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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.

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