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It is a great challenge in chemistry to clarify every detail of reaction processes. In older days chemists mixed starting materials in a flask and took the resul­ tants out of it after a while, leaving all the intermediate steps uncleared as a sort of black box. One had to be content with only changing temperature and pressure to accelerate or decelerate chemical reactions, and there was almost no hope of initiating new reactions. However, a number of new techniques and new methods have been introduced and have provided us with a clue to the examination of the black box of chemical reaction. Flash photolysis, which was invented in the 1950s, is such an example; this method has been combined with high-resolution electronic spectroscopy with photographic recording of the spectra to provide a large amount of precise and detailed data on transient molecules which occur as intermediates during the course of chemical reac­ tions. In 1960 a fundamentally new light source was devised, i. e. , the laser. When the present author and coworkers started high-resolution spectroscopic stud­ ies of transient molecules at a new research institute, the Institute for Molecu­ lar Science in Okazaki in 1975, the time was right to exploit this new light source and its microwave precursor in order to shed light on the black box.

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
The concept of free radicals has long roots in chemistry; according to Herzberg [1.1] it already existed in the 19th century. An identification of a chemically stable free radical triphenylmethyl in the early 20th century gave impetus to the study of free radicals. However, it remained a quite difficult task for chemists to characterize free radicals, because most of them appeared only for a short period of time. A number of simple free radicals, mostly diatomic, were later identified in flames and electrical discharges, by recording and analyzing their emission spectra, e.g., CH, OH, CN, and C2. The advent of new quantum mechanics provided spectroscopy with a sound basis, and brought about remarkable progress in spectroscopic studies of diatomic free radicals in the late 1920s and early 1930s, as compiled by Herzberg in [1.2]. After World War II, Herzberg and his students started high-resolution studies of transient molecules; a new technique of flash photolysis was introduced to generate such species, and emission and absorption spectra were recorded by large grating spectrometers [1.3].
Eizi Hirota

2. Theoretical Aspects of High-Resolution Molecular Spectra

Abstract
A high-resolution molecular spectrum denotes here a spectrum which exhibits well-resolved rotational structures or it corresponds to the rotational transition of a molecule (i.e., the pure rotational spectrum). Although direct transitions among fine and/or hyperfme structure component levels are rarely discussed, they are certainly classified as high-resolution spectra.
Eizi Hirota

3. Experimental Details

Abstract
This chapter describes spectroscopic systems for observing the spectra of transient molecules, by placing main emphasis on the spectrometers which my group has set up and employed to observe high-resolution spectra of transient species. The high reactivity and thus the short lifetime of such molecules require that the spectroscopic methods are highly sensitive. Introduction of lasers as sources has increased the sensitivity, in particular in the IR region, where the light sources hitherto available were of low output. Improvement of detectors has also contributed much to the increase of sensitivity. In conventional optical and IR spectroscopy the resolution has been incompatible with the sensitivity, because to attain high resolution one had to reduce the slit width and thus lose the power entering the detector. This difficulty has been eliminated to a great extent also by using lasers as sources. One might think that high resolution may not be indispensable for the study of transient molecules, rather it may make the spectroscopic method cumbersome to use and may limit its applicability range. That this is not the case is indicated by noting that transient molecules exist only with many other molecules that are either reactants or products of the reaction generating them. High resolution enables us to observe spectra of transient species nearly or completely isolated from much stronger spectra of much more abundant, chemically stable molecules. Obviously, high resolution brings about very precise data on molecular constants including the fine and hyperfine coupling constants characteristic of free radicals. These data are central in understanding the molecular structure of free radicals.
Eizi Hirota

4. Individual Molecules

Abstract
A number of free radicals and transient molecules have been investigated at the Institute for Molecular Science and a few other places using such high-resolution spectroscopic methods as described in Chap. 3, and their spectra thus observed have been analyzed in terms of theoretical expressions for energy levels, as expounded in Chap. 2, to derive molecular parameters which characterize these short-lived molecules well. This chapter is devoted to the results for individual molecules, most of which have been studied at the Institute for Molecular Science.
Eizi Hirota

5. Applications and Future Prospects

Abstract
High-resolution spectroscopy is an ideal tool for identifying chemical species and monitoring their abundance not only in total but also in individual quantum states, and can thus provide very detailed information on various chemical systems. Three representative fields are considered in this chapter, and some speculative discussion on future prospects of high-resolution spectroscopic studies of unstable molecules follows.
Eizi Hirota

Backmatter

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