Theoretical and computational studies of conformation, natural bond orbital and nonlinear optical properties of cis-N-phenylbenzohydroxamic acid

The gas phase conformational and nonlinear optical properties of cis-N-phenylbenzohydroxamic acid (cis-NPBHA) keto and enol forms were studied applying traditional hybrid and long-range-corrected Density Functional Theory (DFT) and time-dependent density functional (TD-DFT) methods. The calculated geometrical parameters for the two isomers agreed satisfactorily with literature ones. The keto form was predicted to be more stable than the enol counterpart by 10.70–12.60 kcal/mol, and the Gibb’s free energies for the conversion: enol ⇌ keto were found to be 11.14 kcal/mol (B3LYP/6-311+G**), 12.53 kcal/mol (CAM-B3LYP/6-311+G**) and 13.28 kcal/mol (ωB97XD/6-311+G**). All the selected functionals have computed larger total hyperpolarizabilities for the enol tautomer compared to those of the keto rival. The traditional hybrid functional yielded higher total hyperpolarizabilities than those of the long-range-corrected ones. The total hyperpolarizabilities were nicely correlated with HOMO–LUMO energy gaps and absorption maxima. The support of these molecular properties by natural bond orbital (NBO) calculations was evaluated and discussed.