Low-energy electron attachment to fused 1,4-cyclohexadiene rings by means of electron transmission spectroscopy and exponent stabilization calculations

Electron transmission spectroscopy is used for determining the energies of vertical electron attachment to the empty pi* orbitals of ethene (1), 1,4-c yclohexadiene (2), 1,4,5,8-tetrahydronaphthalene (3), and 1,4,5,8,9,10-hexa hydroanthracene (4), where the number of ethene double bonds, which interac t through space and through the CH2 bridges, increases along the series. In contrast with the expectations based on a simple perturbational model, the energy of the first anion state is nearly constant on going from 1 to 4. M oreover, the energy splitting between the lowest and the highest anion stat es in the larger molecular systems 3 and 4 is smaller than in 1,4-cyclohexa diene. The experimental data are compared with the empty orbital energies o f the neutral states supplied by HF calculations using both a standard basi s set and one augmented with diffuse functions, using the exponent stabiliz ation method for distinguishing the virtual orbitals which give rise to tem porary anion states. The graphs of virtual orbital eigenvalues versus the e xponent scaling factor display avoided crossings and regions where the pi(* ) molecular orbital energies are relatively stable. The orbital energies de termined in correspondence with the avoided crossings do not reproduce the trends of the resonances observed in the spectra. A better match with exper iment (although not completely satisfactory) is obtained by determining the energies in the region of stability of the graphs. This set of results als o predicts smaller through-space and through-bond interactions. (C) 2000 Am erican Institute of Physics. [S0021-9606(00)01130-2].