MP2/6-31G* structural study of tropone and tropolone molecules

Abstract

Abstract The effects of electron correlation on the molecular structure and stability of tropone and tropolone molecules have been studied by MP2/6-31G* calculations. Geometry optimizations have been carried out for tropone, cis -tropolone and the non-hydrogen bonded orthogonal and trans rotamers of tropolone. The tropone seven-membered ring exhibits an appreciable triene-like character which is slightly smoothed upon OH substitution. The asymmetry of the hydrogen bonding of tropolone, emerging from previous HF/6-31G* studies, is reduced by inclusion of electron correlation, and the proton tunnelling barrier height drastically lowers from 70 kJ mol −1 (HF/6-31G*) to 25 kJ mol −1 (MP2/6-31G*). The structural changes induced by OH torsion, supported by additional MP2/6-31G* geometry optimizations of the analogous systems 1,4-pentadiene-3-one and 2-hydroxy-1,4-pentadiene-3-one indicate that intramolecular CO⋯HO hydrogen bonding is favoured by π-delocalization through the cyclic carbon skeleton. SCF calculations on the centrosymmetrical tropolone dimer indicate that self-association enhances π-delocalization in the troponoid ring and causes vibrational frequency shifts which satisfactorily agree with the changes observed in the FT-IR matrix spectra of isolated tropolone molecules and of solid samples.