The dependence of the spin-Hamiltonian parameters of the [Ti(OH 2) 6] 3+ cation on the mode of water co-ordination

Abstract

Single crystal X-band EPR spectra of Ti(III) doped C( NH 2) 3 M III ( SO 4) 2·6 H 2 O ( Gu[ M III : Ti] SH, M III = Ga III , Al III ) collected at ∼1.9 K, are presented. Principal g values for Gu[Ga:Ti]SH and Gu[Al:Ti]SH of g ∥∼0.965, g ⊥∼0, and g ∥∼0.984, g ⊥∼0, respectively, are derived. The values of g ∥ are significantly less than that previously reported for Cs[Ga:Ti]SH ( g ∥∼1.161) and this work is principally concerned with accounting for the difference. Analytical expressions are derived for the first-order Jahn–Teller coupling coefficients between the states comprising the 2T g (T h ) ground term and the twisting librations of the coordinating water molecules, in the framework of the Angular Overlap Model (AOM); and, values of g ∥ computed by numerical diagonalisation of the vibronic Hamiltonian. On the basis of differences in the mode of water co-ordination alone, g ∥ is calculated to increase from 0.965 (Gu[Ga:Ti]SH) to ∼1.10 (Cs[Ga:Ti]SH), in fair agreement with experiment. The EPR spectra of Gu[Ga:Ti]SH contain well-resolved proton superhyperfine structure. The magnitude of the superhyperfine coupling constant far exceeds that inferred from the EPR spectrum of Cs[Ga:Ti]SH, and this too is rationalised in terms of the different stereochemistries of the [Ti(OH 2) 6] 3+ cation in the two crystal systems.