Abstract
Density functional theory (DFT) using the 6-31G* basis set and two nonlocal exchange-correlation functionals (Becke-Lee-Yane-Parr ［B-LYP］ and the three-parameter compound function of Becke ［B3-LYP］) has been used for the calculation of vibrational force fields of a set of 31 organic molecules including a wide range of functional groups. The calculated force constants have been scaled to experimental vibrational frequencies by using (a) an overall scaling constant and (b) a set of 11 factors paying respect to the different kinds of internal coordinates. The comparison of the scaled fundamental frequencies with experiment shows that density functional theory is a reliable tool for the interpretation of IR spectra. The uncorrected DFT frequencies and force constants approximate the experimental ones in a much more uniform fashion than does Hartree-Fock theory. Nevertheless, the use of multiple scale factors leads to further significant improvement. The scaled B3-LYP results are superior to the B-LYP ones, even though the unsealed B-LYP frequencies are, through error cancellation, slightly better than the B3-LYP ones. The reliability of scaled force fields is demonstrated by comparing the calculated and experimental vibrational spectra of aniline.
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