J . Phys. Chem. 1989, 93, 593-597
593
Torsional Transitions and Barriers to Internal Rotation of Methylhydrazine J. R. Durig,* N. E. Lindsay,+ and P. Groner Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: April 28, 1988)
The far-infrared spectra of methylhydrazine(CH3NHNH2),methylhydrazine-C-d3(CD3NHNH2),and methylhydrazineNJV-d3 (CH3NDND2)in the gas phase have been recorded between 370 and 100 cm-' with a resolution of 0.1 cm-I. The amino torsional fundamentals and several hot bands have been observed and assigned on the basis of the presence of the inner and outer skew conformers. Rotational constants have been determined from the observed K-structure arising from the amino torsional mode of the inner conformer. A potential function for the amino torsion of methylhydrazine has been calculated by using three cosine potential coefficients (in reciprocal centimeters), V, = -1542, V3 = -277, and V4 = 131, and two sine potential coefficients, V3' = -152 and V i = 28, for a cis barrier of 1784 f 58 cm-' (5.10 0.17 kcal/mol) and a trans barrier of 1544 f 57 cm-' (4.41 0.16 kcal/mol), both with respect to the more stable inner conformer. The potential functions for the two isotopic species have also been calculated, and the determined values for the constants are in reasonable agreement with those for the normal molecule.
*
*
Introduction Initially electron diffraction,' far-infrared,, and m i c r ~ w a v e ~ - ~ investigations of hydrazine indicated that hydrazine exists in two equivalent skew conformations with a torsional dihedral angle of approximately f9Oo with respect to the cis conformer (nitrogen lone pairs are eclipsed). Substitution of a methyl group for a hydrogen in hydrazine results in two nonequivalent skew forms, with the terms inner and outer referring to the position of the methyl group. In an early thermodynamic study5 of methylhydrazine, it was assumed that the outer skew conformer is the more stable conformer primarily from steric reasons. It was believed that the interactions between the hydrogens of the methyl group and the rest of the molecule would be less for this form. Aston et aLS compared the observed dipole moment,6 measured in benzene solution, to dipole moments calculated from the vector sum of group moments to support the assumption of the predominance of the outer skew conformer. The Raman spectrum of methylhydrazine was initially reported by Kahovec and Kohlrausch' and later by Axford et a].,' who also investigated the infrared spectrum between 1600 and 650 cm-I. In deither study were the torsional fundamentals observed for the methyl and amino rotors, nor were deuteriated analogues used to support tentative vibrational assignments. A complete investigation of the vibrational spectrum of methylhydrazine and two deuteriated analogues was reported by Durig et al.? with the methyl and amino torsional modes being observed in the lowresolution far-infrared spectrum of the vapor. The barriers to internal rotation were calculated, assuming the outer conformation was more stable, resulting in barriers of 3.67 and 3.72 kcal/mol for the methyl and amino torsions, respectively. Ab initio calculationsI0 at the extended 4-31G level support the predominance of the outer skew conformer. A microwave investigation of methylhydrazine by Lattimer and Harmony'' compared experimental dipole moments with calculated dipole moments, from which it was concluded that the inner skew conformer was more stable than the outer skew conformer. The dipole moments were calculated by using the vector sum of the group dipole moments of RNH, and RNHCH3 where R was an atom or group having an electronegativity similar to that of nitrogen. From relative intensity measurements the energy difference between conformers was determined to be 293 cm-' (838 cal/mol). Recent ab initio studies12 support this assignment of the observed microwave spectra to the inner and outer conformers by comparing theoretical and experimental dipole moment components and nuclear quadrupole coupling constants. Their calculated energy difference between the conformers of 171 cm-' 'Taken in part from the thesis of N. E. Lindsay, which has been submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.
0022-3654/89/2093-0593$01.50/0
(488 cal/mol) in favor of the inner skew conformer supports the results obtained by microwave spectroscopy." Combining the low-resolution far-infrared data with the data on the relative stabilities of the inner and outer skew conformers from the microwave study," Lattimer and HarmonyI3 reexamined the amino torsional potential function. With some reassignment of observed transitions a trans barrier height of 1253 f 25 cm-' (3.58 f 0.07 kcal/mol) and a cis barrier height of 3028 f 300 cm-' (8.66 f 0.86 kcal/mol) were calculated. Since the far-infrared spectrum of methylhydrazine has not been studied with relatively high resolution by Fourier transform interferometry, a new study is timely and worthwhile. This is particularly true in view of the far-infrared spectral studies of hydra~ine'~.'~ where detailed rotational fine structure was reported. Therefore, we reinvestigated the far-infrared spectra of methylhydrazine and two deuteriated modifications and determined potential functions for the internal rotation of the amino group.
