Multipulse methods

Time-resolved fluorescence is perhaps the most direct experunent in the ultrafast spectroscopist s palette. Because only one laser pulse interacts with the sample, the mediod is essentially free of the problems with field-matter time orderings that arise in all of the subsequently discussed multipulse methods. The signal... 

Solid-state NMR has been applied successfully in a few cases for the study of poly(imide) blends. It can be said, however, that the techniques arising from recent advances in the analysis of polymer blends, such as selection techniques based on multipulse methods, and improvements in the modelling methods of spin diffusion, have yet to be applied to the study of blends containing poly(imide)s. It is suggested that these techniques will have an important role to play in future studies of poly(imide) blends, particularly for blends such as impact-modified BMI resins. 

The approach to any structural or mechanistic problem will invariably start with the acquisition of one-dimensional spectra. Since these provide the foundations for further work, it is important that these are executed correctly and full use is made of the data they provide before more extensive and potentially time-consuming experiments are undertaken. This chapter describes the most widely used one-dimensional techniques in the chemistry laboratory, beginning with the simple single-pulse experiment and progressing to consider the various multipulse methods that enhance the information content of our spectra. The key characteristics of these are summarised briefly in Table 4.1. This chapter does not cover the wide selection of techniques that are strictly one-dimensional analogues of two-dimensional experiments, as these are more appropriately described in association with the parent experiment and are found throughout the following chapters. 

First, various advanced multipulse techniques have been developed since the mid-1970s, and nowadays are routinely applicable on spectrometers of the latest generation. Particularly innovative and ingenious among these methods are two-dimensional NMR techniques (506-508) and double quantum transition measurements (INADEQUATE) (507-509), which allow one to determine connectivities between carbon atoms within a molecule. 

For electron transfer processes with finite kinetics, the time dependence of the surface concentrations does not allow the application of the superposition principle, so it has not been possible to deduce explicit analytical solutions for multipulse techniques. In this case, numerical methods for the simulation of the response need to be used. In the case of SWV, a semi-analytical method based on the use of recursive formulae derived with the aid of the step-function method [26] for solving integral equations has been extensively used [6, 17, 27]. 

The oxidation of carbon monoxide has been studied by both the usual step-response and isotopic experiments and by the TAP system (2/7). The general conclusion is that the fast response of the TAP system did not produce any additional mechanistic information to that obtained from step-response experiments. A number of the points discussed in previous paragraphs are mentioned, and it is suggested that the final pattern of multipulse response experiments be termed a pseudo-steady state. A factor not mentioned is that transient IR experiments are valuable with the step-response method but not compatible with the TAP system. 

The development of new ID and 2D pulse sequences enables the spectroscopist to obtain structure and dynamic information about systems that were previously very hard to study. As an example is reported in Fig. 3.2.13 the 2D spectrum of erythromycin A measured with the FIREMAT (Five p Replicated Magic Angle Turning ) technique [30]. The slow spinning speed of 390 Hz produces a spinning sideband pattern for each peak in one dimension, whereas a multipulse sequence in combination with a special processing method produces isotropic lines in the second dimension. 

The chapfer consists of four sections, which are devoted to theoretical and practical issues of the detection of nitrogen-containing substances. Section 2 deals with theoretical aspects of the two most popular multi-pulse sequences multipulse spin-locking—MW-4 and "strong off-resonant comb"—SORC. In spite of the fact that the development of these sequences has enabled a dramatic increase in the sensitivity of NQR methods, until recently a number of theoretical and experimental peculiarities of these sequences have not been studied adequately. The primary issue concerns the sequence SORC, and also behaviour of both sequences in case of close times of spin-lattice and dipolar relaxations, which is especially important for the detection of such a popular explosive as hexogen (RDX). There are also demonstrated some experimental techniques for the detection of some other explosives (PETN and trinitrotoluene (TNT)). 

A number of investigators are now developing pulse-shaping and modulation techniques that are useful with ultrashort laser pulses. These methods will permit preparation of precisely timed and phased multipulse sequences of arbitrary complexity for use in nonlinear spectroscopy. In addition, rather than just exploiting pulse sequences to project coherences into echo intensities and time shifts for spectroscopic purposes, as in the methods discussed above, several investigators are devising pulse sequences to focus wavefimctions onto... 

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