Influence on isotope effect calculations of the method of obtaining force constants from vibrational data

Reduced isotopic partition function ratios (s/sub 2//s/sub 1/)f are employed in the calculation of isotope effects on thermodynamic equilibrium constrants. The quadratic force constants of the molecular force field are needed to evaluate (s/sub 2//s/sub 1/)f. Often these force constants are directly deduced from observed fundamentals in vibrational spectra and the (s/sub 2//s/sub 1/)f values so obtained are labeled (ANHARM). In a theroretically more valid procedure that is more difficult, one corrects observed fundamentals for anharmonicity on the basis of observed overtone and combination bands and then deduces force constants from these observed harmonic frequencies. The (s/sub 2//s/sub 1/)f values obtained from these force constants are labeled (HARM). (HARM) values and (ANHARM) values are evaluated and the isotope effects calculated with these values are discussed. It is concluded that the consistent use of (ANHARM) values in such calculations is a valid procedure.