Decoupling channel

The sample solution is usually placed in a glass tube of diameter 5, 8,10, or 15 mm, which is rotated perpendiculary to the magnetic field B0 in electromagnets and along the field 

For 13C experiments, the sample is usually prepared by dissolving the compound to be investigated in a deuterated solvent, which is usually required for field/frequency stabilization. For calibration, a small amount of a reference compound (usually tetra-methylsilane) is added to the sample. 

Coaxially-centered sample tubes for external lock or reference. 

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Fig. 2.—A. Normal, H-N.m.r. Spectrum of Asperlin (1) in Benzene-

Known values of f render Eq. (80), plotted semilogarithmically, quite useful in grouping resonances into series, in clarifying which decoupled channels they belong to, i.e., by which potential they are mainly supported, and in extending the series to obtain the resonance positions and widths of still uncalculated higher members, often more accurately than even elaborate scattering calculations. 

The undesirable off-resonance effects become important in high field spectrometers. Schenker and von Philipsbom114 analysed them and found that they can be partly diminished by phase cycling and composite pulses, but a better solution would be increased rf intensities to shorten the pulses in both observing and decoupling channels. 

Most Varian decoupling parameters start with the letter d to distinguish them from the transmitter parameters, which start with a t. Bruker uses a 1 (FI channel) to specify the transmitter channel and a 2 (F2 channel) to specify the decoupler channel. The following parameters can be examined by entering dg (Varian) or eda (Bruker) ... 

High-power rf fields (ca. 100 kHz) are essential both for the observing and decoupling channels. 

Two situations now confront the inorganic chemist—interest in H X direct and H,X indirect spectroscopy in these cases the X decoupler pulses will be on the first decoupler channel or interest in samples where neither nuclide is proton In this case, the X decoupler pulses will be on the second decoupler channel, variously denoted, second decoupler channel , Y channel , G , etc. Consult the manufacturer s apphcations department for advice if you are not sure. 

X, Y Indirect Spectroscopy. The easiest way to measure the 90° pulse on the second decoupler channel is to use the same sample as above, but switch the X nucleus from the first to the second decoupler channel. Load the appropriate, manufacturer supplied pulse program on Bruker machines this is DECP90F3. Proceed as above. 

Reference to the decoupler channel(s) is often used when referring to these additional channels but this should not be taken too literally as they may only be used for the application of only a few pulses rather than a true decoupling sequence. This nomenclature stems from the early developments of NMR spectrometers when the additional channel was only capable of providing noise decoupling , usually of protons. 

Historically, the presaturating rf as been applied via the decoupler channel , traditionally the second rf channel of the spectrometer. However, decoupling is a misleading term in the context of the NOE since spectra are acquired fully J-coupled, hence the term presaturating rf is used throughout... 

Figure 9.21. Soft pulse calibration sequences for (a) amplitude and (b) hard/soft phases differences for selective pnlses applied on the indirect (decoupler) channel. SL is a spin-lock applied to decouple A and X during the soft pulse.

The pulse calibration on the decoupler channel cannot be performed using the direct method since the pulse which is to be calibrated is transmitted on a different rf channel and resonance frequency to the observed nucleus. To calibrate the rf pulse on the decoupler channel it is necessary to determine the pulse length from the effect the pulse has on the nucleus observed on the transmitter/receiver channel. This indirect calibration is achieved by using a coupled IS spin system and transferring the detectable antiphase... 

The s)mchronization of the NMR pulse sequence and the timing of the illumination poses little problems because modern spectrometers usually have one or several spare lines alternatively, unused transmitter or decoupler channels can serve for that purpose. A recent, fairly t)q)ical software implementation for a pulsed Nd YAG laser, which requires one trigger signal for its flash lamp and another for its Q-switch, is discussed in Ref. 71. 

Early theoretical work on predissociation employed a vibration-to-translation (V-T) mechanism, which predicted 8 x 105 ns,208 which is clearly inconsistent with Table V. In later work, Ewing introduced the rotational channels of the HF fragments with a vibration-to-translation-and-rotation (V-T,R) mechanism. A first-order perturbation theory treatment involving decoupled channels, which is not particularly reliable, showed an increase of only one order of magnitude in the predissociation rate with inclusion of rotation 209 in fact, however, x was now calculated209,210 to be 2 x ns due to other changes in... 

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