Heteronuclear 2D correlation

Gelsevirine (2) was first isolated in 1953 from G. sempervirens as a minor component (3). Its structure was later elucidated on the basis of mass spectrometry as well as H-NMR and 13C-NMR studies (4). Gelsevirine has been found to be the predominant alkaloid in G. rankinii (24), and it was claimed that some of the previously reported 1 H-NMR and 13C-NMR data should be revised. Thus the previous assignments of H-16, H-15, H-14a, H-14e, and H-6 for gelsevirine should be changed to H-15, H-14a, H-16, H-6, and H-14e, respectively, from the evidence of the more accurate homonu-clear 2D COSY experiments. Similarly, from the heteronuclear 2D correlation spectrum, the assignments for C-16, C-15, C-6, and 1V-CH3 should be revised to C-15, C-16,1V-CH3, and C-6, respectively. 

General symmetry principles for rotor-synchronized pulse sequences in MAS solid-state NMR have been presented. The synunetry theory has been extended to the case of generalized Hartmann-Hahn sequences, in which rotor-synchronized r.f. irradiation is applied simultaneously to two isotopic spin species. The symmetry theory has been used to design pulse sequences which implement heteronuclear dipolar recoupling at the same time as decoupling homonuclear spin-spin interactions, and which also suppress CSAs. Experimental demonstrations of heteronuclear 2D correlation spectroscopy, heteronuclear MQ spectroscopy, and the estimation of intemuclear dipolar couplings have been given. 

A variety of methods have already been described in Chapter 4 that allow the editing of the ID spectrum of the heteronuclear spin, for example those based on spin-echoes or polarisation transfer, so providing valuable information on the numbers of attached protons. They do not, however, provide any direct evidence for which protons are attached to which heteronucleus (X-spin) in the molecule and for this heteronuclear 2D correlations are widely used to transfer previously established proton assignments onto the directly bonded heteronucleus or, on occasions, vice versa. This may then provide further evidence to support or reject the proposed structure as being correct, or may provide assignments that can be used as the basis of further investigations. In... 

The processes that occur during the evolution period are probably the most important in describing the effect of the complete pulse sequence. During this period coherence can evolve, coherence can be selectively manipulated or coherence transfer can occur. Coherence manipulation can be the inversion of the coherence order (WATERGATE experiment) or in a l S spin system a phase shift depending upon signal multiplicity (APT or SEMUT experiment). In the case of heteronuclear IS spin systems the creation of antiphase coherence and subsequent polarization transfer using a INEPT or a DEPT unit can be used in multiplicity edited experiments or heteronuclear 2D correlation experiments. In transient NOE experiments such as ROE and TROESY, coherence... 

The focus of the chapter then shifts to heteronuclear 2D correlation (HETCOR) experiments. The evolution of single quantum coherence (SQC) of two different nuclei in these experiments with a coherence transfer step provides the basic formula for these correlations. In WISE (wide line separation), a wide dipolar... 

Another excellent example of the application of NMR techniques in the structure determination of natural products is the structural resolution of rhizobactin, a siderophore isolated from Rhizobium meliloti (354). The proton homonuclear 2D-correlated spectrum revealed the four separate coupled units, whereas the heteronuclear 2D-correlated spectrum established the assignments (Fig. 2.43). Together, these spectra revealed that rhizobactin is composed of one unit each of ethylene diamine, alanine, lysine, and L-malic acid. The sequencing of... 

Magnesium ethanol adducts of the type MgCl nEtOH (n < 6) are especially effective as the precursors of high activity Zeigler-Natta catalysts. Many of these solids are mixtures of two phases. Solid-state NMR (heteronuclear 2D correlated) of MgCl -nEtOH ( < 6), have provided detailed information on the phase composition and molecular-level structures that are present. Such information also provides an explanation for the high activities of catalysts derived from these solids. 

Some of the most important 2D experiments involve chemical shift correlations between either the same type of nuclei (e.g., H/ H homonu-clear shift correlation) or between nuclei of different types (e.g., H/ C heteronuclear shift correlation). Such experiments depend on the modulation of the nucleus under observation by the chemical shift frequency of other nuclei. Thus, if H nuclei are being observed and they are being modulated by the chemical shifts of other H nuclei in the molecule, then homonuclear shift correlation spectra are obtained. In contrast, if C nuclei are being modulated by H chemical shift frequencies, then heteronuclear shift correlation spectra result. One way to accomplish such modulation is by transfer of polarization from one nucleus to the other nucleus. Thus the magnitude and sign of the polarization of one nucleus are modulated at its chemical shift frequency, and its polarization transferred to another nucleus, before being recorded in the form of a 2D spectrum. Such polarization between nuclei can be accomplished by the simultaneous appli-... 

