Composite pulse principle

The use of composite pulses is subject to several counter-indications, the principle of which is the fact that they last much longer than their simple prototypes. This makes their employment problematic in the case of rigid solids, as well as in the detection of sub-sequences which rely on extremely closely-spaced echoes. 

Although in principle the simple scheme presented in Fig. 5.59 should provide TOCSY spectra, its suitability for practical use is limited by the effective bandwidth of the continuous-wave spin-lock. Spins which are off-resonance from the applied low-power pulse experience a reduced rf field causing the Hartmann-Hahn match to breakdown and transfer to cease. This is analogous to the poor performance of an off-resonance 180° pulse (Section 3.2.1). The solution to these problems is to replace the continuous-wave spin-lock with an extended sequence of composite 180 pulses which extend the effective bandwidth without excessive power requirements. Composite pulses themselves are described in Chapter 9 alongside the common mixing schemes employed in TOCSY, so shall not be discussed here. Suffice it to say at this point that these composite pulses act as more efficient broadband 180 pulses within the general scheme of Fig. 5.60. 

Piezoelectricity is the operating principle of quartz watches. In these devices, a tiny crystal of quartz oscillates at a frequency of 32 kHz in response to an electrical charge generated from the battery. In general, the overall size and composition of a piezoelectric crystal will affect its oscillation frequency. Since quartz loses very little energy upon vibration, the integrated circuit (IC, see Chapter 4) within a watch is used to reduce the repeatable oscillations into electric pulses, which are displayed as hours, minutes, and seconds on the watch face. 

The principle of this pulse method and its general equations are easily extended to the case of several components in a mixture. The method was used by Lindholm et al. [24] to determine the quaternary isotherms of the enantiomers of methyl- and ethyl-mandelate on the chiral phase Chiral AGP. One of the serious roadblocks encountered in the use of the pulse tracer method is that the amplitudes of most of the system peaks decrease rapidly when the plateau concentration increases. Since the signal noise increases in the same time, it becomes rapidly impossible to make any accurate measurements of the retention time of these peaks. On the basis of fundamental work by Tondeur et al. [114], the origin of this variation of the relative intensity of the system peaks was explained by Forss n et al. [47], who then derived an effective rule to determine the composition of a perturbation pulse that generates system peaks that are detected easily. The concentrations of the components in the injected perturbation pulse should... 

NMR observations basically contain spin relaxation processes which are associated with molecular motions with different specific frequencies in a given system. For quantitative measurements to determine the compositions of the system or selective measurements of particular components with different relaxation parameters, it is essential, therefore, to understand the principle of the relaxation mechanism. When our interest is focused on molecular motions, spin relaxation parameters such as spin-lattice relaxation time T, spin-spin relaxation time T2, and the nuclear Overhauser enhancement (NOE), are directly measured as a function of temperature or field frequency by using appropriate pulse sequences. Such temperature or frequency dependencies of the spin relaxation parameters are analyzed in terms of appropriate models to obtain detailed information of molecular motions with frequencies of Hz in the system. In this chapter, the basic theories and analyses... 

Cheng and English edited ACS symposium series which covers the solution and solid state NMR investigations for dendrimers, cellulose, polyurethane, polyolefins biopolymers, copolymers and so on. Spiess described a historical overview of role of NMR spectroscopy in polymer science. Newmark summarizes the two dimensional and pulsed gradient diffusion NMR experiments and their applications to polymers Shit et al. reviewed the analysis of polymer molecular weight and copolymer composition by NMR. Sasanuma summarized the the analysis of polyethers and polysulfides by NMR and theoretical calcula-tions Ardelean et al described the principle and its applications of diffusion studies by NMR. Roy et al summarized the structural analysis of Novolak resins by multidimensional NMR. Reviews about NMR study of surfactant polymer blends and the structural elucidation of supramocules are published. 

---