LONG RANGE C-H COUPLING CONSTANT MEASUREMENT
While the simpleHMBC experiment provides the evidence for connectivity, i.e., that certain carbons and protons are two or three bonds apart, it provides insufficient information about the magnitude of these couplings. The EXSIDE experiment is one method of determining the value of two or three bond C-H couplings which typically range from 0 up to 12 Hz. In this experiment, a 2D carbon proton spectrum is obtained where cross peaks appear in the same position as in an HMBC spectrum, that is, at the shift of the corresponding carbon and proton. The cross peak is split in the carbon dimension and this splitting is the J(C-H) coupling,* scaling factor, where this scaling factor is chosen by the user. This allows for easy interpretation in comparison with other methods for J determination. A drawback, however, is that only one proton can be selected at a time(or, actually, one group of non-coupled protons) making the determination of many couplings tedious and time-consuming.
Shown below is a EXSIDE spectrum of sucrose in which the anomeric proton (H-1) at 5.35 ppm has been selected. Cross peaks are shown to its neighboring carbons, C-3 and C-5 at about 73 ppm, and across the glycosidic bond to carbon C`'-2 (105 ppm). The splitting at 105 ppm is 70 Hz which has been scaled by a factor of 20 indicating a coupling of 3.5 Hz.