ABINITIO DETERMINATION OF CHEMICAL SHIELDING IN A MODEL DIPEPTIDE

Ab initio chemical shift calculations have been carried out on planar model forms of the simple dipeptide glycylglycine with both balanced 6-311G(d, p) and mixed (locally dense) 6-311G(d, p)/3-21G bases. Both the structure and the calculated shifts show internal hydrogen bonding to occur in the zwitterionic form, while no such effects are seen when calculations are performed on the crystal structure hydrochloride whose carboxyl group is non-planar. The locally dense basis set approach on the amide group nuclei provides results that agree well with those using the balanced and larger basis set, and provides a considerable saving of CPU time.