Dipolar spectrum

Figure 46 shows the simulated spectra for a series of different N—H bond lengths at a rotor frequency of 2.0 kHz. The sideband pattern is sensitive to the N—H bond length. The relative intensity of SSBs to the centre peak decreases with longer N—H distance. It is therefore possible to determine the N—H bond length within an accuracy of 0.01 A by careful comparison of the integral ratio of the centre peak to the sideband intensities of the dipolar spectrum obtained experimentally with that of the simulated spectra. 

Inspection of Eq. (56) immediately shows that Fourier transformation is not suitable to produce the dipolar spectrum S bij) from S t), as the time-domain signal is not oscillating harmonically with the frequency of the dipolar coupling. Vogt et al. have shown that application of the so-called REDOR asymptotic rescaling (REDOR-AR) permits use of Fourier transformation to generate Siby) from the asymptotic form of S t) for t — 00, which they calculated as... 

Fig. 5 (a) The dipole-dipole interaction between two spins A and B depends inter alia on the angle 9 between the spin-spin-vector and the external magnetic field Bq. (b) This angular dependency can be observed for oriented samples as shown with this experimental data for a biradical in a liquid crystal under different orientations, (c) Simulated dipolar spectrum for a macroscopically isotropic sample (Bake pattern). Adapted from [66], copyright Wiley-VCH Verlag GmbH and Co. KGaA. Reproduced with permission... 

Figure 1. Aromatic part of the phase-sensitive 2D resolved dipolar spectrum of the liquid crystal 4 -methoxybenzylidene-4-n-butylaniline (MBBA) in the nematic phase. The spread in the ardirection is governed by the proton-carbon dipolar couplings C+ carbon chemical shifts) while the spread along the horizontal axis is determined by the carbon chemical shifts, exclusively. The corresponding proton-decoupled ID spectrum is sketched at the top of the 2D spectrum for clari ation. Projection of the 2D spectrum onto the

In the most simple case, where only term A needs to be considered, the dipolar spectrum is independent of the resonance freqnencies of the two spins and is a simple Pake pattern (Eig. 2a). If term B also needs to be considered, both the shape of the pattern changes and the singularities shift (Eig. 2b). In this situation, numerical simulations may be required to extract Vdd and to determine the distance r. 

The primary result of a pulsed ELDOR measurement is a distribution of dipolar cou-phngs d. This information is contained either in the primary time-domain data (variation of echo intensity as a function of dipolar evolution time t) or in the dipolar spectrum obtained from these data by Fourier transformation. Time- and frequency-domain data contain exactly the same information, since Fourier transformation is a linear operation. However, some features are easier to recognize in time domain (e.g., quahty of least-squares fitting of the data) and others in frequency domain (e.g., orientation selection by missing parts of the Pake pattern). 

In the proton-detected local field (PDLF) [38-40] experiment (Figure 4-3b), magnetization evolves under the local field of rare spins during L and is subsequently transferred to spin for detection. Hence, PDLF dipolar spectrum is governed by two-spin dipolar interactions, which leads to a superposition of dipolar doublets. 

High-resolution dipolar spectra can also be obtained for solids. If there are large chemical shifts between magnetically dilute spins, as in the carbons in glycine (NH CHzCOO ), CP with proton decoupling splits each C line into a triplet because of the dipolar interaction with the N nucleus. From this high-resolution dipolar spectrum it is possible to determine the internuclear distances and do temperature-dependent motion studies. Cross polarization can be extended to 3-spin systems such as H, C, In double cross polarization, magnetization... 

The angular brackets denote an average that depends on the molecular motion. For dipolar H- C splittings, values as high as 40 kHz are possible. However, a doublet is not observed for semicrystalline polymers. The observed result is generally a broad Gaussian-shaped resonance. The dipolar spectrum is featureless because of the many interactions of all of the spins with each other. Because of the distance dependence of... 

The tenn slow in this case means that the exchange rate is much smaller than the frequency differences in the spectrum, so the lines in the spectrum are not significantly broadened. Flowever, the exchange rate is still comparable with the spin-lattice relaxation times in the system. Exchange, which has many mathematical similarities to dipolar relaxation, can be observed in a NOESY-type experiment (sometimes called EXSY). The rates are measured from a series of EXSY spectra, or by perfonning modified spin-lattice relaxation experiments, such as those pioneered by Floflfman and Eorsen [20]. 

Adducts from various quaternary salts have been isolated, in reactions with aldehydes, a-ketoaldehydes, dialkylacylphosphonates and dialkyl-phosphonates, isocyanates, isothiocyanates, and so forth (Scheme 15) (36). The ylid (11) resulting from removal of a Cj proton from 3.4-dimethyl-S-p-hydroxyethylthiazolium iodide by NEtj in DMF gives with phenylisothiocyanate the stable dipolar adduct (12) that has been identified by its NMR spectrum and reactional product, such as acid addition and thiazolidine obtention via NaBH4 reduction (Scheme 16) (35). It must be mentioned that the adduct issued from di-p-tolylcarbodiimide is separated in its halohydrogenated form. An alkaline treatment occasions an easy ring expansion into a 1,4-thiazine derivative (Scheme 17) (35). 

Fig. 12. Effect of a strong exchange interaction on the shape of the EPR spectrum displayed by a pair of centers A and B having identical g vEilues, = 1.89, g, = 1.96, g = 2.07, and rotated magnetic axes according to xjly, yglx, zJIzy,. (a) 9 GHz spectrum calculated with J = 0 (b) and (c) spectra calculated with J = 25 X 10 cm at 9 and 35 GHz, respectively. The spectra were calculated as described in Ref. 192) without including any dipolar terms, with the linewidths ui = cr, = oi = 0.01.

Thus, identification of all pairwise, interproton relaxation-contribution terms, py (in s ), for a molecule by factorization from the experimentally measured / , values can provide a unique method for calculating interproton distances, which are readily related to molecular structure and conformation. When the concept of pairwise additivity of the relaxation contributions seems to break down, as with a complex molecule having many interconnecting, relaxation pathways, there are reliable separation techniques, such as deuterium substitution in key positions, and a combination of nonselective and selective relaxation-rates, that may be used to distinguish between pairwise, dipolar interactions. Moreover, with the development of the Fourier-transform technique, and the availability of highly sophisticated, n.m.r. spectrometers, it has become possible to measure, routinely, nonselective and selective relaxation-rates of any resonance that can be clearly resolved in a n.m.r. spectrum. 

The ROESY spectrum affords homonuclear transverse nOe interactions as cross-peaks between the various dipolarly coupled hydrogens. This... 

The NOESY spectrum of buxatenone shows four cross-peaks, A-D. Cross-peak B represents the dipolar coupling between the most upfield C-19 cyclopropyl proton (8 0.68) with the most downfield olefinic proton (8 6.72). This could be possible only when the double bond is located either between C-1 and C-2 or between C-11 and C-12. The possibility of placing a double bond between C-11 and C-12 can be excluded on the basis of chemical shift considerations, since conjuga-... 

The NOESY spectrum of 7-hydroxyfrullanolide reveals the spatial proximities between the various protons. Cross-peak D arises from the gemi-nal coupling between the exomethylenic geminal protons (8 5.71 and 6.06). Dipolar interaction between the 06 proton (8 4.97) and the allylic methyl protons (8 1.64) is inferred from cross-peak C. This interaction is possible only when the C-6 proton is a-oriented. The C-1/3 and 02/8 protons (8 1.31 and 1.45, respectively) exhibit cross-peaks... 

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