Degree spin-lattice relaxation time

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

On-line SFE-NMR coupling was also reported [151,152], SFE provides some degree of separation by means of solubility and affinity to the matrix. This offers the possibility of transferring analytes directly from the extraction into the NMR probe. Drawbacks in the acquisition of SFE-NMR and SFC-NMR spectra are the elongated spin-lattice relaxation times 7) of protons and the pressure dependence of H NMR chemical shifts [153]. 

The spin-lattice relaxation times of essentially all the protons in vindoline, measured by a Fourier transform method, show values reflecting the degree of steric interaction with other protons. (276) A wealth of coupling constant data has been made available by first order analysis of the 300 MHz NMR spectrum of vindolinine [458]. (277) H NMR shifts for the metabolite of vindoline [459] (278) are shown on the structure. 

The pulse width is an important factor in the measurement of pulsed spectra. The optimal pulse-width may be estimated21 from the equation cos a = exp(— TJT), in which a is the pulse width (in degrees), Tt the spin-lattice relaxation-time (in s), and T the pulse-repetition time (in s). For monosaccharides in 20% aqueous solution, values of the protonated carbon atoms are22 1 s at 30°. Using 8 k of computer memory for the acquisition, and a sweep width of 5-6 kHz, T becomes 0.6-0.8 s, and the equation gives an optimum pulse-width of 60°. In Fig. 1 is shown a series of spectra measured at different pulse-widths, all other variables being kept constant. The best s/n is seen to correspond to a 63° pulse. If, 3C-n.m.r. spectra are recorded for very concentrated solutions, or impure samples, the Tj values may become small, and, in such cases, a 90° sample pulse will be optimal. 

As we pointed out in Section 4.11, lanthanide shift reagents owe their utility partly to the fact that the electron spin-lattice relaxation time for the lanthanides is very short, so that NMR lines are not exceptionally broad. On the other hand, there are shiftless paramagnetic reagents that shorten both Tx and T2 to a moderate degree without causing contact or pseudocontact shifts. 

Fig. 18. Temperature and hydration dependence of NMR relaxation. Variation with temperature of the proton spin-lattice relaxation time, T, at 60 MHz of polycrystalline lysozyme with various degrees of hydration. —, hydration with HjO 0—0, hydration with D2O. From Andrew (1985).

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. 

The pulsed NMR apparatus (12 MHz) and methods of procedure have been described previously (10, 13). The spin-spin relaxation time, T2, equivalent to the inverse absorption line width, is a time constant for the exponential decay toward internal equilibrium in the nuclear spin system. The spin-lattice relaxation time, Tu is the time constant for the exponential decay toward equilibrium between the nuclear spins and all other degrees of freedom of the system. The data are presented in Figure 1. Note that at higher temperatures T2 is much lower than Ti. 

The self-diffusion coefficients of toluene in polystyrene gels are approximately the same as in solutions of the same volume fraction lymer, according to pulsed field gradient NMR experiments (2fl). Toluene in a 10% cross-linked polystyrene swollen to 0.55 volume fraction polymer has a self-diffusion coefficient about 0.08 times that of bulk liquid toluene. Rates of rotational diffusion (molecular Brownian motion) determined from NMR spin-lattice relaxation times of toluene in 2% cross-linked ((polystytyl)methyl)tri-/t-butylphosphonium ion phase transfer catalysts arc reduced by factors of 3 to 20 compai with bulk liquid toluene (21). Rates of rotational diffusion of a soluble nitroxide in polystyrene gels, determined from ESR linewidths, decrease as the degree of swelling of the polymer decreases (321. 

All NMR experiments were performed on a Varian XL-200 spectrometer at 50.31 MHZ. Relevant instrument settings include 90 degree pulse angle, 1.0 second acquisition time, 0.5 second pulse delay, 238.5 ppm spectral width, and broad band proton decoupling. About 40,000 transients were collected for each spectrum. Temperature was maintained at 40 C. Spin-lattice relaxation time (Tl) and Nuclear Overhauser Enhancement (NOE) values for all C-13 NMR resonances were carefully measured to determine the optimum NMR experimental conditions. The spectral intensity data thus obtained were assured of having quantitative validity. 

Because of the fluorescence mechanism (lifetime Tp = 0.5 msec), particles which occupied the empty levels in the past have transferred to the remaining five (out of six) nuclear levels (Fig. 5b), all of which have spin-lattice relaxation times, T3, of at least one minute. The sudden onset of Pr F spin-spin coupling during field sweep, or the sudden application of NMR, allows the transient transfer of population back into the empty laser-bleached levels, thus momentarily restoring some degree of laser absorption. After these processes two out of the six levels are empty (Fig. 5c) and the steady-state transmission is the same as before. 

The mechanism for spin-lattice relaxation is as follows. All paramagnetic species in the sample have an associated magnetic field surrounding them with which each of the other paramagnetic species may interact. In liquids, the random molecular collisions that constitute Brownian motion permit these local magnetic fields to fluctuate a fluctuation that occurs at the resonant frequency will induce a radiationless transition. The spin-lattice relaxation is characterized by a spin-lattice relaxation time, T, which thus effectively controls the degree of saturation. 

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