Theoretical Studies Of Magnetic Resonance Shielding Constants

Nuclear magnetic resonance(NMR)is so important that it has become a standard technique for giving key information about the structure and composition.The theoretical calculation of NMR parameters is a necessary tool for the prediction of NMR spectroscopy.Moreover,the methods and technology continue to develop to enlarge the limit for size of molecules.In recent years,more and more combination methods have been applied to the theoretical calculation of the NMR shielding constants.The FPA-M(Focal point analysis for magnetic properties)method has successfully calculated the shielding constants of various small molecular systems,and its accuracy has reached the level of CCSD(T)/CBS.However,the FPA-M method need to calculate the shielding constants at MP2/cc-pV5Z and CCSD(T)/cc-pVTZ,and it can only calculate the systems with six heavy atoms.This work consists of the following parts:(1)On the base of FPA-M,two methods which can reduce the computational cost are studied:(a)The FPA-M-HF method is obtained by replacing the MP2 method with the HF method.The results show that the FPA-M-HF method can accurately calculate the shielding constants for 13C,15N,170 and 19F atoms,and its accuracy with cc-pCVnZ and pcJ basis stes is better than CCSD(T)/cc-pV5Z;with cc-pVnZ basis set it can achieve the best balance between calculation cost and accuracy,(b)The FPA-M-D method is obtained by replacing the VTZ basis set with VDZ basis set.The results show that this method can greatly reduce the computational complexity,but its accuracy is lower than CCSD(T)/cc-pV5Z.(2)In order to decrease computational cost and further enhance computing speed,FPA-M-H method is put forward.The nuclear magnetic resonance value approximates the HF/CBS number adding the[?e(CCSD(T))-?e(HF)]difference.Then,the ?e(CCSD(T))can be obtained by a lower basis set(e.g.cc-pVDZ)at CCSD(T)level adding a correction term,decided by a difference between large(e.g.cc-pVTZ)and low basis set(e.g.cc-pVDZ)at MP2 level.This composite method can reach the accuracy of CCSD(T)/cc-pV5Z for 13C,15N,17O and 19F atom and can calculate the system containing 16 heavy atoms.This is of great significance to further guide the calculation of biological acromolecules.(3)The FPA-M-H and the XO method(extended ONIOM method)developed by our research group are effectively combined to calculate the shielding constant of macromolecules.In the XO method,the boundary effect can be reduced by adjusting the size of the molecular fragments to overlap.In this paper,the optimal block scheme of six macromolecules is calculated and the shielding constants are calculated,which demonstrate the usefulness of this composite method to predict NMR of macromolecules.