Martine Castellà-Ventura

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 Hartree–Fock levels (RHF, ROHF) and the density functional theory approach at the B3-LYP and UB3-LYP levels, using the 6–31G(+*) basis set, were applied to predict the molecular structures, the energetic properties (proton affinity, electron affinity, bond dissociation energy and rotational barrier height) and the vibrational properties (harmonic wavenumbers, force fields and potential energy distributions) of six species susceptible to be involved in the photoreduction of various isotopomers of 4,4′-bipyridine: three isoelectronic closed-shell systems (the neutral molecule 44BPY, the N-monoprotonated cation, 44BPYH+ and the N,N′-diprotonated cation 44BPYH22+) and three isoelectronic open-shell systems of their reduced forms (the anion radical 44BPY•-, the N-monohydro radical 44BPYH• and the N,N′-dihydro cation radical 44BPYH2•+). The stabilities of these species are discussed on the basis of computed electronic energies, including zero-point vibrational energies, by B3-LYP and HF (including MP2 electron correlation energies) methods. Our calculations show that (i) all the species are stable at both the HF and 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‖ and V⊥ than do MP2 calculations, underestimating V⊥ and overestimating V‖, and (iii) the B3-LYP method predicts slightly better structures and harmonic vibrational wavenumbers than the HF method. However, using both methods, there is good agreement between theory and experiment, concerning 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 agreement allows us to validate the calculated structures of all species studied.