NMR signals

Doddrell D M, Pegg D T and Bendall M R 1982 Distortionless enhancement of NMR signals by polarization transfer J. Magn. Reson. 48 323-7... 

The sinc fiinction describes the best possible case, with often a much stronger frequency dependence of power output delivered at the probe-head. (It should be noted here that other excitation schemes are possible such as adiabatic passage [9] and stochastic excitation [fO] but these are only infrequently applied.) The excitation/recording of the NMR signal is further complicated as the pulse is then fed into the probe circuit which itself has a frequency response. As a result, a broad line will not only experience non-unifonn irradiation but also the intensity detected per spin at different frequency offsets will depend on this probe response, which depends on the quality factor (0. The quality factor is a measure of the sharpness of the resonance of the probe circuit and one definition is the resonance frequency/haltwidth of the resonance response of the circuit (also = a L/R where L is the inductance and R is the probe resistance). Flence, the width of the frequency response decreases as Q increases so that, typically, for a 2 of 100, the haltwidth of the frequency response at 100 MFIz is about 1 MFIz. Flence, direct FT-piilse observation of broad spectral lines becomes impractical with pulse teclmiques for linewidths greater than 200 kFIz. For a great majority of... 

The amplified signal is passed to a double-balanced mixer configured as a phase-sensitive detector where the two inputs are the NMR signal (cOq) and the frequency of the synthesizer (03. gf) with the output proportional to cos(coq - co gj.)t + 0) + cos((coq + + 9). The sum frequency is much larger than the total bandwidth of the... 

Once a slice has been selected and excited, it is necessary to encode the ensuing NMR signal with the coordinates of nuclei within the slice. For each coordinate (x andy) this is achieved by one of two very closely related means, frequency encoding or phase encoding [1]. In this section we consider the fonner and in the next, the latter. In tlie section after that we show how the two are combined in the most coimnon imaging experiment. 

Generally, however, the application of tracer methods remains a rarity compared to methods which directly exploit the motion sensitivity of the NMR signal. The detection of motion is based on the sensitivity of the... 

While the stick plot examples already presented show net and multiplet effects as separate phenomena, the two can be observed in the same spectrum or even in the same NMR signal. The following examples from the literature will illustrate real life uses of CIDNP and demonstrate the variety of structural, mechanistic, and spin physics questions which CIDNP can answer. 

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

Figure B2.4.1 illustrates this type of behaviour. If there is no rotation about the bond joining the N, N -dimethyl group to the ring, the proton NMR signals of the two methyl groups will have different chemical shifts. If the rotation were very fast, then the two methyl enviromnents would be exchanged very quickly and only a single, average, methyl peak would appear in the proton NMR spectrum. Between these two extremes, spectra like those in figure B2.4.1 are observed. At low temperature, when the rate is slow, two...

The observable NMR signal is the imaginary part of the sum of the two steady-state magnetizations, and Mg. The steady state implies that the time derivatives are zero and a little fiirther calculation (and neglect of T2 tenns) gives the NMR spectrum of an exchanging system as equation (B2.4.6)). 

A simple, non-selective pulse starts the experiment. This rotates the equilibrium z magnetization onto the v axis. Note that neither the equilibrium state nor the effect of the pulse depend on the dynamics or the details of the spin Hamiltonian (chemical shifts and coupling constants). The equilibrium density matrix is proportional to F. After the pulse the density matrix is therefore given by and it will evolve as in equation (B2.4.27). If (B2.4.28) is substituted into (B2.4.30), the NMR signal as a fimction of time t, is given by (B2.4.32). In this equation there is a distinction between the sum of the operators weighted by the equilibrium populations, F, from the unweighted sum, 7. The detector sees each spin (but not each coherence ) equally well. 

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

The aromatic shifts that are induced by 5.1c, 5.If and S.lg on the H-NMR spectrum of SDS, CTAB and Zn(DS)2 have been determined. Zn(DS)2 is used as a model system for Cu(DS)2, which is paramagnetic. The cjkcs and counterion binding for Cu(DS)2 and Zn(DS)2 are similar and it has been demonstrated in Chapter 2 that Zn(II) ions are also capable of coordinating to 5.1, albeit somewhat less efficiently than copper ions. Figure 5.7 shows the results of the shift measurements. For comparison purposes also the data for chalcone (5.4) have been added. This compound has almost no tendency to coordinate to transition-metal ions in aqueous solutions. From Figure 5.7 a number of conclusions can be drawn. (1) The shifts induced by 5.1c on the NMR signals of SDS and CTAB... 

Spm-spm splitting of NMR signals results from coupling of the nuclear spins that are separated by two bonds (geminal coupling) or three bonds vicinal coupling)... 

NMR The H NMR signals for the hydroxyl protons of phenols are often broad and their chemical shift like their acidity lies between alcohols and carboxylic acids The range is 8 4-12 with the exact chemical shift depending on the concentration the solvent and the temperature The phenolic proton m the H NMR spectrum shown for p cresol for example appears at 8 5 1 (Figure 24 4)... 

Coupling constant J (Section 13 7) A measure of the extent to which two nuclear spins are coupled In the simplest cases It IS equal to the distance between adjacent peaks in a split NMR signal... 

Below 100 °C tri-t-butyldiaziridinimine (176) only undergoes inversion at the exocyclic nitrogen, as evidenced by coalescence of NMR signals at about 50 °C. Heating for 1 h to 150 °C, however, results in clean formation of ( )-azoisobutane and t-butyl isocyanide. 

Figure 18.16 One-dlmenslonal NMR spectra, (a) H-NMR spectrum of ethanol. The NMR signals (chemical shifts) for all the hydrogen atoms In this small molecule are clearly separated from each other. In this spectrum the signal from the CH3 protons Is split Into three peaks and that from the CH2 protons Into four peaks close to each other, due to the experimental conditions, (b) H-NMR spectrum of a small protein, the C-terminal domain of a cellulase, comprising 36 amino acid residues. The NMR signals from many individual hydrogen atoms overlap and peaks are obtained that comprise signals from many hydrogen atoms. (Courtesy of Per Kraulis, Uppsala, from data published in Kraulis et al.. Biochemistry 28 7241-7257, 1989.)...

Indirect or scalar coupling of nuclear spins through covalent bonds causes the splitting of NMR signals into multiplets in high-resolution NMR spectroscopy in the solution state. The direct or... 

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