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PART 2

METHODS AND TEMPORAL REGIMES

1. WHAT IS MATRIX ISOLATION?

If one peruses the literature on reactive intermediates, or discusses the subject with colleagues interested in that field, one soon finds that the term ‘‘matrix isolation’’ means different things to different people, so some semantic clarification appears to be in order at the outset.

The term ‘‘matrix isolation’’ was coined by George Pimentel who pioneered this field1,2 together with George Porter.3 Pimentel intended this term to refer to a method whereby a substrate is mixed with a large exess of an (unsually unreactive) host gas and is condensed on a surface that is sufficiently cold to assure rapid solidification of the material. In this way, one ends up with a sample where (ideally) each substrate molecule is immobilized in a cavity surrounded by one or more layers of inert material and is thus ‘‘isolated’’ from the other substrate molecules in a ‘‘matrix’’ of the host gas.

In the course of time, the term matrix isolation came to be applied in a more general sense, encompassing a range of techniques where guest molecules are trapped in rigid host materials and are thereby prevented from undergoing diffusion. Such host materials may be, for example, crystals, zeolites or clays, polymers, boric acid glasses, or cryptands. However, most relevant in the present context are studies of reactive intermediates in frozen solutions that are often referred to under the heading of ‘‘matrix isolation,’’ but should perhaps more appropriately be referred to as ‘‘low-temperature spectrosopy in rigid media.’’

Such experiments have provided (and continue to provide) much valuable spectrosopic information on many types of reactive intermediates discussed in this volume. In particular, solvents that provide transparent glasses on freezing, such as the ether–pentane–alcohol (EPA) mixture introduced by G.N. Lewis,4 methyltetrahydrofuran (MTHF), or the mixture of CFCl3 and CF2Br CF2Br discovered by Sandorfy5 have proven to be very convenient media for obtaining ultraviolet– visible (UV–vis) absorption spectra of reactive intermediates that can be generated by photolysis or radiolysis at 77 K. On the other hand, many odd electron species (radicals, triplet biradicals and carbenes, radical ions) were characterized by electron spin resonance (ESR) methods in solvents that form polycrystalline matrices long before matrix isolation techniques became widely available.

However, to include this enormous body of work, and the techniques that stand behind it, into the present chapter would surpass its limits. Hence, the reader is encouraged to visit the contributions on carbocations, carbanions, radicals, radical ions, carbenes, sylilenes, nitrenes, and arynes where studies involving frozen solutions will be referred to in their topical context. Thus, this chapter will deal only