Effect of electron correlation on theoretical vibrational frequencies

Abstract Theoretical HF /6‐31 G * (Hartree–Fock, 6‐31 G * basis set) and MP 2/6‐31 G * (second‐order Møller–Plesset, 6‐31 G * basis set) vibrational frequencies based on complete quadratic force fields have been obtained for a set of 36 one‐ and two‐heavy‐atom molecules comprising first‐row elements for which experimental spectroscopic data are available. Frequencies calculated at the HF /6‐31 G * level are an average of 12.6% higher than experimental values. Partial treatment of electron correlation via the perturbation method of Møller and Plesset, terminated at second order, leads to a significant reduction in this error, although theoretical MP 2/6‐31 G * frequencies are still larger than the experimental quantities by 7.3%. Part of the difference may be traced to the restriction of quadratic force fields, as comparison with experimental harmonic frequencies shows deviations of only 9.5% and 4.7% for the two levels, respectively. The calculated frequencies are used in conjunction with the corresponding theoretical equilibrium structures to obtain absolute molecular entropies, which may in turn be used to yield entropies of reaction. These latter quantities are generally in good accord with entropies derived using experimental structures and frequencies.