Theoretical investigations on the structure and vibrational spectra of N-(2-hydroxy-1-naphthylidene)threonine

Hartree-Fock (HF) and hybrid density functional theory (DFT) have been used to investigate the electronic structure and energetics of enol-imine and keto-amine tautomers of N-(2-hydroxy-1-naphthylidene)threonine. Both these tautomers engender six-membered ring due to intramolecular hydrogen bonded interactions. The enol-imine tautomer turns out to be ∼9 kJ mol−1 lower in energy in the HF framework owing to strong O-H⋯N (1.832 Å) interactions compared to N-H⋯O interactions (H⋯O=1.906 Å) in keto-amine tautomer. Consequently the downshift of the CN stretching vibration to 1560 cm−1 in the enol-imine tautomer has been predicted. The intense CO stretching, insensitive to the tautomeric transformation, turns out to be at 1715 cm−1. The effect of solvents on the tautomeric stability has been investigated. Self-consistent reaction field (SCRF) calculations have shown that unlike in the gas phase structure protic polar solvents facilitate the keto-amine form to be of lower energy. Configuration interaction single excitation (CIS) calculations reveal relatively strong O-H⋯N (1.768 Å) interactions in the excited state of enol-imine tautomer.