Decoupler modulation

The signals of a large number of coumarin derivatives [418] were assigned using the single-frequency off-resonance decoupling,. /-modulated spin echoe and INEPT techniques. 

In Section 2-4, we encountered a number of spectral acquisition parameters. The selection of these parameters is relatively direct and straightforward, with the exception of the flip angle a and the decoupler modulation frequency (for WALTZ decoupling). Neither a nor the decoupler modulation frequency is entered directly, and both parameters require that the spectrometer be calibrated. 

The decoupler modulation frequency employed in WALTZ decoupling is 4(752)-... 

Figure 5. Three series of C spectra from a C-F doublet with different F decoupler modulations schemes. In each series, the decoupler offset was incremented in 5kHz step , (a) WALTZ-16, (b) GARP, and (c) WURST-40.

It is also usually possible to remove all the couplings from a particular isotope, e.g. H, provided that one only wishes to observe the spectrum from another isotope, e.g. Either the decoupling frequency is noise-modulated to cover the relevant range of chemical shifts, or else the same decoupling is achieved more efficiently, and with less heating of the sample, by using a carefiilly designed, continuous sequence of... 

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. 

Figure 4.7 Pulse schemes representing separation of decoupling effects from the nOe during X nucleus acquisition. The decoupler is programmed to produce noise-modulated irradiation or composite pulse decoupling at two power levels. Suitable setting of the decoupler may produce either (a) nOe only, (b) proton decoupling only, or (c) both nOe and proton decoupling.

Figure 5.7 (A) Pulse sequence for gated decoupled /-resolved spectroscopy. It involves decoupling only during the first half of the evolution period

The noise-modulated broadband decoupled 13C NMR spectrum of (/)/.)-penicilla-mine is shown in Fig. 5. Two methyl carbons were found to resonate at chemical shifts of 27.7 and 30.2 ppm, in addition to a CH carbon at 64.6 ppm. Two quaternary... 

To execute a sequential solution for a set of modules, you have to tear certain streams. Tearing in connection with modular flowsheeting involves decoupling the interconnections between the modules so that sequential information flow can take place. Tearing is required because of the loops of information created by recycle... 

Decoupling tends to introduce much more separation between concepts than is typically found in an analysis. For example, any aspect of an object s behavior that could conceivably be varied tends to get delegated to a separate plug-in module. Thus, we end up with many more types of objects than were mentioned in the specification. Nevertheless, our refinement techniques (see Chapter 6, Abstraction, Refinement, and Testing) enable us to keep the traceability between the two models. 

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.

Assume a 180° PIP is applied at the centre of the 13CO in the middle of the 13C evolution time for homonuclear decoupling and at the same time a compensating PIP is applied on the other side of the l3C (Fig. 11) to minimize the disturbance to the 13C that may have a transverse or longitudinal magnetization. The two simultaneous PIPs become an amplitude modulated pulse described by 2/i cos (27iAft)Ix, where f is the pulse strength of each 180°... 

---