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

摘要

The IR intensities and Raman activities of the C-H stretching mode as well as some other spectroscopic and structural features of NCH...NCH and F3CH...NCH are calculated in the MP2 approximation with a variety of correlation consistent basis sets.In agreement with earlier studies the traditional hydrogen bond in NCH...NCH 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 F3CH...NCH.The IR intensity of the C-H stretching band in the NCH...NCH dimer markedly increases whereas for the blue-shifting F3CH...NCH 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 dipole moments 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 NCH...NCH and F3CH...NCH is,however,a particular feature of the F3CH...NCH 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.