Dipole moments, spectroscopy, and ground and excited state conformations of cycloalkane-1,2-diones

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Abstract
The dipole moments of a series of α,α,ω,ω-tetramethylcycloalkane-1,2-diones with a ring size varying between 4 and 8 carbon atoms, and of di-t-butylglyoxal, camphorquinone, and homoadamantane-4,5-dione have been measured. The (time-averaged) dihedral angle Φ between the carbonyl groups of these compounds has been calculated from the dipole moments. The HeI photoelectron spectra showed that the ionization potentials of the two different non-bonding MOs (N+ and N–) hardly vary with the ring size and thus with Φ. The u.v. absorption spectra exhibit two bands; the long wavelength band shifts from ca. 330 nm for a substrate with perpendicular diketo-geometry to ca. 450 or 500 nm for a planar transoid and planar cisoid diketo-conformation, respectively. The second absorption band is found for all compounds at ca. 285 nm. The fluorescence and phosphorescence bands shift in a regular way relative to the long wavelength absorption band, viz. the shift is maximal for Φ 90° and minimal for Φ 0 or 180°. The data show in a quantitative way that 1,2-diketones emit from excited states with coplanar diketo-geometry.

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