Molecular orbital treatment of the conformations of composite molecules: Phenylthiophenes

A correlative study of the theoretical determination of the conformation of composite molecules is given. Results of EH, M-I-M and $\text{CNDO}\text{2}$ treatments are compared for the model composite systems; 2- and 3-phenylthiophenes. The systematic failure of the EH method reveals its inadequacy for conformational analysis of composite molecules. The transition energies, oscillator strengths and dipole moments of the two isomers, as calculated by the CNDO method, agree satisfactorily with experiment and confirm a planar equilibrium conformation for the studied composite molecules. The total charge distribution calculated by the CNDO method shows an appreciable σ-charge contribution and a comparatively small charge-transfer contribution. The formal σ-charges on the atoms of the two isomers are shown to be polarized in a direction opposed to that of the π-polarization. The molecular orbitals of the two isomers are found to possess similar nodal properties which indicates that the extent of cross conjugation in the 3-phenyl isomer is very limited.