In homonuclear-shift-correlated experiments, the Ft domain corresponds to the nucleus under observation in heteronuclear-shift-correlated experiments. Ft relates to the unobserved or decoupled nucleus. It is therefore necessary to set the spectral width SW, after considering the ID spectrum of the nucleus corresponding to the Ft domain. In 2D /-resolved spectra, the value of SW depends on the magnitude of the coupling constants and the type of experiment. In both homonuclear and heteronuclear experiments, the size of the largest multiplet structure, in hertz, determines... 

SWi, which in turn is related to the homonuclear or heteronuclear coupling constants. In homonuclear 2D spectra, the transmitter offset frequency is kept at the center of (i.e., at = 0) and F domains. In heteronuclear-shift-correlated spectra, the decoupler offset frequency is kept at the center (Fi = 0) of thei i domain, with the domain corresponding to the invisible or decoupled nucleus. 

The matrix obtained after the F Fourier transformation and rearrangement of the data set contains a number of spectra. If we look down the columns of these spectra parallel to h, we can see the variation of signal intensities with different evolution periods. Subdivision of the data matrix parallel to gives columns of data containing both the real and the imaginary parts of each spectrum. An equal number of zeros is now added and the data sets subjected to Fourier transformation along I,. This Fourier transformation may be either a Redfield transform, if the h data are acquired alternately (as on the Bruker instruments), or a complex Fourier transform, if the <2 data are collected as simultaneous A and B quadrature pairs (as on the Varian instruments). Window multiplication for may be with the same function as that employed for (e.g., in COSY), or it may be with a different function (e.g., in 2D /-resolved or heteronuclear-shift-correlation experiments). 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

The basic pulse sequence employed in the heteronuclear 2D shift-correlation (or HETCOR) experiment is shown in Fig. 5.40. The first 90° H pulse bends the H magnetization to the y -axis. During the subsequent evolution period this magnetization processes in the x y -plane. It may be considered to be made up of two vectors corresponding to the lower (a) and higher (/3) spin states of carbon to which H is coupled. These two... 

Figure 5.40 (A) Pulse sequence for the 2D heteronuclear shift-correlation experiment. (B) Effect of the pulse sequence in (A) on H magnetization vectors of CH.

Fig. 10.13. 2D J-resolved NMR spectrum of santonin (4). The data were acquired using the pulse sequence shown in Fig. 10.12. Chemical shifts are sorted along the F2 axis with heteronuclear coupling constant information displayed orthogonally in F . Coupling constants are scaled as J/2, since they evolve only during the second half of the evolution period, t /2. 13C signals are amplitude modulated during the evolution period as opposed to being phase modulated as in other 13C-detected heteronuclear shift correlation experiments.

Homonuclear as well as heteronuclear 2D shift correlation experiments ( H/ H-COSY, H/ C-COSY, H/C COSY- H/ H-TOCSY), involving the perturbation of either one or two types of nuclei respectively and in the heteronuclear case including both the conventional, direct C detection, as well as the more sensitive, indirect ( inverse or reverse ) H-detection. 

The experiments are used to correlate H- and C chemical shifts. The C detected "C/ H-COSY experiment (Fig. 3.29a) is the most popular heteronuclear 2D experiment available with older type spectrometers, not yet equipped with inverse" H detection mode. Due to the surviving geminal couplings in FI the signals of methylene groups with non-equivalent protons are immediately recognized. 

The modified heteronuclear shift-correlation experiment described by Bauer et al,54 has the advantage of giving 2D-spectra with proton shifts in the F, dimension and long-range C,H splittings. However, its overall sensitivity is low because of relaxation during relatively long delays, and the resolution obtained is only to the nearest Hertz. 

So far we have discussed homonuclear 2D H,H-shift correlation spectroscopy (H,H-COSY) as well as heteronuclear 2D C,H-shift correlation spectroscopy (C,H-COSY, or C,H-HSC). Let us now consider homonuclear 2D C,C-shift correlation spectroscopy. 

HOM2DJ, 2D homonuclear 7-resolved experiment HSC, 2D heteronuclear shift correlation Hz, unit of frequency (cycles per second)... 

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