Theoretical Research On The Halogen Bonds

Halogen bonding is the noncovalent intermolecular interaction between halogen atoms (lewis acids) and neutral or anionic Lewis bases. Most of the energetic and structural features found in the hydrogen-bonded complexes are reproduced in halogen-bonded complexes as well. By virtue of its strength selectivity, and directivity, halogen bonding has led to a number of applications in fields as diverse as molecualr recognition, enantiomer's separation, crystal engineering, and supramolecular architectures. Particularly, the utilization of this specific interaction in the context of drug design is nowadays coming to light. Based on the similarity of the hydrogen bond and halogen bond, one Lewis base can supply electrons to the hydrogen atom or halogen atom, so competitive effect can be found between hydrogen bond and halogen bond. There may exist two or more intermolecular interactions in one complex, and these interactions influence each other:both become stronger, or weaker, or one be stronger and one be weaker. Surely this cooperativity can be found between hydrogen bond and halogen bond. In the formation of R-X…Y halogen-boned complex, the length of the R-X bond can be shortened or lengthened, with the vibrational frequency shift. The former can be called blue-shift halogen bond, and the latter is referred to as red-shift halogen bond. The substituent group plays a very important role in the halogen-boned complex. Generally speaking, electronwithdrawing group weakens the electrondonating ability of the Lewis base and enhances the positive electrostatic potential of the halogen atom; electrodonating group enhances the electrondonating ability of the Lewis base and weakens the positive electrostatic potential of the halogen atom. The thesis is of quantum chemical study on the competitive and cooperative effects between hydrogen bonds and halogen bonds, the chemical origin of the blue-shift halogen bonds, and the influence of the substituent group on the strength of the halogen bonds.The main research contents of this thesis are as follows:1. The H…πand X (X=F, Cl, Br, I)…πinteractions between hypohalous acids and benzene are investigated at the MP2/6-311++G(2d,2p) level. We investigated the molecular electrostatic potential of the hypohalous acids and the interaction energies of the complexes. NBO analysis and AIM analysis have been performed to study the orbital interaction, the charge transfer and the topological properties. The main conclusion are as follows:(1) HOCl,HOBr,and HOI can form hydrogen and halogen-bonded complexes with C6H6, but there is only hydrogen bonding interaction between HOF and C6H6. This can be attributed to the electrostatic potential of the hypohalous acids. HOF has only one positive sites and the other hypohalous acids have two. (2) The strength of the H…πinteractions is contradictory to the MEP sequence, which is dominated by the dispersion interaction; the strength of the X…πinteractions follows the MEP sequence, which is dominated by the electrostatic interaction. (3)There exists competition between hydrogen bonding and halogen bonding. The hydrogen bonding is stronger than the halogen bonding except the HOI…C6H6 complex. (4) There exists charger transfer from C6H6 to HOX, and BCPs can be found between hydrogen or halogen atoms and carbon atom.2. The cooperativity between hydrogen and halogen bond in the XY…HNC…XY (X, Y=F, Cl, Br) complexes have been studied at the MP2/aug-cc-pVTZ level. Two hydrogen-bonded dimers, five hydrogen-bonded dimers, and ten trimers were obtained. The hydrogen-and halogen-bonded interaction energies in the trimers are larger than those in the dimers, indicating both the hydrogen bonding and the halogen bonding are enhanced. The binary halogen bonding plays the most important role in the ternary system. The hydrogen bonding influences the magnitude of the halogen bonding interaction much more than the hydrogen bonding in the trimers with respect to the dimers. Our calculations are consistent with the conclusion that the stronger noncovalent interaction has a bigger effect on the weaker one. The variation of the vibrational frequency in the HNC molecule has been considered. The NH antisymmetry vibration frequency has a blue shift, whereas the symmetry vibration frequency has a red shift. A dipole moment enhancement is observed in the formation of the trimers. The variation of the topological properties at BCP have been obtained using the AIM method, which is consistent with the results of the interaction energy analysis.3. We calculated a series of halogen-bonded complexes at the MP2/aug-cc-pVDZ level. The halogen bond donor is CFnH3-nCl, and the halogen bond acceptors are NH3, H2O, H2S, and Br-. Ten stable halogen-bonded complexes were obtained and the C-Cl bond length was contracted in all of these complexes. We carried out AIM and NBO analysis under the MP2/aug-cc-pVDZ optimized structures. The contracted C-Cl bond implies the enhancement of the bond strength. The variation of the electron density at the bond critical point of C-Cl bond correlates well with△r(C-Cl). A balance among intra-and inter-hyperconjugation and rehybridization determined the contracted C-Cl bond.4. We investigated the MenH3.nY(Y=N, P; n=0,1,2,3)…XF(X=Cl, Br) halogen-bonded complexes at the MP2/6-311++G** level. We discussed the electrondonating ability of 0, S, N, P from two aspects:molecular electrostatic potential and the orbital energies of the lone pairs, and some corresponding results can be explained. Compared with the 0 and S series, the halogen-bonded complexes containing N and P are very different. Very large amount of charge transfer and increment of dipole moment have been obtained in our calculations. The F-X bond length in the MenH3-nP…XF(X=Cl, Br) complex is larger than the corresponding MenH3-nN…XF(X=Cl, Br) complex. Deformation energy must be added to correct the interaction energy in order to obtain the regular intermolecular strength.5. We investigated the enhancing effect of halogen bonding induced by the transition metal Cu, Ag, and Au in the M-CH2-X…CIF (M=Cu, Ag, Au; X=F, Cl, Br) complexes. We obtained two optimized Cu-CH2-X…CIF and Ag-CH2-X…ClF structues, and only one Au-CH2-X…CIF structure. The comparison of the interaction energy between these complexes and the CH3X…CIF complex indicates that the substitution of the transition metal enhances the strength of the halogen bonding. The NBO results indicates that there exists an interaction between the lone pairs of the halogen atom and the antibonding orbital of the F-Cl and C-M bond. AIM analysis have been carried out to disclose the nature of these noncovalent interactions.