Paramagnetism in Diamagnetic Molecules

Abstract

The experimental and theoretical determinations of the Van Vleck paramagnetic contribution (χhf) to the susceptibility of diamagnetic molecules are considered. The data on the hydrogen molecule are reviewed, and the mean square orbital angular momentum is recalculated by using Newell's wave function. The H2 calculations are interpreted as indicating that the main contributions to χhf are from energy levels directly adjacent to the double ionization limit (to H2++) at 51 ev. A corresponding calculation is made for methane by using approximate wave functions and it is found that, contrary to the H2 molecule work, L2 is quite insensitive to the wave function employed. An effective excitation energy of 30 ev is derived from an experimental value of χhf obtained from rotational magnetic moment data. Similar data for CH3F are considered and explained by the inductive effect of the F atom upon the C–H bond dipole moment. Approximate rigid-charge calculations of χhf and Pascal's rule are discussed, and the higher hydrocarbons are treated by the above procedures. It is conjectured that, in general, the energy levels near the various ionization limits in a molecule are the principal contributors to χhf, and also, that relatively low effective excitation levels (order of magnitude about 50 ev) might be expected.