Decoupling references

This section highlights some essential patterns and shows how they serve the major objectives of decoupling. References to more-detailed discussion on these patterns can be found in the Bibliography. 

In a B,C spin system, the multiplet of spin B contains the /(B, C) coupling constant among other couplings. This coupling can be determined by comparison of two spectra a coupled spectrum in which the J(B, C) coupling evolves and a decoupled reference experiment in which the J(B,C) is removed. 

Plasma polymerization is a vapor phase-activated (by plasma) A/D-coupled CVD. Parylene polymerization is a vapor phase-activated (by thermal process) A/D-decoupled CVD. Cascade arc torch (CAT) polymerization is a vapor phase-activated (by plasma) A/D-decoupled CVD. General CVD is a surface-activated (by thermal process) A/D-coupled CVD. HWCVD is a hot wire activated A/D-decoupled CVD. Coupled and decoupled refer only to the spatial separation. 

For more complex multiplets, the separation of the outer peaks of the multiplet can be compared to that in the H spectrum to indirectly extract the additional spUtting or, alternatively, a simple fitting procedure taking the internal sateUite y (CH) component as decoupled reference multiplet can be apphed. Qn the other hand, a double-selective ID version of a refocused HSQMBC experiment has been also proposed for the fast and accurate measurement of specific y(CH) values from pure IP ID multiplets [149]. 

An alternative way of acquiring the data is to observe the signal. These experiments are referred to as reverse- or inverse-detected experiments, in particular the inverse HETCOR experiment is referred to as a heteronuclear multiple quantum coherence (HMQC) spectmm. The ampHtude of the H nuclei is modulated by the coupled frequencies of the C nuclei in the evolution time. The principal difficulty with this experiment is that the C nuclei must be decoupled from the H spectmm. Techniques used to do this are called GARP and WALTZ sequences. The information is the same as that of the standard HETCOR except that the F and F axes have been switched. The obvious advantage to this experiment is the significant increase in sensitivity that occurs by observing H rather than C. 

All P.M.R. spectra were measured with a Varian HA 100 spectrometer operating in the frequency-sweep mode with tetramethylsilane as the reference for the internal lock. The double and triple resonance experiments were performed using a Hewlett Packard 200 CD audio-oscillator and a modified Hewlett Packard 200 AB audio-oscillator (vide infra). Spectra were measured using whichever sweep width was required to ensure adequate resolution of the multiplets under investigation, generally 250 or 100 Hz, and sweep rates were selected as necessary. Extensive use was made of the Difference 1 and Difference 2 calibration modes of the instrument, both for the decoupling experiments and for the calibration of normal spectra. 

Heteronuclear two-dimensional /-resolved spectra contain the chemical shift information of one nuclear species (e.g., C) along one axis, and its coupling information with another type of nucleus (say, H) along the other axis. 2D /-resolved spectra are therefore often referred to as /,8-spectra. The heteronuclear 2D /-resolved spectrum of stricticine, a new alkaloid isolated by one of the authors from Rhazya stricta, is shown in Fig. 5.1. On the extreme left is the broadband H-decoupled C-NMR spectrum, in the center is the 2D /-resolved spectrum recorded as a stacked plot, and on the right is the con tour plot, the most common way to present such spectra. The multiplicity of each carbon can be seen clearly in the contour plot. 

When both G12 = G21 = 0, the system is decoupled and behaves identically to two single loops. When either G12 = 0 or G21 = 0, the situation is referred to as one-way interaction, which is sufficient to eliminate recursive interactions between the two loops. In such a case, one of the loops is not affected by the second while it becomes a source of disturbance to this second loop. 

The theory behind both of these experiments, and in particular the DEPT experiment, is rather complicated, so that we refer you to NMR textbooks for details. The important feature of both is that the carbon signals appear to have been simply broad-band decoupled, but that according to the multiplicity they appear either in positive (normal) phase or in negative phase, according to their multiplicity. 

Decoupling The saturation of a particular signal or signals in order to remove spin coupling from those signals. Also referred to as spin decoupling. 

Figure 26 Proposed HMBC pulse sequences68 for editing into two subspectra according to the number of directly attached protons being odd or even Filled and open bars refer to 90° and 180° pulses, respectively, while the dashed open boxes represent 13C decoupling. Recommended phase cycle 0 — x, —x, x, — x 02 = x, x, 4(—x), x, x ...

We also remark that Eq. (5.44) may be decomposed into separate sets of equations for the odd and even ap(t) which are decoupled from each other. Essentially similar differential recurrence relations for a variety of relaxation problems may be derived as described in Refs. 4, 36, and 73-76, where the frequency response and correlation times were determined exactly using scalar or matrix continued fraction methods. Our purpose now is to demonstrate how such differential recurrence relations may be used to calculate mean first passage times by referring to the particular case of Eq. (5.44). 

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