Infrared Spectra of H-Bonded Systems : Anharmonicity of the H-Bond Vibrations in Cyclic Dimers

Abstract

We present a theoretical simulation of the infrared spectra of the stretching vibrations of the H atom in hydrogen‐bonded carboxylic acid dimers. We have refined a model previously proposed where the H‐atom stretching vibration in an H‐bond X–H···Y is coupled to the “hydrogen‐bond vibration” X^→–H^→···Y^← (we suppose that the H‐atom vibration is harmonic with a frequency of oscillation depending on the X···Y distance), by the introduction of an anharmonic term (Morse potential) in the X^→–H^→···Y^← motion. This model gives a value for the frequency of oscillation of the X^→–H^→···Y^← motion in agreement with the observed value (in contrast to the previous model where the X^→–H^→···Y^← motion was assumed to be harmonic), simulates in terms of delta functions the experimental spectrum rather well, and predicts the isotope effect completely. We conclude that the coupling of the H‐atom stretching vibration with the hydrogen‐bond vibration X^→–H^→···Y^← together with the strong anharmonicity of the latter motion are characteristic features of stretching vibrations in hydrogen bonds, at least for weak and intermediate hydrogen bonds.