Pure-exchange spectra

An alternative to sine-squared modulation of the two-dimensional exchange spectrum to achieve pure-exchange spectra is sine modulation.67 A very similar pulse sequence to that in Fig. 38 is used, as shown in Fig. 40, and the experiment again requires recording spectra for several different t... 

Fig. 40. The pulse sequence required to produce sine-modulated two-dimensional exchange spectra for static (non-spinning) samples, in order to achieve pure-exchange spectra.67...

Spectra with the form of the two terms on the right-hand side of Eq. (18) can be obtained by two extra modulation periods of length t in the exchange pulse sequence, one before and one after rm. The sine.sine and cosine.cosine terms of Eq. (18) are selected by suitable choices of the phases of the flip back pulses, labelled a and b in Fig. 38. The spectrum of the form of Eq. (18) is produced by the pulse sequence in Fig. 38(a), for non-spinning samples, while the spectrum of the form 1/2 S is produced using the sequence of Fig. 38(b), which matches that in Fig. 38(a) in terms of pulses and delays and so should produce a matched intensity spectrum, so that when the relevant spectra from the pulse sequence in Fig. 38(a) are subtracted from it, the desired pure-exchange spectrum is obtained. The A periods in both sequences are simply Hahn echoes, implemented to achieve non-distorted powder patterns in both spectral dimensions.2... 

In Section 12.3 it was described how, with the help of a two-dimensional exchange spectrum, domain sizes in the iPP/EP blend are estimated. There an estimated value for the Xe diffusion coefficient was used. Now with experimental data from Table 12.2 the average EPDM domain size for the iPP/EPDM blend can be calculated. It was assumed that the structure of the EPDM in the blend is the same as in the pure material, (i.e., the Xe diffusion coefficient in the EPDM domains in the blend is equal to the measured D for pure EPDM and likewise for iPP), then for Xe in the EPDM domain during A = 1.2 seconds is 20 grn. In the same time for Xe in the iPP matrix is approximately 5 grn. These distances for a diffusion time of 1.3 milliseconds, the inverse of the frequency difference 770 Hz of the two lines in the Xe NMR spectrum of the blend, are 0.6 and 0.2 grn, respectively. The average size of the EPDM domains in the iPP/EPDM blend is... 

Fig. 14. (a) Eull reference spectrum 5 0, spectrum S after CODEX dephasing and pure-exchange... 

CODEX spectrum /S.S = Sq — S of natural abundance PMMA at 300 K, for Tni = 500ms. The intensity of A S is low since only a minority of the side groups (35%) are undergoing flips, (b) Series of pure-exchange CODEX spectra of PMMA at 300 K, = 6.5 kHz, as a function of as indicated. Large-amplitude side group and smaller-amplitude backbone motions are observed. (Adapted with permission from Ref. 662.)... 

Fig. 9.16 Pure-exchange CODEX C NMR spectra, recorded for a sample of amorphous PMMA, (at natural abundance in C) at 300 Kwith different A CP MAS spectrum is shown at the top. (Reproduced by permission of the American Chemical Society from [77].)...

When accelerated sufficiently, amplitude-frequency modulation in the absence of dephasing results in signal monochromatization, just like in the case of pure frequency modulation. Before the spectrum collapses, exchange between branches causes their broadening, but after collapse it provides their coalescence into a single line at frequency... 

If you take a pure sample of ethanol, and run its NMR spectrum in dry CDCI3, the hydroxyl proton will appear as a well-defined triplet, which couples to the adjacent -CH2-, rendering it a multiplet. This is because the hydroxyl proton remains on the oxygen for relatively long periods of time, as there is nothing in the solution to entice it off, i.e., exchange (if any) is said to be very slow on the NMR timescale (less than about 1 s). 

FIGURE 11.6 EPR of the copper dimer in pure copper acetate powder. Strong exchange coupling gives an S = 0 ground state and an S = 1 excited state at 2J 300 cm 1. At T = 48 K the triplet is hardly populated, and the spectrum is dominated by a trace of monomeric copper. 

In principle, the estimation of conformational equilibrium in a piperidine derivative by protonation is very simple. If nitrogen inversion is slow compared to deuteronation, if H+/D+ exchange does not occur in the salts, and if H+/D+ exchange is unimportant in the mixing process, then the relative proportion of the salts corresponds to that of the conformers. Thus the spectrum of pure dry cis-3,5-dimethylpiperidine in deuterotrifluoroacetic acid (unchanged after 2 days) showed a septet at 8 2.70 for the 2,6-axial protons in 112 and 113. This was interpreted as a triplet for 112 and a quartet for 113 and analysis of the multiplets gave 54% 112 and 46% 113 (AG"... 

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