Theoretical Study Of Multifurcated Bent Hydrogen Bonds And Intermolecular Interaction

In present paper, ab initio quantum mechanics method is employed to investigate the multifurcated bent hydrogen bonds and intermolecular interaction. The results show that multifurcated hydrogen bonds are mostly blue shifting H-bonds. Topological properties of electron densities of these H-bonds are found to be very different from those of linear H-bonds. The main results are as follows:1. H-bonding angle∠YHX has an important effect on the electronic properties of the H-bond Y…HX, such as intra- and intermolecular hyperconjugations and rehybridization, and topological properties of electron density. We studied the bifurcated bent H-bonds of the proton donors H₂CO and H₂CF₂ with the proton acceptors Cl^- and Br^- at the four high levels of theory: MP2/6-311++G(d,p), MP2/6-311++G(2df,2p), MP2/6-311++G(3df,3pd) and QCISD/6-311++G(d,p), and found that they are all blue-shifted. These complexes have large interaction energies, 9～20 kcal mol^-1, and large blue shifts, Ar(CH) = -0.003～-0.006 (?) andâ–³v(CH) = 40～90 cm^-1. The topological properties of the electron density of the bifurcated H-bonds Y…H₂CZ are similar to those of the usual linear H-bonds, there is a bond critical point between Y and each hydrogen, and a ring critical point inside the tetragon YHCH.2. The intermolecular interactions between H₂PF, H₂POH, H₂AsF, H₂AsOH and (X, Br^-, HF, HCl, HBr were studied at the four levels of theory: MP2/6-311++G(d,p), MP2/6-311++G(2df,2p), MP2/6-311++G(3df,3pd) and QCISD/6-311++G(d,p), and found that in these compleces the HX(X=P,As) bonds are all blue-shifted. The complexes of the anion acceptors Cl^- and Br^- have large interaction energies, 25?100kJ/mol, and large blue shifts,â–³r(HX) =-0.006?-0.010(?) andâ–³v(HX) = 10～60cm^-1. The natural bond orbital analysis shows that blue shifts of the HX bonds in these systems are mainly caused by three factors: rehybridization; indirect intermolecular hyperconjugation, that electron density from n(Y) of the proton acceptors is transferred not toσ^*(PH, AsH), but toσ^*(PZ, AsZ) of the donors; intramolecular hyperconjugation. The topological properties of electron densities of these systems are found to be very different from those of usual H-bonds, that the intermolecular bond critical point, which represents a closed-shell interaction, is not between Y and H, but between Y and P(As), and so these interactions can not be exactly classified as H-bonding.3. Ab initio quantum mechanics methods were applied to investigate the PH₂…Y and AsH₂…Y intermolecular interactions between H₂PF, H₂POH, H₂AsF, H₂AsOH and N₂, CO, OC. 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 two P-H…Y or As-H…Y (Y= N₂, CO, OC) intermolecular interactions in each of the twelve complexes. AIM analysis shows that the topological properties of electron density for these intermolecular interactions have essential differences from those for usual H-bonds, where the bond critical point is not between H and Y, but between P and Y or As and Y, so these intermolecular interactions can not be classified as H-bonds. The NBO analysis shows that three factors lead to the formation of these intermolecular interactions: (1) intermolecular hyperconjugation in a bent configuration is very small and can be neglected; (2) the electron density in the N, 0, C lone pair is transferred toσ^*(P-Z) orσ^*(As-Z) (Z=F, O) antibonding orbital mostly. (3) there exists rehybridization.4. Ab initio quantum mechanics methods were applied to investigate the CH₃…Y bent hydrogen bonds in the complexes Cl^-…H₃CF, Cl^-…H₃CCl, Cl^-…H₃CBr, Br^-…H₃CF, Br^-…H₃CCl and Br^-…H₃CBr. The MP2 level of theory with the four basis sets 6-311++G(d,p), 6-311-H-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 NBO analysis shows that four factors lead to the formation of these blue-shifted H-bonds: (a) intermolecular hyperconjugation in a bent 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 toσ^*(C-H) but to theσ^*(C-X) (X=F,Cl,Br) antibonding orbital; (d) decrease of intramolecular hyperconjugation leads to decrease of occupancy inσ^*(C-H). AIM analysis 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.