Slow spinning

In paramagnetic materials, the relaxation frequency is in general determined by contributions from both spin-lattice relaxation and spin-spin relaxation. Spin-lattice relaxation processes can conveniently be studied in samples with low concentrations of paramagnetic ions because this results in slow spin-spin relaxation. Spin-spin relaxation processes can be investigated at low temperatures where the spin-lattice relaxation is negligible. Paramagnetic relaxation processes have... 

Figure 9. Li MAS NMR spectra of the ordered spinel [Lio.5Zno.5]tet[Lio.5Mni,5]oct04 and LizMnOs. The asterisks indicate the spinning sidebands of the resonances from Li in the octahedral site in the spinel and the 2b site of Lij-MnOs (i.e., the site due to Li in the Mn layers). The local environments for Li in these sites and the Li in the Li layers of LijMnOs are shown. These spectra were acquired at a moderately slow spinning speed of 8.7—8.8 kHz (cf Figure 6) to increase the intensity seen in the spinning sidebands (relative to the isotropic resonance), improving the accuracy of the analysis of the sideband manifolds.

Nitrous Oxide, Ozone, and Nitrogen Dioxide.28 I(52Py2) undergoes slow, spin orbit relaxation in the presence of nitrous oxide (Table IX) rather than chemical reaction according to... 

FIGURE 28. 39.758-MHz CPMAS 29Si NMR spectrum of solid 1,1,1-trimethyltriphenyldisilane. (a) Slow spinning (206 Hz), (b) Fast spinning (3.2 kHz). The two spinning sideband manifolds in (a) are indicated by different symbols. Experimental conditions contact time 8 ms, recycle time 10 s, number of transients 2090 (a) and 405 (b). Reproduced by permission of the Royal Society of Chemistry from Reference 140... 

Table 2 Observed principal values of 13C NMR chemical shifts of the L-alanine residue Cp-carbons for peptides containing the L-alanine residues from 13C CP/MAS slow-spinning NMR measurements.

By slow spinning band distillation in each case (R=CH3, CgHg) the two compounds corresponding to the two 31p nmr signals were separated with the major product in both instances having the downfield Up nmr chemical shift (R=CH3 6 31p + 75.1, 36% yield R=C H, 6 31P + 65.9, 58% yield). These compounds were identified spectroscopically ( H and 13c nmr, MS, IR) and by microanalysis as the expected 1,2-oxaphosphol-4-ene derivatives of the reaction of eq (1). ... 

Figure 3 Effect of spin rate on the spectrum of a highly anisotropic nucleus, (a) Static sample (bj slow spinning (c) faster spinning (After ref. 1, page 222)...

Even at large AG, > 0 where excitation of the triplets is practically irreversible (ko 0), the difference between Eqs. (3.828) and (3.831) does not disappear. The Stem-Volmer constants Kcb are identical to the kinetic rate constants kc b only when they both are smaller than ko- This is true near the point of inflection in the %r(AG,-) dependence, which is equally far from the maxima of both constants. Near this point kb kc -C ko, and there should be no difference between the fast and slow spin conversions. This is the point of intersection of all the curves shown in Figure 3.103. As can be seen from this figure, accounting for the spin state is not an essential factor in electrochemiluminescence. This is because the electrochemical preparation of ions is not spin-selective. 

Fig. 16. Pulse sequence used in slow-spinning version of DECODER experiment. Each of the solid rectangles represents a 90° pulse. Standard CYCLOPS and spin-temperature alternation were used for phase cycling, (b) Pulse sequence used in the 3D experiment the phase cycling for the t part was similar to Grans170. (Adapted from Lewis et al.260 with permission.)...

One of the main advantages of 13C-detected INADEQUATE is that, unlike 1H-detected INADEQUATE, it provides information about connectivity of quaternary carbons. However, quaternary carbons are handicapped by slow spin-lattice relaxation. In order to improve their sensitivity, the addition of relaxation agents has been proposed.9 It should be noted, however, that this procedure should be carried out with caution, as it may degrade signal strength by quenching the nuclear Overhauser effect.10 One study showed that a moderate... 

For completeness, it is already mentioned here that the relatively high intensity of the origin hne II at T = 1.3 K (Figs. 13 and 16 a) is not in accordance with a Boltzmann distribution for two states which are separated by 7 cm. It will be pointed out in Sects. 4.2.7 and 4.2.8 that the emission from state II cannot be frozen out. This is a consequence of the relatively slow spin-lattice relaxation from state II to state I (compare also the Refs. [24,62,64,65]). 

In conclusion, it is possible for Pt(2-thpy)2, to separate the emission spectra that are super-imposed in time-integrated spectra by time-resolved emission spectroscopy. It is important that one also obtains a low-temperature (1.3 K) emission spectrum from a higher lying state with the corresponding high spectral resolution. This possibility is a consequence of the relatively slow spin-lattice relaxation. Or vice versa, since the monitored time-resolved emission spectra are clearly assignable to different triplet substates, these results nicely support the concept of a slow spin-lattice relaxation as developed above. Moreover, the results presented reveal even more distinctly a triplet substate selectivity with... 

By monitoring excitation spectra with a time-resolved detection of the emission, briefly called time-resolved excitation spectroscopy , it is possible, to identify specific relaxation paths. Although, these occur on a ps time scale, only measurements with a ps time resolution are required. It is shown that the relaxation from an excited vibrational state of an individual triplet sublevel takes place by a fast process of intra-system relaxation (on the order of 1 ps) within the same potential surface to its zero-point vibrational level. Only subsequently, a relatively slow crossing to a different sublevel is possible. This latter process is determined by the slow spin-lattice relaxation. A crossing at the energy of an excited vibrational/phonon level from this potential hypersurface to the one of a different substate does not occur (Fig. 24, Ref. [60]). This method of time-resolved excitation spectroscopy, applied for the first time to transition metal complexes, can also be utilized to resolve spectrally overlapping excited state vibrational satellites and to assign these to their triplet substates. 

Nanosecond pump probc time-resolved resonance Raman experiments were carried out with two Nd YAG lasers (At=7 ns) which provide the pump- (532 nm, 4 mJ) and probe pulses (416 nm, 100 mJ), a triple polychromator equipped with an intensified photodiode array, and a slow spinning cell (25tpm). For every sample, spectra were taken for delay times of At=-20, 0, 20,100 and 500 ns, 1,10,100 and 500 is, and 1ms. ... 

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