A theoretical study of blue-shifting hydrogen bonds in π weakly bound complexes

In this work, a theoretical study about vibrational blue-shifting hydrogen bonds in pi weakly bound complexes formed by acetylene (C2H2 center dot center dot center dot HCF3), ethylene (C2H4 center dot center dot center dot HCF3), cyclopropene (C3H4 center dot center dot center dot HCF3), tetrahedrene (C4H2 center dot center dot center dot HCF3) and fluoroform (HCF3) is presented. In these systems, the formation of the (pi center dot center dot center dot H) interaction occurs through the charge transfer from hydrocarbons to fluoroform via contact between their pi bonds and hydrogen atoms, respectively. By taking into account calculations performed at the B3LYP/6-311++G(d,p) level of theory, geometry results indicate a shortening of H-C bond of HCF3, where in infrared vibrational analysis, this structural observation is known as a blue-shifting stretch mode. In other words, it was observed that the H-C stretch frequency is shifted to upward wavenumbers accompanied by a reduction on the absorption intensity. Energetically, these pi complexes are weakly bound because their intermolecular energies are very low, varying from -1.3 kJ mol(-1) to -4.7 kJ mol(-1). Moreover, a theoretical explanation for blue-shifts on H-C bonds of the fluoroform was presented through the evaluation of the ChelpG atomic charges, by which the quantification of charge transfer was used in order to justify the strengthening on (pi center dot center dot center dot H) hydrogen bond as follows: C2H2 center dot center dot center dot HCF3 > C2H4 center dot center dot center dot HCF3 > C3H4 center dot center dot center dot HCF3 > C4H2 center dot center dot center dot HCF3. (C) 2009 Elsevier B.V. All rights reserved.