An investigation on the structure and group vibrations of balenine molecule by matrix isolation IR spectroscopy, DFT and MP2 based calculations

Abstract

Stable conformers of neutral balenine were scanned through molecular dynamics simulations and energyminimizations using Allinger’s MM2 force field. For each of the found minimum-energy conformers,geometry optimization and thermochemistry calculations were performed by using B3LYP, MP2,G3MP2B3 methods, 6-31G(d), 6–311++G(d,p) and aug-cc-pvTZ basis sets. The calculation results haveindicated that balenine has about twenty stable conformers whose relative energies are in the range of0–9.5 kcal/mol. Three of these are thought to provide the major contribution to matrix isolation IR spectraof the molecule. Our solvent calculations using the polarized continuum model revealed the stable zwitterionstructures which are predicted to dominate IR spectra of balenine in water and heavy water (D2O)solvents. Pulay’s SQM-FF method was used in scaling of the harmonic force constants and vibrationalspectral data calculated for the neutral and zwitterion structures. These refined calculation data togetherwith those obtained from anharmonic frequency calculations enabled us to correctly interpret the matrixisolation IR spectrum of balenine and the tautomerism-based changes observed in its KBr IR and solution(D2O) IR spectra. The results revealed the crucial role of conformation and zwitterionic tautomerism onthe structure and vibrational spectral data of the molecule.