Photochemical studies of alkylammonium molybdates. Part 8. Location of protons interacting with the paramagnetic electron in a single crystal of photoirradiated [NH3Pri]6[Mo8O26(OH)2]·2H2O

Abstract

The e.s.r. spectrum of a photoirradiated single crystal of [NH₃Prⁱ]₆[Mo₈O₂₆(OH)₂]·2H₂O at room temperature reveals the interaction of two magnetically equivalent protons with the paramagnetic electron localized at the octahedral site where a hydroxide group is co-ordinated originally: one of the two protons is the hydroxide proton. Another proton originates from a hydrogen-bonding [NH₃Prⁱ]⁺ proton which is transferred to a bridging oxygen atom trans to the hydroxide group. Analysis of the e.s.r. parameters g, ⁹⁵Mo and ⁹⁷Mo hyperfine, and ¹H superhyperfine tensors has established that the maximum eigenvector of the ¹H superhyperfine tensors with all positive signs lies close to the Mo^V⋯ H⁺ direction and that the superhyperfine couplings with the proton nuclei arise from a direct participation of the H 1s orbital in the semioccupied molecular orbital. Plausible locations of the protons, which give equal spin density in two hydrogen 1s orbitals, have been estimated by extended-Hückel calculations on a neutral fragment model [NH₃][H₂Mo^VO₃(OH)(H₂O)₂], for the molybdenum(V) site. The calculated semioccupied molecular-orbital consists mainly of the extensively mixed 4d orbitals, the oxygen and nitrogen 2p orbitals, and the two hydrogen 1s orbitals. The two protons at this site are situated symmetrically at a distance of ca. 1.9 Å from the paramagnetic molybdenum atom and 1.43 Å from the nearest oxygen atom. The geometry of the site corresponds to the saddle zone in the full potential energy surface, which can be regarded as a transition state for the solution photochemistry.