Experimental Section Commercially obtained methylhydrazine (Aldrich Chemical) and methylhydrazine-N,N'-d3 (Merck, Sharp & Dohme) were purified by using a low-temperature fractionation column. The purified samples were collected directly into a 20-cm cell for vapor-phase studies in the far-infrared spectral region. Methylhydrazine-C-d3 was preparedI6 by adding 22.5 mL of an-
(1) Morino, y.; Iijima, T.; Murata, Y. Bull. Chem. SOC.Jpn. 1960, 33, 46. (2) Yamaguchi, A.; Ichishima, I.; Shimanouchi, T.; Mizushima, S. J . Chem. Phys. 1959, 31, 843. (3) Kasuya, T.; Kojima, T. J . Phys. SOC.Jpn. 1963, 18, 364. (4) Tsunekawa, S . J. Phys. SOC.Jpn. 1976, 41, 2077. ( 5 ) Aston, J. G.; Fink, H. L.; Janz, G. J.; Russell, K. E. J. Am. Chem. Soc. 1951, 73, 1939. (6) Ulich, H.; Peisker, H.; Audrieth, L. F. Ber. Dtsch. Chem. Ges. E 1935, 68, 1977. (7) Kahovec, L.; Kohlrausch, K. W. F. Z. Phys. Chem. Abt. B 1937, 38, 96. (8) Axford, D. W. E.; Jam, G. J.; Russell, K. E. J . Chem. Phys. 1951, 19, 704.
(9) Durig, J. R.; Harris, W. C.; Wertz, D. W. J. Chem. Phys. 1969, 50, 1449. (IO) Radom, L.; Hehre, W. J.; Pople, J. A. J . Am. Chem. SOC.1971,93, 289.
(11) Lattimer, R. P.; Harmony, M. D. J . Chem. Phys. 1970, 53, 4575. (12) Yamanouchi, K.; Kato, S.; Morokuma, K.; Sugie, M.; Takeo, H.; Matsumura, C.; Kuchitsu, K. J . Phys. Chem. 1987, 91, 828. (13) Lattimer, R. P.; Harmony, M. D. J. Am. Chem. SOC.1972,94,351. (14) Tsuboi, M.; Overend, J. J . Mol. Spectrosc. 1974, 52, 256. (15) Tsuboi, M.; Hamada, Y.; Henry, L.; Chazelas, J.; Valentin, A. J. Mol. Spectrosc. 1984, 108, 328.
0 1989 American Chemical Society
Durig et al.
594 The Journal of Physicol Chemisfry, Vol. 93, No. 2, 1989
TABLE I: Observed Wavenumbem (em-') for the Q-branch K-stmchre of the 315- and 290-em-' Bands of CH,NHNH, J' '.K K: 3" K." ':K obsd obsd - calcd obsd IS 15 13 2 nn17 -?79 -, .774 .- . . .I d. 2 -0.018 2 IS 13 I5 12 4 338.005 4 4 6 0 6 2 8 2 8 4
15
II
15 15
IO
IO
9
IO 15
10
8 8 7 7 6 6
10
15
5
IO
5 5
4 3 4 4 5 5
4 0 6 2 2 4 2 6 2 6
5
10
4 IO 4
IO
8
15
8
2
IO
8
15
11
15 in 15 15 I5
9 9 IO
IS
6 6 7 7 8 8 9 9 IO
15
12
15 15 15
I2 I1 in
IO I5
IO
10
I5 10 15 10 15
IO 6
IO 6 6 6 6 10 6 10 15
IO 15
IO 15
IO I5 10 15
9 9
8 8 7 7 6 6 6 5
5
2 6 6 4 4
I5 I5 10
9
6 IO 6 6 6 6
4 4 3 4 5
IO
5
6
6 6 6 7 7 8
10 15
15
IO
15
IO I1 I2 13
4 6 6 2 8 2 8 4 IO 4 10 6 2 6 2 4 2 2 6 0 4 IO 4
8 2 8 2 6 0 6 4 4 2
hydrous hydrazine (Fisher Scientific) dropwise to 25 g of iodomethaned, (Aldrich Chemical) at 0 OC. The mixture was stirred under an atmosphere of dry nitrogen. When the addition was complete, the system was allowed to warm to room temperature for approximately 30 min. The solid material was brought into solution by gently heating the mixture. Excess hydrazine was distilled from this mixture, and the product was collected in the temperature range between 65 and 95 "C at 1 atm. The distilled product was purified by using a low-temperature vacuum fractionation column to yield 1,l-dimethylhydrazine-d, (-40%), methylhydrazine-C-d, (-SO%), and 1,2-dimethylhydrazine-d6 (
