Quantum-Chemical Study of the Molecular Forms of Vanillin

摘要

Equilibrium structural parameters, dipole moments, relative energies, vibration spectra, and ionization potentials of various conformers of benzoid (B), p- and o-quinoid (Q), and deprotonated anionic forms of vanillin in a vacuum and in a polarizable medium with dielectric properties of water were calculated by the quantum-chemical method (U)B3LYP/cc-pVTZ. The intramolecular hydrogen bond in free vanillin benzoid molecule Bl is characterized by the energy of 4 kcal mol~(-1) and by the barrier of cleavage 8 kcal mol~(-1) that correspond to the conformational transition B1 -> B2. The energy of the optimal p-quinoid conformer (Ql) is 22 kcal mol~(-1) larger than the energy of Bl in a vacuum and 15 kcal mol~(-1) in water. Thep-quinoid forms greatly exceeds the benzoid forms by dipole moment, electrophilicity, and nucleophilicity. The structure calculated for the anion is intermediate between benzoid and p-quinoid forms, and the calculated energy of polarization stabilization in water is 57 kcal moF1.