The theoretical investigation of solvent effects on the relative stability and 15N NMR shielding of antidepressant heterocyclic drug

Abstract

The density functional theory (DFT) and Tomasi’s polarized continuum model (PCM) were used for the investigation of solvent polarity and its dielectric constant effects on the relative stability and NMR shielding tensors of antidepressant mirtazapine (MIR). The obtained results indicated that the relative stability in the polar solvents is higher than that in non-polar solvents and the most stable structure was observed in the water at the B3LYP/6-311++G (d,p) level of theory. Also, natural bond orbital (NBO) interpretation demonstrated that by increase of solvent dielectric constant, negative charge on nitrogen atoms of heterocycles and resonance energy for LP(N₁₀) → σ* and π* delocalization of the structure’s azepine ring increase and the highest values of them were observed in water. On the other hand, NMR calculations showed that with an increase in negative charge of nitrogen atoms, isotropic chemical shielding (σ_iso) around them increase and nitrogen of piperazine ring (N₁₉) has the highest values of negative charge and σ_iso among nitrogen atoms. NMR calculations also represented that direct solvent effect on nitrogen of pyridine ring (N₁₅) is more than other nitrogens, while its effect on N₁₉ is less than other ones. Based on NMR data and NBO interpretation, it can be deduced that with a decrease in the negative charge on nitrogen atoms, the intramolecular effects on them decrease, while direct solvent effect increases.