On the Molecular and Electronic Structure of Spiroketones and Half-Molecule Models

Abstract

The equilibrium structures of three polyketones based on the 2,2‘-spirobiindan skeleton (1,1‘-dione, 1,3,1‘-trione, and 1,3,1‘,3‘-tetraone), their “half-molecule” fragments (1-indanone and 2,2-dimethylindan-1,3-dione), and the indandione dimer (2,2‘-dimethyl-[2,2‘]-biindenyl-1,3,1‘,3‘-tetraone) were investigated using the density functional theory model B3LYP/6-31G(d,p). The results matched the X-ray experimental data that are available for one of the spiroketones. The electronic structure of these ketones was investigated by means of their spectroscopic properties. The NMR 13C chemical shifts, calculated by the continuous-set-of-gauge-transformations formalism with the B3LYP/6-311+G(2d,p) method, were fairly consistent with NMR observations, in particular for the carbonyl, spiro, and quaternary carbons. The He(I) photoelectron spectra were measured and interpreted by means of ab initio outer-valence-Green's-function calculations. The theoretical results consistently reproduced the energies and splittings of the uppermost bands. These bands were associated with the phenyl π orbitals and the n(CO) lone-pair orbitals of the keto groups. Electron transmission spectroscopy, with the support of calculated π* virtual orbital energies, was employed to characterize the empty levels. Strong mixing between the phenyl and carbonyl π* fragment orbitals gave rise to stable anion states. Temporary anion states with mainly carbonyl character were observed in the 1.5−2.5 eV energy range. In the spiroketones, their energy splittings increase with the number of carbonyl groups present in the molecules and indicate the occurrence of through-space interactions between the two perpendicular indan halves.