Chemical shift delta scale

PMR spectrum of />ethoxyacetanilide in CDC13. Spectrum obtained at a magnetic field of 1.4092 tesla and a radiofrequency of 60 MHz. The chemical shift scale is the delta scale relative to the methyl... 

We say that each group of protons has a characteristic chemical shift. The measurement of the chemical shift helps to identify the type of group responsible for the absorption and indicates what groups are present in the molecule. The chemical shift of a group of lines is expressed in terms of the 8 scale (delta scale), which measures the difference in absorption frequency between the sample (v) and a standard (v°) ... 

So protons with the same chemical shift are called equivalent protons. Non equivalent protons have different chemical shifts expressed by 8 (Delta) pr x (Tau) scales. Since majority of organic compounds have protons resonating at lower fields than the protons of tetramethylsilane, 8 for chemical shift for TMS has been given a value zero, a scale has been given in which most proton resonances are of the same sign and any protons or set of protons absorbing at a field lower than TMS is given a positive value for 8. 

The displacement of a signal from the hypothetical position of maximum shielding is called its chemical shift, notated as S (delta) and measured in parts per million (ppm). As indicated on Fig. 12-4, the zero of the 6 scale is conventionally located at the signal produced by the H s of tetramethylsilane (TMS), (CHj)4 Si. This compound serves because its H-signal is usually isolated in the extreme upheld region. Clues to the structure of an unknown compound can be obtained by comparing the chemical shifts of its spectrum to the d values in such tabulations as Table 12-3. Some generalizations about molecular structure and proton chemical shift in H nmr (pmr) arc ... 

In essence, the chemical shift of a nucleus such as proton ( I I) is its resonance frequency. It is usually expressed in parts per million (ppm) relative to a standard. The most common standard is tetramethylsilane [(CH3)4Si, TMS] which defines 0 on the delta (8) scale and 10 on the older, less used t scale. A small amount of TMS is typically added to the NMR solution to be examined. The presence of an internal standard minimizes experimental variations. This is particularly important because the chemical shift is typically a change of only a few hertz per megahertz, hence the part per million (ppm) scale. The separation of peaks will be greater in hertz at higher field but spectra obtained at different field strengths are comparable on the ppm scale. Common reference standards are listed in Table 6.32. 

It is convenient to divide the subject into four sections. The first two, on chemical shift reagents and on relaxation studies, deal with techniques of line assignments and other facets of steroid behavior. The third, on substituent effects, lays the background for predicting steroid 13C chemical shifts and interactions of substituents with the steroid framework. In the fourth section the use of 13C NMR to solve problems in steroid stereochemistry is discussed. All chemical shift data are reported on the delta scale. 

Conformational analyses of JOM-13 and [L-Ala3]DPDPE have proven to be critical for the determination of the bioactive conformation of enkephalin-like peptides at the delta receptor. H-NMR studies of JOM-13 in aqueous solution revealed that this tetrapeptide exists in two distinct conformations on the NMR time scale as evidenced by two sets of resonances [63]. Large differences in the observed chemical shifts and coupling constants for the D-Cys2 residue in the two conformers suggested that the major differences between the two NMR conformers reside in the disulfide portion of the molecule however, a paucity of conformationally informative nuclear Overhauser enhancement (NOE) interactions precluded the development of a detailed structural model from the NMR studies. In order to develop such a model a thorough conformational analysis of JOM-13 was undertaken, in which the NMR data were complemented by x-ray diffraction results and by molecular mechanics calculations [64]. The results indicate that the 11-... 

The most common scale of chemical shifts is the 8 (delta) scale, which we will use (Figure 13-8). The signal from tetramethylsilane (TMS) is defined as 0.00 ppm on the 8 scale. Most protons are more deshielded than TMS, so the 8 scale increases toward the left of the spectrum. The spectrum is calibrated in both frequency and ppm 8. 

The difference (in ppm) between the resonance frequency of the proton (or carbon nucleus) being observed and that of tetramethylsilane (TMS). Chemical shifts are usually given on the 5 (delta) scale, in parts per million downfield from TMS. (p. 568)... 

The position on the chart at which a nucleus absorbs is called its chemical shift. The chemical shift of TMS is set as the zero point, and other absorptions normally occur downfield, to the left on the chart. NMR charts are calibrated using an arbitrary scale called the delta ((5) scale, where 1 5 equals 1 part per million (1 ppm.) of the spectrometer operating frequency. For example, if we w ere measuring the NMR spectrum of a sample using an instrument operating at... 

An NMR spectrum plots the intensity of a peak against its chemical shift measured in parts per million (ppm). The common scale of chemical shifts is called the 5 (delta) scale. The proton NMR spectrum of tert-butyl methyl ether [CH30C(CH3)3] illustrates several important features ... 

The variation in resonance frequency due to the electronic environment of a nucleus is called the chemical shift. Chemical shifts on the delta scale are defined by... 

The most commonly used scale is the 5 delta) scale. The position of the tetramethylsilane signal is taken as 0.0 ppm. Most chemical shifts have B values between 0 and 10 (minus 10, actually). A small B value represents a small downfield shift, and a large B value represents a large downfield shift. 

By agreement, most workers report chemical shifts in delta (S) units, or parts per million (ppm), of the main spectrometer frequency. On this scale, the resonance of the protons in TMS comes at exactly 0.00 ppm (by definition). 

Chemical shifts are measured along the spectrum axis using a delta (5) scale, in units of parts per miUion (ppm). When comparing one signal with another ... 

The NMR spectrum is calibrated using tetramethylsilane (Mc4Si), which produces a single low frequency peak in both H and C NMR spectra. This is set as a chemical shift value of 0 on the delta (5) scale, where 13 = 1 part per million (ppm) of the operating frequency of the spectrometer. Almost all other absorptions occur at higher frequency to this signal, typically 0-10 ppm for H NMR and 0-210 ppm for NMR. 

Delta (8) scale (Section 14.1B) A common scale of chemical shifts used in NMR spectroscopy in which the absorption due to tetramethylsUane (TMS) occurs at zero parts per million. 

As we have also mentioned, chemical shifts are most often measured with reference to the absorption of the protons of TMS (tetramethylsilane). A small amount of TMS is usually added to the sample, and its signal establishes zero on the delta (5) scale. 

---