A study of the molecular structure and spectroscopic properties of tetrahydro[4]beltene and related compounds Original Research Article

The equilibrium structure of hexacyclo[8.7.0.03,8.05,15.06,13.012,16]heptadeca-1(10),5-diene-3,8-dicarboxylic anhydride, a tetrahydro[4]beltene with two σ-conjugated double bonds, and of two derivatives, the first with only one double bond and the second with a closed annular belt, was calculated at the RHF/6-31G** ab initio level of theory. The results, consistent with the available X-ray experimental data, account for an efficient homoconjugation in the two compounds with an open belt. The NMR 13C chemical shifts were analyzed by means of ab initio CSGT (continuous set of gauge transformations) calculations performed with the B3LYP/6-311+G(2d, p) DFT-HF hybrid functional model. The He(I) photoelectron spectra were measured and interpreted by means of ab initio OVGF (outer valence Green function) calculations, which give an overall consistent reproduction of the energies and splittings of the uppermost bands. These are associated with the π(CC) and/or Walsh orbitals of the equatorial belt and with the n(CO) lone pair orbitals of the anhydridic moiety. Electron transmission spectroscopy was employed to characterize the low-lying temporary anion states of tetrahydro[4]beltene. The single-crystal X-ray structure was also determined for the tris(demethano)pagodane derivative.