Deuterium field lock

A Varian XLFT-100 Fourier Transform nmr Spectrometer interfaced with a Varian 620-L minicomputer with magnetic tape storage provided high-resolution, proton-decoupled spectra of natural abundance carbon-13 at 25.2 MHz. For identification of carbon peaks, chloroform-d solutions of surfactant (solubility about 20 wt%) were prepared. Chloroform-d also served for a deuterium field lock. Samples of surfactant in water or decane were placed... 

Field lock. Syn. deuterium field lock, lock, lock. The holding constant of the strength of the applied magnetic field through the monitoring of the Larmor frequency of one nuclide (normally H, but possibly F) In the solution and making small field strength adjustments. 

The deuterium line of the deuterated solvent is used for this purpose, and, as stated earlier, the intensity of this lock signal is also employed to monitor the shimming process. The deuterium lock prevents any change in the static field or radiofrequency by maintaining a constant ratio between the two. This is achieved via a lock feedback loop (Fig. 1.10), which keeps a constant frequency of the deuterium signal. The deuterium line has a dispersion-mode shape i.e., its amplitude is zero at resonance (at its center), but it is positive and negative on either side (Fig. 1.11). If the receiver reference phase is adjusted correcdy, then the signal will be exactly on resonance. If, however, the field drifts in either direction, the detector will... 

Figure 1.10 (a) The dispersion mode line should have zero amplitude at resonance, (b) The deuterium lock keeps a constant ratio between the static magnetic field and the radiofrequency. This is achieved by a lock feedback loop, which keeps the frequency of the deuterium signal of the solvent unchanged throughout the experiment. 

The deuterium lock prevents changes in the static field (Bq) and radiofrequency (B,) by maintaining a constant ratio between the two. It therefore ensures long-term stability of the magnetic field. If the lock is not applied, a drastic deterioration in the shape of the NMR lines is expected, due to magnetic and radioffequency inhomogeneities, (a) With deuterium lock... 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

The ideal solvent should contain no protons and be inert, low boiling, and inexpensive. Deuterated solvents are necessary for modem instruments because they depend on a deuterium signal to lock or stabilize the B0 field of the magnet. Instruments have a deuterium channel that constantly monitors and adjusts (locks) the B ) field to the frequency of the deuterated solvent. Typically, H NMR signals are in the order of 0.1 to several Hz wide out of 300,000,000 Hz (for a 300 MHz system), so the B ) field needs to be very stable and homogeneous. 

One aspect of the experimental conditions under which these spectra were obtained is important to understand so that the spectra can be rationally interpreted. For solubility and stability purposes, peptides are generally dissolved in buffered water. Recall from Chapter 3 that compounds prepared for NMR experiments are almost always dissolved in deuterated solvents. The need for deuterated solvents is so that the spectrometer can remain stable for the duration of the experiment by way of the field/frequency lock. The lock signal comes from the deuterium NMR signal of... 

Before we acquired the spectrum to the right, the field homogeneity was carefully adjusted by maximizing the deuterium lock signal but the methyl singlets are split into distorted doublets. In fact, every line in the spectrum is identically split. What is producing this effect ... 

Suitable solvents are those which dissolve lignin to the maximum extent possible without interfering significantly with the lignin NMR signals. Usually the solvent is deuteriated. The solvent deuterium NMR resonance is usually used for the field-frequency lock signal. The most common lignin solvents and their relevant... 

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