Isoelectronic Homologues and Isomers:  Tropolone, 5-Azatropolone, 1-H-Azepine-4,5-dione, Saddle Points, and Ions.

Computational studies of 12 64-electron homologues and isomers of tropolone in the S0 electronic ground state are reported. Three minimum-energy structures, tropolone (Tp), 5-azatropolone (5Azt), and 5-H-5-azatropolonium (5AztH+), have an internal H-bond and planar Cs geometry, and three, tropolonate (TpO-), 5-azatropolonate (5AzO-), and 1-H-azepine-4,5-dione (45Di), lack the H-bond and have twisted C2 geometry. All 6 substances have an equal double-minimum potential energy surface and a saddle point with planar C2v geometry. The energy for the gas-phase isomerization reaction 45Di → 5Azt is near +4 kJ mol-1 at the MP4(SDQ)/6-311++G(df,pd)//MP2/6-311++G(df,pd) (energy//geometry) theoretical level and around −20 kJ mol-1 at lower theoretical levels. The dipole moments computed for 45Di and 5Azt are 9.6 and 2.1 D, respectively, and this large difference contributes to MO-computed free energies of solvation that strongly favor—as experimentally observed—45Di over 5Azt in chloroform solvent. The MO-computed energy for the gas-phase protonation reaction 45Di + H+ → 5AztH+ is −956.4 kJ mol-1, leading to 926.8 kJ mol-1 as the estimated proton affinity for 45Di at 298 K and 1 atm. The intramolecular dynamical properties predicted for 5Azt and 5AztH+ parallel those observed for tropolone. They are therefore expected to exhibit spectral tunneling doublets. Once they are synthesized, they should contribute importantly to the understanding of multidimensional intramolecular H transfer and dynamical coupling processes. [ABSTRACT FROM AUTHOR]