Effect of an electric field on a methane molecule

In zeolite cavities the strong electric field due to the ionic charges induces forbidden IR bands and splitting of degenerate vibrational modes. This has been observed on the CH4 molecule orientated in tripod configuration in adsorption sites of NaA and CaA zeolites. The two components of the v 3 splitting have similar intensities for a given temperature but evolve differently with the temperature, corresponding possibly to a variable distance of the molecule with respect to the site. We have calculated the infrared intensity, i.e., |∂M/∂Q|2, taking into account the permanent and the induced moment . The calculated intensities for the electric field directed along one of the CH bonds are plotted versus the field amplitude value and compared with the experimental data. For CH4 in NaA this leads to the assignment of each component (A and E symmetry) and to the conclusion that the derivatives of the permanent moment and of the polarizability tensor of CH4 with respect to the normal coordinate Q 3 have opposite signs. In CaA below 120 K the evolution is not clearly understood.