46

0.00

-

26886.29 26884.34

3-2 1-1 2-1

36 045*54 36 045.20 36043.95 30, S - ~ O , Z 37 090.76 319 3-2 1, z 35 055.46 %,2-22,1 37 741.82 3z41-22,0 38 392.80 See the corresponding footnotes t o Table

-0.02 -0.40 -0.21 -0.22 -0.11 I.

TABLE IV : Observed and Calculated Frequencies (MHz) of C~S-CH,’~ONO transition

__

S-J F-F l l , l - O a ~ o 2-1 2oqz-10q1 21,z-11,1 1-1 3-2 2-1 219,-11q0 3-2 21,z-10,1 1-0 3-2

b

Vobsd calcd

;

Vobsda

hc

25 092.15 25 602.52 2 3 964.13 2 3 962.16

25 091.91

0.18 -0.03

23 962.00

-0.03

27 625.36 27 625.54 36 158.41 36.157.2736 156.88

0.01 0.01

1-1 37 938.68 - 0.48 35 828.81 -0.36 39 441.85 -0.34 3z91-2 2,o a-c See the corresponding footnotes to Table I. 30,3-20,z 3 1 , 3-2

1,

z

TABLE V: Observed and Calculated Frequencies (MHz) of c ~ ~ - ’ ~ C H , O N O transition J‘--J 21~z-1a~1

F’-F 3-2 1-1 2-1

b

Vobsda

hc

36 569.18

36 568,81

’obsd calcd

;

0.00

36 37 35 38 34

567.57 30, 3-2 a, z 610.45 -0.27 313 3-2 1,z 550.30 -0.37 32,2-22,1 266.5d 0.3 62,4-61,5 380.48 0.28 Hypothetical center oba Accurate to +0.2 MHz. tained using main-species coupling constants from ref 1. Rigid-rotor frequencies from constants of Table X. Accurate to i 0 . 5 MHz.

transitions in cis-CH,ONO have been calculated using the previously determined quadrupole coupling constants (x, = 1.39(3), Xbb = -4.86(3), and xCc= 3.47(3) MHz)’ and the theory of Bragg and Golden.18 The A , B , and C rotational constants given in Table X were derived using explicit rigid-rotor expressions for the 21,2-11,1,21,1-11,0, and 21,2-10,1 transitions. The spectral assignment of the various isotopic species of cis-methyl nitrite is shown in Tables 11-IX and the rotational constants are given in Table X. Quadrupole hyperfine splittings for cis-CD,ONO have been analyzed to give xaa = 0.97(20), Xbb = -4.47(3), and xcc= 3.50(3)

TABLE VI: Observed and Calculated Frequencies (MHz) of c~s-'~CH,O'~NO

transition J'- J ~1,1-00,0 20,2-10,1 21,2-11,1 22,0-21,1 3O

"obsd calcd

"

"obsd'

I, 2

3212-22,1

a Accurate to f 0.1 MHz. from constants of Table X.

transition J'-J ll~l-Oo,o

20,2-10,1 21,,-11,1

2132-10,1

32,1-2,,0

1-1 2-'1 3-2 3-2 1-1 2-.1 2-2 3-2 4-3 2-1

v&da

25 25 25 24 22 22

521.14 518.99 517.59 230.88 787.90 785.88

hc

b

25 518.75

"obsd

;

"calcd

0.09 -0.14

22 785.79

0.00

36 123.43 36 123.08

0.00

36 121.97 37 065.29 37 064.58

-0.24

-0.34 36 022.87 -0.06 34 099.35 - 0.20 32,2-22,1 36 543.91 a-c See the corresponding footnotes to Table I. 30, 3-20, 1, 3-2

3-2 2-1 21,1-11,0 2-2 3-2 2-1 1-0 1-1 21,2-10,1 1-0 3-2 1-1 2-1 22,o-2 1, I 1-1 3-3 2-2 30,3-20,2

31,3-2 I , 2 32,2-22,1 32,1-22,0 3-2 4-3 2-1 a-c

37 064.80

0-1 2-1 1-1

20,2-10,1 21,z-11,1

Rigid-rotor frequencies

-

F'-F 0-1 2-1 1-1

J'-J ll,l-Oo,o

"obsd

F'--F

~ 0 , 1 - ~ 0 , 0

TABLE VI1 : Observed and Calculated Frequencies (MHz) of c~s-s-CH[,DONO

transition

TABLE VIII: Observed and Calculated Frequencies (MHz) of cis-as-CH,DONO

;

0.09 -0.12 0.11 -0.23 - 0.44 -0.23 -0.19

25 291.67 25 090.96 23 526.42 38 155.22 37 232.32 35 190.34 37 884.56

3-20,2 I ' 3-2

1475



2

1 2 714.11 24 251.08 24248.84 24 247.47 25 242.49 23 673.60

"hc

;

"calcd

0.22 24248.65

0.11

2 3 673.48

0.08 -0.03

27 182.09

27 182.17

-0.14

27 180.17 35 209.64 35 208.51

36 208.13

0.01

34 788.85

-0.41

27 183.48

35 207.03 34790.17 34 789.21 34787.53 37 413.55 35 399.66 38 141.43 38 869.79 38 86g.02

-0.31 -0.31 -0.18 38 869.27

-0.29

See the corresponding footnotes to Table I.

TABLE IX: Observed and Calculated Frequencies (MHz) of cis-CD,ONO

transition

1,2

MHz. Comparison of the (Xbb - xcc)value with that of cis-CH30N0 gives a value of Xab = 2.8 & 0.6 MHz for the latter, using the angle of rotation (3.5') derived from the structural determination. A , B , and C for CH30I5NO, CH30N180,CH3180N0,as-CH2DON0,and CD,ONO were derived as for the main species. For 13CH30N0 and 13CH3015N0,a consistent set of rotational constants was obtained using the 21,2-10,1,or ll,l-Oo,o, 30,3-20,2and 31,3-21,2 transitions by transfer of distortion shifts from cisCH30N0. In a similar ]procedure the 21,2-11,1, 21,2-10,1, and 30,3-20,2transitions were used for s-CH2DON0. Some A-E splittings and forbidden E lineslg were measured for cis-CH3015N0(see Table 11). An IAM analysis using the Woods computer programm gave a methyl group V3barrier of 2090 cal mol-' (see Table XI) in good agreement with the main species value reported in ref 1. Structure. The Ic-Z,-Ib terms for the isotopic species (see Table X), which vary little with heavy atom substitution, clearly indicate the molecule to have C, symmetry with a planar heavy atom skeleton. The substitution a and b coordinatesz1for all the atoms in cis-methyl nitrite were calculated using Kraitchman's equations.2z Of the four possible combinations of data, the set obtained from AI,/ is considered representative and is given in Table XII. Irrespective of which combination of AI'S is used, the Kraitchman coordinates do not meet the first-moment condition Crnial:= 0. All the coordinates involved are large (>0.5 A), and the discrepancy is beyond experimental error and must be due to zero-point vibrational effects. [A similar but more severe problem was encountered in the structural determinationz3of N203which also has a large amplitude torsional motion about the central bond.] Changes of up to 0.005 A in the a coordinates of heavy

