FT-NMR spectrometer

Figure Bl.12.3. Schematic representation showing the components of a pulse FT NMR spectrometer.

The key dimension m NMR is the frequency axis All of the spectra we have seen so far are ID spectra because they have only one frequency axis In 2D NMR a stan dard pulse sequence adds a second frequency axis Only pulsed FT NMR spectrometers are capable of carrying out 2D experiments... 

Nuclear Magnetic Resonance. In 1994 there were three principal vendors of nmr instmmentation in the U.S., Bmker Instmments (Billerica, Mass.), JEOL USA, Inc. (Peabody, Mass.), and Varian Associates (Palo Alto, Calif.). Details of instmmentation are best obtained directly from manufacturers. A schematic illustrating the principal components of a ft/nmr spectrometer is shown in Eigure 3. 

Carbon-13 nmr. Carbon-13 [14762-74-4] nmr (1,2,11) has been available routinely since the invention of the pulsed ft/nmr spectrometer in the early 1970s. The difficulties of studying carbon by nmr methods is that the most abundant isotope, has a spin, /, of 0, and thus cannot be observed by nmr. However, has 7 = 1/2 and spin properties similar to H. The natural abundance of is only 1.1% of the total carbon the magnetogyric ratio of is 0.25 that of H. Together, these effects make the nucleus ca 1/5700 times as sensitive as H. The interpretation of experiments involves measurements of chemical shifts, integrations, andy-coupling information however, these last two are harder to determine accurately and are less important to identification of connectivity than in H nmr. 

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

Proton NMR and deuteron NMR spectra of soluble fractions and spent solvent mixtures were obtained by using a JE0L FX60Q FT NMR Spectrometer. A flip angle of 45° was used which corresponds to 14 ms for and 75 ms for 2H. The pulse repetition times were 6.0 and 9.0 s, respectively. Chloroform-d was used as the NMR solvent, and chloroform was used as the 2H NMR solvent. 

The most abundant isotope of earbon ( C) cannot be observed by NMR. is a rare nucleus (1.1% natural abundance) and its low concentration coupled with the fact that has a relatively low resonance frequency, leads to its relative insensitivity as an NMR-active nucleus (about 1/6000 as sensitive as iff). However, with the increasing availability of routine pulsed FT NMR spectrometers, it is now common to acquire many spectra and add them together (Section 5.3), so C NMR spectra of good quality can be obtained readily. 

The 90 MHz H-NMR spectrum of benzoic acid shown in Figure 7 was obtained in deuterated chloroform using a Hitachi R-1900 FT-NMR spectrometer. Chemical shifts were measured relative to tetramethylsilane and assignments for the observed bands are found in Table 4. Due to the relatively low resolution of the 90 MHz NMR spectrometer, the only H-H... 

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. 

CJ3 NMR spectra provide information on the skeleton of organic compounds. The low isotopic abundance (1.1 percent) and the rather small magnetic dipole moment of C13 make the spectra difficult to study with a conventional spectrometer. However, FT NMR (Section 8.3) makes natural-abundance C13 studies relatively easy to do. The availability of FT NMR spectrometers has led to a rapid growth of C13 work. 

Separate work on a Fourier transform (FT) NMR spectrometer revealed the presence of two peaks from the solvents from both within and outside the swollen gel [104], See below for a discussion of the origin of the NMR linewidths. The chemical shift of the olefinic peaks was found to shift down field with increasing crosslink density, and hence a modified method for determining H% was introduced. In this paper they also introduced the first 13C NMR measurements of swollen rubber blends, and again found a systematic increase in linewidth with increasing crosslink density. The higher resolution in the 13C spectrum compared with NMR allows the potential of more detailed information on rubber mixtures. 

Fourier transform NMR spectroscopy. The FT NMR spectrometer delivers a radio-frequency pulse close to the resonance frequency of the nuclei. Each nucleus precesses at its own resonance frequency, generating a free induction decay (FID). Many of these transient FIDs are accumulated and averaged in a short period of time. A computer does a Fourier transform (FT) on the averaged FID, producing the spectrum recorded on the printer. 

Spectroscopic Measurements. NMR Spectra. The NMR spectra were recorded at 298°C with a JEOL-PSIOO FT NMR spectrometer operating at 99.5 MHz. The autoreductions were done directly in the NMR tube, generally with 0.4-ml samples of 10-mM iron porphyrin solutions containing a 20-100 molar excess of the substrate. Samples were routinely prepared in a nitrogen atmosphere. 

Commercially available FT NMR spectrometers (e g, Bruker, Vanan, GE, JEOL) are designated by their H operating frequency ranges (Table 5 4 1) Satisfactory nC NMR spectra of lignin can be obtained with 200 and 250 MHz spectrometers Higher field provides greater sensitivity and is used when enhanced sensitivity is a critical requirement The following instrument specifications are important... 

In most fields of physical chemistry, the use of digital computers is considered indispensable. Many things are done today that would be impossible without modem computers. These include Hartree-Fock ab initio quantum mechanical calculations, least-squares refinement of x-ray crystal stmctures with hundreds of adjustable parameters and mar r thousands of observational equations, and Monte Carlo calculations of statistical mechanics, to mention only a few. Moreover computers are now commonly used to control commercial instalments such as Fourier transform infrared (FTIR) and nuclear magnetic resonance (FT-NMR) spectrometers, mass spectrometers, and x-ray single-crystal diffractometers, as well as to control specialized devices that are part of an independently designed experimental apparatus. In this role a computer may give all necessary instaic-tions to the apparatus and record and process the experimental data produced, with relatively little human intervention. 

FT-NMR spectrometer, with 10- or 15-mm-diameter probe capability if possible gas NMR tube with concentric Teflon valve (e.g., J. Young NMR-10) 1-L reaction flask with greaseless stopcock 0- to 1-bar pressure gauge vacuum line Ultra-Torr fittings microsyringes hot plate and oil bath with 0 to 200°C thermometer liquid N2 and ice baths benzoyl chloride and bromide, D2O (95 percent) fume hood. 

Figure 20 H NMR spectra of M-SbA and M-SbA exchanged with increasing quantities, 0.5 to 5.4 meq/g, of Li. The numerical values in parentheses indicate uptake of Li+ in meq/g, and the H NMR spectra were measured with a high-power wide band FT-NMR spectrometer 90°, pulse 1.0 ps.

The complexes of 4-[Co(chxn)3]Cl3 5H20 and (Co(en)3]Cl3 3H20 were prepared by the standard method. The sodium sulfate and the potassium oxalate used were guaranteed reagents of Wako Pure Chemical Industries, Ltd. The Co and NMR spectra were measured with a JEOL GX-270 FT NMR spectrometer operating at 64.1 and 270.1 MHz, respectively. The temperature of the sample solution was controlled at 27iO.S C. The solvents... 

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