Spin-lattice relaxation pulse delay determination

Fig. 1. Pulse sequences for determining spin-lattice relaxation time constants. Thin bars represent tt/2 pulses and thick bars represent tt pulses, (a) The inversion-recovery sequence, (b) the INEPT-enhanced inversion recovery, (c) a two-dimensional proton-detected INEPT-enhanced sequence and (d) the CREPE sequence. T is the waiting period between individual scans. In (b) and (c), A is set to (1 /4) Jm and A is set to (1 /4) Jm to maximize the intensity of IH heteronuclei and to (1/8) Jm to maximize the intensity of IH2 spins. The phase cycling in (c) is as follows 4>i = 8(j/),8(-j/) 3 = -y,y A = 2(x),2(-x) Acq = X, 2 —x), X, —X, 2(x), —x, —x, 2(x), —x, x, 2 —x),x. The one-dimensional version of the proton-detected experiment can be obtained by omitting the f delay. In sequence (d), the phase 4> is chosen as increments of 27r/16 in a series of 16 experiments.

Relaxation parameters provide valuable information about molecular motions. The spin-lattice relaxation time T is usually determined by the so-called inversion recovery pulse sequence (65). The experiment comprises a set of spectra with different interpulse delays, and Tx is determined by fitting the signal intensities for a given nucleus to Eq. 2, where A and B are constants, x is the respective interpulse delay, and /,is the intensity measured at that delay ... 

Figure 2, Partial 100-MHz 1H NMR spectra of methyl fi-D-glucopyranoside, showing a two-pulse nonselective inversion-recovery determination of the spin-lattice relaxation rates. AU spectra were monitored as follows pulse width (90°) — 67 ftsec number of transients — 8, pulse delay = JO sec, acquisition time = 4 sec, data points = 4096, spectral width — 500 Hz, and sensitivity enhancement — 1.5 sec. The time interval (sec) between the 180°- and 190°-pulses are indicated to the right of the respective spectra.

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...

CpD and CpH were determined hy I l-T Ml NMR after gas-phase transfer and napping. Such procedures arc notorious for losses. Further. 5-s delays between pulses, the method used, ate probably inadequate — small, rigid liydtocarhon molecules lend to have unusually long spin-lattice relaxation times. No experiments validating the method were reported [ I We lind increases of a faclor of two in the aicas ol cyclopropane peaks, relative to other components ol reaction mixtures, when the pulse delay is increased Irom 5 to 60 s X8. ... 

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. 

The SPEMAS technique has the advantage to be quantitative if the recycle delay is well calibrated as a function of the spin-lattice relaxation times (TO previously measured on each signal. However, CP experiment is not much applied for quantitative aspects because the signal intensities are not only dependant of the total nuclei concentrations but also of their dynamics. In order to get quantitative reliable data fi om CPMAS data, a series of spectra have been recorded in order to exactly determine the match of optimum cross polarization, the proton decoupling power, the pulse width and the delay times. These parameter calibrations are essential before further analyses even if they take a long time. Concerning the spin-echo experiment, the low intensity of signals for few transients excluded precise parameter calibration. Anjnvay, measurements of spin-spin relaxations times (T2) is necessary for reliable quantification of spin-echo spectra. 

The stimulated ESEEM experiment is performed at X-band ( 9.5 GHz), which is optimal for echo modulation induced by deuterium nuclei. The stimulated echo is observed after application of three microwave pulses, with the sequence nl2-r-nl2-T-nl2-r-echo. Pulse durations typically are 8 or 16 ns. To maximize the deuterium modulation, the interval r between the first and second pulses is set to t = 1/2vd, where is the deuterium nuclear Larmor frequency. Because is close to 2.2 MHz at X-band, r is close to 220 ns. ESEEM is recorded by scanning the second time delay T. The upper limit for this decay time T ax is determined by echo decay from spin-lattice relaxation typically it is around 10 ps in... 

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