J-Resolved

Another useful-sounding technique is the proton /-resolved experiment in which chemical shift and coupling information are separated into two different dimensions. This is equivalent to turning the peaks sideways and looking down on them from above so that viewing them in the x direchon, they all appear as singlets, but in the y direction, they reveal their multiplicities. 

This would be very useful indeed, particularly where overlapping multiplets are concerned. Unfortunately, in the very circumstances where the technique would be most useful, it tends to fall over with strong artefacts becoming intrusive in strongly coupled systems. 

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Homonuclear teclmiques such as J-resolved spectroscopy also exist for rotatmg all multiplets tlirough 90°, to resolve overlaps and also give a ID spectrum from which all homonuclear couplings have been removed [26]. 

Aue W P, Kharan J and Ernst R R 1976 Homonuclear broadband decoupling and two-dimensional J-resolved NMR spectroscopy J. Chem. Phys. 64 4226-7... 

Generally, the most powerful method for stmctural elucidation of steroids is nuclear magnetic resonance (nmr) spectroscopy. There are several classical reviews on the one-dimensional (1-D) proton H-nmr spectroscopy of steroids (267). C-nmr, a technique used to observe individual carbons, is used for stmcture elucidation of steroids. In addition, C-nmr is used for biosynthesis experiments with C-enriched precursors (268). The availability of higher magnetic field instmments coupled with the arrival of 1-D and two-dimensional (2-D) techniques such as DEPT, COSY, NOESY, 2-D J-resolved, HOHAHA, etc, have provided powerful new tools for the stmctural elucidation of complex natural products including steroids (269). 

Figure 2.3. J-resolved two-dimensional C NMR spectra series of a-pinene (1) [in (CDsbCO, 25 °C, 50 MHz],...

There are basically three main types of 2D NMR experiments J-resolved, shift correlation through bonds (e.g., COSY), and shift correlations through space e.g., NOESY). These spectra may be of homonuclear or heteronuclear type involving interactions between similar nuclei (e.g., protons) or between different nuclear species (e.g., H with C). 

Three-dimensional NMR spectra, like 2D NMR spectra, may be broadly classified into three mtyor types (a) 3D J-resolved spectra (in which the... 

A detailed analysis of the proton high field NMR spectra of tomato juice and pulp has recently been acquired [15]. The combination of suitable selective and two-dimensional techniques (J-resolved, COSY, TOCSY, DOSY, etc.) was used for... 

The two axes (dimensions) in our 2D spectra are thus both frequency axes. We shall see as we continue that we can adjust our experiment so as to choose different types of frequency information. An early experiment, known as the J-resolved experiment, was designed in such a way that one axis was the (proton or carbon) chemical shift axis and the other the one-bond proton-carbon coupling constant. Flowever, this experiment is not generally very useful for structural determination, so that we shall not discuss it here. 

J-resolved spectroscopy Two-dimensional techniques, both homo- and heteronuclear, that aims to simplify interpretation by separating chemical shift and coupling into the two dimensions. Unfortunately prone to artifacts in closely coupled systems. 

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

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.

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]. 

J-Resolved Constant Time r Experiment for the Determination of the Phosphodiester Backbone Angles a and f 172... 

For the measurement of cross-correlated relaxation rates, there are mainly three methods that have been used in practice. In the /-resolved constant time experiment, the multiplet Hnes exhibiting differential relaxation are resolved by the f couplings, and the line width is translated into intensity in a constant time experiment (Fig. 7.19a,d). In the J-resolved real time experiment the line width of each multiplet line is measured instead (Fig. 7.19b, d). This experiment has been applied so far only for the measurement of... 

J-Resolved Constant Time Measurement of Cross-Correlated Relaxation Rates... 

Fig. 7.20 J-resolved constant r HN (CO)CA experiment for the measurement of cross-correlated relaxation rates, especially rcNH The experiment has two 90° (15N) pulses (shaded part) simulta-...

Fig. 7.21 Traces through co2 of the J-resolved constant r HN(CO)CA of rhodniin. The different multiplet patterns obviously indicate different geometries of the HN and Ha,Ca vectors for these residues.

As an example of the measurement of cross-correlated relaxation between CSA and dipolar couplings, we choose the J-resolved constant time experiment [30] (Fig. 7.26 a) that measures the cross-correlated relaxation of 1H,13C-dipolar coupling and 31P-chemical shift anisotropy to determine the phosphodiester backbone angles a and in RNA. Since 31P is not bound to NMR-active nuclei, NOE information for the backbone of RNA is sparse, and vicinal scalar coupling constants cannot be exploited. The cross-correlated relaxation rates can be obtained from the relative scaling (shown schematically in Fig. 7.19d) of the two submultiplet intensities derived from an H-coupled constant time spectrum of 13C,31P double- and zero-quantum coherence [DQC (double-quantum coherence) and ZQC (zero-quantum coherence), respectively]. These traces are shown in Fig. 7.26c. The desired cross-correlated relaxation rate can be extracted from the intensities of the cross peaks according to ... 

Fig. 7.26 J-resolved constant r C,H-HSQC experiment (a) to measure the cross-correlated relaxation rates in RNA with a geometry given in b.

These results suggest that the signals arise from dipole-dipole coupled protons. Kreis et al. confirmed this finding by measurements using one-dimensional zero- and double-quantum filtering, two-dimensional J-resolved spectroscopy, two-dimensional constant time COSY and longitudinal order separation... 

Figure 2.6 2D J-resolved solid-state NMR spectrum of Hf( CH2Bu )3/Si02 (soo) (a) and of Zr("CH2Bu )3/Si02-(8oo) (b) trace extracted along the tB dimension of the 2D J-resolved spectrum at S = 106 and 93 ppm, respectively.

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