Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

332 DETECTION AND ESTIMATION OF INTERMEDIATESConsideration will be given now to the requirements of a matrix that radicalstabilisation may occur and to the ways in which radicals may be introduced intothe matrix. In order that stabilisation of active species may occur, it is desirablethat the matrix material(i) is inert with respect to the active species;(ii) is sufficiently rigid to prevent diffusion of the active species; and(iii) has sites or holes suitable for the accommodation of the active species.The vapour pressure of the matrix must be low at the operating temperature(< 10- ’ torr), although it may be convenient if the vapour pressure is high enoughat room temperature to permit handling as a gas. Further restrictions on thematrix material are imposed by the method of detection which is to be employed.In particular, for spectroscopic investigation the matrix material must not absorbappreciably, and scattering must be reduced to a minimum. Some degree of compromisemay have to be accepted in the properties required. Thus glasses of etherisopentane-alcohol(EPA) are clear, and may be used for near-ultraviolet studies oftrapped aromatic radicals2”, although the glass may absorb too strongly for usein infrared studies, and it may be too reactive to act as a good matrix for aliphaticradicals. On the other hand, the inert gases and nitrogen, which are chemicallyideal, frequently form highly scattering microcrystalline deposits when frozen.The temperature at which the matrix is formed may affect its scattering properties:Becker and PimentelZ9* find, for example, that xenon forms a fairly transparentmatrix if deposited at 60” K, but a highly scattering one if formed below 50°K.Scattering may be less of a problem in infrared studies than it is in visible or ultravioletexperiments, although against the reduced scattering must be set the muchsmaller oscillator strength of vibrational, rather than electronic, transitions.Radicals may be trapped in inert gas matrices in one of two main ways. First,the radicals may be prepared by a gas-phase reaction (i.e. by photochemical,thermal, electrical or chemical processes) and allowed to condense in the presenceof the matrix material on a cold surface. Secondly, a matrix containing a precursorof the radical may be deposited, and the radical produced in the matrix by asolid-phase reaction. Photolysis of the precursor is the most controlled way offorming the radical, although the precursors are then restricted to photosensitivematerials. Again, the excess energy available in a photochemical reaction may besufficient for the radical(s) to escape from the matrix cage and primary recombinationmay occur. Gamma ray or X-ray radiolysis, or electron bombardment, ofthe solid is not restricted to photosensitive precursors, nor is the excess energylikely to be insufficient to allow the radicals to escape from the cage. In general,however, radiolysis and electron bombardment are too indiscriminating in theirattack, and photolysis remains the method of choice.Radicals may be produced also by secondary reactions with a reactive matrixof the radicals fist produced. For example, Milligan and Jacox2” have preparedHCO (DCO) by the photolysis of HI (DI) or H,S (D,S) in a matrix of solid carbon