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 ), dispiro[2.0.2.4]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.