NMR tetramethylsilane

The molar diamagnetic susceptibility of thiazole and some derivatives was initially determined by the classical Curie-Cheneveau method (5,315,316) and later confirmed by a method (317) based on the difference of NMR proton chemical shift of a sample of tetramethylsilane immersed in the liquid to be investigated, according to the shape (cylindrical or spherical) of the sample tube (Table 1-47) (318),... 

Just as chemical shifts in H NMR are measured relative to the protons of tetramethylsi lane chemical shifts m NMR are measured relative to the carbons of tetramethylsilane Table 13 3 lists typical chemical shift ranges for some representative types of carbon atoms In general the factors that most affect chemical shifts are... 

Tetramethylsilane (TIMS) (Section 13 4) The molecule (CH3)4Si used as a standard to calibrate proton and carbon 13 NMR spectra... 

By trapping PX at liquid nitrogen temperature and transferring it to THF at —80° C, the nmr spectmm could be observed (9). It consists of two sharp peaks of equal area at chemical shifts of 5.10 and 6.49 ppm downfield from tetramethylsilane (TMS). The fact that any sharp peaks are observed at all attests to the absence of any significant concentration of unpaired electron spins, such as those that would be contributed by the biradical (11). Furthermore, the chemical shift of the ring protons, 6.49 ppm, is well upheld from the typical aromatic range and more characteristic of an oletinic proton. Thus the olefin stmcture (1) for PX is also supported by nmr. 

Proton nmr. In the simplest experiment, the sample and a small amount of a reference compound such as tetramethylsilane [75-76-3] (TMS), are placed in a tube, usually of 5-mm diameter. Typical samples may be a neat Hquid or a solution containing as Htde solute as 0.01 mg/cm. The... 

J3 4 = 3.45-4.35 J2-4 = 1.25-1.7 and J2-5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. C chemical shifts for thiophene, from tetramethylsilane (TMS), are 127.6, C 125.9, C 125.9, and C 127.6 ppm. 

FIGURE 13.7 The200-MHz H NMR spectrum of chloroform (HCCb). Chemical shifts are measured along the x-axis in parts per million (ppm) from tetramethylsilane as the reference, which is assigned a value of zero. 

Chemical shift (Section 13.4) A measure of how shielded the nucleus of a particular atom is. Nuclei of different atoms have different chemical shifts, and nuclei of the same atom have chemical shifts that are sensitive to their molecular environment. In proton and carbon-13 NMR, chemical shifts are cited as 8, or parts per million (ppm), from the hydrogens or carbons, respectively, of tetramethylsilane. 

FIGURE 4.16 Proton NMR spectra of several amino acids. Zero on the chemical shift scale is defined by the resonance of tetramethylsilane (TMS). (Adaptedfrom Atelrkh Library of NMR Spectra. ... 

To define the position of an absorption, the NMR chart is calibrated and a reference point is used. In practice, a small amount of tetramethylsilane [TMS (CH )4Si] Is added to the sample so that a reference absorption peak is produced when the spectrum is run. TMS is used as reference for both l H and 13C measurements because it produces in both a single peak that occurs upfield of other absorptions normally found in organic compounds. The ]H and 13C spectra of methyl acetate in Figure 13.3 have the l MS reference peak indicated. 

The NMR chart is calibrated in delta units (5), where 15=1 ppm of spectrometer frequency. Tetramethylsilane (TMS) is used as a reference point because it shows both 1H and 13C absorptions at unusually high values of the applied magnetic field. The TMS absorption occurs at the right-hand (upfield) side of the chart and is arbitrarily assigned a value of 0 5. 

Chemical shift (Section 13.3) The position on the NMR chart where a nucleus absorbs. By convention, the chemical shift of tetramethylsilane (TMS) is set at zero, and all other absorptions usually occur downfield (to the left on the chart). Chemical shifts are expressed in delta units. 5, w here 1 5 equals 1 ppm of the spectrometer operating frequency. 

Deshielding (Section 13.2) An effect observed in NMR that causes a nucleus to absorb downfield (to the left) of tetramethylsilane (TMS) standard. Deshielding is caused by a withdrawal of electron density from the nucleus. 

