JOURNAL OF MAGNETIC RESONANCE, Series A 119, 134–138 (1996) ARTICLE NO. 0063 Increased HMBC Sensitivity for Correlating Poorly Resolved Proton Multiplets to Carbon-13 Using Selective or Semi-selective Pulses

Correlations based on scalar coupling between protons ity, in much the same way as the use of selective 1H pulses in the refocused INEPT (7, 8) experiment can be used toand carbons separated by two or three chemical bonds are very useful for structure determination of organic molecules. transfer magnetization more effectively through long-range 1H – 13C couplings (9, 10).In particular, the heteronuclear multiple-bond correlation ex-periment (HMBC) has become widely used for this purpose Figure 1 shows three different implementations of the 1H-selective HMBC experiment. Selection of the appropriate(1). This experiment enjoys the increased sensitivity re-sulting from detection of the NMR-sensitive 1H spin, and, scheme depends on the particular application. Several of the experimental details will be discussed later, but first wein contrast to most of the 13C-detected analogs, long-range correlation intensities are not affected by the size of the briefly discuss when to use which pulse sequence. The scheme of Fig. 1A is the simplest and is identical to theone-bond JCH couplings. The HMBC experiment requires a relatively long delay for obtaining 1H magnetization in regular HMBC pulse scheme, except that the 1H RF power is attenuated and the carrier position is selected such thatantiphase with its long-range coupled 13C, prior to generation of multiple-quantum coherence. Similarly, after the evolu- the proton of interest, but not the protons J-coupled to the proton of interest, experiences the 1807 pulse. If the 1H oftion period, 1H magnetization becomes observable only after it is rephased with respect to its long-range 13C coupling interest is sufficiently separate from its coupling partners i