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1 Polish Journal of Chemistry

1 2007

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Infrared and Raman Intensities in Proper and Improper Hydrogen-Bonded Systems

Jabłoński M., Sadlej A.

Polish Journal of Chemistry

2007

Vol. 81, nr 5-6

767-782

The IR intensities and Raman activities of the C–H stretchingmode as well as some other spectroscopic and structural features of NCHźźźNCH and F3CHźźźNCH are calculated in the MP2 approximation with a variety of correlation consistent basis sets. In agreement with earlier studies the traditional hydrogen bond in NCHźźźNCH results in the elongation of the C–H bond distance in the proton donor and the red shift of the corresponding vibration frequency. The opposite changes are found in F3CHźźźNCH. The IR intensity of the C–H stretching band in the NCHźźźNCH dimer markedly increases whereas for the blue-shifting F3CHźźźNCH complex the IR intensity of the corresponding band is significantly reduced. The Raman activity of the C–H stretching band in both investigated complexes increases upon the hydrogen bond formation. These results are analysed in terms of the interaction induced dipolemoments and polarizabilities, and their derivatives with respect to the C–H bond distance. The difference between IR intensities of the C–H stretching band calculated for NCHźźźNCH and F3CHźźźNCH is, however, a particular feature of the F3CHźźźNCH complex and cannot be used to distinguish between traditional and improper hydrogen bonds. Neither the Raman activities offer such a possibility. The present study of interaction induced properties shows that the blue shift of the X–H frequency in systems with improper hydrogen bonds is primarily due to particular features of the electronic structure of the proton donor.