Tetramethylsilane, chemical shift

Just as chemical shifts in NMR are measured relative to the protons of tetramethylsilane, chemical shifts in NMR are measured relative to the carbons of tetramethylsilane. Table... 

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),... 

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... 

TABLE 7.47 Proton Chemical Shifts of Reference Compounds Relative to tetramethylsilane. 

TABLE 7.50 Estimation of Chemical Shifts of Alkane Carbons Relative to tetramethylsilane. 

TABLE 7.60 Carbon-13 Chemical Shifts of Deuterated Solvents Relative to tetramethylsilane. 

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. 

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. ... 

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. ... 

FIGURE 4.17 A plot of chemical shifts versus pH for the carbons of lysine. Changes in chemical shift are most pronounced for atoms near the titrating groups. Note the correspondence between the p. values and the particular chemical shift changes. All chemical shifts are defined relative to tetramethylsilane (TMS). (From Suprcnant, H., ct at., 1980. 

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. 

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 ... 

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. 

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. 

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. 

It is important to note the orientation dependence of the shielding constant, cr, and the fact that shielding is proportional to the applied field, whence the need for chemical shift reference materials such as tetramethylsilane. 

Infrared spectra were obtained with a Perkin-Elmer 1800 and a Nicolet Magna-IR 750 FTIR spectrophotometer, and the absorption frequencies are reported in wave numbers (cm4). NMR spectra were obtained with BZH-300 and CA-F-300 Bruker FTNMR 300 MHz spectrometers. Chloroform-d was used as solvent, and all chemical shifts are reported in parts per million downfield (positive) of the standard. H-NMR and 13C-NMR chemical shifts are reported relative to internal tetramethylsilane, while 19F-NMR chemical shifts are reported relative to internal fluorotrichloromethane, Rf values were obtained from silica gel thin-layer chromatography developed with a mixture of 1.5 mL methylene chloride and three drops of acetone. The number of hydrate water molecules was calculated from the integration of H-NMR spectra. 

Fig. 6.—13C-N.m.r. Spectra of A, /3-Limit Dextrin of Rabbit-Liver Glycogen (aqueous solution ambient temperature chemical shifts based on tetramethylsilane) and B, Waxy-barley Amylopectin [in D,0 at 70° chemical shifts (8C) based on external tetra-methylsilane].

DzO. [Temperature 70° chemical shifts (8C) based on external tetramethylsilane.]... 

Fig. 10.—13C-N.m.r. Spectrum at 68 MHz of Branched a-D-Glucan from Streptococcus mutans OMZ-176. [Solvent, DsO, pD 14, at 40° chemical shifts are based on external tetramethylsilane (8C = 0).]...

Fig. 12.—13C-N.m.r. Spectra of A, /3-D-Glucan of Endosperm of Naked Barley (25.2 MHz D20 at 90° chemical shifts expressed as 8C, relative to internal DSS) and B, Li-chenan (62.9 MHz in Me2SO-2H8 at 100° chemical shifts expressed as 8C relative to tetramethylsilane inset lines, corresponding to cellulose and laminaran spectra, are also presented.)...

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