Tetramethylsilane shift reference

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. 

Sc, carbon chemical shift, referred to tetramethylsilane (8 = 0) (cf. Sect. I) SCS, substituent-induced chemical shift, or substituent effect difference between S s of a given carbon atom in a monosubstituted and the respective unsubstituted parent molecule (cf. Sect. Ill) NAE, nonadditivity effect nonadditivity of individual SCSs in disubstituted molecules (cf. Sect. IV) ICS, intramolecular-interaction chemical shift = NAE (cf. Sect. IV) A, polarization effect difference in S s of sp2 carbon atoms in a double bond (cf. Sect. IV-C) LEF, linear electric field (cf. Sect. II-B-3) SEF, square electric field (cf. Sect. II-B-3). 

Using a somewhat different approach, Bucci (62) interpreted these substituent influences in terms of a combination of electronegativity and the number m of lone electron pairs at the substituent or at that atom which is directly attached to the a-carbon atom, respectively, and derived the empirical expressions [5] and [6] for, 3C chemical shifts referred to tetramethylsilane (8 = 0) ... 

Any heteronuclear signal of a solvent or an added reference substance can be used for referencing I3C shifts. For example, 13C shifts can be directly measured relative to a deuterium signal of the deuterated solvent usually required for field/frequency stabilization. However, homonuclear shift references such as the l3C signals of tetramethylsilane (TMS), carbon disulfide, benzene, cyclohexane, 1,4-dioxane or the easily localizable mul-tiplet signals of deuterated solvents (Fig. 2.22) are predominantly applied in 13C NMR. 

For 13C shift/structure correlations and for tabulations of ppm values one generally accepted reference should be used. Carbon disulfide, which appears in the low field region of, 3C spectra, was widely used in the early literature [73a, b]. Later, tetramethylsilane (TMS), known from proton NMR, became the generally accepted carbon-13 shift reference, particularly because of some parallels in the behavior of H and 13C shifts. 

Both were recorded over 5000 Hz range in deuterated dimethylsulphoxide on FT-80 A-80 MHz NMR spectrometer using tetramethylsilane as reference standard. The carbon chemical shift value are assigned on the basis of signal multiplicity, chemical shits and the comparison with the chemical shits of model compounds. 

When preparing a sample it is common practice to add a suitable compound to act as an internal chemical shift reference in the spectrum, and the selection of this must be suitable for the analyte and solvent. In proton and carbon NMR, the reference used in organic solvents is tetramethylsilane (TMS, 0.0 ppm) which has a number of favourable properties it has a sharp 12-proton singlet resonance that falls conveniently to one end of the spectrum, it is volatile so can be readily removed and it is chemically inert. In a few cases this material may be unsuitable such as in the study of silanes or cyclopropanes. For routine... 

Chemical shifts are quoted in t units (deuteriochloroform) downfield from the tetramethylsilane internal reference. All resonances are singlets unless stated otherwise. 

Tetramethylsilane (TMS) is the most commonly used chemical shift reference compound for H and C NMR. In both types of spectra TMS gives a single sharp resonance at lower frequency than most other proton or carbon resonances. The TMS is usually added to the sample solution as an internal reference. The accepted sign convention is for chemical shifts to high and low frequency of the reference peak to be positive and negative, respectively. Figure 9 shows the 250 MHz H spectrum of methyl acetate... 

As mentioned in Section 2.3.1, the almost universally adopted primary reference for chemical shifts in NMR spectra is the compound tetramethylsilane (TMS). Incidentally, TMS is also employed as the primary chemical shift reference in H and Si NMR [18]. Some old reports give chemical shifts relative to benzene or carbon disulfide, but the use of the TMS resonance as the zero chemical shift mark is now commonplace. Unfortunately, TMS is highly volatile, toxic, and flammable, which makes problematic its use as a direct reference sample. A simple alternative is to use another sample with a known spectrum as an external and secondary reference. In the case of solid-state NMR there are many known secondary standards, such as adamantane (higher-frequency peak at 38.6 ppm), hexamethylbenzene (higher-frequency peak at 132.2 ppm), and glycine (higher-frequency peak at 176 ppm) [4,15,98]. 

The integrals of the aliphatic hydrogen band and of the aliphatic carbon band must be corrected for the NMR absorption line due to the internal chemical shift reference tetramethylsilane (0.0 ppm chemical shift in both H and spectra). 

Tetramethylsilane, American Chemical Society (ACS) reagent internal chemical shift reference for H and C NMR spectra. 

If tetramethylsilane was used as an internal chemical shift reference, subtract the portion of integral contributed by the NMR absorption line of TMS (0.0 ppm in the H NMR spectrum) from the total integral value for Region B. 

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

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. 

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. 

Deshielded proton would give the resonance signal upfleld and a shielded proton would absorb down field. These shifts in the NMR signals are what are known as Chemical shifts. These shifts are measured with reference to a standard which is tetramethylsilane (TMS). 

The preferred standard for both the hydrogen nucleides and 13 C is tetramethylsilane (TMS). Indeed it is quite feasible to refer all shifts to the proton resonance of TMS, but most spectroscopists prefer a homonuclear reference. 

IR spectra were taken on an Analect RFX-30 FTIR spectrophotometer neat between NaCI or KBr plates or as KBr disks. 1H NMR spectra were recorded on a Nicolet NT-360 (360 MHz) or on a Varian VXR-200 (200 MHz) spectrometer. All chemical shifts are reported in parts per million (8) downfield from internal tetramethylsilane. Fully decoupled 13C NMR spectra and DEPT experiments were recorded on a Varian VXR-200 (50 MHz) spectrometer. The center peak of CDCI3 (77.0 ppm) was used as the internal reference. 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

Chemical shifts are expressed in p.p.m. downfield from tetramethylsilane as internal reference, with... 

In CDCI3 or DMSO-t/g we normally choose tetramethylsilane (TMS), (CH3)4Si, as our reference, and this is given a reference value of 0 Hz so that, in the above example, the signals which occur at 100 Hz and 500 Hz, at magnetic fields of 100 MHz and 500 MHz respectively, have a chemical shift of 8 1.0 in both cases. 

Spectrum calibration, for which a reference compound such as tetramethylsilane (TMS) or - less preferably - the solvent signal is used, allows the chemical shifts of the investigated compound to be compared with those of reference compounds, available in the literature or in commercial data bases. 

For line shift measurements with the eight-pulse cycle between 180°K and room temperature, the reference was acetyl chloride. Its frequency was measured relative to a spherical tetramethylsilane (TMS) sample at room temperature, and all results are reported relative to this TMS on the r scale, (r = a + 10 ppm, where a is the signed chemical shift used in solid state NMR.) At lower temperatures, the reference was a single crystal of Ca(OH)2, oriented in the magnetic field such that the major axis of its proton chemical shift tensor was parallel to the external field (19). Thus, it is assumed that the proton chemical snift of the Ca(OH)2 remained unchanged as the temperature was varied. 

---