Electronic structure, laboratory experiments

Since the science presented here would never materialize without productive interactions between theory and experiment, it is certainly appropriate to dedicate this book to the practitioners of experimental chemistry who do not hesitate to regard electronic structure calculations as an integral part of their investigations and to the vanguards of molecular quantum mechanics who do not shy away from visiting research laboratories where matter rather than its abstract representations is studied. 

In addition to the activity documented above there has been a tremendous amount of activity in the development of more traditional experiments for the physical chemistry laboratory. Some of these experiments are improvements on older methods, some involve new systems, and some involve new types of analysis. There are far too many of these experiments to discuss individually, but all of them will be found listed in tables below. They have been divided roughly into spectroscopy and the electronic structure of matter, thermodynamics, including thermochemistry and properties of liquids, solids and solutions, and kinetics, including photochemistry. 

This exercise affords students an opportunity to measure actual UV and FTIR absorption spectra of several organic solutes while comparing overall differences in UV and FTIR spectra for chemically similar and dissimilar organic solutes. The laboratory experiment focuses on the influence of delocalization of electron density and the nature of UV absorption spectra. The experiment also focuses on the relationship of organic functional group analysis and FTIR absorption spectra. A review of an appropriate text which reviews the theory of UV vis and FTIR absorption spectroscopy and molecular structure gives an appreciation of what will be observed in the laboratory (3). 

Electron nuclear double resonance is a powerful tool for the study of the electronic structure of triplet states because of its high precision. ENDOR linewidths can be as narrow as 10 kHz, which represents an increase in resolution of better than six orders of magnitude over that which can be obtained optically. The technique is particularly useful when combined with hf methods owing to the first-order nature of the hyperfine interaction in the presence of a field. Although such experiments are difficult, the information obtained is unique. Accordingly, the hf EPR (or ODMR) spectrometer has been modified for ENDOR operation in several laboratories. In order to illustrate the power of the method, we discuss here some recent optically detected hf ENDOR experiments on (njr ) benzophenone and its iso-topically labeled derivatives (Brode and Pratt, 1977, 1978a,b). The results, although incomplete, show considerable promise for the ultimate determination of the complete spin distribution in this prototype triplet state. 

EPR encompasses a wide range of powerful methods to probe the geometry and electronic structure of paramagnetic metal complexes. Techniques that were until recently available only in a few specialized laboratories are becoming more widely available in commercial spectrometers. The challenge to the experimentalist is to select the set of experiments that will most directly address the particular question of interest. 

Chapter 16, "Variation of the Fundamental Constants as Revealed by Molecules Astrophysical Observations and Laboratory Experiments, by Victor Flambaum and Mikhail Kozlov, describes the application of precision molecular spectroscopy to the study of a possible variation of the fundamental constants. The authors show that molecular spectra are mostly sensitive to two dimensionless constants, namely the fine-structure constant and the electron-to-proton mass ratio. The chapter discusses the results which follow from the astrophysical observations of the optical and microwave spectra of molecules, as well as possible laboratory experiments with molecules. Although the accuracy of the laboratory results cannot yet compete with that of the astrophysical observations, there are significantly improved experiments in progress that are likely to reverse this situation soon. The ideas behind these experiments and the preliminary experimental results so far obtained are discussed in detail. 

The advantages of SEXAFS/NEXAFS can be negated by the inconvenience of having to travel to synchrotron radiation centers to perform the experiments. This has led to attempts to exploit EXAFS-Iike phenomena in laboratory-based techniques, especially using electron beams. Despite doubts over the theory there appears to be good experimental evidence that electron energy loss fine structure (EELFS) yields structural information in an identical manner to EXAFS. However, few EELFS experiments have been performed, and the technique appears to be more raxing than SEXAFS. 

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