Lanthanide Shift Reagents in 13C Nuclear Magnetic Resonance: Quantitative Determination of Pseudocontact Shifts and Assignment of 13C Chemical Shifts of Steroids

Abstract

The ^l3C n.m.r. chemical shifts of some monofunctional keto- and hydroxy-steroids were monitored as a function of the concentration of added shift reagent, Pr(fod)₃ or Eu(fod)₃, and the observed induced shifts were fitted to the geometric quantities in the one-term pseudocontact shielding equation. The correlation between experimental and calculated shifts was quantitative for all carbons except those closest to the complexing site, where contact interactions are most noticeable. With Pr(fod)₃ the carbonyl and hydroxyl carbons and the alpha carbons usually had to be excluded from the calculations in order to optimize the correlation between predicted and observed induced shifts for the remaining nuclei and to derive reasonable adduct geometries. With Eu(fod)₃, contact shifts were also noticeable at the beta carbons and the europium shift reagents are therefore less useful in ¹³C n.m.r. spectroscopy. It is shown that the application of shift reagents, in conjunction with off-resonance decoupling, permits the assignment of practically all carbon chemical shifts of the steroids studied.