Extensive ab initio calculations are reported for the optimized geometries of the methyl, ethyl, isopropyl, and tert-butyl radicals. In addition, vibrational frequencies and infrared and Raman intensities are computed and compared with experimental infrared spectra of the series of radicals. The theoretical calculations are used to assign experimental vibrational spectra and elucidate the radical structure. © 1991, American Chemical Society. All rights reserved.