AB-INITIO COMPUTATIONS OF THE GEOMETRICAL, ELECTRONIC, AND VIBRATIONAL PROPERTIES OF THE GROUND-STATE, THE ANION-RADICAL, AND THE N,N'-DIHYDRO CATION-RADICAL OF 4,4'-BIPYRIDINE COMPARED TO TRANSIENT RAMAN-SPECTRA

The geometrical and electronic structures, the force constants, the vi brational frequencies, and the potential energy distributions for vari ous isotopomers of 4,4'-bipyridine (44BPY) in the ground state S-0 and for the corresponding anion radical 44BPY(.-) and N,N'-dihydro cation radical 44BPYH(2)(.+) have been computed by the 3-21G(+) ab initio m ethod, with the RHF formalism for the ground state S-0 and with the RO HF formalism for the ionic species. The theoretical results have been compared to the available vibrational data. The very good agreement be tween calculations and experiment has allowed the determination of the conformations of the S-0 state and of the ion radicals. The ground st ate is characterized by an aromatic structure in which the two pyridyl rings are linked in a twisted conformation, and the ionic species pre sent a quinoidal planar distortion, more marked in the cation radical than in the anion radical.