Deuterium resonance

The carbon-13 NMR spectrum of griseofulvin (Figure 3) was obtained at ambient temperature in DMSO-d containing TMS as internal reference utilizing Varian Associates XL-100-15 spectrometer equipped with Fourier accessories The system was locked to the deuterium resonance frequency of the solvent, and operated at a frequency of 25.2 MHz for carbon-13. The chemical shifts are reported ( c, ppm.) from the Internal standard TMS. 

The interaction of the OEC with magnetic nuclei (e.g. from substrate water) can also be studied by ENDOR and ESEEM techniques. Despite some initial controversy, there is now agreement that after H20/D20 exchange, proton resonances disappear and/or deuterium resonances can be detected for the S2 state.13-418,446 449,469 471 Thus, exchangeable protons exist at 3-6 A from the cluster. 

Proton and fluorine are the most frequently studied nuclei in the solid state NMR of polymers. However, useful information can be obtained from deuterium resonance, (2H I = 1). In the presence of nuclei with spin I > 1/2, which possess a quadrupolar moment, the most dominant interaction occurs between the quadrupolar moment and an electric field gradient tensor V, generated by the C—D bonding electrons. Consequently, the only dominant interaction, besides Hz (Eq. (1)), will be described by the quadrupolar Hamiltonian, which for a single spin has the form ... 

In the former case, an internal lock is established at the deuterium frequency of the solvent by adjusting the frequency of the lock transmitter until it matches that frequency. The operator typically observes a decreasing number of interference-pattern sine waves as the lock transmitter frequency approaches that of the deuterium nuclei in the solvent. A null appears when the two frequencies are identical the operator then turns the lock control to On. On most modem spectrometers, autolocking procedures are also available that search for the deuterium resonance and automatically lock the spectrometer when the signal is found. 

The lock channel regulates the field by monitoring the dispersion mode deuterium resonance rather than the absorption mode signal that is usually considered in NMR, and aims to maintain the centre of this resonance at a constant frequency (Fig. 3.45). A drift in the magnetic field alters the... 

Figure 3.45. The spectrometer lock system monitors the dispersion mode signal of the solvent deuterium resonance. A shift of the resonance frequency due to drift in the static field generates an error signal that indicates the magnitude and direction of the drift, enabling a feedback system to compensate this.

The first procedure for locking is to establish the resonance condition for the deuterium signal, which involves altering either the field or the frequency of the lock transmitter. Of these two options the latter is preferred since it avoids the need for changing transmitter frequencies and is now standard on modem instruments. Beyond this, there are three fundamental probe-dependent parameters that need to be considered for optimal lock performance. The first of these is the lock transmitter power used to excite the deuterium resonance. This needs to be set to the highest usable level to maximise the signal-to-noise ratio but must not be set so high that it leads to lock saturation. This is the... 

Although the lock level is used as the primary indicator of field homogeneity, it is not always the most accurate one. The lock level is dependent upon only one parameter the height of the deuterium resonance. This, whilst being ratber sensitive to the width of the main part of the resonance, is less sensitive to changes in the broad base of the peak. The presence of such low level lumps can be readily observed in the spectrum (particularly in the case of protons) but for this to be of use when shimming, the spectrometer must be able to... 

Choice of Solvent. The most appropriate solvent for NMR studies of polymers would allow a range of polymer concentrations to be investigated, be free of overlap problems and hopefully provide a signal for internal lock. Not all of these conditions can usually be met as many high molecular weight polymers pose solubility problems and can be examined in only a limited number of solvents. Deuterium resonance is the typical choice for an internal lock signal on most modern NMR spectrometers. Unfortunately, the majority of available deuterated solvents are poor solvents for many addition polymers such as the polyolefins while it is generally possible to find a number of appropriate deuterated solvents for many of the condensation polymers. The... 

The presence of only one deuterium resonance in the evaporated product suggests that a monomethylated derivative is present MS confirms this condition. This finding implies that the intermediate picks up protons from the solvent or other source. To investigate this matter, we performed the reduction in [2H8]THF (with 4.3 K/dibenzothiophene) and found only biphenyl as a CH3I quench product after 24 h. MS analysis of the 42-h quench product showed appreciable deuterium incorporation mlz = 154, intensity... 

Fig. 1.—Proton Magnetic Resonance Spectra (at 100 MHz) of Methyl 4,6-0-Benzylidene-2-deuterio-a -D-riho-hexopyranoside in Solution in Deuteriochlorofomi. [The normal spectrum is shown along the bottom of the Figure. The insets show the effect of irradiation at the frequency of the deuterium resonance.]...

Most FT-NMR spectrometers require the use of deuterated solvents because the instruments use the deuterium resonance signal as a lock signal, or reference signal, to keep the magnet and the electronics adjusted correctly. Deuterium is the isotope of hydrogen and can easily substitute for it in... 

Figure 5. a, NMR spectra (at 100 MHz, in CCI under pressure) of a mixture of gaseous propane, propene, and cyclopropane, b, Proton noise-decoupled NMR spectra (at 15.4 MHz, in CCl under pressure) of some of the reaction products obtained from the electrolysis of the [2,2-d,] butyrate ion Expansion, proton-coupled deuterium resonances. (Reproduced from Ref. 6. Copyright 1980, American Chemical Society.)... 

Ja,a, are now all H-D coupling and first order. These cause unresolvable splittings to occur which were removed by a strong irradiation at the deuterium resonance frequency (Bloom and Shoolery, 1955), Both of these studies of deuteriated cyclohexane are in agreement and suggest a virtually temperature-independent AG+ value of 10-3-10-5 kcal mole , JH+ = 10-9 + 0-6 kcal mole and AS+ = 2-9 + 2-3 e.u. A transmission coefficient of 0-5 is assumed in the inversion process. The r ults obtained by Harris and Sheppard (1961) have been recalculated and the values of JH+ and AS= now agree with the results on deuteriated cyclohexane. 

Despite the impressive field stability provided by superconducting magnets, they still have a tendency to drift significantly over a period of hours, causing NMR resonances to drift in frequency leading to a loss of resolution. To overcome this problem, some measure of this drift is required so that corrections may be applied. On all modem spectrometers, the measurement is provided by monitoring the frequency of the deuterium resonance of the solvent. The deuterium signal is collected by a dedicated observe spectrometer within the instrument that operates in parallel with the principle channels, referred to as the lock channel or simply the lock. 

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