Theoretical investigation of hydrogen bonding effects on oxygen, nitrogen, and hydrogen chemical shielding and electric field gradient tensors of chitosan/HI salt Original Research Article

A density functional theory study has been carried out to calculate the 17O, 15N, 13C, and 1H chemical shielding as well as 17O, 14N, and 2H electric field gradient tensors of chitosan/HI type I salt. These calculations were performed using the B3LYP functional and 6-311++G (d,p) and 6-31++G (d,p) basis sets. Calculated EFG and chemical shielding tensors were used to evaluate the 17O, 14N, and 2H nuclear quadruple resonance, NQR, and 17O, 15N, 13C, and 1H nuclear magnetic resonance, NMR, parameters in the cluster model, which are in good agreement with the available experimental data. The difference in the isotropic shielding (σiso) and quadrupole coupling constant (CQ) between monomer and target molecule in the cluster was analyzed in detail. It was shown that both EFG and CS tensors are sensitive to hydrogen-bonding interactions, and calculating both tensors is an advantage. A different influence of various hydrogen bond types, N–H⋯I, O–H⋯I, and N–H⋯O was observed on the calculated CS and EFG tensors. On the basis of this study, nitrogen and O-6 are the most important nuclei to confirm crystalline structure of chitosan/HI. These nuclei have large change in their CS and EFG tensors because of forming intermolecular hydrogen bonds. Moreover, the quantum chemical calculations indicated that the intermolecular hydrogen-bonding interactions play an essential role in determining the relative orientation of CS and EFG tensors of O-6 and nitrogen atoms in the molecular frame axes.