VIBRATIONAL-SPECTRA OF ALL-TRANS-1,2,5,7-OCTATETRAENE

The vibrational spectra of all-trans-1,3,5,7-octatetraene have been st udied with nb initio Hartree-Fock (I-IF) and Moller-Plesset second-ord er perturbation (MP2) calculations. The MP2-predicted spectra are in g ood agreement with experiment, clearly showing the importance of the e lectron correlation effect. The basis set dependency is notable. Relat ive to the experimental data, the mean absolute percentage deviations of vibrational frequencies predicted by HF/DZ, MP2/DZ, and MP2/6-31G are 12.4%, 3.4%, and 4.4%, respectively. The higher frequencies tend t o be overestimated, whereas the lower frequencies do not. Thus, expone ntial scaling is more suitable than constant scaling. With exponential scaling, the root mean square (rms) frequency deviations of the MP2/D Z and MP2/6-31G results from the experimental results are 29 and 22 c m(-1) respectively, while with constant scaling the two values are 41 and 26 cm(-1), respectively. Therefore, although the MP2 results with the polarization basis sets seem to predict the frequencies in terms o f the mean absolute percentage deviations less accurately than those w ithout the polarization basis sets, the former becomes much more consi stent than the latter when the frequencies are scaled. Exponential sca ling can be a useful approach in that it turns out to be better than c onstant scaling which has been commonly used.