Purcells Effect

H. Y. Carr and E. M. Purcell, Effects of diffusion on free precession in nuclear magnetic resonance experiments. Phys. Rev., 1954, 94, 630-638. 

Bloembergen N, Purcell E M and Pound R V 1948 Relaxation effects In nuclear magnetic resonance absorption Phys. Rev. 73 679-712... 

In electron spin echo relaxation studies, the two-pulse echo amplitude, as a fiinction of tire pulse separation time T, gives a measure of the phase memory relaxation time from which can be extracted if Jj-effects are taken into consideration. Problems may arise from spectral diflfrision due to incomplete excitation of the EPR spectrum. In this case some of the transverse magnetization may leak into adjacent parts of the spectrum that have not been excited by the MW pulses. Spectral diflfrision effects can be suppressed by using the Carr-Purcell-Meiboom-Gill pulse sequence, which is also well known in NMR. The experiment involves using a sequence of n-pulses separated by 2r and can be denoted as [7i/2-(x-7i-T-echo) J. A series of echoes separated by lx is generated and the decay in their amplitudes is characterized by Ty. ... 

N. Bloembergen, E. M. Purcell, R. V. Pound 1948, (Relaxation effects in nuclear magnetic resonance absorption), Phys. Rev. 73, 679-718. 

Purcell, E. M. (1978). The effect of fluid motions on the absorption of molecules by suspended particles, J. Fluid Mech., 84, 551-559. 

T2 measurements usually employ either Carr-Purcell-Meiboom-Gill (CPMG) [7, 8] spin-echo pulse sequences or experiments that measure spin relaxation (Tlp) in the rotating frame. The time delay between successive 180° pulses in the CPMG pulse sequence is typically set to 1 ms or shorter to minimize the effects of evolution under the heteronuc-lear scalar coupling between 1H and 15N spins [3]. 

Methods for measuring Ti and T2 are discussed in Chapter 5 of reference 21. Suffice it to say here that understanding the method for measuring T2 (the Carr-Purcell pulse sequence or spin-echo method) becomes important for discussing two-dimensional NMR spectra. When spin-spin coupling is present, a modulation of spin echoes is produced, and it is this fact that is important in 2-D NMR spectroscopy. Nuclear relaxation rates and mechanisms become important when discussing the effect of paramagnetic metal centers on NMR spectroscopy. 

The interaction of the herbicides atrazine and 2,4-D with HSA was investigated by Purcell et al. (45) using the equilibrium-mixture mode. The study was based on the suggestion that HSA serves as carrier protein to transport the herbicides to target organs to exhibit toxicological effects in vivo. The results were combined with data gained from spectroscopic methods. 

This section reviews the molecular shape descriptors developed by Amoore, Allinger, Simon et al. and Testa and Purcell. The illustrative examples discussed refer to the odour similarity and cardiotoxic aglycones. One has stressed the methods based on the reference structure because, correctly formulated, these methods seem to offer promising perspectives to model the steric effects in biological systems. Finally, a short discussion of possible connections between steric and other substituent constants (relevant in the context of multicollinearity in QSAR) is included. 

Tardif et al. (1992, 1993 a, 1997) have developed a physiologically based toxicokinetic model for toluene in rats (and humans—see Section 4.1.1). They determined the conditions under which interaction between toluene and xylene(s) occurred during inhalation exposure, leading to increased blood concentrations of these solvents, and decreased levels of the hippurates in urine. Similar metabolic interactions have been observed for toluene and benzene in rats (Purcell et al., 1990) toluene inhibited benzene metabolism more effectively than the reverse. Tardif et al. (1997) also studied the exposure of rats (and humans) to mixtures of toluene, we/a-xylene and ethylbenzene, using their physiologically based pharmacokinetic model the mutual inhibition constants for their metabolism were used for simulation of the human situation. 

Noting the similarity between NMR relaxation and dielectric relaxation effects, Bloembergen, Purcell, and Pound (35) adapted the Debye theory for the latter to the problems of the former to obtain equation (2). 

First publications on nuclear magnetic resonance and isotope effects appeared almost at the same time. The first NMR spectra, independently recorded in two laboratories by Bloch and Purcell groups, were published in the same issue of Physical Review in 1946.1/2 One year later the idea of isotope effects was introduced by Bigeleisen.3 Isotope shifts on shielding were first discussed by Gutovsky 12 years later.4... 

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