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An investigation of solvated electrons in hexamethylphosphoramide Flynn, Garry John

Abstract

This thesis is concerned with a detailed investigation of the nature, yield, stability, and reactivity of solvated electrons in hexamethylphosphoramide (HMPA). Evidence is presented for their formation by radiolysis and for their comparatively long-lived existence in this highly polar aprotic solvent in which ordinary anions are particularly weakly solvated. By means of microsecond pulse radiolysis methods it was shown that the solvated electron in HMPA has a intense broad structureless absorption band with a band maximum at 2200 ± 100 nm, a band half-width of 3600 cm⁻¹ and maximum molar absorptivity of (3.2 + 0.5) X 10⁴ M⁻¹ cm⁻¹. The spectrum is similar to that found in solutions of sodium metal dissolved in HMPA. Radiolysis studies provided a value of 2.2 ± 0.2 for the free-ion yield of solvated electrons. This was established by both pulse and steady-state methods and is approximately what one would expect on the basis of the liquid's dielectric constant when compared with other solvents. An interesting deduction from the steady-state Ɣ-radiolysis results was that N₂O does not scavenge electrons on a one-to-one basis in this solvent, but rather produced more than one N₂ molecule per electron scavenged. The yields of N₂ from N₂O scavenging of electrons in HMPA/water mixtures are also reported for the full composition range. Electron decays were studied spectro-photometrically and its rate of reaction with N₂O, pyrene, anthracene and other additives are reported in the thesis. When stable solutions of solvated electrons in HMPA (alkali metal solutions) were themselves irradiated, a net decrease in electron concentration occurred, indicating that e⁻HMPA reacts not only with its concomitant positive ion but also with other radiolysis products. Pure HMPA produces hydrogen and methane when irradiated, with yields of 3.3 ± 0.3 and 0.29 ± 0.03 respectively. The H₂ yield was shown to comprise a non-scavengable molecular process (whose yield is 1.4 ± 0.1) and a scavengable part (yield 1.9 ± 0.2) that clearly involved solvated electrons.

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