Tetramethylsilane standard

X-ray diffraction patterns were recorded on a Siemens D5000 diffractometer using CuKa radiation. Thermogravimetric and differential thermal analysis curves were recorded on a Setaram Setsys 12 thermal analysis station by heating in an argon atmosphere from 25 to 1200 -C at a rate of 5 min". Samples were used untreated. The Pt content was determined by the Service Central d Analyse, CNRS (Vernaison, France) and the microanalyses (C, H) were performed at Complutense University (Madrid, Spain). Na isotherms were determined on a Micromeritics ASAP 2000 analyzer. H MAS NMR, Si MAS NMR and C CP MAS NMR spectra were recorded at 400.13, 79.49 and 100.61 MHz, respectively, on a Broker ACP-400 spectrometer at room temperature. An overall 1000 free induction decays were accumulated. The excitation pulse and recycle time for H MAS NMR spectra were 5 ps and 3 s, respectively, those for Si MAS NMR spectra 6 ps and 60 s and those for C CP MAS NMR spectra 6 ps and 2 s. Chemical shifts were measured relative to a tetramethylsilane standard. Prior to measurement, if necessary, samples were dehydrated in a stove at 423 K for 24 h. 

The C NMR spectra were obtained on a Varian CFT-20 NMR spectrometer at room temperature and at a concentration of blends at -10% (w/v) in CDCI, employing a tetramethylsilane standard, assigned a resonance position of zero PPM (6 scale), as an internal standard. 

Tetramethylsilane is used as an internal standard in NMR, as it is both chemically stable and widely soluble. The chemical shift - the shift in ( Si) - is measured in ppm a positive sign indicates that the Si resonance has been shifted to a higher frequency (corresponding to a lower magnetic field). The spectra may be made for neat liquids (in some cases), or in concentrated solutions in tetrachloromethane. The position of the tetramethylsilane standard may be determined before and after the resonance of the unknown has been recorded. 

MHz), (500 MHz) and NMR (500 MHz) spectra were recorded on JEOL ECP-500. Chemical shifts are reported in d ppm, referenced to internal tetramethylsilane standard for H NMR, external trifluoroacetic acid standard (<5-76.5 ppm) for NMR and internal 85 % H3PO4 standard for P NMR. ESI-mass spectra were measured on JEOL JMS-700 M Station. Elemental analyses were performed by Perkin Elmer 2400II. 

Freshly distilled phosphorus trichloride (87 mL, 1 mol), anhydrous ethyl ether (200 mL), and dry pyridine (81 mL, 1 mol) are mechanically stirred in a three-necked flask (1 L) under a dry argon atmosphere at-78°C. Distilled 3-hydroxypropionitrile (68 mL, 1 mol) in anhydrous ethyl ether is added dropwise over ca. 2 h to the cold mixture maintained at-78°C. The suspension is then allowed to warm to ambient temperature and stirred overnight under argon. The precipitate is filtered and washed with ethyl ether (100 mL). The filtrate is concentrated under reduced pressure, and the crude reaction product is fractionally distilled under vacuum affording 2-cyano-ethoxy-dichlorophosphine as a colorless liquid (bp 78°C at 0.6 mm Hg) in 55% yield (95 g, 0.55 mol) (12,18). ip-NMR (CDCI3) 178.8 ppm downfield from the external 80% H3PO4 standard (20). H NMR (CDCI3) 5 in ppm 2.7 (t, 2H) 4.4 (2t, 2H) relative to an internal tetramethylsilane standard (20). 

Fig. 4. Proton magnetic resonance spectral assignments for A batrachotoxinin A, B batrachotoxin, and C homobatrachotoxin. Chemical shifts in ppm for deuterochloroform with a tetramethylsilane standard s singlet d doublet t triplet q quartet br broad. Coupling constants are in parentheses in cycles per second. Spectra (100 MHz) are depicted by Tokuyama et al. (251) and in Fig. 5. Capital letters A—M refer to assignments in Fig. 5. Values for batrachotoxinin A differ somewhat from those reported later for synthetic and natural compound by Imhof et al. (144,145). It appears likely that the values of Imhof et al. correspond to the free base and that the earlier values of Tokuyama et al. were for mixtures of free base and cationic form present in the CDCI3. Certain earlier assignments (257) have been revised in light of the detailed examination of the spectrum of batrachotoxinin A by Imhof etal. (145)...

