VIBRATIONAL ANALYSIS OF SOME TRANSIENT SPECIES IMPLICATED IN THE PHOTOREDUCTION OF 4,4'-BIPYRIDINE BASED ON AB-INITIO AND DENSITY-FUNCTIONAL CALCULATIONS

The conventional ab initio method at the closed and open restricted Ha rtree-Fock levels (RHF, ROHF) and the density functional theory approa ch at the B3-LYP and UB3-LYP levels, using the 6-31G(+) basis set, we re applied to predict the molecular structures, the energetic properti es (proton affinity, electron affinity, bond dissociation energy and r otational barrier height) and the vibrational properties (harmonic wav enumbers, force fields and potential energy distributions) of six spec ies susceptible to be involved in the photoreduction of various isotop omers of 4, 4'-bipyridine: three isoelectronic closed-shell systems (t he neutral molecule 44BPY, the N-monoprotonated cation, 44BPYH(+) and the N,N'-diprotonated cation 44BPYH(2)(2+)) and three isoelectronic op en-shell systems of their reduced forms (the anion radical 44BPY(.-), the N-monohydro radical 44BPYH(.) and the N,N'-dihydro cation radical 44BPYH(2)(.+)). The stabilities of these species are discussed on the basis of computed electronic energies, including zero-point vibrationa l energies, by B3-LYP and HF (including MP2 electron correlation energ ies) methods. Our calculations show that (i) all the species are stabl e at both the HF acid B3-LYP levels, except 44BPY(.-), which is found to be unstable relative to 44BPY + e at the HF level, (ii) the B3-LYP method gives systematically opposite effects on the rotational barrier heights V-parallel to and V-perpendicular to than do MP2 calculations , underestimating V-perpendicular to and overestimating V-parallel to, and (iii) the B3-LYP method predicts slightly better structures and h armonic vibrational wavenumbers than the HF method. However, using bot h methods, there is good agreement between theory and experiment, conc erning not only the absolute wavenumbers but also the isotopic shifts for each compound, and the wavenumber shifts on going from the parent molecules to their reduced forms for each isotopomer. This general agr eement allows us to validate the calculated structures of all species studied. (C) 1998 John Wiley & Sons, Ltd.