Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT

ABSTRACT - Internet Electron. J. Mol. Des. June 2006, Volume 5, Number 6, 331-344

Resonance Energy and Intramolecular Hydrogen Bond Strength in 2-Halo-Phenols and 2,4-Dihalo-Malonaldehyde. A Theoretical Study Giuseppe Buemi Internet Electron. J. Mol. Des. 2006, 5, 331-344

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Abstract:
The aim of the paper is to examine the ring opening, ring closure and π-delocalization energy terms of the title compounds in order to account for the different trend of the hydrogen bridge strength with the halogen atom electronegativity on changing the basis set adopted for its evaluation. Such terms were obtained through partition of the hydrogen bond energy according to an extension of the approach suggested by Grabowski. Geometries and energies were calculated by the ab initio method using several basis sets of different extension and the G03W computation package. Analysis of the obtained results evidences that the ring opening energy terms predicted by the simplest basis sets show opposite trend with respect to those predicted by the more extended bases. The ring closure energies correlate very well with the hydrogen bond strength. It seems that the less extended bases overestimate the energy of the strained chelate ring. Excellent agreement is found between the experimental stretching mode frequencies of the OH group and the theoretical ones calculated at B3LYP/6-311++G(d,p) level. The HOMO surfaces of the chelate and open conformations are also presented. E_HB correlates with the OH frequency shifts better than with the electronegativity of the halogens series and agree with the experimental estimates deduced from the temperature dependence of the infrared OH stretching band intensities better than with those estimated from the far IR OH rotational frequencies.

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