Referencing spectra 3 scale

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.

The l3C nmr spectrum reproduced in Figure 3 was obtained from a D20/Na0D solution of calcium leucovorin on a Varian XL100 Spectrometer. The assignments on the TMS scale and referenced to dioxane are listed below ... 

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... 

Other reference substances are available and can be used as well. Sometimes the resonance of the actual solvent can serve as a reference. Preferably, the referencing method used for the sample should be the same as that used for the authentic reference sample (or library spectrum), or at least the scale... 

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... 

Figure 3. Calculated partial photolonlzatlon cross sections for the fourdlpole-allowed channels In K-shell photolonlzatlon of N2 (Mb = 10 °cm ). Note that the energy scale Is referenced to the K-shell IP (409.9 eV) and Is expanded twofold In the discrete part of the spectrum.

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-... 

Fig. 4 Comparison between UPS and IPES spectra measured from an Alqr thin film top) and INDO-simulated UPS and IPES spectra bottom). The energy scale is referenced to the Fermi level. A compression factor of 1.2 is used to simulate the UPS spectrum and the EWHM is set to 0.5 eV prior to compression. The vertical bars at the bottom of the graph refer to the calculated energies of the molecular orbitals. Reprinted with permission from [133]. Copyright 2000 Elsevier B.V.

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. 

A mass spectrum is a compilation of ions of measured mass plotted against the measured intensities of the ion signals. The mass scale (given in units of mass-to-charge ratio, referenced to the mass... 

As any compound in contact with the IRE produces a spectrum, clearly the only reasonable choice for a background or reference spectrum is the bare IRE. Under no circumstances should the clamp be closed on the IRE without the sample and used as a reference. Even with IREs that are used for liquid analysis, the single-beam spectrum of the clean IRE should be used for the background. For example, if the solution is aqueous and it is desirable to remove the water absorption, two separate background spectra should be recorded. One should be the absorbance spectrum of the aqueous solution referenced to an empty ATR cell, and the other should be the absorbance spectrum of water also ratioed to the spectrum of the empty ATR cell. A scaled subtraction of the water spectrum from the solution spectrum will generally produce better results than a ratio of the spectrum of the aqueous solution to that of pure water. This is because the solute displaces water from the solution, so that the amount of water that contributes to the solution and water spectra will not be identical, so they may not ratio correctly. Scaled subtraction, however, can correct for the difference provided that the solute does not cause a shift of the water band. 

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