Melting points, measured in open capillary tubes using a Thomas-Hoover melting point apparatus, are uncorrected. Elemental analyses were performed by Galbraith Laboratories, Knoxville, Tennesee. % and 19C-NMR spectra were generally obtained with an IBM AF-100, if necessary the higher field Bruker WP-200 or AM-400 NMR spectrometer were employed. Chemical shifts are given in parts per million (ppm) on a a scale downfield from tetramethylsilane (TMS). Infrared spectra were recorded with a Perkln-Elmer 283 spectrophotometer. 

All solvents and reagents were reagent grade or were purified before use. IR spectra were recorded on a Perkin-Elmer 983 NMR spectra were recorded at 200 MHz on a Varian XL-200 using tetramethylsilane as an internal reference. Inherent viscosities were determined at a concentration of 0.5g/dL using a Canon-Fenske viscometer at 30°C. Monomer and polymer synthesis has recently been described (10). 

Structural Characterization. 13C-NMR Spectra of PGG glucan preparations (15 mg/mL in 0.5 M NaOD) were recorded with a Bruker Model AC 200 at 50.3 MHz and all chemical shifts were expressed in parts per million downfield from an internal tetramethylsilane (TMS) standard. 

Spectroscopic Analysis. Infrared (IR) spectroscopic analysis was performed on a Beckman Microlab 620 MX computing spectrometer. Samples were cast on a sodium chloride pellet or made into a pellet with potassium bromide. and 13C NMR spectra were obtained using a JEOL HNM-FX 270 MHz Fourier transform NMR spectrometer. Samples were dissolved in deuterium chloroform and chemical shifts were referenced to an internal standard of tetramethylsilane. 

NMR measurements also provide information on the coordination of the ligands in the uranyl polymers. Solid-state I c-NMR confirms the coordination modes of the carboxylate ligands to the uranyl ion that is, both monodentate and bidentate carboxylate coordination modes are evident. The uranyl dicarboxyl ate polymers which possess two moles of coordinated DMSO exhibit two carbon-13 carbonyl resonances, one at about 175 ppm downfield from tetramethylsilane (TMS) and one at about 185 ppm. The polymers which possess only one mole of coordinated DMSO exhibit only the carbonyl peak near 185 ppm. Based on other known coordination compounds, the 175 ppm peak can be assigned to monodentate carboxylate and the 185 ppm peak to bidentate carboxylate. Thus, 7-coordination predominates in the polymers with either one or two moles of solvent coordinated to the uranyl ion, which is consistent with the infrared results reported elsewhere (5). 

Diphenoxy-2-propanol (3) was prepared from phenol and epichlorohydrin (l-chloro-2,3-epoxypropane), as previously described,8 and recrystallized three times from 2-propanol to yield white crystals, m.p. 82-82.5°C. The nmr spectrum in CDC1- (Varian EM-390 spectrometer) exhibited resonances (in ppm (6) relative to tetramethylsilane) at 3.0 (1H, doublet,... 

Polymer Characterization. Proton NMR spectra at 300 MHz were obtained from a Varian HR-300 NMR spectrometer. Deutero-benzene and spectrograde carbon tetrachloride were used as solvents. The concentration of the polymer solutions was about 1-5%, Carbon-13 NMR spectra were obtained from a Varian CFT-20 NMR spectrometer, using deuterochloroform as the solvent for the polymers. The concentration of the solutions was about 5%. Chemical shifts in both proton and carbon-13 spectra were measured in ppm with respect to reference tetramethylsilane (TMS). All spectra were recorded at ambient temperature. 

The carbon-13 NMR spectra of miconazole nitrate were obtained using a Bruker Instrument operating at 75, 100, or 125 MHz. The sample was dissolved in DMSO-d6 and tetramethylsilane (TMS) was added to function as the internal standard. The 13C NMR spectra are shown in Figs. 9 and 10 and the HSQC and HMBC NMR spectra are shown in Figs. 11 and 12, respectively. The DEPT 90 and DEPT 135 are shown in Figs. 13 and 14, respectively. The assignments for the observed resonance bands associated with the various carbons are listed in Table 4. 

Standard Bruker Software was used to execute the recording of DEPT, COSY, and HETCOR spectra. The sample was dissolved in DMSO-d6 and all resonance bands were referenced to tetramethylsilane (TMS) internal standard. The H NMR spectra of primaquine diphosphate are shown in Figs. 5-8. The H NMR assignments of primaquine diphosphate are shown in Table 3. 

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