Bridged diquaternary salts of di-2-pyridyl ketone and their radical cations: crystal structure, raman and resonance raman studies

Abstract

Raman spectra are reported for two bridged diquaternary salts of di-2-pyridyl ketone. The crystal structure of one parent dication is also reported. Crystal data: C 13H 12Br 2N 2O, M r = 372.05, a = 9.369(1), b = 11.036(4), c = 7.115(1) Å, α = 97.60(2), β = 90.12(1), γ = 67.79(1)°, V = 674.20 Å 3, triclinic, space group P 1 ; Z = 2, D m = 1.83, D x = 1.832 g cm −3, MoKα, λ = 0.71069, Å, μ = 63.54 cm −1, F(000) = 364, T = 292 ± 1 K, R = 0.033, R w = 0.032 for 1330 ( I &> 2.5σ) reflections. Resonance Raman spectra of the coloured radical cations obtained from the diquaternary salts by one-electron reduction are also reported. Changes in the frequency of the carbonyl stretching vibration of the parent dications are correlated with changes in the conformation of the conjugated system. Changes in the Raman spectrum of a parent dication between the solid state and aqueous solution appear to arise from a modification of molecular geometry in solution. The decrease in frequency of the carbonyl stretching mode on radical formation is attributed to a decrease in the carbonyl bond order, due to delocalisation of the odd electron.