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The geometrical and electronic structures, the force constants, the vibrational frequencies, and the potential energy distributions for various isotopomers of 4,4‘-bipyridine (44BPY) in the ground state S0 and for the corresponding anion radical 44BPY•- and N,N‘-dihydro cation radical 44BPYH2 •+ have been computed by the 3-21G(+*) ab initio method, with the RHF formalism for the ground state S0 and with the ROHF formalism for the ionic species. The theoretical results have been compared to the available vibrational data. The very good agreement between calculations and experiment has allowed the determination of the conformations of the S0 state and of the ion radicals. The ground state is characterized by an aromatic structure in which the two pyridyl rings are linked in a twisted conformation, and the ionic species present a quinoidal planar distortion, more marked in the cation radical than in the anion radical.

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