Inorganic solution chemistry at elevated pressures

Abstract

Recent interest in pressure effects on inorganic systems in solution has been centred upon the use of volumes of activation ΔV‡ as criteria of reaction mechanism. Work in our laboratory has sought to determine whether ΔV‡ is indeed a useful parameter in this respect, i.e., whether it is substantially independent of pressure and reaction conditions and whether it can be quantitatively predicted for suitably "simple" reactions. For solvent exchange on metal ions (the simplest conceivable substitution process), a semi-empirical model predicts ΔV‡ for limiting dissociative (bond breaking) and associative (bond making) mechanisms in water, but experimental values lie between these extremes, vindicating the Langford–Gray "interchange" model in which associative and dissociative processes are viewed as being concerted. For adiabatic electron transfer reactions of the outer-sphere type (the simplest conceivable oxidation–reduction process) in water, an adaptation of Marcus theory accounts for the essentially pressure-independent ΔV‡ satisfactorily, and failures of such predictions can be ascribed to complications such as nonadiabaticity or the incursion of inner-sphere (ligand-bridged) reaction pathways. The theory is less successful in nonaqueous solvents. Experimental methods used for these and related studies include high pressure adaptations of nuclear magnetic resonance, UV–visible spectrophotometry, stopped-flow techniques, cyclic voltammetry, and sampling under pressure.