James P. Bloxsidge

Tritium nuclear magnetic resonance spectroscopy. Part VI. Tritiated steroid hormones

The regio- and stereo-specificity of the labelling in a series of tritiated steroid hormones has been examined by 3H n.m.r., which also yields quantitative information on the distribution of the tritium between the labelled sites. Complete analysis is thus readily achieved non-destructively. Hydrogen chemical shifts for various skeletal sites are provided for the first time. The specificity of the methods of labelling steroids with tritium by catalytic reduction, catalysed exchange, and tritiodehalogenation are discussed.

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

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 φT=φ1.442 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.

Tritium nuclear magnetic resonance spectroscopy. Part II. Chemical shifts, referencing, and an application

With the aid of compounds partially monotritiated by base-catalysed exchange with labelled water, corresponding triton and proton chemical shifts measured from internal tritiated water under the same conditions are shown to be the same. Applying this result and using the accurate resonance frequencies of the two nuclei measured at constant field on many of the compounds in organic solvents, a mean value, 1·06663975 (±3 × 10–8), has been obtained for the ratio of the Larmor frequencies, ωT/ωH. This value enables an accurate ghost reference for any 3H n.m.r. spectrum to be derived from the observed 1H n.m.r. frequency of the normal internal reference. The convenience of 3H n.m.r. spectroscopy for solving mechanistic problems is illustrated in respect of the Reimer–Tiemann reaction.