Hydrogen bonds anharmonicity influence

T. Di Paolo, C. Bourderon, and C. Sandorfy, Model calculations on the influence of mechanical and electrical anharmonicity on infrared intensities Relation to hydrogen bonding. Can. 

Apart from the bilinear connection between acoustic and optical vibrations, the usual phonon anharmonicity takes place in the chain. The acoustic vibrations modulate the distances between A atoms and thus influence the frequency of proton vibrations on hydrogen bonds. The proton vibration frequency can be presented as... 

Thus, the adduced estimation shows that under the indicated values of parameters the anharmonicity can appreciably influence the proton current in compounds with hydrogen-bonded chains. 

V. Stefov, Lj. Pejov, B. Soptrajanov. The influence of NH... n hydrogen bonding on the anharmonicity of the n(N-H) mode and orientational dynamics of nearly continuously solvated indole. J Mol Struct 555 363-373, 2000. 

The shift to lower frequencies of the A-H stretch (ws) band is very often taken as the primary characteristic of a hydrogen bond. The maximal shifts of Vj bands may be as much as 3000 cm , as observed in some acid salts of carboxylic acids. The frequency shift results from the anharmonicity increase of the A-H stretching potential itself, and the anhar-monic coupling of the Vs and v modes. The influence of anharmonicity on the IR spectral changes of A-H stretching bands has been studied by Sandorfy and co-workers. Their results show that the strength of the hydrogen bond determines the amount of anharmonicity. 

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