Inverse experiment

In a selective-inversion experiment, it is the relaxation of the z magnetizations that is being studied. For a system without scalar coupling, this is straightforward a simple pulse will convert the z magnetizations directly into observable signals. For a coupled spur system, this relation between the z magnetizations and the observable transitions is much more complex [22]. 

HC HMQC (heteronuclear multiple quantum coherence) and HC HSQC (heteronuclear single quantum coherence) are the acronyms of the pulse sequences used for inverse carbon-proton shift correlations. These sensitive inverse experiments detect one-bond carbon-proton connectivities within some minutes instead of some hours as required for CH COSY as demonstrated by an HC HSQC experiment with a-pinene in Fig. 2.15. 

Inverse experiments Heteronuclear shift-correlation experiments in which magnetization of the less sensitive heteronucleus (e.g., C) is detected through the more sensitive magnetization (e.g., H). 

Technically, the inverse experiment used to be very demanding because the excess of protons not coupled to the nucleus of interest (e.g., protons coupled to the almost hundred-fold excess of 12C instead of 13C) needed to be suppressed. Originally, this was achieved by the use of elaborate phase-cycling schemes, but today the coherence pathway selection by gradient pulses facilitates this process. 

The selective inversion experiment for slow chemical exchange... 

Selective inversion experiments for the determination of slow exchange rates are analogous to the saturation-transfer method in that they involve selective manipulation of one of the exchanging sites, while observing the subsequent effect on the second site as a function of time [48, 69, 70]. Chemical exchange, if present, will provide an alternative route to normal spin relaxation processes which a spin system undergoes, if perturbed at the start of an experiment. The rate of relaxation will depend on both the exchange rate k and the spin-lattice relaxation rate (Ti) (fig. 5). 

The results of the inverse experiment, viz. counter-diffusion of benzene into H-ZSM-5, previously loaded with ethylbenzene from an ethylbenzene/helium stream at 415 K, are displayed in Fig. 6. 

A variety of inverse experiments which require a time delay for transfer of magnetization between H and " Cd spins, such as H-i Cd HMQC, " Cd-edited H- H COSY or H-" Cd hetero-TOCSY experiments and 2D H- N HMQC without Cd excitation pulses, are applied for successful identification of histidines coordinated to Cd(II) metal . The latter technique provides geometrical information for the metal coordination sphere through the determination of 7H-cd and 7n-Cc1 for " Cd-bound imidazole rings . 

The phase behaviour established for concentrated aqueous solutions of PEO-PPO-PEO copolymers has its counterpart in PEO/PBO copolymer solutions. A phase diagram for PE058PB0i7PE0M based on tube inversion experiments is shown in Fig. 4.14 (Luo et al. 1992). The hard gel is isotropic under the polarizing microscope and can be characterized as a cubic phase formed from spherical micelles of a similar size to those in the dilute micellar solution. 

The most robust analysis methods involve direct comparison of the AI (q) term with theoretical computations. Agreement between data and prediction then validate the models used. The application of inversion approaches involves taking Fourier transforms of the data to yield the set of vectors connecting scattering particles. However, one must be cautious when interpreting the results of these inversions. Experiments are currently underway to measure the Fj2ons term independently, which will allow us to extract the pure ion—DNA cross-term which is more straightforward to interpret. 

SIN for less sensitive nuclei to excite one kind of nucleus and detect another kind with a better magnetogyric ratio in the same experiment. This is done in inverse experiments which are described in Chapters 5 and 6). 

In summary, for this area to move forward, methods need to be introduced to provide information additional to 2H lineshapes when studying more complex systems. Calculations such as those just described are one real possibility experimental possibilities include use of magic-angle spinning 2H NMR, and using the full anisotropy of 2H 7j measurements and 13C NMR. A very nice illustration of the use of several different techniques in a motional study examined the slow alkane motions in a urea/alkane inclusion compound via 2H relaxation measurements, selective inversion experiments and two-dimensional 2H exchange.125... 

ID ll spectrum, and crosspeaks are arranged symmetrically around the diagonal. There is only one radio frequency channel in a homonuclear experiment, the H channel, so the center of the spectral window (set by the exact frequency of pulses and of the reference frequency in the receiver) is the same in If and F (Varian tof, Bruker ol). The spectral widths should be set to the same value in both dimensions, leading to a square data matrix. Heteronuclear experiments have no diagonal, and two separate radio frequency channels are used (transmitter for F2, decoupler for F ) with two independently set spectral windows (Varian tof and dof, sw, and swl, Bruker ol and o2, sw(If), and sw(I )). Heteronuclear experiments can be further subdivided into direct (HETCOR) and inverse (HSQC, HMQC, HMBC) experiments. Direct experiments detect the X nucleus (e.g., 13C) in the directly detected dimension (Ff) using a direct probe (13C coil on the inside, closest to the sample, H coil on the outside), and inverse experiments detect XH in the To dimension using an inverse probe (XH coil on the inside, 13C coil outside). 

HETCOR has been largely replaced by the far more sensitive inverse experiments, HSQC and HMQC. Because HETCOR is a 13C-detected experiment, it is called direct ... 

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