Virtual coupling effects in heteronuclear chemical-shift correlation spectroscopy

Heteronuclear chemical-shift correlation (HETCOR) spectroscopy ( I, 2) has become an important 2D NMR technique in the study of complex organic and bioorganic molecules. The cross peaks in the 2D spectra normally arise due to the existence of a nonvanishing heteronuclear scalar coupling. However, virtual coupling between ‘H and 13C spins (3- 7)) arising due to strong coupling between two or more protons, can result in the appearance of spurious peaks in the HETCOR spectra. The commonly used HETCOR pulse sequences are shown in Figs. 1 A and 1 B. Sequence 1 A provides 2D spectra with complete suppression of heteronuclear couplings in both dimensions, although ‘H- ‘H scalar couplings remain in wl. Sequence 1B with a BIRD sandwich (8) at the center of the evolution period provides improved resolution and sensitivity by virtue of its ability to suppress scalar interactions between all but the nonequivalent geminal protons. Complete ‘H- ‘H decoupling ( 9-12) in the HETCOR spectra can provide further improvements and sequence 1 C with its origin in the widely used COLOC sequence (13) is one of the simplest sequences that can provide such spectra with signal intensities dependent only on the homonuclear couplings. In situations involving a wide range of homonuclear ‘H- ‘H couplings sequence 1 C can be conveniently modified by replacing the 180” (C) pulse at the center of the constant time T by a BIRD sandwich, to remove the dependence of the crosspeak intensities on the homonuclear couplings.