Decoupling effect

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.

The spectrum at the bottom of Fig. 16 is obtained with the double adiabatic decoupling pulse, one located at —23.2 kHz and the other at 23.2 kHz. The BSFS is compensated and sidebands are eliminated by the compensating pulse. In addition, the amplitude of the peak is higher than that in the middle, showing a better decoupling effect. Similar results were obtained for 13C off-resonance <5 ranging from —3 to 3 kHz, where < /A/<0.13 can be treated as close to on-resonance. 

In the two-step process the two reactors are coupled by the same separation system. Phenol gives azeotropes with both cyclohexanone and cyclohexanol. The relative volatility of cyclohexanone to cyclohexanol is very low at normal pressure, but it rises significantly under high vacuum. Alternative separation schemes are evaluated based on direct and indirect sequences. Both are equivalent in energy consumption, although the indirect sequence is more suitable by a decoupling effect. 

Figure 8a. Decoupling effect on oc-CHg proton signals in Tetralin and its derivatives. Irradiation at 1.77 ppm.

Conversely, the decoupling effect of v2 requires only that v2 be on during FID acquisition. Therefore, if v2 is on during the presaturation delay but off during FID acquisition, most of the NOE enhancement would be preserved but all decoupling would be lost. The net result is a coupled spectrum with improved signal-to-noise ratio. 

Leaching is accelerated mainly by high-amplitude US however, medium- or even low-amplitude US can be a better choice when the decoupling effect makes near-maximum amplitudes impractical [14,15]. Such is the case with the removal of nitropolycyclic aromatic hydrocarbons from soil by CUSAL [16], which requires an output amplitude of... 

As an alternative to the above method for eliminating the NOE an instrumental technique is available. This depends upon the realization (247) that the time-dependent behaviour of the NOE and of spin decoupling are different. Thus the NOE takes a time comparable for Tj to build up or to decay after application or removal of a rf field, whereas spin decoupling effects appear or disappear almost instantaneously. Consequently if the proton decoupler is gated off immediately prior to... 

This decoupling effect verifies Grigolini s prediction of 1976 made in the context of vibrational relaxatiorf using RMT. 

Both vibrational and rotovibrational relaxation can be described analyti-caDy as multiplicative stochastic processes. For these processes, RMT is equivalent to the stochastic Liouville equation of Kubo, with the added feature that RMT takes into account the back-reaction from the molecule imder consideration on the thermal bath. The stochastic Liouville equation has been used successfully to describe decoupling in the transient field-on condition and the effect of preparation on decay. When dealing with liquid-state molecular dynamics, RMT provides a rigorous justification for itinerant oscillator theory, widely applied to experimental data by Evans and coworkers. This implies analytically that decoupling effects should be exhibited in molecular liquids treated with strong fields. In the absence of experimental data, the computer runs described earlier amount to an independent means of verifying Grigolini s predictions. In this context note that the simulation of Oxtoby and coworkers are semistochastic and serve a similar purpose. 

For finite y , the decay of the oscillation envelope becomes slower as the frequency increases. This is the Grigolini decoupling effect in its simplest form and is qualitatively in agreement with the computer simulations of Section II. Note that there is no decoi ling effect in Markovian systems (i.e., those where -> oo). 

When y - A2 the equivalent of the microscopic time y is = y/ -Decoupling effects are present when Uj = F. To obtain an approximate value of F we can use the experimental data as follows. First, we evaluate the value of decay of the oscillation envelopes of the angular velocity autocorrelation function as a function of Equation (14) shows that this is, approximately, a Lorentzian, the linewidth of which provides the approximate expression for F. The agreement with the numerical decoupling effect is quantitatively good when the ratio ai/uf is assumed to be equal to 8.5. Simple Markovian models cannot account for decoupling effects. 

We believe that the arguments above should convince the reader that the interesting phenomenon detected by Carmeli and Nitzan is another manifestation of the decoupling effect, well understood at least since 1976 (see ref. 86). The only physical systems, the dissipative properties of which are completely independent of whether or not an external field is present, are the purely ideal Markovian ones. Non-Markovian systems in the presence of a strong external field provoking them to exhibit fast oscUlations are characterized by field-dependent dissipation properties. These decoupling effects have also been found in the field of molecular dynamics in the liquid state studied via computer simulation (see Evans, Chapter V in this volume). 

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