Secondary Deuterium Isotope Effects with Lanthanide(III)-Silver(I) NMR Shift Reagents

Secondary deuterium isotope effects have been observed for nitrogen- and oxygen- containing donors in the presence of lanthanide NMR shift reagents1–3. In each case the shifts observed in the NMR spectrum of the partially deuterated compound were larger than those for the non-deuterated analog. It is generally postulated that the greater inductive effect of deuterium compared to hydrogen causes the deuterated analog to compete more effectively for coordination sites on the lanthanide chelate4; however, steric effects cannot be eliminated as a possible explanation for this behavior. Secondary deuterium isotope effects have also been shown to effect the stability of complexes between silver(I) and olefins5. Once again the complex stability imcreases as the degree of deuterium substitution imcrease. Gas chromatographic stationary phases containing Ag(I) have been used to separate deuterated compounds from their less deuterated analogs6–9. In all cases the deuterated compound exhibited a longer retention time. In this paper the first obvservation of a secondary deuterium isotope effect in the presence of binuclear lanthanide (III) silver(I) NMR shift reagents10 is reported.