uobsdu

J'-J 1 i,1-00,0

-

F'-F 0-1 2-1

1-1 21,2-11,1 1-1 - -

uobda

22 22 22 21

549.64 541.57 546.36 712.98

"hcb

Vobsd "calcdC

22 547.41

0.01

21 711'14 21 709.94

21 711.06

0.00

2 4 662.28 24660.32 32 666.50 32 665.41

24 662.37

2-1

3-2

21ql-11,0

2I, 2 - 1

0,l

41,3-40s4 51,4-50,5 6135-60,6 51,4-51q5 61,5-61,6

2-2 3-2 1-1 1-0 3-2 1-1 5-5 4-4 3-3 6-6 5-5 4-4 7-1 6-6 5-5 6-6 5-5 4-4 7-7 6-6 5-5

20 684.94 20 686.80 20 684.48 26 149.10 26 151.19 26148.68 3 3 139.84 33 142.10 33 139.49 21 903.90 21 906.61 21 903.40 30 311.87 30 314.46 30 311.45

-0.01

32 665.10

0.00

20 685.44

-0.65

26 149.68

-1.76

33 140.50

-3.83

21 904.67

-2.40

30 312.62

-4.62

a Accurate to 1.0.05 MHz. Hypothetical centers obtained using the following determined coupling constants: xaa = 0.97(20) and Xbb - xcc = -7.97(5) MHz. Rigidrotor frequencies from constants of Table X.

atoms are required to restore the balance. The bond distances and angles are not seriously affected by these changes or how the changes are actually implemented; for this reason the Kraitchman coordinates have been retained with an increased uncertainty reflected in the structure given. The b axis is much less of a problem; there is one


1476


TABLE X: Rotational Constants (MHz)? Moments of Inertia, and Inertial Defects (amu A') of cis-CH,ONO Isotopic Species

A B C Ia Ib IC

I , -Ia - Ib

CH,ONO

CH,O"NO

CH30NL80

CH,"ONO

20272.38 7437.81 5630.58 24.9293 67.9469 89.7555 - 3.1207

20054.83 7368.62 5574.13 25.1997 68.58 49 90.6645 - 3.1201

19761.81 7152.89 5427.79 25.5734 7 0.65 34 93.1089 -3.1179

19559.16 7364.32 5532.57 25.8383 68.6249 91.3455 -3.1177

I

A B C Ia Ib IC

I,

-

I,

-

Ib

I

-

3CH30N0 20051.83 7249.75 5505.66 25.2035 69.7094 91.7921 - 3.1208

'CH30' 'NO

sym-CH,DONO

asym-CH,DONO

CD,ONO

19842.55 7179.22 5449.03 25.4693 70.3943 92.7460 -3.1176

20216.25 6879.16 5302.21 24.9985 73.4648 95.3142 - 3.1491

187 68.75 7234.05 5479.79 26.9265 69.8 605 92.2254 - 4.561 6

17488.55 6534.51 5058.85 28.8975 77.3395 99.8994 -6.3376

" Accuracy of rotational constants, relative to main species, A +0.2, B k0.05, and C 10.05 MHz. TABLE XI: Internal Rotation Parameters" for cis-CH,O' 5 N 0 A , = -14.74 MHz I , = (3.18) amu A 2 s = 56.8 V,= 2090 cal mol''

A = 20054.83 MHz B = 7368.62 MHz C = 5574.13 MHz h a = 0.5892'

" Used t o fit A, E splittings with Woods IAM computer program, ref 20. From structure. TABLE XII: Principal Axis Coordinates ( A ) and Structural Parameters for cis-CH,ONO b

a

C

V

1

,

I

terminal0

-1.1721 N -0.8039 bridging 0 0.5848 C 1.3320 Hi 2.3590 Ha 1,0754 crniai = -0.122 amu A

-0.5940 0.5289 0.6898 -0,5377 (-0.1725)" -1.1321

Ib

Figure 1. Structure of cis-methyl nitrite.

kO.8919

1

?rnibi = 0.030 amu A 1

Frniaibi = 0.176 arnu A ' 1

crni(bi2 + c i 2 ) = 24.8577 (cf. I,= 2 4 . 9 2 9 3 ) a m u A ' 1

crni(ai2 t ci2)= 67.3274 (cf. I b = 67.9469) amu A * 1

Crni(ai2 t b i z ) = 88.9783 (cf. I , = 89.7555) arnu A ' 1

O = N = 1.182(5) A N-0 = 1.398(5) A 0-C = 1.437(5) A C-Hi = (1.09) A C-Ha = 1.102(10) A

LONO = 114.8(5)" LNOC = 114.7(5)" LOCHi = 101.8(15)" LOCH, = 109.9(5)" LH,CH,= 108.1(15)0

" Coordinate calculated from assumed

C-Hi bond length

of 1.09 A , see text.

small coordinate that of the in-plane hydrogen which is poorly determined as lbHl = 0.2806 8 by substitution and has therefore been obtained by assuming an in-plane C-H bond length of 1.09 8. Only slight adjustment of the heavy atoms is then required to meet the Cmibi = Crniaibi= 0 conditions. The a and b coordinates of the out-of-plane hydrogen atom can be calculated in one way only from the available data. The out-of-plane hydrogen coordinate was obtained using eq 3, given by Laurie and H e r ~ c h b a c h and , ~ ~ the CH30NO/CD30N0 data. A' - A was assumed to be AI,

-

AI, - AIb = (A' - A)

-

-

mH(cH)') (3)

