The Carr—Purcell Pulse Sequence

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 use of the Carr-Purcell pulse sequence for T.2 has only recently been studied but it does have the advantage of the new variable, pulse separation. 

The technique for obtaining reliable values of Tj involves the application of fmlher short rf-pulses to the nuclear spin system such that spin echoes are obtained. In this review, we shall discuss briefly the Carr-Purcell pulse sequence for producing spin echoes with the modification introduced by Maiboom and Gill . After the initial 90° pulse along the x-direction in the rotating frame (cf. first section of this chapter), pulses with cc = = 180° are applied along the... 

Fig. 14. Dependence of the relaxation times T2. and the fractions of protons with different mobility (f.) for unsaturated polyester on the curing time, as measured from broad line NMR ( ), Hahn spin-echo ( ) and Carr-Purcell pulse sequence (O)- Symbol x indicates the initial distribution of styrene and unsaturated polyester protons (adapted from Ref. S5))...

NMR measurements were performed using a Bruker pulsed spectron ter operating at 60 MHz. The protons transverse magnetic relaxation function was analysed from spin echoes which were formed by applying Carr-Purcell pulse sequences to the spin system. 

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. ... 

Measurement of a true T2 can be obtained using a spin-echo pulse sequence, such as the Carr-Purcell-Meiboom-Gill (CPMG) sequence, which minimizes the loss of phase coherence caused by inhomogeneities (Kemp, 1986). 

Spin-spin relaxation times T2 are determined by the Carr-Purcell-Meiboom-Gill spin echo pulse sequence and provide information about slower molecular motions (66,67). 

Phase considerations intrude even in the simplest experiments of observing an FID or an echo. Accurately adjusting the phases of rf pulses can be very important, particularly in experiments involving trains of pulses such as the Carr-Purcell Meiboom-Gill train or the multiple pulse line narrowing sequences. In other sections we have considered how phase shifts originate and how to cope with them. 

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