Gated spin-echo

Attached Proton Test (APT), Gated Spin-Echo (GASPE), and... 

Figure 2.4 (A) Pulse sequence for the gated spin-echo (GASPE) or attached proton test (APT) experiment. (B) Effect of the pulse sequence on the C magnetization vectors of a CH group.

A number of papers have looked at the development of relationships between base stock composition as measured by NMR and either physi-cal/chemical properties or their performance.22 27 Most of this work has been focused on group II and III base stocks, with less or little attention paid to solvent extracted ones. These have all relied on various techniques to simplify the spectra and the assignments of peaks and make peak integration more reliable. These have many acronyms,23 for example, GASPE (gates spin echo), PCSE (proton coupled spin echo), INEPT (insensitive nuclei enhancement by polarization transfer), DEPT (distortionless enhancement by polarization), QUAT (quaternary-only carbon spectra), 2D COSY (two-dimensional homo-nuclear spectroscopy), and HETCOR (heteronuclear shift correlated spectroscopy)]. Table 4.10 provides an example of some of the chemical shift data generated26 and employed in this type of work, and Adhvaryu et al.25 were able to develop the correlations between base stock properties and carbon types in Table 4.11, whose main features correspond to intuition (e.g., the values of API and aniline points are both decreased by aromatic carbon and increased by the... 

F ure 2.4 (A) Pulse sequence for the gated spin-echo (GASPE) or attached proton... 

Figure 5.2 (a) Single slice from coronal, respiratory-gated spin-echo MR images of a... 

Figure 5.7. (A) The gated spin-echo (GASPE, or Attached Proton Test APT) pulse sequence. (B) Effects of a gated spin-echo sequence on a CH doublet, (a) At equilibrium (b) after the 90° pulse (c) after first period t, angle a = (Q — coo)t radians (d) after the 180° pulse and broad band decoupling (e) after final period t.

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

Fig. 2.42. I3C NMR spectra of D-camphor in tetradeuteriomelhanol at 15.08 MHz (a). /-modulation of aliphatic carbon signals depending on the decoupling delay z, a verification of Fig. 2.41 (b) proton broadband decoupled spectrum (c-e). /-modulated spin-echo experiments with z = 4, 6, and 8 ms for CH multiplicity analysis (f-g) spectra with off-resonance (0 and gated decoupling of protons (g) for comparison.

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

Since JCH coupling is only in operation during one t,/2 period in the J-modulated spin-echo sequence with gated proton decoupling, as drawn in Fig. 2.51, the second Fourier transformation provides only one half of the actual Jal magnitudes. Therefore, resolution of smaller couplings will be poor. 

Fig. 5.1. C NMR spectra of 5a-cholcstan-3-onc in dcutcriochloroform (50 mg/0.5 mL) (a) proton broadband-decoupled, 400 scans (b). /-modulated spin-echo experiment for quaternary carbon selection, 1000 scans (c-e) CH, Cl I2, and CH3 subspectra generated from linear combination of three DEPT experiments (see Section 2.9.3.2), 200 scans per experiment (f) gated proton-decoupled experiment for comparison.

Many workers have in fact used density matrix methods for the calculation of line shapes and intensities in multiple resonance experiments, and two excellent reviews of the background theory are available. (49, 50) In addition there is also a simple guide (51) to the actual use of the method which is capable of predicting the results of quite elaborate experiments. Major applications have included the calculation of the complete double resonance spectrum from an AX spin system which gives 12 transitions in all (52) an extremely detailed study of the relaxation behaviour of the AX2 systems provided by 1,1,2-trichloroethane and 2,2-dichloroethanol (53) the effects of gating and of selective and non-selective pulses on AB and AX spin systems and the importance of the time evolution of the off-diagonal elements of the density matrix in repetitively pulsed FT NMR and spin-echo work (54) the use of double resonance to sort out relaxation mechanisms and transient responses (55) the calculation of general multiple resonance spectra (56) and triple resonance studies of relaxation in AB and AX spin systems. (57)... 

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