Cluster expansion of the wave function. Valence excitations and ionizations of pyridine

Abstract

The SAC (symmetry-adapted cluster) and SAC-CI theories are applied to the calculations of the valence excitations and the ionizations of pyridine. The active space consists of 85 MOs (35π and 50 σ MOs). For the valence π→π* and n→π* excited states, S1(1 1B2), S2(1 1A1), S3(2 1B2), Sn(1 1B1), T1(1 3A1), and Tn(1 3B1), the calculated excitation energies are higher than the experimental values by within ∼0.7 eV. Although the S4(2 1A1) state is experimentally considered not to be split from S3, the present calculation predicted it to be higher than S3 by 0.31 eV. The average discrepancy from the experimental data is 0.55 eV without including S4. The transition energies and some other properties are predicted for the valence π→π* excited states, T2(1 3B2), T3(2 3A1), T4(2 3B2) and for the n→π* excited states, S′n(1 1A2) and Tn(1 3A2), for which the experimental values are unavailable so far. The following two observations seem to be interesting. (i) The valence π→π* excited states of pyridine are understood from those of benzene, reported previously, as a result of perturbation, an alteration of one C–H group to an isoelectronic nitrogen atom. The typical V states, S3 and S4, show large σ-reorganization effect, which is detected as the change in the transition energy and some one electron properties. (ii) For the relative positions between the π→π* and n→π* excitations, our results are as follows. The lowest singlet excited state is n→π*[Sn(1 1B2)]. The lowest triplet state is π→π*[T1(1 3A1)] and n→π*[Tn(1 3B1)] exists higher than T1 by 0.15 eV. Experimentally, the lowest singlet excitation is confirmed to be n→π*, but two controversial assignments exist for the lowest triplet state: π→π* or n→π*. For the ionization potential, the experimental spectra is reproduced to within ∼0.6 eV. The nature of the first three peaks are calculated as nππ, which is supported by some recent experimental studies. The similarity and difference in the satellite peaks of benzene and pyridine are pointed out.