Vibrational Predissociation Spectrum of the Carbamate Radical Anion, C5H5N-CO2−, Generated by Reaction of Pyridine with (CO2)m−

Abstract

We report the vibrational predissociation spectrum of C₅H₅N-CO₂⁻, a radical anion which is closely related to the key intermediates postulated to control activation of CO₂ in photoelectrocatalysis with pyridine (Py). The anion is prepared by the reaction of Py vapor with (CO₂)_m⁻ clusters carried out in an ionized, supersonic entrainment ion source. Comparison with the results of harmonic frequency calculations establishes that this species is a covalently bound molecular anion derived from the corresponding carbamate, C₅H₅N-CO₂⁻ (H⁺). These results confirm the structural assignment inferred in an earlier analysis of the cluster distributions and photoelectron spectra of the mixed Py_m(CO₂)_n⁻ complexes [J. Chem. Phys. 2000, 113 (2), 596−601]. The spectra of the (CO₂)_m⁻ (m = 5 and 7) clusters are presented for the first time in the lower energy range (1000−2400 cm⁻¹), which reveal several of the fundamental modes that had only been characterized previously by their overtones and combination bands. Comparison of these new spectra with those displayed by Py(CO₂)_n⁻ suggests that a small fraction of the Py(CO₂)_n⁻ ions are trapped entrance channel reaction intermediates in which the charge remains localized on the (CO₂)_m⁻ part of the cluster.