Electronic and Molecular Structure of Simple 3,3′-Bicyclopropenyls. Photoelectron Spectroscopy and Model Calculations

The electronic and molecular structure of 3,3′-bicyclopropenyl ( 1 ) and its alkyl derivatives 3,3′-dirnethyl-3,3′-bicyclopropenyl ( 2 ), dispiro 2.0.2.3]nona-1,5-diene ( 3 ), dispiro2.0.2.4]deca-1,5-diene ( 4 ), dispiro 2.0.2.5]undeca-1, 5-diene ( 5 ), and dispiro 2.0.2.6]dodeca-1, 5-diene ( 6 ) are studied by means of photoelectron spectroscopy and model calculations. Through-bond' effects in model compound 1 are analyzed in detail, illustrating a general difficulty with NDO models. Low-energy photoelectron bands of 2–6 can be assigned to ejection of electrons from cyclo-propenyl π- and Walsh-orbitals. Strong ‘through-bond’ coupling leads to splitting of the π-bands in the range 1.0–1.5 eV, while the strongly conformation-dependent splitting of the Walsh-bands allows conclusions concerning the preferred torsional angles. The preference of a gauche-conformation is predicted for 2 in the gas phase.