Evaluation of NMR Chemical Shift by Fragment Molecular Orbital Method

NMR chemical shifts are evaluated by the Fragment Molecular Orbital (FMO) method. Several practical approximations for the method are tested for water oligomer and poly-chloroprene. The FMO method reproduces the value by the ab-initio method without approximation and the experimental values. The adjacent approximation in fragment pair correction is found to be useful for qualitative prediction of NMR chemical shift in large bio-molecular systems, while monomer approximation fails in predicting the shielding constants of nuclei especially near the division for fragmentation. Preliminary application of the method with low level basis set to a bio-molecule indicates encouraging results for using the method as a theoretical tool in molecular biology.