Chemical structure and intra-molecular effects on NMR-NQR tensors of harmine and harmaline alkaloids

Abstract

Density functional theory (DFT) methods were used to analyze the effects of molecular structure and ring currents on the NMR chemical shielding tensors and NQR frequencies of harmine and harmaline alkaloids in the gas phase. The results demonstrated that NMR tensors and NQR frequencies of ¹⁵N nuclei in these compounds depend on chemical environment and resonance interactions. Hence, their values are obviously different in the mentioned structures. The interpretation of natural bond orbital (NBO) data suggests that in harmine structure, the lone pair participation of N₉ in π-system electron clouds causes to development of aromaticity nature in pyrrole ring. However, the chemical shielding around N₉ atom in harmine structure is higher than in harmaline, while in harmaline structure, lone pair participation of N₂ in π-system electron clouds causes to development of aromaticity nature in pyridine ring. Hence, chemical shielding around N₂ atom in harmaline structure is higher than in harmine. It can be deduced that by increasing lone pair electrons contribution of nitrogen atoms in ring resonance interactions and aromaticity development, the values of NMR chemical shielding around them increase, while χ and q_zz values of these nuclei decrease.