CW spectrometers

The NMR measurements were carried out at room temperature by using a CW spectrometer constructed at the Department of Physics, Technical University in Kosice with working frequency 10.545 MHz. The attained derivative recordings were averaged out of a few accumulated repetitions and the average was used for the next evaluation of the derivative recordings. 

The NMR spectra were taken on a JEOL JNM-MH-100 (CW) spectrometer using tetramethylsilane as an internal standard. 13C spin-lattice relaxation time of the polymer was measured by the inversion-recovery Fourier transform method on a JNM-FX100 FT NMR spectrometer operating at 25 MHz. 

Recorded at 30°C with a 60-MHz CW spectrometer which allowed the spectrum to be recorded within 20 sec from dissolution. The checkers recorded the spectrum under more leisurely conditions in C6D6, where the complex, although poorly soluble, is more stable. 5... 

The proton magnetic resonance spectrum (Figure 4) of cimetidine was recorded for a deuterated dimethyl sulfoxide solution containing approximately 100 mg/ml of the compound with tetramethylsilane as the internal standard. The spectrum illustrated was obtained using a Perkin-EImer R32 proton magnetic resonance (CW) spectrometer. The resonances are ... 

Two types of spectrometer are used to display nitrogen resonances the continuous wave (cw) spectrometer, which detects the absorption line or its derivative, and the pulse apparatus on which free precession decays or echo signals are observed. 

The majority of the earlier photo-CIDNP experiments were carried out in a cw spectrometer with the sample subjected to continuous uv irradiation. The first consideration was therefore how to get light into the nmr probe as a relatively high light-absorbed intensity was required to generate the radicals at a... 

FIG. 13 Effect of phasing on signal shape. The TMS peaks on both forward and backward scans are quite high and narrow, with good ringing and perfect symmetry. Ringing is seen only on CW spectrometers. 

Due to the high receptivity of F, E nf correlation spectroscopy became available very early in the history of NMR spectroscopy. Using the INDOR technique on a cw-spectrometer, McFarlane and coworkers gave the first report whatsoever on the use of "X, "Y correlated spectroscopy for indirect observation via of the resonances in complexes of the type WF6 0R and WOp4L. Beside the first chemical shift determination for a low-7 transition metal, which was at that time not feasible by any other technique, further the indirect determination of y( W, P), and the assignment of relative coupling signs, was reported. 

We have been concerned with the precision and accuracy of NMR data of polymers since we first started NMR studies on polymers.1-4 Using continuous-wave (CW) spectrometers, the effects of measurement conditions including temperature, sample concentration and radiofrequency (rf) field strength, were examined using several polymer and copolymer samples. Since our research group have been deeply involved in stereospecific polymerization of methacrylates, one of the main concerns about NMR measurement was the precision of tacticity determination by NMR. The errors in determining the tacticity of poly(methyl methacrylate) (PMMA) and those in the results of polymerization of methyl methacrylate (MMA) by radical and anionic initiators were examined and found to be satisfactorily small.4 Although there... 

H NMR spectra of the PMMA in CDC13 were measured on 26 spectrometers, whose resonance frequencies ranged from 90 to 500 MHz. Chemical shift data of OCH3 protons scatter only within 0.01 ppm, with a few exceptions. The standard deviation is 0.0038 ppm for 19 data obtained from FT instruments and 0.0169 ppm for 7 data obtained from CW spectrometers. Standard deviations for a-CH3 and CH2 proton shifts are less than 0.01 ppm. The chemical shift of water, which was incidentally introduced into sample solutions, scatters from 1.56 to 1.76 ppm. If the temperatures of measurements are almost constant, the water contents in the sample solutions are estimated to be from 0.07 to 0.20%. Relative intensity data, 3(CH2 + a-CH3)/OCH3, agreed with the theoretical value of 5. Standard deviations of the intensity measurements are larger for the... 

ESR measurements were performed with the cw spectrometer ERS 300 (ZWG, Berlin) equipped for in-situ investigations with a flow reactor and a gas as well as liquids (with vaporization) supplying system [12,13]. 0.4 g catalyst was applied each and the reaction conditions were comparable to catalytic runs as described above. 

Proton NMR was the first type of NMR spectroscopy to become available to the researcher. The high natural abundance of the NMR-active H-nucleus (-100%) and its relatively high sensitivity allowed early low-field continuous-wave (CW) spectrometers, with little in the way of signal-averaging capabilities, to successfully produce spectra from milligram quantities of small organic molecules. Such instruments,... 

The terms upfield and downfield, which came into use when continuous wave (CW) spectrometers were used (before the advent of Fourier transform spectrometers), are so entrenched in the vocabulary of NMR that you should know... 

Although FT NMR can become a major tool for wide line NMR spectroscopy, cw spectrometers will still be necessary to record spectra which are so broad that the deadtime is comparable to or greater than T2 from samples which do not give rise to echoes. In the frequency domain, this means that the (sinx)/x wiggles shown in the last figure will have frequencies and amplitudes comparable or greater than the peak itself, thus obscuring the desired spectrum. See IV. B. 3. and VI.D.4. for other pulse techniques to overcome the deadtime problem. 

Two general types of NMR spectrometers are currently in use, continttoiis-wavc (CW) and pulsed, or (F I -NMR), spectrometers. All early studies were carried out withC W instruments. In about 1970. however. F I -NMR spcciromeiers became available commercially, and now this type of insirurneiii dominates the market. In both I ypes of instruments, the sample is positioned in a powerful magnetic field that has a SI rength of several tesla. CW spectrometers are... 

Fig. 9. Schematic diagrams of the major components of cw and pulsed EPR-ENDOR instruments. The sample is in a resonant microwave cavity, situated between poles of a magnet and surrounded by a temperature-control system (not shown). The structure of the circulator directs microwaves from the source to the cavity, and from the cavity to the detection system. A radio frequency synthesizer provides rf to coils situated around the cavity. Note that this diagram shows an arbitrary orientation of the rf coils. For convenience the magnetic field modulation coils are not shown for the cw spectrometer. For the pulsed EPR spectrometer (B), fast switches (ovals) are used to control pulse timing for the rf and microwave pulses, as well as to protect the detector. For simplicity, several features including the timing circuitry are not shown. The signal from the detector is sent to a boxcar integrator. Both spectrometers are computer-interfaced for data collection and storage. Further details may be found elsewhere.

Two methods are useful to measure slow motion spin label spectra saturation transfer (ST-EPR) and two-dimensional electron spin echo spectroscopy (2D ESE). In the ST-EPR experiment, " a cw spectrometer is operated at high microwave power, and this causes partial saturation of the nitroxide spectrum. As the field is scanned, molecular motion carries this saturation to nearby regions of the spectrum, yielding spectra that are very sensitive to the rotational correlation time. One of the great advantages of ST-EPR is that little instrumentation is required beyond the conventional cw EPR instrument. Most any laboratory equipped to perform EPR can also perform ST-EPR. 

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