ANALYSIS OF TORSIONAL SPECTRA OF MOLECULES WITH 2 INTERNAL C(3-UPSILON) ROTORS .26. A FAR-INFRARED STUDY OF 1,1-DIMETHYLHYDRAZINE

The infrared spectra of 1,1-dimethylhydrazine, (CH3)2NNH2, and two iso topomers, (CD3)2NNH2 and (CH3)2NND2, have been recorded in the region between 600 and 100 cm-1. Very rich and complex spectra were obtained and analysis of the data has been carried out. The interpretation of t he spectra arising from the two methyl torsional modes of the -d0 comp ound was carried out using a semi-rigid model, and the resulting poten tial function obtained is V30 = 1685 +/- 12 cm-1 (4.82 +/- 0.04 kcal m ol-1); V03 = 1827 +/- 16 cm-1 (5.22 +/- 0.05 kcal mol-1); V60 = -92 +/ - 5 cm-1 (-0.26 +/- 0.02 kcal mol-1); V06 = -41 +/- 6 cm-1 (-0.12 +/- 0.02 kcal mol-1) and V33' = -51 +/- 5 cm-1 (-0.15 +/- 0.01 kcal mol-1) . Ab initio gradient calculations were carried out employing the 3-21G and 6-31G basis sets, as well as the 6-31G* basis set with electron correlation at the MP2 level. The structural parameters, conformationa l stability, and three-fold barriers to internal rotation have been de termined and the gauche conformer is calculated to be more stable than the trans form by 783 cm-1 (2.24 kcal mol-1) with the MP2/6-31G basi s set. These calculations were also used to re-evaluate the previously reported assignment of the fundamental modes, and to obtain a potenti al function for the asymmetric torsion. All of these results are discu ssed and compared with corresponding quantities for some similar compo unds.