Relaxation time measurements experiments

Spin—lattice relaxation is the time constant for the recovery of magnetiTation along the z-axis in a NMR experiment. Various methods are available for the measurement of spin lattice relaxation times. The interested reader is referred to the series of monographs echted by Levy on Carbon-13 NMR spectroscopy [44, 45] for more details. The energy transfer between nuclear moments and the lattice , the three-dimensional system containing the nuclei, provides the mechanism to study molecular motion, e.g. rotations and translations, with correlation times of the order of the nuclear Larmour frequencies, tens to hundreds of MHz. We will limit our chscussion here to the simple inversion-recovery Tj relaxation time measurement experiment, which, in addition to providing a convenient means for the quick estimation of Tj to establish the necessary interpulse delay in two-dimensional NMR experiments, also provides a useful entry point into the discussion of multi-dimensional NMR experiments. 

Sohaublin S, FIdhener A and Ernst R R 1974 Fourier speotrosoopy of non-equilibrium states. Applioation to CIDNP, Overhauser experiments and relaxation time measurements J. Magn. Reson. 13 196-216... 

Any NMR field-cycling (FC) relaxometry experiment presumes that the sample is subject to a magnetic field of various intensities for time intervals of varying durations. More specifically, between the various intervals of a relaxation-time measurement, the external magnetic field induction... 

When the primary electron donation pathway in photosystem II is inhibited, chlorophyll and p-carotene are alternate electron donors and EPR signals for Chl+ and Car+ radicals are observed.102 At 130 GHz the signals from the two species are sufficiently resolved to permit relaxation time measurements to be performed individually. Samples were Mn-depleted to remove the relaxation effects of the Mn cluster. Echo-detected saturation-recovery experiments were performed with pump pulses up to 10 ms long to suppress contributions from cross relaxation and spin or spectral diffusion. The difference between relaxation curves in the absence of cyanide, where the Fe(II) is S = 0, and in the presence of cyanide, where the Fe(II) is S = 2, demonstrated that the relaxation enhancement is due to the Fe(II). The known distance of 37 A between Fe(ll) and Tyrz and the decrease of the relaxation enhancement in the order Tyrz > Car+ > Chl+ led to the proposal of 38 A and > 40A for the Fe(II)-Car+ and Fe(II)-Chl+ distances, respectively. Based on these distances, locations of the Car+ and Chl+ were proposed. 

The design of a pulsed experiment is shown in Figure A. 1.4. In a one-dimensional experiment, ti, the amount of time Bappid is left on may be constant (as in obtaining a simple 1H spectrum) or it may be varied (e.g., in an inversion recovery experiment that measures the T relaxation time). In experiments where t is varied, each FID acquired over time t2 is processed and treated as an individual data point in a manner dependent on the experiment. 

We have utilized in the last two sections the fact that the bulk diffusion coefficient D is related to NMR relaxation times through the magnetic field gradient. The precision in the determination of D is directly related to the precision of the relaxation time measurement and of the gradient G. Therefore, it is important to determine G as accurately as possible. Very often, this is the weak link in the experiment, a fact which is not always recognized. 

The block diagram in the last section shows the spectrometer with the CAMAC modules identified. Several different experiments requiring different pulse sequences can be performed easily with such a system. A moderately complicated example is a spin-lattice relaxation time measurement in the time domain on a poly crystalline intermetallic sample containing 1=3/2 nuclei. Since a non-cubic 1=3/2 system has unequally spaced levels, special techniques must be used for relaxation time measurements (see III.C.3.) and we adopt the procedure of Avogadro and Rigamonti (1973) to initialize the populations before the magnetization recovery. 

In the last three decades, nuclear magnetic resonance has become a powerful tool for investigating the structural and physical properties of matter. Today, nuclear magnetic resonance is the physical method most widely used in analytical chemistry. For special applications, e.g. relaxation time measurements, there is available a variety of modifications of the basic nuclear magnetic resonance experiments such as pulse and spin-echo methods. In the course of this development and when electronic computers were provided at a reasonable price, Fourier transform spectroscopy was applied to nuclear magnetic resonance in the middle of the sixties. At that time, Fourier methods were already used to a large extent in far infrared spectroscopy (see Refs. and references cited therein). 

We will discuss the SSNMR most common experiments used to investigate pharmaceutical compounds, including one- and two-dimensional experiments and relaxation time measurements. Additionally, we will mention some aspects of first principle calculation. Also we highlight the importance of NMR as a multinuclear technique. We will focus on SSNMR applied for charaaerization of new developed active pharmaceutical compounds and formulated drugs. We will pay attention to some particular topics as polymorphism, complexes with cyclodextrin and an emerging issue in pharmaceutical industry as it is the development of cocrystals. NMR crystallography is also discussed. 

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