Theoretical Research Of Intermolecular Organofluorine Hydrogen Bonds

Organofluorine compounds possessing unique physicochemical properties are widely used in pharmaceutical chemistry,agrochemicals,and functional materials.Even the organic fluorine-hydrogen bond strength is relatively weak,it plays an important role in the field of crystal packing,functional materials,medicinal chemistry,etc.The detailed study of organic fluorine hydrogen bonds is helpful to better understand the unique behavior of organic fluorine,and effectively regulate the application of organic fluorine hydrogen bonds in the above various fields.The purpose of this thesis is to systematically study the effects of various factors such as substituents,molecular configurations,and electronic structures of C atoms on the strength of organic fluorine-hydrogen bonds based on ab initio quantum chemical methods to provide a theoretical basis for the synthesis of functional materials and drugs.This paper is divided into the following four chapters:In chapter 1,we introduced the background and significance of the research,introduces the types of intermolecular interactions,the research status of organic fluorine hydrogen bonds and our research plan.In chapter 2,the second-order Moller-Plesset perturbation method and a serial of analytical methods used to study the properties of weak interactions were introduced.Such as electrostatic potential(ESP),electron density topological analysis(AIM),natural bond orbital theory(NBO),non-covalent interaction index(NCI),and Energy Decomposition Analysis(EDA)method for localized molecular orbitals.In chapter 3.an investigation of C-F…H-O hydrogen bonds on the complexes CHnXCHnF…H2O(n=0.1,2;X=H,F.Cl,Br)was performed at the MP2/aug-cc-pVTZ level.We found that the electron-withdrawing halogen substituents on the vicinal carbon reduce the ability of the F atom to act as a hydrogen bond acceptor and the effect of vicinal F.Cl.and Br substituent on the strength of C-F…H-O is comparative.Based on the ESP analysis,we found that the electron-withdrawing halogen substituents(F/Cl/Br)on the vicinal carbon cause the fluorine atom to be less ESP minimum value to the same degree.The position of the substituents on the vicinal carbon affects the strength of the C-F…H-O interaction.The electron withdrawal of substituents with stagger conformation in 1-fluoro-ethane and cis configuration in 1-fluoro-ethene much weakens the interaction of C-F…H-O due to the hyperconjugative interaction between?(C-F)and ?*(C-X)compared to other structures.We found that there is a good linear relationship between electron density at the BCP of the F…H and wiberg bond indexes(WBI)as well as between natural bond-bond polarizability(NBBP)and WBI,which indicates that the magnitude of NBBP and WBI could be a good indicator of the strength of the hydrogen bond.In chapter 4,ab initio calculations were performed on CH.XCHnF…HF(n=1,2;m=n,m=n-1;X=H,F)and CHnF…HF(n=2,3)at the MP2/aug-cc-pVTZ level to investigate the effects of the electronic structure of carbon atom in C-C-F on the organofluorine hydrogen bonds.Different with the conventional hydrogen bonds,we found that instead of carbon rehybridization and hyperconjugative effects,the magnitude of fluorine atomic volume in these systems plays important roles in determining the strength of the C-F…H-F organofluorine hydrogen bonds;the carbanion-mediated organofluorine hydrogen bond could be very strong,which is contrary to the notion that the organofluorine hydrogen bonds are weak hydrogen bonds.The lone pair electrons at both the proximal and the vicinal carbon remarkably strengthen the C-F…H-F hydrogen-bond interaction to ca.20 to 25 kcal/mol.Due to the high electronegativity of fluorine atom,it easily attracts the excess electron from the proximal and vicinal carbon,which results in the increase of its volume and negative charge.The large volume of fluorine atom gives rise to the large polarizability energy.and the large negative charge favors the large electrostatic ones,both of which contributed to the high interaction energy.In these organofluorine hydrogen-bond complexes,the electrostatic contribution is more dominant with the energy two times higher than the polarizability energy.