-0.015 amu 8' as suggested in ref 24 but in this case, as

for nitrosomethane,lj hydrogen and deuterium were constrained to the same position to give a better estimate of the out-of-plane substitution coordinate. The structure given in Table XI1 and Figure 1has been calculated direct from the Kraitchman coordinates (apart from the small b coordinate of the in-plane hydrogen). The errors on the structural parameters, given in parentheses in Table XII, include a contribution incurred from the difficulties discussed above. The bond lengths and angles involving heavy atoms are probably accurate to f0.005 A and f0.5", respectively, for comparison with other substitution structures. The present determination agrees satisfactorily with the structure given3 by Endo and Kamura; the inclusion of oxygen-18 data mainly improves the determined N-0 bond length and CON angle, decreasing the former by 0.017 8 and increasing the latter by 0.7'. The conformation of the methyl group has been determined in the present work from the partially deuterated species to be that shown in Figure 1 (analogous to methyl formatelo and methyl nitratez5)with the in-plane C-H bond trans to the nitrosyl oxygen. Furthermore, the methyl group is tilted by 5.4 f 1.5' away from the nitrosyl oxygen atom, where the tilt is defined as the angle between the C-0 bond and the perpendicular from the carbon atom to the basal plane of the CH3 pyramid. A similar tilt of 4.8 f 1" was observed in methyl nitrate.16 trans-Methyl Nitrite. Microwave Spectrum and m = 0 Assignment, The predicted microwave spectrum of trans-methyl nitrite was based upon rotational constants given in ref 1. The observed spectrum was found to be characteristic of a near prolate rotor ( K = -0.98) in so far that groups of a-type, R-branch lines occurred a t integral

The Journal of Physical Chemistry, VoL 83, No. 11, 1979


1477

+

multiples of B C. However, the presence of an unusually large number of strong absorption lines with similar Stark effects in these regilons immediately suggested the low barrier to internal rotation as previously reported. At the low barrier limit, the internal rotation levels are preferably labeled by the free rotation quantum number m. For the free rotor states with m a multiple of 3 the f m degeneracy is removed by the V3 barrier; therefore the lowest m states for which strongly barrier dependent splittings appear in the microwave spectrum of an asymmetric rotor correspond to m = f3. For asymmetric tops with v6 barriers (e.g., CH3BF,26and CH3N0,27)these splittings can be very large because the v6 term directly connects the nearly degenerate diagonal terms fK, -m with f K , +m for m = f3. However for V3barrier molecules this near degeneracy is removed and splittings of rotational transitions in m = f3 states are much smaller (typically a few MHz), the V3 terms only connecting states m with m f3. Curl et al.13studied the microwave spectrum of CH,NCO and determined the V3barrier to be 49 f 3 =fl, cal mol-l from the splitting of the J = 3 4, m = f 3 transitions. Lett and Flygare12 reanalyzed the spectrum of CH3NC0 and determined the V3value to be 83 f 15 cal mol-l from the splitting of the AJ = 1, = 0, m = f 3 transitions. They assigned the microwave spectrum of CIH3NCSI and determined V3for this molecule to be 304 f 50 cal mol-l, also from the splitting of K-, = 0, m = rt3 transitions. The torsion-rotation energy levels for a molecule with a plane of symmetry and a single, symmetric internal rotor can be calculated from the following rigid top-rigid frame Hamiltonian2” H = A‘P? B’Pb2 4- cPc2- 2F 1 pgPgp

-

+

g=a,b

F&’aPb(PaPb+ P$a)

+

+ Fp2 + v(a)(4)

where A’ = A pg =

+. Fp?

B’ = B

+ Fpb2

AgIcx/Ig p = - ih(a/aa)

and

V ( a )= 1/*V3(1- cos 3a) A, is the direction cosine of the top axis with the gth principal axis and F is the reduced rotational constant for the methyl torsion. The nonvanishing matrix elements of H in a basis of symmlatric top and free-rotor product wave functions are given by (JKMmIHIJKMm) = A’K2 + y2(B’ + C ) [ J ( J+ 1 ) - K 2 ]- 2Fp,Km + Fm2 (JKMmlHlJK f 1Mm) = Fpb[:Gpa(2K f 1) - m ] [ J ( Jt 1) - K ( K f l)I1/’ (JKMmlHlJK rt 2Mm) = Y4(B’- C) X [ J ( J 1) - K ( K f l ) ] 1 / 2 [ J + ( J 1 ) - ( K f 1)(K f 2)I1I2

+

(JKMmlHIJKMm f 3 ) =: -V3/4 (5) Terms in V3/2which are diagonal in all quantum numbers cancel for microwave transitions and have not been included. Note that there are some errors in the equivalent matrix elements given in ref 12. Off-diagonal elements in m due to the barrier lead to an infinite matrix. If the barrier is very low a Van Vleck perturbation technique can be used to achieve an adequate diagonalization in the m quantum number.29 For trans-methyl nitrite, however, the matrix was diagonalized directly after a reasonable truncation in m. The same procedure had been adopted

Figure 2. J = 2

-

3 transition in trans-CH,ONO (28.8-29.2 GHz).

for CH3NC012J3and CH3NCS.I2 The matrix factors into two, an A block consisting of basis functions with m equal to zero or a multiple of three and an E block for the remaining m v a l u e ~ . In ~ ~the present work a computer program was written which sets up the A and E matrices separately for J,,, = 4 using a maximum of nine m functions for each symmetry block. The size of each matrix to be diagonalized was therefore 9(2J + 1 ) which leads to a 81 X 81 matrix for the J = 4, A or E levels. In the later stages of the work, we also used the low barrier internal rotation program written by Anderson3I with identical results for the transition frequencies. The latter computation involves diagonalizing the pure torsional part of the Hamiltonian (4) in a free rotor basis by taking full advantage of the Al, A2, and E symmetry factoring. This is then followed by setting up the complete Hamiltonian in the representation of the direct product with the matrices of the asymmetric rotor. This procedure leads to a faster execution time. In addition relative intensities of transitions and Stark effects are calculated. The a type, J = 2 3 transition in trans-CH30N0 is shown in Figure 2. The strong line to low frequency was assigned to the K-, = 0, m = 0 transition on the basis of spectral position and Stark effect. Similar a-type, Rbranch = 0, m = 0 transitions ( J = 0 through 3) were subsequently found which closely followed a rigid-rotor analysis. Further confirmation of the m = 0 assignment came from the a-type, R-branch, K1= 1, m = 0 lines which were found nearly symmetrically placed about the = 0, m = 0 transitions. Also a t each J J + 1 transition, further a-type K_l = 0 lines were observed and assigned by their slow, second-order Stark effects. These lines were of comparable intensity to the m = 0 transitions but displaced to higher frequency (see Figure 2). These undoubtedly originate from rotational transitions in excited m states, their shifts from the m = 0 line being similar to the observed shifts of higher m, = 0 transitions in CH3NCS12and CH3NC0.12J3Also some of these K-, = 0 transitions do not closely follow a rigid-rotor treatment, indicative of higher m states. The measured m = 0 transitions for trans-CH30N0are given in Table XI11 and have been fitted using an effective rigid-rotor Hamiltonian. The measured lo,l-Oo,o transition gave B + C and the splitting of the = 1,J = 1 2 transitions gave 2(B-C) after correcting for quadrupole hyperfine structure using the coupling constants xaa= 0.63, X b b = -3.95, and xcc= 3.32 MHz for the trans molecule given in ref 1. The derived B and C rotational constants are given in Table XVI. The rather large discrepancies between the observed and calculated frequencies for the a-type, AJ = 1, K-, =

