Coherence transfer experiments

B.2 HETERONUCLEAR (USUALLY - C) EXPERIMENTS B.2.1 Coherence-Transfer Experiments... 

NMR spectra have been used frequently to elucidate and/or confirm the structures of these heterocycles, but little or no systematic study had been done. A detailed study of l3C NMR spectra by distortionless enhancement by polarization transfer (DEPT), inverse H-I3C coherence transfer experiments (HMQC and HMBC) and by INADEQUATE of factor F0 has been reported <91JBC9622>. The 13C NMR spectra of a series of pyrido[4,3-J]pyrimidines were interpreted on the basis of a detailed study of other analogues <91JCS(P2)1559>. Carbon-13 NMR spectroscopy has been used to indicate the site of alkylation of pyrido[2,3-c]pyridazin-4-ones <90CPB3359>. 

Coherent transfer experiments can roughly be divided into two classes pulse-interrupted free-precession experiments and Hartmann-Hahn-type experiments (Ernst et al., 1987). Examples of homo- and heteronuclear pulse-interrupted free-precession coherence transfer are COSY (correlation spectroscopy Aue et al., 1976), RELAY (relayed correlation spectroscopy Wagner, 1983), and INEPT (insensitive nucleus enhancement by polarization transfer) transfer steps (Morris and Freeman, 1979 Burum... 

Here the generic term Hartmann-Hahn experiment is used for polarization- or coherence-transfer experiments that are based on the Hartmann-Hahn principle (see Section II), that is, on matched effective fields that are created by a rf irradiation scheme. These experiments may be classified according to the following practical and theoretical aspects (see Fig. 6) that are related to properties of samples, spin systems, coherent magnetization transfer, effective Hamiltonians, multiple-pulse sequences, and incoherent magnetization transfer ... 

If the coupling constants are known in advance, the total mixing time can be reduced in multiple-step selective coherence-transfer experiments by using the selective homonuclear analog of the optimized heteronuclear two-step Hartmann-Hahn transfer technique proposed by Majumdar and Zuiderweg (1995). In this technique [concatenated cross-polarization (CCP)] a doubly selective transfer step (DCP) is concatenated with a triple selective mking step (TCP). For the case of a linear three-spin system with effective planar coupling tensors, a CCP experiment yields complete polarization transfer between the first and the third spin and the total transfer... 

Because liquid state Hartmann-Hahn transfer relies on J couplings, Hartmann-Hahn experiments yield important information for the elucidation and assignment of complicated 7-coupling networks. Hartmann-Hahn transfer has a number of advantages compared to coherence-transfer experiments based on pulse-interrupted free precession most importantly, it is often more efficient, particularly if the linewidth is comparable to or... 

A number of "X, ""Y coherence transfer experiments involving polarization transfer from or to quadrupolar nuclei were performed by using both INEPT and the universal polarization transfer (UPT) scheme (Fig. 1(a)) which is a generalization of DEPT. The use of the INEPT method is unproblematic. It can be applied without modification for polarization transfer from any number of equivalent spin-half nuclei to a nucleus of arbitrary spin, and a report on successful measurement of C, D INEPT spectra also demonstrated its utilization for magnetization transfer from a quadrupolar to a spin-half nucleus. Application of the UPT scheme requires appropriate adjustment of the variable pulses <(> and 9 to the spin and number of the... 

TOCSY spectra have been reported often [2,27,30-40], and have been valuable in finding minor structures in lignins [34,41]. The older relayed coherence transfer experiments (where magnetization is transferred sequentially from one proton to the next coupled proton) provided similar data, but is seldom used today these experiments were useful for assignments in the eight isomers of trimeric lignin 3-ether models [11]. 

A perennial problem with coherence transfer experiments is the choice of coupling constant to use for calculating the free precession period prior to the coherence transfer step. The same problem arises when calculating the refocusing delay after a coherence transfer step prior to data acquisition under decoupling. 

Historically, NMR experiments such as cross-polarization [87], DEPT [88], and INEPT [89] have been used to transfer spin polarization from abundant spins (such as protons) to a dilute spin system. The few examples of coherence transfer experiments involving quadrupolar nuclei do make use of magnetization transfer from protons to nuclei such as Mo 190], " Na (91], and O 192], but, as noted in previous sections, zeolites and the related AIPO4 structures contain many quadrupolar nuclei but few protons. Nevertheless, it is still possible to perform coherence transfer between other spin pairs in these systems, for example, between Si (/ = 1/2) and Al (/ = 5/2) in zeolites, or between P (/ = 1/2) and Al (/ = 5/2) in AIPO4S. 

The inverse-detected 2D NMR experiments that have been discussed to this point have all been discrete, single-purpose experiments, e.g. correlating protons with their directly bound heteronucHde (typically or N). There are another class of inverse-detected 2D NMR experiments that are generally referred to as hyphenated 2D experiments. These are experiments that first establish one type of correlation, followed by an additional experiment segment that then pursues a further spectroscopic task. Predecessors of the inverse-detected variants of these experiments were the HC-RELAY (proton—carbon heteronuclear relayed coherence transfer) experiments pioneered by Bolton [151—155]. Examples of these include, but are by no means hmited to HXQC-COSY and -TOCSY [156—158], -NOESY [159], -ROESY [160], and more recent gradient variants [161] etc., where X = S (single) or M (multiple) quantum variants of the experiments. 

Friedrich J, Davies S, Freeman R (1987) Shaped selective pulses for coherence-transfer experiments. J Magn Reson 75 390-395... 

A new 3D spin-state-selective coherence transfer experiment has been intro-... 

Hubrich M, Jeschke G, Schweiger A. 1995. The generalized hyperfine sublevel coherence transfer experiment in one and two dimensions. J Chem Phys 104 2172-2184. 

An example of the 2D-heteronuclear relayed coherence transfer experiment is provided by the 2D contour plot of 2-acetonaphthalene shown in Figure 5.78. The ID proton and NMR spectra are drawn on the axes. The resonances due to direct (nonrelayed) coupling interactions are indicated with an asterisk, and they were determined by a conventional COSY experiment. The cross peak marked A shows a direct coupling between C-3 and the protons corresponding in position on the vertical axis to the cross peak A (see dashed line). The cross peak at B shows that the carbon marked 4 is indirectly coupled to the same protons. Therefore carbon 3 must be adjacent to carbon 4. 

In the relayed coherence transfer experiment the first step is an initial homonuclear magnetization transfer (i.e., from proton A to proton B). The coherence is then relayed to a third nucleus, which may be a proton (homonuclear relay) or a different nucleus (e.g., heteronuclear relay). Recently, another variation of the relay experiment has been reported which involves transfer of coherence to a heteronuclear spin as the first step ( H-X heteronuclear shift correlation). The coherence is then relayed to a third nucleus, a proton (X- H coherence transfer), which is the nucleus observed. ... 

G. King and P.E. Wright, Application of two-dimensional relayed coherence transfer experiments to H-NMR studies of macromolecules, J. Magn. Resonance 54, 328 (1983). 

Figure 18 shows the application of relayed coherence transfer to phos-phothreonine, an AMQjX spin system (Bolton and Bodenhausen, 1982b). The contour plot of the two-dimensional data set in Fig. 18 has correlated all the protons of phosphothreonine with the P resonance. Figure 19 compares the simulated H spectrum of phosphothreonine with a phase-sensitive cross-section, two-dimensional data set. Quantitative analysis of coupling constants can be obtained for protons not directly coupled to the P nucleus. The relayed coherence-transfer experiment makes possible analysis... 

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