Quantum Chemical Studies On Bond Dissociation Energies And Hydrogen Bonding Application To Organocatalysis

A sound knowledge of Bond Dissociation Energy (BDE) is fundamental to understanding chemical processes. Using theoretical methods, BDEs of various compounds have been obtained. The structure-activity relationship has been studied. A new composite ab initio method has been developed and the precision for BDE calculation was improved. Organocatalysis is the hot spot in organic chemistry. The applications of hydrogen bonding interaction in organocatalysis were investigated,and the essence of the hydrogen bonding was further disclosed.Ten DFT methods were compared for calculating the carbon-hydrogen bond dissociation energies of hydrocarbons. It was found that the newly developed DFT method BMK gave the best results. Subsequently the carbon-hydrogen bond dissociation energies of 172 hydrocarbons including alkanes, unsaturated chain hydrocarbons, alicyclic hydrocarbons, and aromatic hydrocarbons were calculated systematically using the BMK method. The root of mean square error was as low as 7.95 kJ·mol^-1 and the correlation coefficient was 0.985. The NBO analysis revealed that the C-H BDEs of hydrocarbons were dependent on the hybridization of the parent compound, the hybridization of the radical, and the spin density of the radical. A good quantitative relationship among BDEs and the three parameters was found. Furthermore, a good correlation was observed between the carbon-hydrogen bond lengths and the C-H BDEs of alkanes as well as unsaturated hydrocarbons.On the basis of systematic examinations it was found that the BMK functional significantly outperformed the other popular density functional theory methods including B3LYP, B3P86, KMLYP, MPW1P86, O3LYP, and X3LYP for the calculation of BDEs. However, it was also found that even the BMK functional might dramatically fail in predicting the BDEs of some chemical bonds. To solve this problem, a new composite ab initio method named G3//BMK was developed by combining the strengths of both the G3 theory and BMK. G3//BMK was found to outperform the G3 and G3//B3LYP methods. It could accurately predict the BDEs of diverse types of chemical bonds in various organic molecules within a precision of ca. 1.2 kcal/mol.In the theoretical studies on hydrogen bonding application to organocatalysis, two kinds of the organocatalysis system by hydrogen bonding activation were studied using the MP2 method in detail, the one is ureas or thioureas with different carbonyl compounds, and the other is ureas or thioureas with imines. The structures of hydrogen bonding compounds were discussed and the NBO analysis further disclosed the essence of the hydrogen bonding interaction. In addition, the substituent effects were investigated. Two possible hydrogen bonding complexes conformations, cis and trans, were discussed and the energy gaps between them were analyzed, in combination with the catalytic reactions.