-

-

-

1470

The Journal of Physical Chemistry, Vol. 83,No. 11, 1979


TABLE XIII: Observed' and Calculatedb m = 0 Frequencies (MHz) for trans-Methyl Nitrite Isotopic Species transition J'- J

CH30N1so

CH,ONO

F'-F

vobsd

1-1

9 614.92 19 227.95 1 8 868.53

0.00 0.49 - 3.07

2-1 3-2

1 8 866.79

- 3.06

1 8 865.73

-2.96

19 587.95d 19 586.7gd

- 3.10

19 585.10d 28 837.15 28 298.93 29 378.99 28 824.65 28 831.79 38 440.66 37 729.55 39 169.57 38 431.31 38 449.44

-2.98 1.91 -4.26 - 4.37 -20.11 -22.49 4.76 - 5.28 - 5.44 -26.52 - 32.17

~0,1-~0,0

20,2-LI, I 21?2-11,1

Avc

-

18 321.12

0.49

- 3.08

27 26 27 27 27 36 35 37

477.83 988.04 969.72 465.90 471.67 629.67 982.15 290.93

CH301'NO

-A vc

*obsd

1.77 - 3.78 - 3.77

-17.98 -20.03 4.05 - 4.68 -4.76

Avc

*obsd

9 575.80 1 9 149.59

0.00 0.49

1 8 782.69

-2.98

19 514.55

- 2.98

28 719.32 28 172.73 29 270.47 28 707.81 28 715.41 38 283.05 37 561.21 39 024.70

1.94 -4.22 - 4.25 -19.59 -22.01 4.89 -5.15 -5.32

Calculated frequencies from constants of Table XVI and quadrupole coupling constants Measured using radio frequency-microwave double resonance.

vCalcd,

TABLE XIV: Observed' and Calculatedb m = 0 Frequencies (MHz) for trans-Methyl Nitrite Isotopic Species

-____

transition

J'-J 20,2-~0,1 21,2-11,1

21,1-11,0

,CH,ONO

F'-F

"obsd

CH3'*0N0 A uC

1-1 2-1 3- 2 2-2 1-0 2- 2 2- 1 3-2 1-1

30.r20, 2 28 045.65 1.91 -3.83 1 , 3 - 1~, 2 27 534.47 28 559.84 -3.82 3 1,2-2 1, t 28 033.54d -18.78 32,2-22,1 28 040.29 -20.60 32,1-22,0 37 387.37 5.71 40,4-30,3 41,4-31,3 36 710.71 - 4.54 38 077.21 -5.15 41,3-31,~ 37 376.61 -24.81 42,3-3 2 , z - 29.46 4~,~-32,i 37 393.39 See the corresponding footnotes t o Table XIII.

2 transitions in trans-CH,ONO (up to 32 MHz) are mainly due to a breakdown of the rigid-rotor treatment even for the m = 0 state. [If the barrier were zero, then the matrix elements of the Hamiltonian (4)for m = 0 would reduce to those of a rigid rotor.] However, it was useful to derive effective B and C rotational constants in this way for predicting the spectra of the various isotopic species. The b-type spectrum for trans-CH,ONO, which is expected to be more severely affected by the low barrier internal rotation, has not been investigated in the present work. It was decided therefore to base the structural determination on isotopic shifts of the B and C rotational constants derived from the a-type spectra. A vibrational satellite series of a-type, R-branch, K-, = 0 transitions has also been measured for trans-CH30N0. These are listed in Table XV together with the shifts from the ground-state absorptions. The transitions accurately follow a rigid-rotor analysis and have therefore been assigned to m = 0 transitions in an excited vibrational state. Relative intensity measurements give a vibrational fre-

-_ Avc

vobsd

1 9 111.30

0.49

1 8 732.51

- 2.86

CD30N0 vobsd

Avc

16 983.34 16 696.44 16 694.75

0.00 -10.69 -10.63

16 693.67

-10.55

weak 1 9 488.94

-2.87

obscured 1 7 254.42

- 10.68

weak 28 661.30 28 097.17 29 232.15

1.98 -4.06 - 3.88

1 7 252.68 25 471.25 25 041.09 26 880.77

-10.63 0.93 -15.71 -15.75

38 204.83

5.26

33 954.62 33 386.46 34 505.66

- 20.43 - 20.82

38 973.14

-4.98

2.94

quency of 230 f 30 cm-', almost certainly the torsion about the central 0-N bond. The measured a-type, m = 0 frequencies for the various isotopic species of trans-CH,ONO are listed in Tables XI11 and XIV and effective rigid-rotor B and C rotational constants derived from these spectra are given in Table XVI. B + C values for trans-CH,ON1sO and -CH3lsONO were determined from the 20s-10,1 transitions after allowing for a small distortion shift equal to that of the main species. A similar procedure was adopted for trans13CH30N0using the 30,3-20,2 transition. The K-, = 1lines were analyzed for effective B-C values to give a consistent set of constants for the isotopic species. The microwave spectrum of trans-CD,ONO was mainly studied for the internal rotation problem (see next section). For tr~ns-~~CH,ON the 0 , radio frequency-microwave double-resonance technique32was useful for assigning the = 2, J = 2 3 and J = 3 4, a-type transitions. Some excited state K_, = 0, m = 0 transitions have been measured for trarzs-CH3015N0and -CH30N180,analogous to

-

-



1479

TABLE XV: Observed K - , = 0, rn = 0 Frequencies and Shifts (MHz) of a Vibrationally Excited State for trans-Methyl Nitrite Species mat4

