Simplification and assignment of carbon-13 NMR spectra with spin-echo fourier transform techniques

Abstract

A series of spin-echo Fourier transform (SEFT) experiments are described with which 13C NMR spectra can be simplified and resonances assigned to methyl, methylene, methine, and quaternary carbon sites. The second half of the echo from a standard two-pulse spin-echo sequence is collected as the free-induction decay. Gated broadband proton decoupling is used to modulate the phase and intensity of the 13C resonances. The assignment of resonances to carbon sites is done with three SEFT spectra measured with the decoupler gated off during one of the two evolution periods. The first spectrum, measured with an evolution time of 1/ J sec between the 90 and 180° pulses, provides all the chemical shift information of the standard broadband decoupled spectrum and in addition distinguishes methyl and methine carbons from methylene and quaternary carbons. The other two SEFT spectra identify quaternary carbons and distinguish methine carbons from methyl carbons. Since the resonances are decoupled to singlets, assignments for large molecules can be made with more certainty than with off-resonance decoupled spectra. Subspectra containing only resonances from methine and methyl carbons or quaternary and methylene carbons, 1H coupled or decoupled, are obtained by spin-echo difference or addition experiments. The fully coupled subspectra are shown to be of use for the measurement of 13C 1H coupling constants. The estimation of one-bond coupling constants by analysis of the intensity modulation of 13C resonances in SEFT spectra measured as a function of the length of the evolution period is also described. The SEFT experiments are demonstrated with spectra for cholesterol.