Referencing, chemical shift scale

Referencing of heteronnclear NMR spectra can be a source of confusion. In an effort to establish universal NMR standards, and hence chemical shift scales, lUPAC have recently adopted a procedure in which all NMR spectra are, effectively, referenced to the H signal of TMS. ... 

Chemical shift scale is a parts per million (ppm) scale. The H and C scale is referenced to 4-trimethyl silane (TMS) at Oppm... 

A iH NMR spectrum is a graph of resonance frequency (chemical shift) vs. the intensity of Rf absorption by the sample. The spectrum is usually calibrated in dimensionless units called "parts per million" (abbreviated to ppm) although the horizontal scale is a frequency scale, the units are converted to ppm so that the scale has the same numbers irrespective of the strength of the magnetic field in which the measurement was made. The scale in ppm, termed the 6 scale, is usually referenced to the resonance of some standard substance whose frequency is chosen as... 

The use of a secondary reference and/or employment of external referencing is a result of some practical considerations. Aspects considered range from principal factors like sample solubility, boiling point or signal overlap to trivial matters of convenience such as routine or tradition in a laboratory. In practice, the chemical shift S, measured relative to a secondary reference as an internal reference, is converted into the 6 scale according to the equation 2... 

A different method of referencing is used in the E-scale of chemical shifts, sometimes referred to as universal referencing . (We shall reserve the term absolute scale , which is also used in this context24, for a different concept see below.) In this scale the chemical shift is given as the absolute frequency of the signal that would be observed in a magnetic field in which the XH NMR signal of TMS appears exactly at 100 MHz. Thus... 

Referencing, as explained (Section III), significantly affects the accuracy of the reported chemical shifts it can introduce a systematic error. If any secondary reference is employed, the relationship used for the conversion should be given, including the numerical value (6s). Statements like chemical shifts measured relative to external hexamethyldisiloxane and internal octamethylcyclotetrasiloxane were scaled to TMS 6-scale leave the 6 values with a large additional uncertainty, perhaps 1-2 ppm. 

TMS cannot be used in aqueous solution because it is not water soluble. For a chemical-shift reference, a water-soluble equivalent (such as sodium d4-3-trimethylsilylpropanoate ((CH3)3SiCD2CD2C02-Na l, TSP )) can be added the single H peak is defined as zero ppm in water. The water peak itself can also be used as a chemical-shift reference, but care must be taken to correct for the temperature dependence of its chemical shift. Referencing of 13C and 15N chemical shifts can be done by using an accurate H reference. If the exact chemical shift is known at the center of the H spectral window (usually the water resonance), the precise radio frequency can be calculated for the zero point of the XH chemical-shift (ppm) scale. For example, on a 600 MHz spectrometer with a reference frequency of 600.13231564 MHz and a water chemical shift of 4.755 ppm ... 

The isotropic shift and the quadrupole induced shift (QIS = Co(p)/Co(l), with Cs defined in Table 2.4) can readily be obtained from the data. Firstly, the 2D spectrum must be referenced correctly. To obtain the correct ppm scale in the FI dimension one must set the spectrometer frequency in the FI dimension to pVo. The shift reference is most readily set at the carrier frequency, i.e. the shift in ppm at the carrier frequency in the multiple quantum dimension Ft is the same as the shift in ppm at the carrier frequency in the single quantum dimension F2. This is valid for data obtained with or without delayed acquisition and processed with or without shearing. The isotropic chemical shift is the same for both the single quantum and the multiple quantum dimensions. The QIS is different in both dimensions, however, and is given by... 

The H and C chemical shifts reported in this review are referenced to tetramethylsUane (TMS) as the internal standard on the 8 scale (ppm). Liquid NH, has been used as the external standard for the " N chemical shifts, with the following values employed to correct the chemical shifts originally reported relative to other commonly used standards liquid NH3 (0.0 ppm), 1 M urea in DMSO (77.0 ppm), 1 M HNO, in H2O (377.3), liquid MeNO, (381.7 ppm). Liquid nitromethane and 1 M urea in DMSO-Jg are frequently used as secondary external standards for measurements in coaxial systems or in separate experiments. A more detailed discussion can be found in recently published review articles on nitrogen NMR spectroscopy. " 2 ... 

The goal of most NMR spectroscopy is a chemical-shift spectrum, because this interaction is the source of NMR s remarkable utility as a molecular characterization tool. While a given nuclide is said to precess at the Larmor frequency, the exact precession frequency for a particular nucleus depends on its chemical environment. Chemical-shift frequencies scale with Bq, so field-independent ppm units are generally preferred in order to facilitate comparison among spectra recorded at different fields. Because chemical shifts (commonly designated by 8) are usually of only a few hertz, relative to the megahertz-scale Larmor frequency, they are usually expressed as parts per million, or ppm. In a 4.7-T magnet, will resonate at 50 MHz. A peak separation of 50 Hz corresponds to a chemical shift of 1 ppm (50 Hz/50 MHz). In a 9.4-T field ( C 100 MHz), these same peaks will be 100 Hz apart. While the difference, in frequency units, has doubled, the chemical shift is still 1 ppm (100 Hz/lOO MHz). The chemical-shift 5 scale is referenced relative to the frequency of a standard material. For example, in H, and Si NMR, the reference is tetramethylsilane (TMS), whose reso-... 

Curiously in the case of 59, the N spectrum was referenced to nitro-methane, while for 60 the spectrum was referenced to ammonia. Expressing both N chemical shifts on the same scale (ammonia = 0 ppm), the chemical shifts of the nitrogen resonances were 209.1 and 152.0 ppm, respectively, which is a relatively large difference in chemical shift for the introduction of a hydroxyl substituent in 60, which is fairly far removed from N1. 

Figure Bl.11.9. Integrated 250 MHz H NMR spectrum of dilute propan-1-ol in dinrethylsulfoxide solvent. Here, the shift order parallels the chemical order. Arr expansion of the H2-I nrultiplet is included, as is the implicit frequency scale, also referenced here to TMS = 0.

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