"obsd

transition

CH,ONO

CH,O1'NO

CH,ON1sO

II

18 338.46 19 160.88 20,2-~0,1 19 239.78 (1'1.83) (11.29) (17.34) obscured 30,3-20,228 854.92 28 736.27 ( 1'7.7 7) (16.95) 40*4-30,338 464.50 38 305.78 36 664.88 (23.84) (22.73) (35.01)

m=+2

I

the main species (see Table XV). Excited Methyl Torsional States and Barrier to Internal Rotation. Excited m state, = 0 lines a t the J = 2 3 transition in trans-CH,ONO are shown in Figure 3 under higher resolution. The pattern of four lines a t 29 182.78,29 185.22,29 190.75, and 29 204.17 MHz occurred a t every a-type, R-branch transition and closely followed a rigid-rotor analysis. A very similar pattern of four lines was also found for t r a n ~ - C H , O ' ~ Nand 0 -CH30N180(see Table XVII for the measured frequencies). The pair of lines to lower frequency of this pattern and of equal intensity was assigned to the J = 2 3, K-' = 0, m = f3 transitions. The corresponding m = f 3 splitting of the 200-10,1transition was used to derive V3for trans-CH30N0 as follows. The angle between the top axis and a-principal axis was taken to be 28.5' from the structural analysis (see next section). I , for the methyl top was assumed to have the value 3.113 amu A2. The rigid B and C rotational constants appropriate to the torsion-rotation Hamiltonian (4) were obtained by constructing the energy matrix using an approximate V3barrier and the above geometric factors. B' and C were then fitted to the observed m = 0, 20,2-10,1 transition and the splitting of the m = 0, K-l = 1lines, for J = 1 2, by diagonalization of the Hamiltonian matrix. As expected, the rigid out-of-plane C rotational constant hardly changes from the effective m == 0 value (see Tables XVI and XIX). The A rotational constant could then be calculated from the planarity relation

-

-

-

I,

+ 1, - I , = 2Cm,c,'

= I,

(6)

by assuming zero inertial defect. Finally the V3barrier was fitted to the 20,2-10,1,m = f 3 splitting holding all other parameters constant. V, for trans-CH30N0 was determined t o be 29 f 3 cal mol-'. The quoted error on V3 allows for an estimated error of 0.1 MHz for the measured splitting which causles an error in V3 of 1 cal mol-'. Also considered were uncertainties of 0.5" in the top angle and 0.1 amu A2 in I , causing V3to change by 1 and 0.5 cal mol-', respectively. The 20,2-101, m = f 3 splittings for trans-CH30L5N0and -CH30NlB0gave V3 barriers of 29 and 27 cal mol-', respectively, in close agreement with the main species values even though the splittings vary considerably with isotopic species (see Tables XVII and XIX). The top angles 0, needed for these isotopes were computed from the [structure of trans-CH30N0 given in = 0 the next section. If the stronger J = 2 3, transitions in trans-CH,ONO a t 29 190.75 and 29 204.17

-

-

Flgure 3. Excited rn state lines, K-, = 0, J = 2 3 in trans-CH,ONO at 29.2 GHz (Stark field 0.5 kV cm-', zero-field absorption upward).

MHz (see Figure 3) are taken as the split m = f3 lines, then very inconsistent barrier values would be obtained for the three isotopic species. A V3barrier value of 188 cal mol-' previously reported,' determined from separation of the m = 0 and m = fl lines, would require the m = f 3 splitting of the K_l = 0 lines a t the J = 2 3 transition to be ca. 90 MHz and in the same region as the present m = f3 assignment. However careful searches have revealed no possible K-, = 0 candidates meeting these requirements. Further confirmation of our m = f 3 assignment comes from the fit over the remaining m states. Model calculations showed that all the excited m states for the a-type, A J = 1, = 0 transitions would follow a rigid-rotor treatment except the m = f l transitions. At the J = 1 2 transition for trans-CH30N0, a strong K-l = 0 line was observed at 19 480.75 MHz which is slightly t o higher frequency of the pattern of four lines referred to previously. At the J = 2 3 l,ransition, the four-line pattern occurred as expected for a rigid rotor but there was no strong K-' = 0 line to higher frequency (see Figure 3). Instead a strong K-l = 0 line appeared to lower frequency a t 29 156.58 MHz and this had a microwave/microwave double-resonance connection with the line a t 19 480.75 MHz. These two lines were accordingly assigned to the m = f 1, 3 0 , ~ - - 2and ~ , ~20,2-10,1transitions respectively. All the measured AJ = 1, = 0 transitions for trans-methyl nitrite and isotopic species are contained in Table XVII. The pattern of excited m-state transitions for t r ~ n s - C H ~ 0 ' ~ N and O -CH30N180was similar to the

-

-

-

TABLE XVI: Effective Rotational Constants ( M H Z ) and ~ Moments of Inertia (amu A ' ) for trans-Methyl Nitrite Isotopic Species ~

CH,C)NO _ . 4987.49 4627.43 Ib 101.3287 IC log.:! 13 1 Obtained from raw analysis _

_

B C

a

_

_

CH,ON'*O

CH,O"NO I3CH,ONO 4744.26 4970.87 4846.28 4417.03 4604.94 4504.49 106.5237 101.6676 104.2812 114.4153 109.7466 112.1938 of spectra, see Table XIX for refined constants.

CH,'80N0

4967.53 4589.26 101.7359 110.1215

CD,ONO 4386.26 f 0.03 4106.35 t 0.03 115.2180* 0.0006 123.0718 * 0.0007

1480



TABLE XVII: Observed and Calculated K-,= 0 Frequencies (MHz) for trans-Methyl Nitrite Species transition m vobsda A V ( I ) ~ A V ( I I ) ~A V ( I I I ) ~ CH,ONO 20,2-10,1 0 19 227.95 -0.03 -0.03 -0.01 0.10 1.64 t 3 1 9 454.92 21.52 - 3 19456.26 21.56 0.14 1.63 + 2 19459.96 14.29 4.77 -1.26 0.00 + 4 1 9 467.38 36.42 -1.66 +1 1 9 480.76 41.83 39.45 -2.37 3Q,3-20,2

