Line in nmr

In appearance, the PHIP phenomenon closely resembles those due to CIDNP [16], another phenomenon, which also gives rise to emission and enhanced absorption lines in NMR spectra. However, CIDNP is the consequence of the occurrence of free radicals, and previously has frequently been considered unequivocal proof for free radical reactions. 

Aimr coupling constant, measure of the splitting of lines in NMR measurements ... 

A method for narrowing lines in NMR spectra and obtaining data for nuclei of low natural abundance (see NMR spectroscopy)... 

The position of a line in NMR spectroscopy is the most important piece of information when solving an unknown spectrum. The difference in frequency of resonance of different groups of atoms is a very small fraction of the total magnetic field and, as stated above, the absolute position of resonance cannot be determined due to variations in the strength of... 

The lines in NMR spectra are very narrow - linewidths of 1 Hz or less are not uncommon - so the magnetic field has to be very homogeneous, meaning that it must not vary very much over space. The reason for this is easily demonstrated by an example. 

There is a second infrared absorption feature at 2080 cm" that is also due to hydrogen and attributed in part to S1H2 sites (dihydride), where two hydrogens are bonded to the same tetrahedrally coordinated silicon. However, some of the intensity in this absorption may also be due to Si-H sites (Paul, 1980). For this reason, there appears to be no one-to-one correspondence between the broad line in NMR and the 2080 cm absorption peak in the infrared absorption. 

CIDNP is not a direct method of detection of the structure of radical species the information of hy-perfine interaction (hfi) constants, g-factors, and lifetimes of paramagnetic intermediates, is obtained on the basis of the analysis of the phases and intensities of polarized lines in NMR spectra, as well as from CIDNP kinetics, and the dependence of polarization efficiency on the magnetic field strength applied to the sample during reaction. However, the simplicity and reliability of the identification of polarized lines in the NMR spectra ensure high plausibility of CIDNP-based structural and kinetic information. 

The principal dilTerence from liquid-state NMR is that the interactions which are averaged by molecular motion on the NMR timescale in liquids lead, because of their anisotropic nature, to much wider lines in solids. Extra infonnation is, in principle, available but is often masked by the lower resolution. Thus, many of the teclmiques developed for liquid-state NMR are not currently feasible in the solid state. Furthemiore, the increased linewidth and the methods used to achieve high resolution put more demands on the spectrometer. Nevertheless, the field of solid-state NMR is advancing rapidly, with a steady stream of new experiments forthcoming. 

Figure B2.4.3. Proton NMR spectrum of the aldehyde proton in N-labelled fonnainide. This proton has couplings of 1.76 Hz and 13.55 Hz to the two amino protons, and a couplmg of 15.0 Hz to the nucleus. The outer lines in die spectrum remain sharp, since they represent the sum of the couplings, which is unaffected by the exchange. The iimer lines of the multiplet broaden and coalesce, as in figure B2.4.1. The other peaks in the 303 K spectrum are due to the NH2 protons, whose chemical shifts are even more temperature dependent than that of the aldehyde proton.

If the radiofrequency spectmm is due to emission of radiation between pairs of states - for example nuclear spin states in NMR spectroscopy - the width of a line is a consequence of the lifetime, t, of the upper, emitting state. The lifetime and the energy spread, AE, of the upper state are related through the uncertainty principle (see Equation 1.16) by... 

The example of B5H9 serves to show how the chemical shift may be used as an aid to determining the stmcture of a molecule and, in particular, in deciding between alternative stmctures. There are many examples in the literature of this kind of application which is reminiscent of the way in which the chemical shift in NMR spectroscopy may be employed. However there is one important difference in using the two kinds of chemical shift. In XPS there are no interactions affecting closely spaced lines in the spectmm, however close they may be. Figure 8.15 illustrates this for the C lx lines of thiophene. In NMR spectroscopy the spectmm becomes more complex, due to spin-spin interactions, when chemical shifts are similar. 

In reeent years, tire use of elevated temperatures has been reeognised as a potential variable in method development. Witlr inereased temperature, aqueous-organie mobile phases separations ean improve, viseosity deereases and diffusion inereases so baek pressures are redueed. At higher temperatures (usually with superheated water > 100 °C under modest pressures) water alone ean be used as the mobile phase and eair provide unique separation opportunities. The absenee of an organie solvent enables the use in HPLC of alternative deteetors sueh as FID or on-line LC-NMR using deuterium oxide as the eluent. 

When the alcohol 1 is dissolved in fluorosulfonic acid at — 136°C and then allowed to warm to — 110°C, it gives rise to a cation having a C-NMR spectmm consisting of five lines in the intensity ratio 2 1 2 2 2. Suggest possible stmctures for this cation, and discuss any stabilizing features which might favor a particular stmcture. 

The following compounds all show a single line in their 1H NMR spectra. List them in expected order of increasing chemical shift ... 

AH C-C bonds are equivalent one resonance line in both and l3C NMR spectra. 

Variable temperature NMR studies of [IrCl2(PMe2Ph)4]+ (XXV L = PMe2Ph) show a broad line in the 31P spectrum at room temperature but on cooling to 80°C a mass of sharp lines are observed, owing to three to four rotational isomers (Figure 2.89) the predominant ABCD pattern is... 

In NMR the shape of the absorption curve can be most readily quantified in terms of the even moments of the line shape. For an absorption line described by f(H), the nth moment is defined by... 

Another point of contention has been the extent to which, if any, SbFj is reduced to SbFs upon intercalation. Although chemical analyses have shown an F Sb ratio of 5 1 (Lll, M5), Sb Mossbauer measurements (B24) indicated partial reduction of Sb(V) to Sb(III). On the other hand, mass-spectral measurements as a function of temperature (S15) showed only SbFs, evolved in stages, with no fluorocarbons emitted at any time. The latter are usually an indication of partial reduction of the intercalant and fluorination of the graphite host. Wide-line, F-NMR chemical-shifts are consistent with either SbFj or SbFe, but not with SbFs, but the occurrence of fluorine exchange could produce minor amounts of trivalent species (FI 1) this point is thus still controversial, and will be alluded to again. 

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