Identification of isolated methyl groups by detection of cross-correlation effects using polarization transfer

It is known that cross-correlation effects can induce nonexponential proton magnetization decay in some isolated methyl groups on large molecules in liquids. Here it is shown theoretically and confirmed experimentally that these cross correlations can also give rise to a more instantly recognizable effect, a negative methyl 13C signal in an otherwise accurate methine carbon subspectrum obtained from an editing polarization transfer sequence. Although this technique offers a new method for detecting directly and studying cross-correlation spectral densities, and thus the underlying molecular dynamics, this paper is concerned mainly with its potential for the rapid identification of isolated methyl groups in edited spectra of medium to large molecules.