Decoupler offset in ppm (0 = on resonance)

locked at all times. If this process is repeated over and over (+z to —z, — z to +z, etc.) during the acquisition of the FID, we have a decoupling sequence that can "cover" a very wide range of 13C shifts with relatively low power output. After all, decoupling is just the process of "confusing" the protons by showing them an attached 13 C that is rapidly flipping between the a and states. The effectiveness of adiabatic decoupling (WURST-40) is shown in Figure 11.6 (bottom). With less than half of the average B2 amplitude used for the GARP (rectangular pulse) decoupling scheme, we see good decoupling over a bandwidth of 200 ppm. For modern instruments with the capability of generating rapid strings of shaped pulses on the second (decoupler) channel, this is clearly the method of choice.

A look at direct (1JCH) and long-range (2'3JCH) heteronuclear couplings of glucose in the 1D 1H spectrum will prepare us for the direct (HSQC, HMQC) and long-range (HMBC) 2D experiments. Figure 11.7 shows the downfield region of the 1H spectrum of 1-13C-glucose in D2O, without13C decoupling (top) and with GARP 13C decoupling (bottom). The large (160-170 Hz)1JCH couplings are collapsed in the lower spectrum and only the homonuclear (3 JHH) couplings remain. In the upper spectrum the small amount of residual 12C-bound 1H is seen at the center of the wide 13C-bound 1H doublet. The upfield region of the 1H spectrum is shown in Figure 11.8. On the right-hand side, the ^-glu-2 proton shows up as a ddd (J = 9.4, 7.9, 6.2 Hz) without 13C decoupling (top) and as a dd (J = 9.4, 7.9 Hz) with 13C decoupling. The 6.2 Hz coupling can be assigned to the 2JCH coupling between H-2 and C-1 in ^-glucose. This is a relatively large CH coupling that would give rise to a strong crosspeak in a 2D HMBC spectrum. In the center portion two CH couplings are evident: a 2.3 Hz coupling on the right side (C-1 to H-5 of ^-glucose) added to the ddd pattern of H-5^ and a barely resolved 1.3 Hz coupling in the middle (C-1 to H-3 of ^-glucose) added to the triplet pattern of H-3^. In the left portion a 1.9 Hz CH coupling can be identified (C-1 to H-5 of a-glucose). All of these are 3 JCH couplings—relatively small because the protons are in axial positions and have a gauche relationship to C-1. These would probably show up as weak HMBC crosspeaks or would be lost in the noise if the proton already has complex splitting like H-5a. The arrows indicate peaks that are broadened by coupling to 13C with couplings too small to be resolved: C-1 to H-2 of a-glucose (middle section) and C-1 to H-3 of a-glucose (left). These couplings are probably too small to show up as crosspeaks in a 2D HMBC spectrum.

0 0

Post a comment