Application of tritium nuclear magnetic resonance spectroscopy to the determination of isotopic fractionation factors in methanol–methoxide solutions

Abstract

Tritium n.m.r. measurements of hydroxy chemical shifts in methanolic solutions of sodium methoxide have been used to determine an isotopic fractionation factor for the inner solvation shell of the methoxide ion and contributions from inner and outer solvation shells to the methoxide ion chemical shift. The identity of protium and tritium chemical shifts and the relationship between tritium and deuterium fractionation factors φ^T=φ^1.442 mean that measurements in MeOH and MeOD double the information available from ¹H n.m.r. measurements alone. The necessary assumption previously made to derive φ from ¹H measurements, that secondary solvation does not contribute to the methoxide ion chemical shift, is shown to be incorrect, but the revised value of φ(0.7) differs only slightly from earlier values, although treatment of the secondary solvation shift as a variable leads to some loss of precision in the definition of φ. At high methoxide concentrations plots of chemical shift against concentration are distinctly curved. Contributions to the curvature from breakdown of the assumption that isotopic atom fractions in the solution as a whole and in the bulk solvent are identical are evaluated.