The Applications Of Valence Bond Theory To Intra- And Inter- Molecular Electron Transfers

The theoretical study of electron transfer(ET)has always been one of the most important research topics in broad areas.One of the key issues in ET is the construction of the diabatic states,which is also a great challenge,as the perfect diabatic state does not exist.As two basic theories in quantum chemistry,molecular orbital(MO)theory is more popular than valence bond(VB)theory currently,because the former is more efficient than the latter.However,the delocalized nature of MO methods leads them more difficult to construct the charge-localized diabatic states.Differently,VB theory starts from nonorthogonal localized orbitals to construct strictly localized wavefunction,which is viewed as the most adequate approach to construct diabatic states.As the simplest variant of ab initio VB theory,block-localized wavefunction(BLW)method can selfconsistently derive the wavefunction for a strictly electron-localized diabatic state and incorporates the efficiency of MO theory.To this end,in this doctoral dissertation,we firstly proposed a new approach to construct the diabatic states explicitly by using ab initio VB methods(chapter 2).Secondly,we studied the nature of the resonance-assisted hydrogen bond(chapter 3)and its application in biological systems(chapter 4-5)by using BLW method.Finally,the role of hyperconjugation in Agostic effect was examined(chapter 6).In Chapter 1,we briefly reviewed the research background and theoretical methods involved in this dissertation.In Chapter 2,a novel approach is proposed to construct the charge-localized diabatic state explicitly within the framework of ab initio VB theory,namely VB blockdiagonalization approach(VBBDA).It is able to produce high-quality diabatic states with a short and compact VB structure expansion.Therefore,we can evaluate the accurate electronic coupling compared to the high-level methods but with less computational effort.Moreover,VBBDA can also be applied to obtain the whole potential energy curves along the reaction coordinates and further to locate the crossing point between two diabatic states where ET occurs.In Chapter 3,we applied the BLW method to study a series of exemplary intramolecular hydrogen bonding,which are linked by ?-conjugation.Intramolecular resonance-assisted hydrogen bond(RAHB)is much stronger than conventional one and it has been extensively accepted and applied both theoretically and experimentally.However,its nature and the role of the resonance is still controversial.Here,we clarify that the RAHB origins from the electron movement from donor to acceptor through resonance.Accordingly,we also provide a direct proof of the resonance-impaired hydrogen bond(RAHB),where the electron moves reversely from acceptor to donor.In Chapter 4,we examined the roles on hydrogen bonding in the association of amides and imides by using BLW methods.As one of the governing forces of protein folding and the structures of nucleic acids,the hydrogen bonds in amide and imide dimers exhibit very different strengths.It was firstly explained by repulsive secondary electrostatic interactions(SEI)due to spectator carbonyl group in imide.However,SEI can rationalized the energetic change but failed to explain the structural difference.Our computational results reveal that all ? induction effect(IE),? resonance effect(RE)along with the SEI are factors are responsible to the different binding energies in the dimers of amides and imides.In Chapter 5,the multiple hydrogen bond in self-assembled systems exhibit very different stability though they share the same numbers and similar types of hydrogen bond.For decades,the secondary electrostatic interaction(SEI)has been regarded as the fundamental cause for the relative strengths of multiple hydrogen bonds.Our computations show that the multiple hydrogen bond in self-assembled complexes is a kind of resonance-assisted hydrogen bond(RAHB)in nature,in which ? resonance only occurs on the monomer.More importantly,the hydrogen bond strengths become nearly identical for various multiple hydrogen bonded dimers with resonance "shut down".In other word,the resonance effect is the driving force for the relative strength of multiple hydrogen bonds.In Chapter 6,the Agostic effect in transition-metal complexes is explored by generalized BLW method.Agostic effect represents the interaction between the metal center and a C-H group on a ligand.It is one of the most important discoveries in the field of organometallic chemistry,because it can activate the inert C-H group.By using the BLW method,we present that the Agostic effect is interpreted in three terms,including the largely destabilizing steric term,stabilizing charge transfer and dispersion terms.Besides,the present study also clarifies the variable roles of the three effects in the Agostic phenomenon.The study is very important for us to understand the nature of the activation of C-H group in transition-metal complexes.In brief,the importance of our works is that we succeed in applying ab initio VB theory to study ET reactions and ET-driven non-covalent interactions systematically.This is achieved by our developed VBBDA and BLW methods respectively,which can derive the strictly charge-localized diabatic wavefunctions.