Model Calculations on the Influence of Mechanical and Electrical Anharmonicity on Infrared Intensities: Relation to Hydrogen Bonding

Abstract

The influence of mechanical and electrical anharmonicity on the intensity of X—H stretching vibrations is investigated by means of model calculations in relation to hydrogen bonding. It is found that while both kinds of anharmonicities have a significant effect on the intensity, they cannot explain the characteristic increase in intensity of the fundamental and the equally characteristic decrease in the intensity of the first overtone. It is shown that not only the first but both these phenomena are due to the large value of the first derivative of the dipole moment in hydrogen bonded systems. This is linked to the possibility that the effect of electrical anharmonicity (d²μ/dQ²) might be cancelled by a high value of (dμ/dQ)_c in the expression of the overtone intensity.