Nuclear Magnetic Resonance Facility (NMR) | Indirect Detection 2D Experiments

Indirect Detection 2D Experiments
Apart from identifying the spin network of each ring the peaks were assigned straightforward using the gradient selected two-dimensional NMR techniques such as gHSQC and gHMBC in combination with the hydride experiment (gHSQC-TOCSY, gHMQC-TOCSY).
HSQC (Heteronuclear Single Quantum Correlation) spectroscopy is an inverse chemical shift correlation experiment that yields exactly the same information as the HETCOR. The advantage of HSQC is that the nucleus with the highest gamma (¹H) is detected, and so it is possible to obtain the highest sensitivity. The challenge of an inverse chemical shift correlation experiment, however, is that the large signals from 1H not coupled directly to a ¹³C nucleus must be suppressed in a difference experiment.
HMBC (Heteronuclear Multiple Bond Correlation) spectroscopy is a modified version of HSQC suitable for determining long-range ¹H-¹³C connectivities. Since it is a long-range chemical shift correlation experiment, HMBC provides basically the same information as COLOC (also long range HETCOR) but it has a higher sensitivity, since it is an inverse experiment.
HETCOR spectrum
gHMBC spectrum
gHMBC spectrum-expansion
gHMBC spectrum-expansion 1
gHMBC spectrum-expansion 2
gHMBC spectrum-expansion 3
gHMBC spectrum-expansion 4
2D HSQC-TOCSY spectroscopy
2D HSQC-TOCSY spectroscopy (Heteronuclear Single Quantum Coherence Total Correlation Spectroscopy) is a powerful technique for identifying spin systems. The single quantum HSQC sequence filters out all protons not spin-coupled to a ¹³C, while addition of the TOCSY sequence distributes this information to all protons and thereby allows the detection of long-range as well as direct ¹H-¹³C correlations.
In other words, the HSQC-TOCSY combines two very useful experiments. The HSQC part of the experiment correlates protons with their directly bound carbons and the TOCSY part of the experiment relays the magnetization around the proton spin system. The extent of the relay is dependent on the mixing time in the TOCSY experiment. The observation of how the magnetization transfer varies with mixing time thus gives valuable information about the network of couplings in the molecule.
The gHSQC-TOCSY spectrum of 1 with a mixing time of 36 msec, showed relayed responses between the proton pairs that are ortho to each other. The contour plot of gHSQC-TOCSY revealed the following pairs of cross peaks: C-20 (d 119.98)/H-20 (d 7.37); C-14 (d 124.51)/H-13 (d 7.34); C-1 (d 123.49)/H-2 (d 6.97); C-23 (d 122.82)/ H-22 (d 6.60). Some direct responses were not removed in the contour plot of the gHSQC-TOCSY spectra.
Finally, the gHSQC-TOCSY spectrum with a mixing time of 80 msec showed that the direct responses have disappeared and we can observe relayed responses only between the para pairs of protons of the ring (A-D). There are strong cross peaks between C-14 (d 124.51) and H-11 (d 9.73) and between C-1 (d 123.49) and H-4 (d 9.29) etc.
gHSQC-TOCSY spectrum (mix = 80 ms)
gHSQC-TOCSY spectrum (expansion)
gHSQC-TOCSY spectrum (expansion 2)
gHSQC-TOCSY spectrum (expansion 1)