Deuterium Isotope Effects in the Electron Spin Resonance Spectra of Naphthalene Negative Ions

Abstract

High-resolution electron spin resonance spectra have been obtained of the anion radicals of naphthalene and of 10 different deuterium substituted naphthalenes. The ratio of the proton (aH) to deuteron (aD) hyperfine splittings was found to be different from the value aH/aD=6.514 expected on the basis of the magnetic moments and spins of the two isotopes and, furthermore, it was also found to be different at the α and β positions. These anomalous aH/aD ratios have been qualitatively interpreted in terms of vibrational effects which alter the sigma—pi interactions at the substituted positions. The data indicate that the sigma—pi parameters (Q's) for the two isotopes and at the α and β positions are all different, even when account is taken of the magnetic moments and spins. Deuterium substitution at one position was found to alter the hyperfine splittings of the protons at other positions. The magnitude and sign of these changes, which depend on the position and number of substituted deuterium atoms as well as on the position of the proton, have been shown to obey an additivity relationship. The changes in splittings are interpreted as resulting from a redistribution of spin density throughout the radicals. Hückel molecular-orbital calculations have been performed in which the resonance integral for bonds adjacent to a deuterium-substituted position have been altered by a small amount in order to account for this redistribution in spin density. Good agreement between the experimental and calculated changes was obtained, while similar calculations in which the presumed weak electron-donating effect of deuterium was accounted for by a change in the Coulomb integral at the substituted position were in systematic disagreement to a significant extent with the spin densities at certain positions. It was concluded that ESR data provide a more direct means of evaluating the vibrational perturbations introduced by deuterium substitution than do studies of chemical kinetics. Some of the spectra were of such high resolution that anomalies were introduced because of second-order shifts. These could not be analyzed by the usual approximations, and an Appendix is included on the detailed procedures which had to be employed.