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 quantu m mechanical force fields to predict the fundamental vibrational frequ encies of related molecules, with acceptable accuracy, are reinvestiga ted. 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 a re rather close to the Hartree-Fock limit. Another requirement is sing let stability of the SCF wave function. The calculation of the vibrati onal frequencies of the rotational isomers of glyoxal, O=CH-CH=O, is p resented as an example. Thus when quantum mechanical force fields obta ined 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 obtai ned for the light s-trans-glyoxal conformer to the force field of the s-cis rotamer gives a wholly satisfactory prediction of the vibrationa l frequencies of the latter. Comparison of the theoretical and experim ental fundamental vibrational spectra of the s-trans and s-cis conform ers of nitrous acid, HONO, shows that the above is probably true for i norganic 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 onl y agreement between the calculated and experimental frequencies of iso topomers.