Heteronuclear Double Resonance Experiments

3 Heteronuclear Double Resonance Experiments. - 5.5.i HSQC HMQC. A number of modifications were proposed for detection of long-range 

A new element for simultaneous indirect detection of C and signals in labelled proteins was proposed by Uhrin et The CT- CHs, VT- N-HSQC sequence combines constant-time carbon evolution with variable delay nitrogen evolution. This is achieved by the variation of the position of a 90° pulse creating transverse coherence within the C constant-time period. The maximum indirect acquisition time for both nuclei is determined by constant-time period set to l/ /(C,C) = 28.6 ms. The method is best suited for detection of CH3 signals due to their slower relaxation. The proposed element was incorporated into NOESY-based experiments resulting in 3D NOESY-CH3NH and 3D HSQC-NOESY-CH3NH sequences. The experiments 

Heteronuclear coherence transfer periods of HSQC or HMQC experiments are often incorporated into semi-constant-time proton chemical shift evolution to reduce the effective relaxation and thus improve sensitivity. This method was further extended by Bazzo et to utilise N chemical shift evolution period of HMQC-NOESY experiment for further reduction of the proton effective relaxation rate. This is achieved by changing the position of refocusing pulse in the middle of the heteronuclear MQ evolution period when evolution is encoded. As the result the MQ evolution period that is 

HMQC-NOESY experiments were described and tested. 

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Numerous ID heteronuclear double resonance experiments of various types (e.g. H Sn, etc.) have been carried out already in the 1960s and... 

Experiments in which both the irradiated and the observed nuclei are protons are called homonuclear double-resonance experiments and are represented by the notation The irradiated nucleus is denoted by braces. When the observed and irradiated nuclei are different nuclides, as in proton-decoupled spectra, the experiment is a heteronuclear double-resonance experiment and is denoted, for example, C H. ... 

In triethylphosphine complexes of the type cis- and trans-[PtCl2(Et3P)2], heteronuclear double-resonance experiments have shown that the signs of the Jpcn - pcch coupling constants are negative and positive, respectively. ... 

Only one example is known for /( Si, Be) = 51 Hz in (/ -C5H5)Be-SiMe3 (65) For tri-coordinate silylboranes, the Si NMR signals are broad owing to partially relaxed Si-"B spin-spin coupling. In one case (66), the magnitude and sign of /( Si,"B) has been determined by H Si heteronuclear double-resonance experiments. Resolved 1 1 1 1 quartets (67) as the result of j( Si,"B) are observed for silylborates (Scheme 16). 

EXAMPLE 12.1 Suppose we were to observe the H spectrum of CH2F2 (Section 8.6.1) at a field strength of 5.87 T, while simultaneously irradiating the fluorines, (a) Does this experiment involve homo- or heteronuclear double resonance (b) What are the values of v, v2, and the difference (Av) between them ... 

In the previous article (1) on heteronuclear double resonance in this series it was suggested that such experiments would soon vie for importance with homonuclear ones. In fact, there has been an almost total eclipse partly as a result of the use of proton decoupling in all work, partly because of improved methods of frequency generation and control which have made experimental distinctions between the two types of experiment much less important. The present review therefore deals with both homo- and hetero-nuclear experiments and includes multiple resonance work also. The seven years up to mid-1978 are covered, although it has been impossible to mention every experiment. Emphasis is laid upon new ideas and developments of technique, with some preference for the more recent work. The Chemical Society specialist reports on NMR spectroscopy have included regular articles on multiple resonance (2,3) and a number of reviews deal with various aspects of the subject. (4-19) It has been decided to omit work on the solid state. 

When the two sets of nuclei are of different isotopes (e.g., C and H), the experiment is described as heteronuclear double resonance. When we observe the signal from nuclei of isotope A while irradiating nuclei of isotope B, we label the resulting spectrum with the shorthand designation A (B). When the two sets of nuclei belong to the same isotope (e.g., both H), the technique is described as homonuclear double resonance. 

The development of the double resonance experiments described in Section 3.8.2 was targeted at spin- /i systems. TRAPDOR and Rotational-Echo Adiabatic Passage Double Resonance (REAPDOR) experiments are designed specifically for quadrupolar nuclei and do not work on systems containing only spin- /2 nuclei. These MAS experiments still rely on the modulation of the heteronuclear dipolar coupling (as in the REDOR experiment) to prevent an echo from refocusing, but the modulation is no longer by 180° pulses. 

ATpairs, and is the basis of many so-called double-resonance experiments used for the structural determination of proteins and of other biological macromolecules, as we shall see later. A variation on the HSQC experiment is the heteronuclear multiple bond correlation (HMBC) experiment. This is a sensitive technique that maybe used to identify heteronuclear and spin-spin coupled nuclei. 

By using double resonance experiments, one can greatly simplify the spectrum coupling between different types of nuclei is confirmed by both the disappearance of the peak for the saturated nuclei and the collapse of the fine structure of the coupled nuclei. Nuclei of the same type can be decoupled, as in the proton-proton example given earlier this is called homonuclear decoupling. It is of course possible to decouple unlike nuclei, such as decoupling this is called heteronuclear decoupling. 

In double resonance experiments, rf-pulses or pulse sequences are applied not only to the resonating 7-spins, but also to the non-resonating S-spins. Magnetic interactions between the resonating and the non-resonating nuclei, especially the heteronuclear magnetic dipole-dipole interaction, can be studied selectively by double resonance techniques. 

It has been shown by Trebosc et that the FS REDOR (Frequency Selective Rotational Echo Double Resonance) experiment can be used for accurate through-space measurements in spin pairs that involve the quadrupolar nuclei. The experiment reveals both heteronuclear dipolar and scalar couplings, which can be re-introduced selectively site after site. As an example, couplings between the aluminium and phosphorus nuclei in the VPI5 zeohte have been measured. They agree very well with those reported in the literature, which validates the authors approach. 

To be exhaustive about ID double resonance NMR experiments, we have to mention the TEDOR (transferred echo double resonance) experiment, which involves transferring magnetization between two heteronuclear spins [45]. Nevertheless, this experiment has been applied only once to Al-based fluoride, on AIPO4-CJ2, a microporous... 

The previous decoupling methods involved coherent spatial averaging to remove the heteronuclear dipolar coupling. The dominant line-broadening mechanism in the H magnetic resonance of solids is normally the H- H homonuclear dipolar interaction, which cannot be removed by the double-resonance experiment if the spectrum is to be observed. In order to obtain narrow H resonances of solid samples, the line-broadening effects of the homonuclear dipolar interactions must be eliminated while the chemical-shift interactions are retained. 

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

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