VIBRATIONAL ANALYSIS OF ALKYL BROMIDES .4. BRANCHED-CHAIN BROMIDES - 1-BROMO-2-METHYLALKANES AND 3-BROMOMETHYLPENTANE

Infrared and Raman spectra were obtained for 1-bromo-2-methylpropane, 1-bromo-2-methylbutane, 1-bromo-2-methylpentane, and 3-bromomethylpentane. Each of these compounds exists as two or more rotational isomers. The propane crystallized in the PC conformation, but the other compounds did not crystallize. Vibrational assignments were made with the aid of normal coordinate calculations. A fifty-seven parameter modified valence force field was used and a least-squares refinement fit 128 frequencies of five conformers of the propane and butane with an average error of 4.4 cm-1. The force constants obtained were used in zero-order calculations for two additional conformers of the butane and five conformers of the two pentanes, with the average error being 4.5 cm-1 for the 221 assigned frequencies. The increase in CBr stretching frequency of the PH conformers from the normal value in alkyl bromides is attributed to the increased contribution of CC stretch and decreased contribution of the CBr stretch. © 1978.