Single-selective relaxation rate

Combinations of non-selective and/or single-selective relaxation-rates, or both, with n.0.e. values may conveniently be performed with reliable results, especially when other methods seem impractical. However, these experiments are time-consuming, as they entail the determination of a rather large number of experimental values. Moreover, the n.O.e. parameters carry their own systematic and random errors, which are magnified in the calculation of interproton distances. The deuterium-substitution method requires specific deuteration at a strategic position, which, in many cases, may be inconvenient or impractical. Also, this technique is valid only when the relaxation rates obtained after deuterium substitution are at least 5% enhanced, relative to the relaxation rates of the unsubstituted compound, and it requires that, for a meaningful experiment, the following condition " be satisfied. 

Stage 1 involves first the determination of the Ri-value of the protons (A) of interest by the two-pulse sequence (10) referred to earlier, in which the perturbing, 180°-pulse is applied in a non-selective fashion to all the proton resonances (we shall refer to this value as the non-selective relaxation rate (ns)). Then the two-pulse sequence is repeated, except that now the 180°-pulse is applied selectively (11), (12) to just the A-resonance, and the recovery back to thermaT equTTibrium of the magnetisation of A is monitored with the usual non-selective 90° pulse this gives a new relaxation rate constant, the single-selective relaxation rate (R ()C) where Vindicates the nucleus which has been subjected to the single-selective 180° pulse). 

The spectra shown in figures 2 and 3 illustrate the determination of the non-selective and single-selective relaxation rates for the anomeric proton of methyl e-D-glucopyranoside, and the resultant data, along with that of the a-anomer and the two trideuteriomethyl glucosides is given in Table I. It is... 

Referring back to the Ri-values given in Table I it is a trivial matter to calculate the relaxation contributions between the methoxyl protons and H-l for the two methyl D-glucopyrano-sides and these values are listed below in Table II. Not surprisingly, there is excellent agreement between the values calculated from the non-selective and single-selective relaxation rates. 

According to these equations, the effect of selectively perturbing the spin states of spins i and j is to isolate the cross-relaxation paths common to these two spins. Combining Eqs. 15 and 19, the individual cross-relaxation terms are readily determined from single-selective and double-selective relaxation-rate measurements, that is. 

Single- and double-selective relaxation-rates, together with n.O.e. experiments, have been used to examine the configuration and conformation of asperlin (1) in benzene solution." " Comparing experimental distances for the proton pairs H-4,H-7 and H-5,H-7 with those obtained from molecular models, it was possible to confirm earlier evidence that the oxirane ring is trans, and also to show that, of the two possible diastereoisomeric forms (49a and 49b), the data are more fully compatible with structure 49a, the... 

However, the relaxation contributions obtained from Eq. 22 were not satisfactorily compared with those obtained from specific, deuterium-substitution experiments and single- and double-selective relaxation-rates. Moreover, the errors estimated for the triple-pulse experiments were very much larger than those observed for the other techniques. This point will be discussed next. 

Selective, spin-lattice relaxation-rates are measured by the inversion-recovery technique. A rather weak, 180° pulse of very long duration (10-50 ms) inverts a multiplet (single-selective) or two multiplets (double-selective) in the spectrum of asperlin (1 see Fig. 2 ) and the recovery of the... 

The effects of Nj and He on the rotational relaxation rate in the CO2 00° 1 upper laser level have been measured by Abrams and Cheo 379), who used the output of a g-switched CO2 laser (1 Kw peak power, 20 nsec) to selectively deplete the population of one rotational level in a flowing CO2, CO2 + He and CO2 + N2 laser amplifier. The relaxation from neighbouring rotational levels into this depleted state showed up as a recovery of the amplifier gain at the corresponding line which was detected as a function of time with a third cw single-frequency CO2 laser. 

Figure 3. Partial 100-MHz 2H NMR spectra of methyl p-D-glucopyranoside, showing the single selective determination of the spin-lattice relaxation rate of H-l using a two-pulse inversion recovery sequence. All spectra were monitored as for those in Figure 2 except for pulse delay — 25 sec. The duration of the selective 180°-pulse was 38 msec (13 Hz bandwidth). The time interval (sec) between the selective 180°- and 90° -pulses are indicated to the right of each...

Single-ion nonradiative decay for Ln3+ diluted into transparent host elpa-solite crystals, where the energy gap is greater than the Debye cutoff, is primarily due to multiphonon relaxation (with rate kmp). In some cases, first order selection rules restrict phonon relaxation between states, such as between Tig and T4g, or between T2g and T5g, CF states for MX63- systems. The dependence of the multiphonon relaxation rate, kmp, upon the energy gap to the next-lowest state (AE) has been investigated for other systems and is given by a relation such as [353, 354]... 

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