Accurate (13)C and (15)N Chemical Shift and (14)N Quadrupolar Coupling Constant Calculations in Amino Acid Crystals: Zwitterionic, Hydrogen-Bonded Systems.

Abstract

EIM (embedded ion method), cluster, combined EIM/cluster, and isolated molecule (13)C and (15)N chemical shielding and quadrupolar coupling constant (QCC) calculations at the B3LYP level with D95**, D95++**, 6-311G**, and 6-311+G** basis sets were done on the amino acids l-alanine, l-asparagine monohydrate, and l-histidine monohydrate monohydrochloride and on the two polymorphs α and γ glycine. The intermolecular interactions that are present in the amino acid crystals are accounted for in the EIM calculations by a finite array of point charges calculated from Ewald lattice sums and in the cluster calculations by a shell of neighboring molecules or molecular fragments. The combined EIM/cluster calculations utilize a cluster of molecules inside an EIM point charge array. The theoretical (13)C and (15)N principal shielding values for the amino acids studied are compared to the experimental principal shift values. In addition, theoretical CN bond orientations in the chemical shift principal axis system (PAS) are compared to the experimental orientations obtained from (13)C-(14)N dipolar couplings. The theoretical QCC at the nitrogen positions are compared to experimental (14)N QCC principal values reported in the literature. The carbon and nitrogen theoretical chemical shielding, the C-N orientations, and the QCCs from the ab initio calculations show improved agreement with the experimental values when the intermolecular interactions are accounted for by EIM or cluster calculations. The EIM (13)C shielding calculations are found to give better agreement with the experimental values than cluster (13)C shielding calculations. However, to achieve good agreement between the theoretical (14)N QCC and the (15)N principal shielding values with the respective experimental values, both intermolecular electrostatic and covalent interactions have to be included explicitly in the EIM/cluster calculations.