On the molecular and electronic structure of spiroketones and half-molecule models

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, l′,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.