Fig. 5. Proton magnetic resonance spectra (100 MHz) for batrachotoxin class alkaloids and assignments. Chemical shifts in ppm (5) for deuterochloroform with a tetramethylsilane standard. Assignments A pyrrole NH B pyrrole 5-H C olefinic proton at C-7 D oleflnic proton at C-16 proton at C-20 FI4-OCH2 and proton at C-11 G one proton at C-15, the other appears at 6 2.3 methylene protons at C-18 / pyrrole 2-CH3 T and / pyrrole 2-CH2CH3 / NCH3 K pyrrole 4-CH3 L 2I-CH3 M I9-CH3 (see Fig. 4)...

Fig. 11. Proton magnetic resonance spectrum (100 MHz) and assignments for dihydroiso-histrionicotoxin. Chemical shifts in ppm for deuterochloroform with a tetramethylsilane standard (Tokuyama, unpublished spectrum)...

Instead of measuring chemical shifts m absolute terms we measure them with respect to a standard—tetramethylsilane ( 113)481 abbreviated TMS The protons of TMS are more shielded than those of most organic compounds so all of the signals m... 

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

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. 

It is convenient to reference the chemical shift to a standard such as tetramethylsilane [TMS, (C//j)4Si] rather than to the proton fC. Thus, a frequency difference (Hz) is measured for a proton or a carbon-13 nucleus of a sample from the H or C resonance of TMS. This value is divided by the absolute value of the Larmor frequency of the standard (e.g. 400 MHz for the protons and 100 MHz for the carbon-13 nuclei of TMS when using a 400 MHz spectrometer), which itself is proportional to the strength Bg of the magnetic field. The chemical shift is therefore given in parts per million (ppm, 5 scale, Sh for protons, 5c for carbon-13 nuclei), because a frequency difference in Hz is divided by a frequency in MHz, these values being in a proportion of 1 1O. ... 

Shielding constants reported in experimental studies are usually shifts relative to a standard compound, often tetramethylsilane (TMS). In order to compare predicted values to experimental results, we also need to compute the absolute shielding value for TMS, using exactly the same model chemistry. Here is the relevant output for TMS ... 

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. 

The standard is typically tetramethylsilane, Si(CH )4, which has a lot of protons and dissolves in many samples without reaction. Each group has a characteristic chemical shift, although the precise value depends on the other groups attached to the group of interest. For instance, if we observe a resonance at 8 = 1, we can be reasonably confident that it arises from a methyl group in an alcohol. 5 ... 

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. 

The spectra were recorded at 250 MHz in CDCI3, using tetramethylsilane as internal standard (TMS = 0). The multiplicities have been added by the reviewer and are based on the coupling constants indicated and examination of the visually reproduced spectra. The C-6 and C-7 protons and the aromatic protons resonating between 2.4 and 1.8 ppm, and 7.9 and 7.2 ppm, respectively, were not differentiated. 

The spectra were recorded using tetramethylsilane as internal standard (TSM = 0). The signals for the protons marked with na were not reported. 

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. 

Greaves et al. [74] used a selected ion-monitoring assay method for the determination of primaquine in plasma and urine using gas chromatography-mass spectrometric method and a deuterated internal standard. After freeze-drying and extraction with trichloroethylene, the sample plus internal standard was reacted with Tri Sil TBT (a 3 3 2 by volume mixture of trimethylsilylimidazole, A/O-bis-(trimethylsilylacetamide and trimethylchlorosilane) and an aliquot injected to the gas chromatograph-mass spectrometer. The gas chromatographic effluent was monitored at m/z 403, and m/z 406, the molecular ions of the bis-tetramethylsilane ethers of primaquine and 6-trideuteromethoxy primaquine. 

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