Substituent Effects On Small Molecules Of Diazene And Imine And Theoretical Studies On Hydrogen Bonding In HNNOH System

At first, B3LYP method is applied to study substituent effects on diazene derivatives(HNNR) and imine derivatives (H2C=NR and HN=CHR), R=H, F, OH, OCH3, OCF3, NH2, NO2, NO, CHO, CN, CH3, CF3. These simple molecules could be taken as prototype of large diazene and imine compounds. Natural bond orbital and Atoms in Molecules are employed to analyze the substituent effects on geometrical structures, atomic charges, relative stabilities of isomers and trans/cis isomerization reactions. In addition, isodesmic reaction is used to study the substituent effects on stabilities of the diazene and imine molecules. In order to choose reliable computational level, eight methods are applied to optimize trans and cis forms of five diazene molecules, respectively using eight basis sets. With comparison of computation, B3LYP/6-311++G** is selected as computational level in this part. In the study, we pay more attention to the correlation of the molecular properties with the Hammett substituent constants and the electronegativities, while the correlation could provide some direct information for the experimenters. In the second part, MP2 method is employed to study the effects of conformation of HNNOH monomer and dimmer on correlation of interaction energy with parameters describing hydrogen bond in dimmer systems. Our observations show that the conformation of the complexes and the constituting monomers could affect the correlation, even break down it. The result suggests that we should consider the effect of conformation when applying parameters describing hydrogen bond to predict stabilities of complex. In addition, the eight topological parameters derived from AIM are applied to determine the hydrogen bond, which is helpful to understand the nature of hydrogen bonding. On the other hand, we discuss effect of basis set superposition error on the geometrical structures and interaction energies of complexes. Our results reveal that it is necessary to use counterpoise-corrected gradient procedure to study intermolecular interactions.In the third part, MP2 and B3LYP methods are employed to study the relative stabilities of conformers of N-Cl glycine using 6-311++G** basis set. Single-point MP4 calculations using the same basis set are carried out in order to confirm the reliabilities of B3LYP and MP2 on energies and relative stabilities of glycine derivatives. NBO and AIM methods are used to analyze hyperconjugative interaction between the molecular orbitals and the nature of bond as well as intramolecular hydrogen bonds. Our observation indicates that the origin of relative stability comes from the balance of the hydrogen bond and the hyperconjugative effect. These observations provide some useful information to understand the effect of electronegative group on the conformation of glycine, further on conformation of protein.