MP2 6-31G-ASTERISK STRUCTURAL STUDY OF TROPONE AND TROPOLONE MOLECULES/

The effects of electron correlation on the molecular structure and sta bility of tropone and tropolone molecules have been studied by MP2/6-3 1G calculations. Geometry optimizations have been carried out for tro pone, cis-tropolone and the non-hydrogen bonded orthogonal and trans r otamers of tropolone. The tropone seven-membered ring exhibits an appr eciable triene-like character which is slightly smoothed upon OH subst itution. The asymmetry of the hydrogen bonding of tropolone, emerging from previous HF/6-31G studies, is reduced by inclusion of electron c orrelation, and the proton tunnelling barrier height drastically lower s from 70 kJ mol-1 (HF/6-31G) to 25 kJ mol-1 (MP2/6-31G*). The struct ural 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 pi-delocalization through the cyc lic carbon skeleton. SCF calculations on the centrosymmetrical tropolo ne dimer indicate that self-association enhances pi-delocalization in the troponoid ring and causes vibrational frequency shifts which satis factorily agree with the changes observed in the FT-IR matrix spectra of isolated tropolone molecules and of solid samples.