Theoretical Study On Spin-spin Coupling Constants Of Directly Bonded Atoms

The structural information of molecular geometric structures and bonds between atoms can be obtained by study on nuclear magnetic resonance (NMR). Therefore, NMR is widely used in physics, chemistry and life science.In the past fifty years, NMR has been a valuable tool in the area of chemistry and biology. However, NMR has never been widely used. Along with the development of quantum chemistry, its simplicity makes it a stringent request that the chemist and the physicist can study the world through the nucleiThe chemical shift and the spin-spin coupling constant are two important parameters of NMR. By now, experimental values of chemical shifts have been very accurate and ab initio methods have also been greatly developed. Therefore, the calculation of spin-spin coupling constants has become a concerned problem of quantum chemists.Big bases, electronic correlation effects and high-level ab initio methods are needed to predict spin-spin coupling constants quantitatively. Nevertheless, good results still cannot be obtained, so reliable semi-empirical methods are the direction people are interested in. According to the equation in the relative references, theoretical calculation on spin-spin coupling constants of directly bonded atoms has been done.Spin-spin coupling constants of C-F C-P(III) C-P(V) C-H have beenstudied and four semi-empirical equations have been obtained. S-characters and net charges of some compounds have been introduced to these equations. The results are basically consistent with the experimental values. Therefore, we get the conclusion that this method can be used to predict and test the experimental values.Forty compounds that have OF bonds in their molecular formula have been divided into four groups. One group has ten, the second has twenty, the third has thirty and the last has forty compounds. Four equations have also been obtained. The results show that the number of the molecules in each has no big influence to the accuracy of the equations. Furthermore, two methods and two basis sets have been compared and there are no differences between the results of the equations. Therefore, the methods and basis sets have no influences to the veracity of the equation.At last, four compounds that are monomers of poly (aryl ether ketone) s have been chosen. They have been optimized and their IR spectra have been calculated. Their s-characters and net charges have been introduced to the equation of spin-spin coupling constants of the OF bond, the equation has been obtained. The results are fundamentally consistent with the experimental values. So we can expect that this method can be used to test the spin-spin coupling constants of the molecules we have known and to predict the constants of big molecules, especially the molecules in biologic area.