Theoretical Study Of Blue-shifting Hydrogen Bond Between π-conjugate Hetero-cyclic Molecules And Some Proton Donors

In present paper, ab initio quantum mechanics method is employed to investigate the origin of red-shifting and blue-shifting hydrogen bond between furan, thiophene, pyrole, pyridine and HCl, C₂H₂, HCX₃(X=F, Cl, Br, I). The main contents are the following:1. 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 Cl—H...N H-bond, which causes a large frequency red shift of 725 cm^-1 for the Cl—H vibration and an elongation 0.0495 A of this bond using the basis set 6-31G(d,p). Two H-bonds are formed between pyridine and HCCl₃: the C—H...N hydrogen bond with an elongation 0.0049 A of the C—H bond and a red shift of 80 cm^-1 for the C—H stretch vibration of HCCl₃, and the C—H...Ï€interaction with a contraction 0.003 A of the C—H bond and a blue shift of 58 cm^-1 for the C—H stretch vibration of HCCl₃ 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 C—H and Cl—H 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 C—H...N H-bond in pyridine—HCCl₃.2. The C—H...N and C—H...Ï€interaction between pyridine and HCX₃(X=F, Cl,Br, I) was investigated by means of quantum chemical method of high level ab initio calculations. For the C—H...N interaction classical H-bonds are formed with an elongation of C—H and a red shift. However, for the C—H...Ï€interaction blue shifting H-bonds are identified with a contraction of C—H and a blue shift. The result of vibrational spectral analysis indicates that it is impossible to confirm blue shifting or red shifting H-bonds by the derivative of permanent dipole moment with respect to C—H stretch of the proton donor only. The NBO analysis show that the competitions of hyperconjugation and rehybridization result in two kinds of H-bonds.3. Ab initio quantum mechanics method is employed to investigate intermolecular interactions between furan, thiophene, pyrole and pyridine as proton acceptor and acetylene and trifluoromethane at B3LYP/6-311++G(d, p), MP2/6-31G(d, p), MP2/6-31+G(d, p), MP2/6-311++G(d, p) levels. For compounds containing acetylene C—H...Ï€red shifting H-bond is formed with C—H bond elongation and a concomitant red shift. However, for compounds containing trifluoromethane, C—H...Ï€blue shifting H-bond is formed with C—H bond contraction and concomitant blue shift. The NBO analysis shows that the C—H bond length in C—H...Ï€is controlled by a balance of three main factors. C—H bond lengthening due toπ→σ*(C—H) hyperconjugative interaction is balanced by C—H bond shortening due to increase of s-character and polarization of the C—H bond and redistribution of electron density in proton donor. In compounds containing acetylene, hyperconjugative interaction dominates which results in red shifting H-bonds. In compounds containing trifluoromethane, the condition is reverse which results in blue shifting H-bonds.