Shift correlation, heteronuclear proton-detected

A H(detected)- C shift correlation spectrum (conmion acronym HMQC, for heteronuclear multiple quantum coherence, but sometimes also called COSY) is a rapid way to assign peaks from protonated carbons, once the hydrogen peaks are identified. With changes in pulse timings, this can also become the HMBC (l eteronuclear multiple bond coimectivity) experiment, where the correlations are made via the... 

HC HMQC (heteronuclear multiple quantum coherence) and HC HSQC (heteronuclear single quantum coherence) are the acronyms of the pulse sequences used for inverse carbon-proton shift correlations. These sensitive inverse experiments detect one-bond carbon-proton connectivities within some minutes instead of some hours as required for CH COSY as demonstrated by an HC HSQC experiment with a-pinene in Fig. 2.15. 

NMR probes are designed with the X-coil closest to the sample for improved sensitivity of rare nuclei. Inverse detection NMR probes have the proton coil inside the X-coil to afford better proton sensitivity, with the X-coil largely relegated to the task of broadband X-nucleus decoupling. These proton optimized probes are often used for heteronuclear shift correlation experiments. 

Direct heteronuclear chemical shift correlation Conceptually, the 2D J-resolved experiments lay the groundwork for heteronuclear chemical shift correlation experiments. For molecules with highly congested 13C spectra, 13C rather than XH detection is desirable due to high resolution in the F% dimension [40]. Otherwise, much more sensitive and time-efficient proton or so-called inverse -detected heteronuclear chemical shift correlation experiments are preferable [41]. 

The first of the proton-detected experiments is the Heteronuclear Multiple Quantum Correlation HMQC experiment of Bax, Griffey and Hawkins reported in 1983, which was first demonstrated using 1H-15N heteronuclear shift correlation [42]. The version that has come into wide-spread usage, particularly among the natural products community, is that of Bax and Subramanian reported in 1986 [43]. A more contemporary gradient-enhanced version of the experiment is shown in Fig. 10.14 [44],... 

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

Hyphenated heteronuclear shift correlation methods Many of the experiments described thus far can be used as building blocks to create more sophisticated pulse sequences. The earliest variants of hyphenated NMR experiments, such as HC-RELAY, were heteronuclear detected [56]. More recent hyphenated heteronuclear shift correlation methods are based on proton detection and include... 

The task of generating a display of heteronuclear X/Y-connectivities with optimum sensitivity can in principle be performed by recording a three-dimensional proton detected shift correlation in which the chemical shifts of both heteronuclei X and Y are each sampled in a separate indirect dimension. Three-dimensional fourier transformation of the data then generates a cube which is defined by three orthogonal axes representing the chemical shifts of the three nuclei 1H, X, Y, and the desired two-dimensional X/Y-correlation is readily obtained as a two-dimensional projection parallel to the axis... 

Torres, A.M., Nakashima, T.T, and Mcclung R.E.D. 1993. J-compensated proton-detected heteronuclear shift-correlation experiments. J. Magn. Reson. Ser. A 102 219-227. 

In the case of an unknown chemical, or where resonance overlap occurs, it may be necessary to call upon the full arsenal of NMR methods. To confirm a heteronuclear coupling, the normal H NMR spectrum is compared with 1H 19F and/or XH 31 P NMR spectra. After this, and, in particular, where a strong background is present, the various 2-D NMR spectra are recorded. Homonuclear chemical shift correlation experiments such as COSY and TOCSY (or some of their variants) provide information on coupled protons, even networks of protons (1), while the inverse detected heteronuclear correlation experiments such as HMQC and HMQC/TOCSY provide similar information but only for protons coupling to heteronuclei, for example, the pairs 1H-31P and - C. Although interpretation of these data provides abundant information on the molecular structure, the results obtained with other analytical or spectrometric techniques must be taken into account as well. The various methods of MS and gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy supply complementary information to fully resolve or confirm the structure. Unambiguous identification of an unknown chemical requires consistent results from all spectrometric techniques employed. 

The development of older heteronuclear 2 D N M R experiments relied on the detection of the heteronuclide, typically C. Quite early it was recognized that detection via a proton or other high sensitivity, high natural abundance nuclide, for example or offered a considerable sensitivity advantage [2,3]. There were, however, experimental challenges to overcome, specifically in the case of shift correlation... 

Heteronuclear chemical-shift correlation experiments usually involve protons as one of the nuclei and can be performed by detecting either protons or the X nuclei (the most common being C Section 6-2). The principal advantage of H-detected experiments is their sensitivity, which is a function of the gyromagnetic ratios (yh/Yx) - terms of the ratios of Larmor frequencies (instead of y s) at, say, 400 MHz for and 100 MHz for C, the benefit of detecting H rather than C is (400/100) / = 8. 

X-nucleus-detected experiments, however, have an advantage that can become important. In heteronuclear, chemical-shift correlation experiments, it is, as a rule, better to detect the nucleus with the more congested spectrum, which is almost always protons. On occasion, however, X-nucleus spectra are more crowded than their counterparts. In this situation, it could be better (if sensitivity permits) to carry out an experiment that detects the X nuclei. 

The two principal H-detected, direct, heteronuclear chemical-shift correlation experiments are HMQC and HSQC. The X-nucleus-detected counterpart is HETCOR, The and X-nucleus spectral widths are reduced in each of these experiments. It is important to remember that the latter should be decreased to contain only the signals of protonated X nuclei. Quaternary carbons, for example, do not participate in these experiments, and their signals should not be included in the reduced spectral windows. 

Heteronuclear chemical-shift correlation experiments can be performed by detecting either protons or the X nuclei (Section 7-8). All of the comments that were made there for the direct, heteronuclear chemical-shift correlation experiments apply equally well to their indirect (or longer range—e.g., two- and three-bond correlation) counterparts. 

Not all these steps may be necessary, and the direct observation of a heteronuclide, such as carbon or nitrogen, can often be replaced through its indirect observation with the more sensitive proton-detected heteronuclear shift correlation techniques. 

Traditional methods of heteronuclear shift correlation follow the scheme of Fig. 6.1b above, relying on the direct observation of the X-spin and with the source nucleus (typically protons) detected indirectly. As discussed at the beginning of this chapter, this general approach is less sensitive than... 

The advantages of the TOCSY experiment have led to the implementation of spinlock sequences in heteronuclear correlation experiments to detect additional homonuclear coupling interaction. In the l C, IR TOCSY-HMQC experiment in addition to cross peaks from one-bond connectivities between a DC and a IR nucleus there are correlation peaks to protons which are coupled to the primary proton by homonuclear H, H coupling. The advantage of this method is that it allows the differentiate between overlapped iR signals of different spin systems provided that the spin systems differ by the carbon shift of at least one carbon atom. This approach is shown by the simulations in the Check it 5.4.2.4. 

---