On the vibrational spectra and conformational stability of 1,1-dimethylhydrazine from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

Variable temperature (2105 to 2150 8C) studies of the infrared spectra (3500–400 cm21) of 1,1-dimethylhydrazine, (CH3)2NNH2, in liquid krypton have been carried out. No convincing spectral evidence could be found for the trans conformer which is expected to be at least 600 cm21 less stable than the gauche form. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from MP2/6-31G(d) ab initio calculations. The predicted infrared and Raman spectra are compared to the experimental ones. The adjusted r0 parameters from MP2/6-311 þ G(d,p) calculations are compared to those reported from an electron diffraction study. The energy differences between the gauche and trans conformers have been obtained from MP2 ab initio calculations as well as from density functional theory by the B3LYP method calculations from a variety of basis sets. All of these calculations indicate an energy difference of 650–900 cm21 with the B3LYP calculations predicted the larger values. The potential function governing the conformational interchange has been predicting from both types of calculations and comparisons have been made. The barrier to internal rotation by the independent rotor model of the inner methyl group is predicted to have a value of 1812 cm21 and that of the outer one of 1662 cm21 from ab initio MP2/6-31G(d) calculations. These values agree well with the experimentally determined values of 1852 ^ 16 and 1558 ^ 12 cm21, respectively, from a fit of the torsional transitions with the coupled rotor model. For the coupled rotor model the predicted V033 (sin 3t0 sin 3t1 term) value which ranged from 190 to 232 cm21 is in reasonable agreement with the experimental value of 268 ^ 3 cm21 but the predicted V33 (cos 3t0 cos 3t1 term) value of 273 to 2139 cm21 is 25% smaller and of the opposite sign of the experimental value of 333 ^ 22 cm21. These theoretical and spectroscopy results are compared to similar quantities of some corresponding molecules. q 2003 Elsevier B.V. All rights reserved.