Theoretical Study On Chemical Reactive Sites And Molecular Structures With Their Properties

Various parameters of molecules can be easily and conveniently obtained through quantum chemistry calculations. Physical or chemical properties of molecules can be described, explained and determined by wavefunction analysis methods. Recently, theoretical study on chemical reactive sites and molecular structures with their properties has been becoming a research hotspot. A large number of theoretical methods used to predict reactive sites have been proposed in recent years, but systematical comparisons and examinations of their validities are still absent unfortunately. Further, correlations between the predicted values and actual reaction rates should also be examined and investigated systematically. In the study on molecular structures with their properties, Traditional theoretical models of molecular structure are often based on the differences of electronegativity between atoms which participate in forming chemical bonds, but the environmental differences among different chemical systems have not been premeditated. So relative theoretical methods still need to be developed and consummated. Main contents of this work are as follows:First, four categories of molecules:carbonyl compounds, aromatic compounds, pyridine derivatives and heterocyclic compounds have been tested. 14 methods of predicting reactive sites have been used and comparisons in detail have also been executed. We found that methods which can reflect the local electron softness, such as condensed dual descriptor, give very good predicting results. But methods which reflect the electrostatic potential, such as electrostatic analysis and atomic charge analysis, often give poor results. For the tested system in this article, the best methods to predict reactive sites are condensed dual descriptor and Hirshfeld atomic charge analysis.Second, based on the previous work,7 methods that can preciously predict reactive sites of nucleophilic and electrophilic reactions have been chosen to analyze the relativities between experimental reaction rates and theoretical predicting results. The results showed that, for aromatic compounds, methods that can reflect the local electron hardness, such as the electrostatic potential above 1.6A of van der Waals surface of molecules and Hirshfeld atomic charge, give good results to describe the relative size of reaction rates. But for methods which can reflect the local electron softness, such as condensed Fukui function and condensed dual descriptor, poor results appear.Third, relationship between bond angles of polar molecules in ground state and bond dipole moment. We use ADCH atomic charge to evaluate the value bond dipole moment, electron localization function and values of localization function at bond critical point to analyze the electronic structure of chemical bonds. Through the analysis of bond angles and bond dipole moment data of covalent molecules and cyclic molecules (both in ground state) formed by the IVA group elements(C, Si, Ge), VA group elements(N, P, As), VIA group elements(O, S, Se), VIIA group elements(F, Cl, Br), under the similar electronic structure of bonds, we found that bond angles increases as the bond dipole moment goes up due to the repulsion effect of bond dipole moment. This discovery may deepen our knowledge of geometric construction of molecules.Last, a new energy extrapolation method has been proposed. Configuration interaction calculation in complete active space is related to the numbers of active electrons and orbitals. However, the configuration interaction energy is not a monotonically decreasing function of these two variables. Thus, the numbers of active electrons and orbitals are not proper variables to be used to extrapolate the configuration interaction energy. In order to solve this problem, we defined a new variable:the maximum number of unoccupied orbitals in the complete active space. We made a series of configuration interaction calculations on singlet, doublet and triplet molecules and simulated their ground state energies with the number of active electrons and the number of maximum unoccupied orbitals. The mean square root errors of these simulations are at the order of 10-6. The accuracy of the extrapolated energies is better than that of MP4 and is also better than that of CCSD for small molecules. The extrapolated full configuration interaction energies are very close to the energy values of full configuration interactions. Furthermore, the extrapolated energies are exploited to optimize the bond distances of several diatomic molecules and to compute the harmonic vibrational frequencies. Their accuracies are better than that of the complete active space self-consistent field.