The Effect On Hydrogen Bonds Of Intramolecular Hyperconjugation

In the present paper,ab initio quantum mechanics method(MP2) is employed to investigate the origin of hydrogen bond among HNO,HNS and HCHO so on,especially the effect on hydrogen bonds of intramolecular hyperconjugation.The main contents are the following:1.Quantum chemistry ab initio methods were applied to study the blue- and red-shifted H-bonds between HNO and(HF)_{1≤n≤3}.In these systems,all of the F…H-N H-bonds are blue-shifted, and the blue shifts are caused by repolarization,rehybridization,and intramolecular hyperconjugation;all of the X…H-F(X = O,N,F) are red-shifted,intermolecular hyperconjugation leads to the red shifts.In these chains of H-bonds,the variations of intermolecular hyperconjugations, electron densities at the bond critical points(BCPs),and strength and frequency shifts of the H-bonds are correlated with each other and have definite rules.2.The blue-shifted H-bonds between HNO(HNS) and(HF)_{1≤n≤3} have been theoretically studied. The calculated results show that in the HNO systems the H-bonds NH…F lead to large contraction of NH bonds and large blue shifts of NH stretch frequency which increase with n,and that in the HNS systems the blue shifts of NH stretch frequency are much smaller than those in the HNO system with the same n.The topological analysis of electron density demonstrates that these interactions are indeed H-bonds.The natural bond orbital analysis shows that in the HNO systems the blue-shifted H-bonds are caused by repolarization and rehybridization and decrease of intramolecular hyperconjugation,but in the HNS systems the reason that the blue shifts of NH bonds are very small is that the ability of hyperconjugation donation and adjustability on electron density inσ^*(NH) of n(S) is much smaller than that of n(O).3.Ab initio quantum mechanics method(MP2) is employed to investigate the H-bonds in the dimmers and tripolymers of HCHO and HNO with the 6-311++G(d,p),6-311++G(2df,2p), 6-311++G(3df,3pd),aug-cc-pVDZ,aug-cc-pVTZ basis sets.In the four complexes,C(N)-H…O blue shifting H-bond is formed where C(N)-H bond contracts and the corresponding stretching frequency blue shifts larger than 87cm^-1.The topological properties of the electron density of the H-bonds C(N)-H…O are usual linear H-bonds,there is a bond critical point between Y and each hydrogen,and a ring critical point inside each complexes.The NBO analysis shows that three main factors lead to the formation of these blue-shifted H-bonds:(a) intermolecular hyperconjugation is very small and can be neglected;(b) decrease of intramolecular hyperconjugation leads to decrease of occupancy inσ^*(C-H) andσ^*(N-H);(c) there exists rehybridization.