Diffusion ordered spectroscopy examples

All the spectroscopic approaches applied for structural characterization of mixtures derive from methods originally developed for screening libraries for their biological activities. They include diffusion-ordered spectroscopy [15-18], relaxation-edited spectroscopy [19], isotope-filtered affinity NMR [20] and SAR-by-NMR [21]. These applications will be discussed in the last part of this chapter. As usually most of the components show very similar molecular weight, their spectroscopic parameters, such as relaxation rates or selfdiffusion coefficients, are not very different and application of these methodologies for chemical characterization is not straightforward. An exception is diffusion-edited spectroscopy, which can be a feasible way to analyze the structure of compounds within a mixture without the need of prior separation. This was the case for the analysis of a mixture of five esters (propyl acetate, butyl acetate, ethyl butyrate, isopropyl butyrate and butyl levulinate) [18]. By the combined use of diffusion-edited NMR and 2-D NMR methods such as Total Correlation Spectroscopy (TOCSY), it was possible to elucidate the structure of the components of this mixture. This strategy was called diffusion encoded spectroscopy DECODES. Another example of combination between diffusion-edited spectroscopy and traditional 2-D NMR experiment is the DOSY-NOESY experiment [22]. The use of these experiments have proven to be useful in the identification of compounds from small split and mix synthetic pools. 

Clearly, a choice had to be made as to which of the many methods available today should be included. This choice is likely biased to some extent by the editor s own preferences and a reader might arrive at the conclusion that another choice would have been better. Some chapters deal with methods of fundamental importance. For example, Chapter 2 provides a practical guide to the determination of binding constants by NMR and UV methods and thus covers an aspect imminently important to the field, which deals with noncovalent binding and weak interactions. Similar arguments hold for the next two chapters on isothermal titration calorimetry and extraction methods. The following chapters on mass spectrometry, diffusion-ordered NMR spectroscopy, photochemistry, and circular dichroism do... 

The second-order calibration example shown next is from the field of environmental analytical chemistry. A sensor was constructed to measure heavy metal ions in tap and lake water [Lin et al. 1994], The two heavy metal ions Pb2+ and Cd2+ are of special interest (the analytes) and there may be interferents from other metals, such as Co2+, Mn2+, Ni2+ and Zn2+. The principle of the sensor is described in detail in the original publication but repeated here briefly for illustration. The metal ions diffuse through a membrane and enter the sensor chamber upon which they form a colored complex with the metal indicator (4-(2-pyridylazo) resorcinol PAR) present in that chamber. Hence, the two modes (instrumental directions) of the sensor are the temporal mode related to the diffusion through the membrane, and the spectroscopic mode (visible spectroscopy from 380 to 700 nm). Selectivity in the temporal mode is obtained by differences in diffusion behavior of the metal ions (see Figure 10.22) and in the spectroscopic mode by spectral differences of the complexes formed. In the spectroscopic mode, second-derivative spectra are taken to enhance the selectivity (see Figure 10.23). The spectra were measured every 30 s with a resolution of 1 nm from 420 to 630 nm for a period of 37 min. This results in a data matrix of size 74 (times) x 210 (wavelengths) for each sample. 

There are further classes of experiment that result in pseudo-2D NMR spectra. These do not have a second frequency axis resulting from Fourier transformation of a variable time, but the second axis is some other parameter. One example is provided by continuous-flow directly coupled HPLC-NMR spectra where the second axis in the pseudo-2D plot is the chromatographic retention time. Another example is diffusion-ordered NMR spectroscopy where the second axis plots the molecular diffusion coefficient associated with each NMR peak, this parameter being derived from the dependence of peak intensities on the square of an applied magnetic field gradient. 

Almost all of these examples involve diffusion of a chemical species measuring diffusion rates has long been a specialty of NMR spectroscopy. The studies of KBr and drawn polyethylene produced unique information in the latter case, the known orientation of the deuterium electric field gradient in C-D bonds is used to determine the orientation, with respect to the magnetic field, of a polymer chain of a uniaxially ordered polyethylene fiber. The real time imaging of the polymerization of methyl methacrylate is very interesting and may represent a major direction for NMR imaging applications to polymer science. 

The various examples of photoresponsive supramolecular systems that have been described in this chapter illustrate how these systems can be characterized by steady-state and time-resolved spectroscopic techniques based on either absorption or emission of light. Pertinent use of steady-state methods can provide important information in a simple vay stoichiometry and stability constant(s) of host-guest complexes, evidence for the existence of photoinduced processes such as electron transfer, energy transfer, excimer formation, etc. Investigation of the dynamics of these processes and characterization of reaction intermediates requires in most cases time-resolved techniques. Time-resolved fluorometry and transient absorption spectroscopy are frequently complementary, as illustrated by the study of photoinduced electron transfer processes. Time-resolved fluorometry is restricted to phenomena whose duration is of the same order of magnitude as the lifetime of the excited state of the fluorophores, whereas transient absorption spectroscopy allows one to monitor longer processes such as diffusion-controlled binding. 

We will analyze the SM spectra and their fluctuations semiclassically using the stochastic Bloch equation in the limit of a weak laser field. The Kubo-Anderson sudden jump approach [58-61] is used to describe the spectral diffusion process. For several decades, this model has been a useful tool for understanding line shape phenomena, namely, of the average number of counts < > per measurement time T, and has found many applications mostly in ensemble measurements, for example, NMR [60], and nonlinear spectroscopy [62]. More recently, it was applied to model SMS in low-temperature glass systems in order to describe the static properties of line shapes [14-16, 63] and also to model the time-dependent fluctuations of SMS [64-66]. 

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