TESTING THE VALIDITY OF SCALING THE QUANTUM-MECHANICAL MOLECULAR-FORCE FIELDS FOR ROTATIONAL ISOMERS

The conditions under which the scaling method can be applied to quantum mechanical force fields to predict the fundamental vibrational frequencies of related molecules, with acceptable accuracy, are reinvestigated. Most of all, transferability of scale factors to the force fields of related organic molecules (especially molecules with heteroatoms) requires that the quantum mechanical calculation gives results which are rather close to the Hartree-Fock limit. Another requirement is singlet stability of the SCF wave function. The calculation of the vibrational frequencies of the rotational isomers of glyoxal, O=CH-CH=O, is presented as an example. Thus when quantum mechanical force fields obtained with a basis set giving results near the Hartree-Fock limit (i.e., HF/6-31G*, HF/6-31G**) are used, transfer of the scale factors obtained for the light s-trans-glyoxal conformer to the force field of the s-cis rotamer gives a wholly satisfactory prediction of the vibrational frequencies of the latter. Comparison of the theoretical and experimental fundamental vibrational spectra of the s-trans and s-cis conformers of nitrous acid, HONO, shows that the above is probably true for inorganic molecules. Agreement between the calculated and experimental vibrational frequencies of rotational isomers is a more stringent test of the transferability of scale factors to related molecules than only agreement between the calculated and experimental frequencies of isotopomers.