Ab initio investigation of hydrogen bonds between pyridine and HCl, CHCl3

The hydrogen bonds of HCl and HCCl3 as the proton donors with pyridine as the acceptor were studied at the MP2 level of theory using the five basis sets 6-31G(d,p), 6-311+G(d,p), 6-311++G(d,p), 6-311++G(2df,2p) and AUG-cc-pVDZ. Pyridine and HCl can only form a ClH…N H-bond, which causes a large frequency red shift of 725 cm−1 for the ClH vibration and an elongation 0.0495 Å of this bond using the basis set 6-31G(d,p). Two H-bonds are formed between pyridine and CHCl3: the CH…N hydrogen bond with an elongation 0.0049 Å of the CH bond and a red shift of 80 cm−1 for the CH stretch vibration of CHCl3, and the CH…π interaction with a contraction 0.003 Å of the CH bond and a blue shift of 58 cm−1 for the CH stretch vibration of CHCl3 using the basis set 6-31G(d,p). In these H-bonds, regardless of which are red-shifted or blue-shifted, the IR intensities of the CH and ClH stretch vibrations increase, and the permanent dipole moment derivatives of the proton donors are positive. The natural bond orbital analysis was carried out, and the concepts of hyperconjugation and rehybridization and the theory of Hobza were applied to account for the origin of these hydrogen bonds. A post-Hartree-Fock wavefunction containing electron correlation in the analysis of the natural bond orbital is required for interpreting the CH…N H-bond in pyridineCHCl3.