0

28 837.15

-0.71

-0.71

t 3 -3 +2 +4

29 29 29 29

182.78 185.22 190.75 204.17

30.95 31.35 15.58 56.81

-0.72 22.16 1.13 -1.18 1.45 -0.78 1.30 -14.64 2.16 -0.31

0 t 3 -3 *2 +4

38440.66 38 911.27 38915.21 38 922.80 38 944.75

-2.13 38.89 39.98 10.06 79.88

-2.13 -2.24 -3.95 -0.82 -2.86 0.14 -8.98 -43.07 3.72 7.05

0 t 3 -3 +2 +4

CH,O' NO 1 9 149.59 -0.03 1 9 3 8 3 . 1 0 21.93 1 9 384.48 21.91 1 9 387.28 13.40 1 9 397.40 38.72 19 406.98 42.74

-0.03 -0.93 -0.95 3.24 -1.92 40.20

28 719.32

-0.75

"-j

1

+ 1 29 156.58 150.02 146.45

4 0 ~ 3 03 ,

20,2-10,1

-tl

30,3-2~,2

0

-0.75

+ 1 29 037.40 152.75 148.94 t 3 -3 +2 +4 40,4-3~,3

29 29 29 29

075.15 077.73 081.90 099.61

31.53 31.94 14.03 60.61

-2.76 -2.35 -1.21 -0.35

t 3 -3 +2 +4

38 283.05 38 767.74 38772.01 38 777.63 38 805.93

20,2-10,,

0 t 3 -3 +2 +4

CH30N1*0 18 321.12 -0.15 1 8 526.97 20.81 18 527.96 20.78 18 531.44 14.63 18 537.82 33.92

-0.15 0.94 0.91 5.80 -1.41

30,3-20,2

0 t 3 -3 +2 +4

27 27 27 27 27

477.83 790.73 792.65 797.95 809.38

-0.69 30.06 30.39 17.42 52.77

-0.69 0.25 0.58 4.17 -0.22

40,4-30,3

0 i-3 -3 t2

36 37 37 37 +4 37

629.67 054.67 058.17 065.36 084.00

-2.01 37.73 39.02 14.62 73.77

-2.01 -2.01 -0.72 -3.04 3.11

0

-2.23 -2.23 39.40 -6.32 40.63 -5.09 7.27 -13.05 85.42 4.14

a Accurate to f 0.05 MHz. Observed minus calculated Obfrequencies obtained using constants of Table XIX. served minus calculated frequencies with inclusion of D J , nonrigidity term, see text. Observed minus calculated frequencies with inclusion of V , and DJ, terms (for CH,O N 0 species only), see text.

main species. The observed minus calculated frequencies (Av(1) of Table XVII) obtained from the torsion-rotation Hamiltonian (4) show that the rigid top-rigid frame model only qualitatively account for the observed spectra. Similar poor fits for CH3NC012J3and CH3NCS'2 have previously been found using this model. In these cases and in the present study, a nonrigidity correction of the form V - Vrigid = 2 ( J + 1)DJ,m2 + 4 ( J + (7) has been applied to the A J = 1, K-l = 0 transitions.

S T A R K FIELD 1 K v cm-'

Fl.5

Im

.Z

m1.4

-

34395

Figure 4. J = 3

34440 Mrlz

4, K.., = 0 higher m-state transitions in transCDBONO (zero-field absorption downward).

Addition of the more important first term in (7) for the = 0 transitions in trans-CH,ONO, -CH,015N0, and -CH30N180with DJ, values of 0.595, 0.635, and 0.552 MHz, respectively, significantly improved the fit for higher m states but there still remain discrepancies for the lower m states (see Av(I1) column of Table XVII). The same conclusions were reached for CH3NC0 and CH,NCS where DJ, was estimated to be 0.9 and 1.23 MHz, respectively.12 A further source of discrepancy between observed and calculated spectra arises through neglect of the v6 term in the barrier potential. Even a small v6 term has a significant effect on the m = f 3 splitting, and hence V , barrier, by directly connecting the m = f 3 states through the matrix element (JKMmlHIJKMm f 6 ) = -1/4Vs. The frequencies of the other m states are relatively insensitive to v6.Nevertheless for truns-CH30N0,a much improved fit, particularly for the m = f l lines, is obtained using V3 = 61 cal mol-', DJ, = 0.595 MHz, and v6 = 6.3 cal mol-l and this is shown in the Av(II1) column of Table XVII (v6 = 3.9 cal mol-' gives an identical fit, merely reversing the m = f 3 assignment). Such a v6 term is physically reasonable since v6 for nitromethaneZ7was determined to be 6.03 cal mol-l. However, discrepancies still remain in the spectral fit which cannot be reduced within the scope of the models described here, for example by tilting the methyl top. In order to confirm the internal rotation analysis for trans-CH30N0, the spectrum of the fully deuterated isotope was studied. The pattern of excited torsional state lines was expected to be significantly altered by the heavier CD, top. Figure 4 shows the higher m-state pattern for J = 3 4, K-l = 0 transitions with their m assignment. All the measured A J = 1,K1= 0 transitions are listed in Table XVIII. The m = f 3 splitting for the 20,2-10,1 transition gave a V3barrier of 46 cal mol-l, assuming I , = 6.36 amu A2 and calculating ea to be 27.7' from the structure of trans-CH,ONO (see Table XIX). The V , value determined for trans-CD30N0 is much less sensitive to the presence of a v6 term; a value of V , = 49 cal mol-' is obtained for an assumed V , of 5.2 cal mol-' as found for CD3N02. This relates well to the V , value of 61 cal mol-' obtained for CH, above, when v6 is taken into account. Again, the rigid top-rigid frame model for internal rotation only qualitatively accounts for the observed spectra (see v&d column of Table XVIII). In this case, even the introduction of the nonrigidity term 2 ( J + 1)DJ,m2 failed to improve the fit for the higher m-state transitions showing that trans-CD,ONO presents much more of an intermediate barrier problem. The m assignment of the = 0 lines in Table XVIII, although based only on the rigid top-rigid frame model, is considered definite except for m = f 4 and f 5 which may be reversed if detailed

-

cis-


and trans-Methyl Nitrite

TABLE XVIII: Observed and Calculated K - , = 0 Frequencies (MHz) for trans-CD,ONO transition 20,2-10,1

10 t 3 i5 -3

16 983.34 17 192.09 17 193.18 17 195.67 17 196.59 17 213.71 17 261.55

16 983.14 17 129.99 17 128.57 17 133.56 17 129.53 17 145.56 17 167.75

25471.25 25 788.25 25 790.21 25 793.81 25 795.56 25 836.22 33 954.62 34 384.66 34 381.28 34 392.38 34 395.13 34 439.05

25472.13 25 695.95 25 693.17 25 701.46 25 694.80 25 715.86 33 958.06 34 263.05 34 258.15 34 270.68 34 260.72 34 283.21

+1 0,2

b

Vobsd"

i2 0, 3-2

Ib

rn

+4

0 +3 i5 -3 i4 +2 0 +3 i5 -3

i4 i2

VCdCd

H

Figure 5. Structure of frans-CH,ONO.

