Linear spin-lattice relaxation time

Spin-lattice relaxation times were measured by the fast inversion-recovery method (24) with subsequent data analysis by a non-linear three parameter least squares fitting routine. (25) Nuclear Overhauser enhancement factors were measured using a gated decoupling technique with the period between the end of the data acquisition and the next 90° pulse equal to eibout four times the value. Most of the data used a delay of eibout ten times the Ti value. (26)... 

Figure 8. Plot of spin-lattice relaxation time against temperature, at 67.9 MHz, for linear and branched PE. Linear PE from Ref. 17, (%) linear PE this work, ( O ) branched PE this work, ( j.

The precipitates of PVPh/PDMA from methanol and acetone solutions were examined by CPMAS NMR [51], and evidence for specific interaction was obtained with a 3 ppm shift in the phenolic carbon resonance peak. The proton spin-lattice relaxation times Tj were shorter than those predicted by a linear model, though the rotating frame spin-lattice relaxation times Tjp of the com-... 

The determiiiation of spin-lattice relaxation time (Tj) and correlation time (r) using NMR spectroscopy (66) provides direct information about the intramcdec-ular motion of cydic peptides. Recently, Fossel et al. (67) determined r values with Cyclo-(Gly2), Qydo-(Gly-Pro), Cyclo-(Pr02), and Cydo- Pro-D-Pro) in D2O solution, and the r values were compared with those for the correspmiding linear dipeptides. 

Redfield limit, and the values for the CH2 protons of his- N,N-diethyldithiocarbamato)iron(iii) iodide, Fe(dtc)2l, a compound for which Te r- When z, rotational reorientation dominates the nuclear relaxation and the Redfield theory can account for the experimental results. When Te Ti values do not increase with Bq as current theory predicts, and non-Redfield relaxation theory (33) has to be employed. By assuming that the spacings of the electron-nuclear spin energy levels are not dominated by Bq but depend on the value of the zero-field splitting parameter, the frequency dependence of the Tj values can be explained. Doddrell et al. (35) have examined the variable temperature and variable field nuclear spin-lattice relaxation times for the protons in Cu(acac)2 and Ru(acac)3. These complexes were chosen since, in the former complex, rotational reorientation appears to be the dominant time-dependent process (36) whereas in the latter complex other time-dependent effects, possibly dynamic Jahn-Teller effects, may be operative. Again current theory will account for the observed Ty values when rotational reorientation dominates the electron and nuclear spin relaxation processes but is inadequate in other situations. More recent studies (37) on the temperature dependence of Ty values of protons of metal acetylacetonate complexes have led to somewhat different conclusions. If rotational reorientation dominates the nuclear and/or electron spin relaxation processes, then a plot of ln( Ty ) against T should be linear with slope Er/R, where r is the activation energy for rotational reorientation. This was found to be the case for Cu, Cr, and Fe complexes with Er 9-2kJ mol" However, for V, Mn, and... 

Low-spin Fe(iii) porphyrins have been the subject of a number of studies. (638-650) The favourably short electronic spin-lattice relaxation time and appreciable anisotropic magnetic properties of low-spin Fe(iii) make it highly suited for NMR studies. Horrocks and Greenberg (638) have shown that both contact and dipolar shifts vary linearly with inverse temperature and have assessed the importance of second-order Zeeman (SOZ) effects and thermal population of excited states when evaluating the dipolar shifts in such systems. Estimation of dipolar shifts directly from g-tensor anisotropy without allowing for SOZ effects can lead to errors of up to 30% in either direction. Appreciable population of the excited orbital state(s) produces temperature dependent hyperfine splitting parameters. Such an explanation has been used to explain deviations between the measured and calculated shifts in bis-(l-methylimidazole) (641) and pyridine complexes (642) of ferriporphyrins. In the former complexes the contact shifts are considered to involve directly delocalized 7r-spin density... 

FIG. I. Spin-lattice relaxation time of Li in 3-9 M LiClin HiOasafunctionofinverse absolute temperature. Circles experimental relaxation time. 7 ," crosses dipolar contribution, obtained from T,"" and the nuclear Overhauser enhancement factor t). The linear least-squaresfit for the latter yields an activation energy of 3-6kcalmol (16)... 

With a chromatographic technique capable of routinely yielding preparative fractions, quantitative and C FT NMR was the major spectroscopic tool used for chemical characterization. The established utility of and C NMR for characterization of coal products is documented well. Unfortunately, high-resolution C FT NMR is not quantitative normally under operating conditions used typically. (It should be noted that quantitative FT NMR measurements also are not obtained routinely. The problem of variable spin lattice relaxation times (Ti s) is present also in FT NMR. In addition, the greater signal intensity of NMR in comparison with C FT NMR poses an additional potential problem of detector linearity in the FT NMR receiver.) For C FT NMR, variable spin lattice relaxation times (Ti s) and nuclear Over-hauser effects (a result of pseudo random noise decoupling) usually... 

Polyesters derived from maleic anhydride and 2,2-di(4-hydroxyphenyl)pro-pane were copolymerised with styrene and then studied by CP/MAS NMR [39] spectroscopy. The three dimensional-crosslinked network formed by the polymerisation was examined using spin-lattice relaxation times in the rotating frame. A correlation between reaction conditions and the structure of the resulting material was found. The degree of residual unsaturation was determined by subtraction of two relaxation times from a linear additivity model used for erosslinked polymer systems. 

Spin lattice relaxation times are calculated from the return of the magnetization to equilibrium using a linear and non-linear least squares analysis of the data. The two analyses yield Tj values within 10% of each other and average values are reported. 

Spin-lattice relaxation times for the protein protons were measured using the 90°-t-90° pulse sequence. Free induction decay amplitude was measured 15-20 microsec after the end of the second pulse by averaging 30 repetitions. To obtain the obvious double exponential relaxation behavior from the water protons, the first pulse of a 180°-t-90° sequence was attenuated so that the 180° pulse width was about 55 microsec while the second pulse remained near 4 microsec. Experimental considerations led us to believe the errors for the protein Ti values and the slow component of the water Ti curve are about 5% although linear least squares fits indicate better precision. 

A three-parameter non-linear procedure for fitting inversion recovery measurements for spin-lattice relaxation times," J. Magn. Resonance 26, 533-536 (1977). 

The conformational analysis of several linear proline-containing hormones has been considerably aided by 1 3C nmr studies. The ratio of cis- and trans-isomers and the spin lattice relaxation times of thyrotropin-releasing factor or hormone (TRF or TRH), L-pyro-glutamyl-L-histidyl-L-prolinamidc, in deuterium oxide, deuterated dimethyl sulfoxide and deuterated pyridine have been determined... 

As discussed in the Introduction, if equivalent information is obtained from both direct and indirect measurements of mechanical properties, then the fluctuation-dissipation theorem is applicable (1). The exact proof of the theorem has already been made. However, an experimental verification is still useful for experimental chemists in an understanding of this theorem. In order to carry out the verification, the spin-lattice relaxation times are con ared with corresponding mechanical properties for fresh silk san les. A linear relationship between the two measurements would then constitute an experimental verification, at least for these sanq>les. It may be noted that experimental verification does not imply a general proof because this is not a rigorous mathematical proof... 

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