Cryogenic Matrix

Authored by: GÖtz Bucher

CRC Handbook of Organic Photochemistry and Photobiology

Print publication date:  March  2012
Online publication date:  March  2012

Print ISBN: 9781439899335
eBook ISBN: 9781466561250
Adobe ISBN:

10.1201/b12252-13

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Abstract

During the last 50 years, matrix isolation spectroscopy (MI) has contributed much to the development of modern physical organic chemistry. It allows the study of a range of interesting molecules, including a variety of reactive intermediates, some excited states, even ions, in an inert and almost noninteracting environment. In a typical MI experiment, a stable photochemical precursor to the reactive molecule of interest is deposited onto a cold spectroscopic window with a large excess of an inert gas, typically argon or other noble gases, but also nitrogen, methane, or carbon dioxide. The precursor is photolyzed, and the reactive intermediate can be investigated by spectroscopy—typically infrared, UV/Vis, and electron paramagnetic resonance (EPR). The advantages of the technique are obvious: the highly inert, rigid, and cold environment not only prevents intermolecular quenching of the reactive molecule to be studied, it also offers very narrow IR band widths, resulting in very high sensitivity and facile interpretation of spectra. Secondary reactions with other molecules can be investigated by adding these to the inert gas used. Annealing of the matrix after photolysis allows for diffusion and thermal reactions to take place. During the last two decades, advances in computer technology and in the design of quantum chemical software have made it possible to reliably calculate IR, UV/Vis, and EPR spectra of small to mid-sized molecules. Comparing spectroscopic information obtained by experiment with calculated spectra meanwhile is standard practice in MI experiments, and nowadays no matrix isolation study is published without extensive supporting calculations. The technique of MI has been reviewed and described authoritatively. 1,2 This chapter will therefore not deal with technical details of the method, which are summarized elsewhere, 2 nor will it provide a comprehensive review of all literature available. It will leave out studies that mainly focus on thermal generation of reactive species (by flash vacuum pyrolysis [FVP]), and it will not cover photochemical reactions of transition metal complexes. It will instead concentrate on recent studies on the photochemical generation of reactive intermediates, and on the photochemistry of reactive species, from an organic chemist’s perspective.

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