The Effect of Isotopic Substitution on Mass Spectra of Molecules

Abstract

A general method for calculating the effect of isotopic substitution on mass spectra is presented. The calculation is based on the notion that the various fragment ions, formed by electron impact on a molecule, are produced by a Franck-Condon mechanism. It is assumed that those bonds will break which possess more potential energy than bond energy at the instant of ionization. The mass spectral data for a molecule are used in conjunction with the ground-state vibrational wave function to evaluate critical bond distances (distances for which the potential energy equals the bond energy). Then the mass spectral pattern for isotopic molecules can be predicted from a knowledge of the ground-state vibrational wave function of the isotopic molecule. The method is applied to the diatomic molecules N14N14, N14N15, N15N15, and C12O16, C13O16. The predicted values compare favorably with previously observed results, however, experimental errors obscure a sharp test of the predictions for these cases. New experimental values are presented for the ratios C13+/C13O216+ and C12+/C12O216+ from carbon dioxide and compared with predictions. Good agreement is found between ratios extrapolated to high ion accelerating voltage and predicted ones.