Photochemical studies of the alkylammonium molybdates. Part 4. Electron spin resonance study of an irradiated single crystal of hexakis-(isopropylammonium) dihydrogen octamolybdate dihydrate

Abstract

A single-crystal e.s.r. study of the formation of Mo⁵⁺ on irradiation of the title compound [NH₃Pr¹]₆[H₂Mo₈O₂₈]·-2H₂O has been carried out at room temperature using 100-kHz modulation. The main line arising from the naturally abundant ⁹⁶Mo(I= 0) is split into three hyperfine lines (1 : 2 : 1) owing to superhyperfine interaction with two equivalent hydrogen atoms. The hyperfine lines due to ⁹⁵Mo and ⁹⁷Mo could not be analysed because of lack of resolution of weaker resonances. The principal components of the g and ¹H superhyperfine tensors are g₁ 1.955, g₂ 1.941, g₃ 1.897, |A₁| 10.14 × 10^–4 cm^–1, |A₂| 9.28 × 10^–4 cm^–1, and |A₃| 8.69 × 10^–4 cm^–1. From the correlation with X-ray crystal-structure data, it is proposed that the formation of Mo^vO₄(OH)₂ in a MoO₆ octahedral site is a highly probable mechanism for the u.v.-induced colouration.