ChemInform Abstract: VIBRATIONAL SPECTRUM AND CONFORMATION OF 2‐METHYL‐2‐NITROPROPANE

Abstract

Abstract The infrared spectra of gaseous and solid 2-methyl-2-nitropropane, (CH 3 ) 3 CNO 3 , have been recorded from 25 to 3500 cm −1 and 80 to 3500 cm −1 , respectively. The Raman spectra of gaseous, liquid and solid 2-methyl-2-nitropropane have been recorded from 20 to 3500 cm −1 and depolarization values have been obtained for the liquid. A complete vibrational assignment for all of the normal modes based on band contours, depolarization values, and group frequencies has been made except for the NO 2 torsion. The vibrational data are consistent with the molecule either being in the bisected conformation or having essentially free internal rotation of the NO 2 group in the fluid phases. Taking into consideration all the available data from the vibrational and rotational spectra, it is probable that the nearly free internal rotation ( V 6 = 71 cm −1 ) can best explain the observed depolarization values, the diffuse infrared band contour for the NO 2 wag, and the far infrared spectrum of the gas. However, from a CNDO/2 calculation, the bisected conformation was found to be more stable than the eclipsed form by 176 cal mol −1 , which is consistent with the Raman depolarization data. From the frequencies for the methyl torsional modes which have been observed in the spectra of the solid phase, a barrier to internal rotation of the methyl groups of 3.93 kcal mol −1 has been calculated. The spectra of the solid are consistent with two molecules per primitive cell. These results are compared to the corresponding quantities for some similar molecules.