Ab initio quantum mechanics methods were applied to investigate the CH₃…Y bend hydrogen bonds in the complexes Cl^－…CH₃F, Cl^－…CH₃Cl, Cl^－…CH₃Br, Br^－…CH₃F, Br^－…CH₃Cl and Br^－…CH₃Br. The MP2 level of theory with the four basis sets 6-311＋＋G(d,p), 6-311＋＋G(2df,2p), 6-311＋＋G(3df,3pd) and QCISD/6-311＋＋G(d,p) and the standard gradient technique were used to optimize the geometries and analyze vibrational frequencies. The results show that there are three C—H…Y (Y＝Cl^－, Br^－) hydrogen bonds in each of the six complexes, and the C—H bond contracts with a concomitant blue shift of the corresponding stretching frequency. The natural bond orbital (NBO) analysis shows that four factors lead to the formation of these blue-shifted H-bonds: (a) intermolecular hyperconjugation in a bend H-bond configuration is very small and can be neglected; (b) there exists rehybridization; (c) the electron density in the Cl^－ or Br^－ lone pair is transferred not to σ*(C—H) but to the σ*(C—X) (X＝F, Cl, Br) antibonding orbital; (d) decrease of intramolecular hyperconjugation leads to decrease of occupancy in σ*(C—H). The analysis using the theory of atoms in molecules (AIM) shows that the topological properties of electron density for these trifurcated bent H-bonds have essential differences from those of usual H-bonds, where the bond critical point is not between H and Y, but between C and Y.

Key words: CH₃…Y bend hydrogen bond, blue-shifted H-bond, electron density topology, natural bond orbital