2D J-resolved 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]. 

Fig. 3.27 2D spectrum (tilted) of peracetylated glucose from a homonuclear 2D J-Resolved experiment. 

Figure 10 Schematic representation of nontilted 2D J-resolved experiment. Projection onto the chemical shift axis recovers the high resolution ID NMR spectrum.

The 2D spin diffusion NMR experiment allows us to examine further the spectral assigments obtained from the ID and the 2D J-resolved experiments [51]. It also provides new details concerning distribution of hydrocarbons in zeolite ZSM-5. Spectral spin diffusion in the solid state involves simultaneous flip-flop transitions of dipolar-... 

In order to isolate and structurally identify the remaining bryostatins earlier detected only in microgram or less quantities and to obtain enough bryostatin 1 for homonuclear correlation (COSY), two dimensional carbon-proton chemical shift correlations ( H- C 2D), 2D-J-resolved experiments, proton-proton differential nuclear Overhauser enhancement (NOEDS, or H-[ H] NOE) experiments, and additional biological evaluation, a 4000-L recollection (1981) of Bugula neritina... 

There are several types of heteronuclear 2D J-resolved experiments (i) the gated decoupler method, (ii) the spin-flip method, (iii) the selective spin-flip method, (iv) the semi-selective spin-flip method, and (v) the use of polarization transfer, e.g., INEPT. In all these experiments, the pulse sequence results in the generation of spin echoes which are modulated during the evolution period by coupling frequencies. 

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. 

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.

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

The presence of anisotropic interactions in solids leads to a greater variety of 2D sequences. One of the simplest is the isotropic/anisotropic correlation, where high resolution is achieved in one dimension while preserving anisotropic information in the second dimension. The dipshift and J-resolved experiments correlate isotropic chemical shift with dipolar and J-coupling information respectively. In these sequences the... 

The pulse sequence, as a variant of the spin echo experiment, also refocuses the spread of frequencies caused by field inhomogeneity, so that some improvement in resolution is obtained. The inset at the lower right of Figure 6-18 shows the normal ID spectra of H-4 and H-5 at the top (Figure 6-18c and e) and the unrotated projection of the 2D J-resolved spectra at the bottom [Figure 6-18d and f, extracted from the projected spectrum (Figure 6-18a) at the top of the 2D display]. The much higher resolution of the 2D resonances is clearly evident. Thus, the procedure is an effective way to measure J accurately, particularly when J is poorly resolved in the ID spectrum. The experiment fails for closely coupled nuclei (second-order spectra). 

At the time of writing Shapiro and his group have shown further that the loss of coupling information which accompanies the spin-echo experiment, can be restored by utilising a 2D J-resolved procedure [132]. Similarly Sprensen and his co-workers have reported the results of 2D H homonuclear correlations to reduce the inhomogeneous line broadening [133]. 

In this section the simple one-pulse experiment and different multi-pulse ID experiments and the 2D coupling resolved experiment will be discussed. By means of the one-pulse experiments several concepts are introduced such as pulse length calibration or delay calculation which must be kept in mind when successfully setting up an n dimensional experiment. The J-resolved 2D experiment is included in this section because it is based on the simple but extremely useful spin echo unit whereby the chemical shift evolution is refocused by a 180° pulse applied in the centre of the free precession evolution. 

A similar structural study was made with angustifoline. The H and l3C NMR spectra of the alkaloid in four different solvents were fully assigned by resorting to 2D H- H and H-I3C COSY and 2D J resolved spectra. Conformational analysis in the solid state and in solution is presented [201]. More recently other studies of the conformational equilibrium of diazatricyclic systems based on one- and two- dimensional H, 3C and 15N NMR experiments were published [202,203]. 

The F, scaling factor can be derived from a 2D C J-resolved experiment on adamantane and the Jc-h quoted in the literature 16]. 

The H-NMR and C-NMR spectra of amodiaquine hydrochloride in DMSO-d6 were determined at 300 MHz and 75.4 MHz respectively on a Bruker AM-300 NMR spectrometer using tetramethylsilane as reference standard. The H-NMR determinations included spin decoupling experiments, 2D J-resolved and COSY-45 measurements (Figure 4-6). The C-NMR spectra comprised DEPT and hetero-nuclear (C-H correlation) measurements (Figure 7-9). The spectral assignments are listed in Table III. 

In addition to the importance of parameter selection on F skew, the ACCORD-HMBC experiment has also been shown to exhibit strong coupling artifact responses similar to those observed in heteronuclear 2D J-resolved spectra. - Finally, the triplet character of responses in the second frequency domain superimposed over the Fi skew was mathematically accounted for by Zangger and Armitage in their development of the ACCORD-HMQC experiment. "... 

D J-resolved NMR experiments are a conceptual amalgamation of two topics discussed above, the /-modulated spin echo and the two-dimensional characteristic of the spin—lattice relaxation experiments. As the name of these experiments implies, scalar coupling information, /, will be displayed in the one frequency domain chemical shift information will be presented in the second frequency domain. The simplest 2D/ experiments sort chemical shift information in the detected time domain, labeled by convention, while the heteronuclear scalar couplings of each carbon are sorted into the indirectly determined time domain, tj (do not be confuse lower case h with the spin—lattice relaxation time, Tj). 

A two-bond scalar J-coupling between the quadrupolar A1 and spin V2 P nuclei of ca. 30 Hz has been determined successfully by Massiot et aV in inorganic solids, such as berlinite (AIPO4), hydrated AIPO4 and VPI-5 molecular sieve using J-Resolved experiments, and subsequently used to acquire 2D J-HQMC heteronuclear correlation. Similar JpM coupling values have been measured in VPI-5 by Trebosc et who applied the PS REDOR (Frequency... 

D NMR techniques like correlation spectroscopy (HETCOR, COSY) and J-resolved experiments (Off-MAS, slow spinning MAS), which play a significant role in the detailed characterization of the structure of an... 

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