Vibrational spectra and structure, ab initio calculations, and conformational stability of 2-methylpropanal

Abstract

The Raman spectra (3600-10 cm −1) of gaseous, liquid and solid and the infrared spectra (3600- 40 cm −1) of gaseous and solid 2-methylpropanal (isobutyraldehyde), (CH 3) 2CHCHO, and the corresponding deuterium compound (CD 3) 2CDCHO have been recorded. Additionally, qualita- tive depolarization ratios have been obtained from the Raman spectra of the liquids. These data have been interpreted for the fluid phases, showing that the gauche conformation (oxygen atom eclipsing a methyl group) is thermodynamically preferred over the high energy trans conformation (oxygen atom eclipsing the secondary hydrogen) and is the only rotamer present in the spectra of the annealed solids. From the relative intensities of the Raman lines of the liquid at 633 cm −1 (gauche) and 551 cm −1 (trans) as a function of temperature, the enthalpy difference is found to be 440±17 cm −1 (1.26±0.05 kcal mol −1). Utilizing rotational constants from previously reported microwave data and structural parameters obtained from ab initio calculations, the r 0 structural parameters are reported for both rotameric forms. The ab initio and r 0 structures for the gauche conformer are consistent with nonequivalent CC bond distances for the isopropyl moiety. A complete vibrational assignment is proposed which is based on infrared band contours, depolarization values, isotopic shifts and group frequencies. The structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio Hartree-Fock gradient calculations utilizing both the 3-21G and 6-31G* basis sets, and to the corresponding quantities obtained for some similar molecules.