One-bond 13C- 1H correlations are easy to spot due to their large value and when joined with a straight line the corresponding cross-peaks form a forward slash '/'. The latter is determined based on the tilt of the cross-peaks, reminiscent to a COSY45 experiment for J HH couplings. For three sitespecifically labeled oligosaccharides, it is demons. The HSQC-HECADE experiment is unique in this regard as it allows to measure not only very small couplings, but also their sign. Sitespecific 13C isotope labeling is a useful approach that allows for the measurement of homonuclear 13C,13C coupling constants. NMR experiments sensitive to the sign of the J coupling are rather rare. Thus, if we know that the sign of nJ CH is negative, then the number of bonds between C and H cannot be three and using the absolute value of the coupling constant we could make an educated guess on whether it corresponds to 2J CH or 4J CH. This has important implications for structure determinations of, for example, natural products, where no a priori assumptions can be made about atom connection pattern. It is well known that 13C- 1H couplings over one and three bonds ( 1J CH and 3J CH)are positive while those over two or four bonds ( 2J CH or 4J CH) are either positive or negative.