TABLE XX: Principal Axis Coordinates (A ) and Structure of trans-Methyl Nitrite a b

-

terminal 0

N bridging 0

" Accurate t o iO.015 MHz. Calculated frequencies obtained using constants of Table XIX. TABLE XIX: Internal Rotation Parameters for trans-Methyl Nitrite and Isotopic Species

B, MHz C, MHz I,,a amu A 2 I b , amu A 2 I,, amu A * I,, amu A' e m b deg F, GHz V,, cal mol-' S

1481

H H

-0.0818 (-0.0876) 0.4379 -0.5029 0.1812 (0.1959) -0.5589 0.7987

cO.890

xmiai = xmibi = xmiaibi = 0.000 xmi(ai2 + ciz)= 101.532 (cf. I b = 102.0268 amu Az)

CH,ONO

CH,015N0

CH,ON"0

CD,ONO

4953.37 4627.32 10.3690 102.0268 109.2158 3.18 28.50 210.2 29 0.64

4936.61 4604.82 10.5564 102.3730 109.7494 3.18 28.66 208.9 29 0.65

4713.70 4416.95 l0.3831 l07.2143 114.4 174 3.18 28.34 210.2 27 0.60

4339.48 4105.50 12.9971 116.4602 123.097 3 6.36 27.13 131.4 46 1.63

a Calculated value from assumed la, see text. lated from the structure of trans-CH30N0.

C

1.6328 (1.6349)" 0.5913 (0.5934) -. 0.466 2 (-0.4641) -1.7287 (-1.7266) -2.5353 -1.8069

C

Calcu-

nonrigidity ejffects are considered. The Stark effect of the m = f l , 20,2-1,,1 transition for trans-CH30N0 has been investigated in some detail since it should be sensitive to the V , barrier.33,34The M = f l lobe was observed t o resolve into two equal components with increasing electric field. The experimental coefficient Av(')/E was 11.9 f 0.1 MHz kV-l cm where A#) is the separation between the M = 1 and M = -1 lobes. The average displacement of the M = +1and -1 lobes from the zero field line, Av('), varied as E z and the second-order coefficient was measured to be 16.36 f 0.04 MHz k V 2 cmz. Theoretical values for these coefficients were calculated for a range of V3barriers from 0 to 150 cal mol-l using pa = 2.18 D and & = 0.92 D, determined from the m = 0 spectra^.^' If V , = 61 cal mol-' (V, = 6.3 cal mol-'), the value which best fits the separation between the 20,2-10,1,m = 0 and f l lines as well as fitting the splitting of the m = ft3 lines, A V ( ~ ) is / Ecalculated ~ to be 16.15 MHz k V 2 cm2 with the product pa& positive. This is in good agreement with the experimental value. However, agreement with the observed first-order coefficient Av(')/E is difficult to achieve even with reasonable changes in the dipole components. For completely free internal rotation, this coefficient is calculated to be 9.2 MHz kV-' cm. It then decreases to a minimum of 3.0 MHz kV-l cm a t V3 = 90 cal mol-' and rises again to 7.6 MHz kV-' cm at V3 = 150 cal mol-l. It is apparent that model errors exist, making it difficult to predict quantitatively the Stark effect

O=N

--

1.164(5) A

N - 0 = 1.41 5( 5) A

LONO = 111.8(5)" LNOC = 109.9( 5)o

0-C = 1.436( 5) A

" Numbers in parentheses are Kraitchman coordinates before adjustments, see text. for m = fl transitions, Despite these difficulties the product y,& is certainly positive giving the orientation (without sign) of the dipole moment, as shown in Figure 5, almost parallel to the C-0 bond. Structure. The structure of trans-methyl nitrite was derived from the rigid B and C rotational constants for the various isotopic species appropriate to the torsion-rotation Hamiltonian (4). For each isotopic species, these rotational constants were obtained by fitting to the R-branch, = 0, m = 0 transitions, sensitive to B + C and the splitting = 1, m = 0 lines, which depend on of the R-branch, B - C. The top angle 0, for each isotope was taken from a preliminary structure determined directly from the m = 0 moments of inertia given in Table XVI. However, the derivation of the rigid B and C rotational constants is relatively insensitive to the exact structure adopted. The rigid B and C rotational constants for trans-CH,ONO, -CH3015N0,and -CH30N180have already been discussed in connection with the internal rotation analysis (see Table XIX). For trans-CH3180N0and -l3CH,ONO, for which no m = f 3 splittings were observed, the main species value of V3was adopted. The a and b coordinates of the nitrogen, terminal oxygen, carbon, and bridging oxygen atoms were calculated from Kraitchman's equationszzfor a molecule with a plane of symmetry, in the form

where I, has been set equal to I, + I, - I b (see eq 6) with negligible error. However the small b coordinate of the terminal oxygen atom is not expected to be accurately

1482


TABLE XXI:

S t r u c t u r a l Parameters of M e t h y l N i t r i t e and Related Molecules

GO, A

c~s-HONO trans-HONO cis-CH,ONO trans-CH,ONO CH,ONO, CH,OCHO a

f COC.

IXON, 0-N, LONO, N=O, deg

104.0 102.1 1.437 114.7 1.436 109.9 1.437 112.7 1.437 114.8' Present work.

A

1.392 1.432 1.398 1.415

deg

113.6 110.7 114.8 111.8

A

1.185 1.170 1.182 1.164

ref

4 4 b b 16

10

located in this way (see Table XX). Initial coordinates for the hydrogen atoms were obtained assuming a regular CH, top with C-H equal to 1.09 A, LHCH equal to 109.5', and the top axis coincident with the C-0 bond. The in-plane C-H bond was further assumed to stagger the lone pair orbitals on the bridging oxygen atom as found for cis-CH,ONO. The assumption about the methyl group conformation has negligible effect on the remaining structural parameters. All four heavy atom a coordinates were slightly adjusted by approximately 0.002 A to fit the Clmlal= 0 condition. This adjustment had to be carried out regardless of other reasonable choices of hydrogen a coordinates. The small terminal oxygen and carbon b coordinates were then calculated to fit Cmlbl = Cmlalbi = 0. The out-of-plane hydrogen coordinate was assumed equal to 0.890 A. The structure calculated in this way represents the minimum change structure from the direct Kraitchman coordinates and should be regarded as that in the hypothetical torsionless state (see Table XX and Figure 5). The heavy atom bond lengths and angles between heavy atoms are judged to be accurate to f0.005 A and 0.5', respectively. The structure and conformation of the methyl group for trans-CH30N0 is not determined in the present work.

