RADICAL-CATION OF 1,3-BUTADIENE - RESONANCE RAMAN-SPECTRUM OF DEUTERATED DERIVATIVES AND IMPROVED FORCE-FIELD

The resonance Raman spectra of the radical cations of three deuterated derivatives of 1,3-butadiene are reported. Together with the data for the parent compound (cf.: Keszthelyi, T.; Wilbrandt, R.; Bally, T. J. Phys. Chem. 1996, 100, 16843), 22 frequencies of totally symmetric fu ndamentals are available to complement 29 previously obtained IR data for the same compounds (Tang, W; Zhang, X.-L., Bally, T. J. Phys. Chem . 1993, 97, 4373) and to generate a new and improved scaled quantum me chanical force field of butadiene radical cation based on B3LYP/6-31G second derivatives. After fitting those with 13 scaling factors, the resulting force field reproduces all 51 observed fundamental vibration s of the four isotopomers with a root-mean-square deviation of 6.03 cm (-1) (10.6 cm(-1) for the 11 strongly anharmonic C-H/C-D stretches, 3. 92 cm(-1) for the remaining 40 modes). It allows tile first experiment ally based prediction of the stretching force constant for the central C-C bond and corrects the previous ones for the terminal C=C stretch. As in the case of the parent compound. evidence is presented for the presence of the s-cis rotamer after ionization whose vibrations are pr edicted very accurately by scaling a B3LYP/6-31G force field of cis-b utadiene(.+) with the optimized scaling factors of the s-trans rotamer .