Abstract: Ab initio quantum mechanics method(MP2) is employed to investigate intermolecular interactions in the complexes of C₅H₅N…HCl(1), C₅H₅N…HCCl₃(2) and C₅H₅N HCCl₃(3) at the 6-31G(d,p), 6-311+G(d,p), aug-cc-pVDZ basis sets. For compounds 1 and 2, Cl—H…N(C—H…N) red shifting H-bond is formed with Cl—H(C—H) bond elongation and a concomitant red shift of the corresponding IR stretching frequency. However, for compound 3, C—H…π blue shifting H-bond is formed with C—H bond contraction and concomitant blue shift of the corresponding IR stretching frequency. It is indicated that both Cl—H and C—H stretch increases in intensity and broadens in the spectral band on formation of compounds. It is concluded that a negative permanent dipole moment derivative of the proton donor is not a necessary condition for the formation of the blue shifting hydrogen bond. The NBO analysis shows that the X—H bond length in X—H…Y is controlled by a balance of three main factors. X—H bond lengthening due to n(Y)→σ*(X—H) hyperconjugative interaction is balanced by X—H bond shortening due to increase of s-character and polarization of the X—H bond and redistribution of electron density in proton donor. In compounds 1 and 2, hyperconjugative interaction dominates which results in red shifting H-bonds. In compound 3, the condition is reverse which results in blue shifting H-bonds.

Key words: Ab initio calculation, Red shifting H-bond, Blue shifting H-bond, Natural bond orbital analysis