Discussion The structural parameters for the O N 0 framework of cis- and trans-methyl nitrite are, as expected, similar to the nitrous acids. One difference, however, is the increase in angle a t the bridging oxygen atom (see Table XXI). Methylation of nitric acid and formic acid also produces only minor structural changes except for the increase in bridging oxygen angle, 102.2'/112.7' in nitric acid35/ methyl nitrate16 and 106.3'/114.8' in formic a~id,~/methyl formate,1° An interesting structural feature is the constancy of the C-0 bond length (1.437 A) for methyl nitrite, methyl nitrate, and methyl formate (see Table XXI). The decrease in C O N and O N 0 bond angles on cis to trans interconversion of methyl nitrite is probably due to reduced steric interaction in the trans molecule. The very low methyl group barrier for trans-CH,ONO would also be consistent with this view. Similar structural changes have been predicted by John and Radomg using ab initio SCF MO theory for the isoelectronic methyl formate molecule but for this system no experimental data for the higher energy trans form is available for comparison. Furthermore, these calculations suggested a low V3barrier for trans-methyl formate with the in-plane C-H bond of the methyl group eclipsing the central 0-C bond. However the determination of the equilibrium conformation of the methyl group with a low barrier to internal rotation, such as trans-CH,ONO, from the spectra of


partially deuterated species, is expected to be a difficult problem. The nitrogen quadrupole coupling constants obtained for cis-CD,ONO have enabled the complete field gradient tensor to be determined. The value of Xab = 2.8(6) obtained for cis-CH30N0 leads to principal axis values of the coupling tensor to be derived as x x = -5.9(5), xz = 2.4(5), and xy = 3.48(3) MHz, with dbx = 20.7(30)'. This value of xn is close to that found for the nitrite ion37and nitrous acid.4 Moreover the x axis makes an angle of 18.5' with the bisector of the LONO (nearer N=O) and might imply the extent to which the lone pair on nitrogen is distended toward those on the central oxygen. The field gradient tensor obtained for the cis isomer can be shown to be hardly altered for trans-CH,ONO, predicting coupling constants for that species to within 0.3 MHz. Acknowledgment. We thank the Science Research Council and the Univeristy of Bristol for a Research Studentship and Scholarship, respectively. We also thank Professor E. B. Wilson, in whose laboratory some of the measurements were made.

References and Notes W. D. Gwinn, R. J. Anderson, and D. Stelman, Bull. Am. fhys. Soc., 13, 831 (1968); D. Stelman, Diss. Abstr., 25, 1609 (1964); R. J. Anderson, /bid., 28, 858-8 (1967). H. W. Brown and D. P. Hollis, J. Mol. Spectrosc., 13, 305 (1964). K. Endo and Y. Kamura, J . Chem. SOC.Jpn., 729 (1977). A. P. Cox, A. H. Brittain, and D. J. Finnigan, J. Chem. Soc., faraday Trans. 2 , 67, 2179 (1971). W. H. Hocking and G. Winnewisser, Z. Naturforsch. A, 31,438 (1976). E. Hirota, J . Chem. fhys., 42, 2071 (1965). J. Nakagawa, K. Kuwada, and M. Hayashi, Bull. Chem. Soc. Jpn., 49, 3420 (1976). E. P. Van Eijck, P. Brandts, and J. P. M. Maas, J . Mol. Struct., 44, 1 (1978). I. G. John and L. Radom, J . Mol. Struct., 36, 133 (1977); L. Radom, W. A. Lathan. W. J. Hehre. and J. A. Poole. Aust. J . Chem.. 25. 1601 (1972); H. Wennerstrom, S.Forsen, and E. Roos, J . fhys: Chem., 76, 2430 (1972). R. F. Curl, Jr., J . Chem. Phys., 30, 1529 (1959). V. W. Laurie and D. R . Lide, J . Chem. fhys., 31, 939 (1959). R. G. Lett and W. H. Flygare, J . Chem. fhys., 47, 4730 (1967). R. F. Curl, V. M. Rao, K. V. L. N. Sastry, and J. A. Hodgeson, J. Chem. fhys., 39, 3335 (1963). H. D. Rudolph and A. Trinkaus, 2 . Naturforsch. A , 23, 68 (1968). P. H. Turner and A. P. Cox, J . Chem. SOC.,Faraday Trans. 2 , 74, 533 (1978). A. P. Cox and S. Waring, Trans. Faraday Soc., 67, 3441 (1971). A. P. Cox and S.Waring, J . Labelled Comp., I X , 153 (1973). J. K. Bragg and S. Golden, Phys. Rev., 75, 735 (1949). D. R. Herschbach and J. D. Swalen, J. Chem. fhys., 29, 761 (1958). R. C. Woods, J . Mol. Spectrosc., 21, 4 (1966). C. C. Costain, J . Chem. Phys., 29, 864 (1958). J. Kraitchman, Am. J . fhys., 21, 17 (1953). A. H. Brittain, A. P. Cox, and R. L. Kuczkowski, Trans. Faraday Soc., 65, 1963 (1969). V. W. Laurie and D. R. Herschbach, J , Chem. fhys., 40, 3142 (1964). W. E. Dixon and E. E. Wilson, Jr., J. Chem. Pbys., 35, 191 (1961). R. E. Naylor, Jr., and E. B. Wilson, Jr., J . Chem. Phys., 26, 1057 (1957). E. Tannenbaum, R. J. Myers, and W. D. Gwinn, J . Chem. fhys., 25, 42 (1956). D. R. Herschbach, J . Chem. Phys., 27, 1420 (1957); C. C. Lin and J. D. Swalen, Rev. Mod. Phys., 31, 841 (1959). S. Siegel, Ph.D. Thesis, Harvard University, 1959. Further factorization of the A block into A, and A, blocks would be possible, due to the plane of symmetry. R. J. Anderson, Ph.D. Thesis, University of California, Berkeley, 1967. F. J. Wodarczyk and E. E. Wilson, J. Mol. Spectrosc., 37, 445 (1971). W. M. Tolles, E. T. Handeiman, and W. D. Gwinn, J . Chem. fhys., 43, 3019 (1965). R. A. Beaudet and E. E. Wilson, Jr., J. Chem. phys., 37, 1133 (1962). A. P. Cox and J. M. Riveros, J . Chem. fhys., 42, 3106 (1965). G. H. Kwei and R. F. Curl, Jr., J . Chem. fhys., 32, 1592 (1960). T. Oja, R. A. Marino, and P. J. Bray, fhys. Lett. A , 26, 11 (1967).

Microwave spectra and structures of cis-and trans-methyl nitrite

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