Rotating cryostat

A method of rotating cryostate (15) is a promising technique for study of unstable radicals. 

Force-field methods, calculation of molecular structure and energy by, 13,1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271 Free radicals, and their reactions at low temperature using a rotating cryostat, study of, 8. I Free radicals, identification by electron spin resonance, 1, 284... 

Free radicals, and their reactions at low temperature using a rotating cryostat, study of,... 

In this chapter we describe briefly the principle and construction of the rotating cryostat and discuss several of the systems which have been studied. 

Two rotating cryostats are in constant use in the authors laboratory and their operation is routine in nature. The 0-ring seal usually lasts for 3-6 months, i.e. about 100 runs of half an hour each. A third rotating cryostat has been built independently, but to the same basic design, at the University of Tennessee (Mamantov e( al., 1966). 

To illustrate the technique we will consider a few examples of free radicals which have been prepared in the rotating cryostat. In particular phenyl and acetyl radicals and methyl-substituted allyl radicals are of interest as they have not been trapped previously or identified with certainty. Since electron spin resonance has been used extensively to detect and identify the free radicals, account of the results will inevitably involve some description and analysis of their spectra, but we wish to focus the main discussion on the conclusions that can be drawn about structure and reactivity of the radicals. For information about the principles of e.s.r. and the interpretation of the spectra of free radicals the reader is referred to review articles and books on the subject (Symons, 1963 Norman and Gilbert, 1967 Maki, 1967 Horsfield, 1967 Carrington and McLachlan, 1967 Ayscough, 1967 Carrington and Luckhurst, 1968). 

The hyperfine splittings in the e.s.r. spectra of radicals of the allylic type are considerably less than those of alkyl radicals, and for radicals trapped in their parent compounds the resolution is insufficient to determine all the hyperfine coupling constants. However, by use of the rotating cryostat, the unsubstituted radical and three methyl-substituted allyl radicals have been prepared in a matrix of adamantane and it has been possible to resolve all the hyperfine couplings. 

There are indications that cr-radicals, such as the phenyl radical may not react with oxygen as readily as the alkyl radicals (Hay, 1967). Attempts to form the phenylperoxy radical in the rotating cryostat have failed, but the results were insufficiently conclusive to determine whether, or not, the failure was due to a low reactivity of the phenyl radical towards oxygen addition,... 

The formation of molecular radical ions by electron transfer reactions between alkali metals and a wide variety of aromatic and other organic compounds in polar solvents is well established. A very large number of radical anions have been prepared by this method and extensive studies of their e.s.r. and optical spectra have been made (Bowers, 1965 Gerson, 1967 Kaiser and Kevan, 1968). In solution the electron transfer reaction will be facilitated by the subsequent solvation of the two ions (or ion pair) by the polar solvent molecules. However, we have observed that similar electron transfer reactions occur readily when alkali metal atoms are deposited on a variety of relatively non polar substances at 77°K in the rotating cryostat. In most cases the parent compound acts as the matrix, though for some radical ions an inert matrix of a non-polar hydrocarbon has been used successfully. It is perhaps surprising that the reactions occur so readily as the energy of solvation of the ions must be quite small in most of these systems as compared with that in the polar liquids. 

In the organic field the experiments using the rotating cryostat have been aimed at the preparation of the radical anions of some aliphatic compounds which had not been prepared previously namely, the radical anions from carboxylic acids and ketones. 

The rotating cryostat has been used to prepare trapped electrons by chemical reaction between alkali metal atoms and ice or solid alcohols at 77°K (Bennett et ah, 1967b, 1967d). The advantage of this technique... 

The infrared spectrum of a deposit prepared with the rotating cryostat is recorded in situ (see Section IIF2) and so the deposit can be removed later and its e.s.r. spectrum recorded. This helps in the assignment of absorption bands to the free radical species as the changes that occur in the infrared spectrum when the sample is warmed can be related directly to the changes in the e.s.r. spectrum. 

From these preliminary results it is apparent that the spectra observed for radicals prepared on the rotating cryostat are consistent with those obtained by the usual transmission methods. 

As described in the experimental section single-step radical-molecule reactions can be studied in isolation and in a very direct way by using the rotating cryostat. The identity of the initial radical is known and the product radical can usually be identified unambiguously by e.s.r. Also the relative amounts of the primary and product radicals can be obtained by analysis of the composite e.s.r. spectrum and thus the extent of reaction can be determined directly. When there is more than one site in a molecule at which reaction can occui, the resulting e.s.r. spectrum, which consists of the spectra of the different product radicals, can often be analysed to give the relative degree of attack at the different sites in the molecule (e.g. as for H-atom addition to an unsymmetric olefin). 

Many of the problems in the study of free radical reactions are associated with the assumptions that have to be made about the radicals present in a system and about the complex sequence of reaction steps that result in the transformation of the initial reactants into products. These problems are clearly minimized in the studies using the rotating cryostat, but there are inherent limitations and difficulties in the use of the cryostat to study radical reactions. Thus, only very efficient reactions can be studied and also the exact conditions under which the reactions occur are not always clearly defined. 

The formation of peroxy-radicals and their subsequent reactions play an important role in the oxidation of hydrocarbons at moderate temperatures. The reaction (17) which in the solid phase must strictly involve a third body (the matrix M) has been studied with the rotating cryostat. 

Radical addition reactions are very difficult to study and despite considerable effort there is still a lack of accurate kinetic results for these reactions. The experiments which have been carried out using the rotating cryostat have nevertheless enabled limiting values of the activation energy to be obtained. From the results the radicals can be divided clearly into two categories, those which react under these conditions and those which do not. 

The reaction of toluene is of interest as toluene has been used extensively in the toluene-carrier gas technique as a means of removing radicals from a reacting system (Szwarc, 1950). The results from the rotating cryostat show that n-heptyl radicals react with toluene at 77°K by abstracting a methyl hydrogen to give the benzyl radical. As expected, addition to the aromatic ring does not occur. 

Experiments have been carried out with the rotating cryostat to study the reaction of 2,6-di-t-butyl-4-methyl phenol (lonol) with n-heptyl and n-heptylperoxy radicals. When lonol was deposited on n-heptyl radicals the e.s.r. spectrum showed that some reaction had occurred at 77°K. When the deposit was warmed slowly the spectrum of the residual n-heptyl radicals disappeared and was replaced by that of the substituted phenoxy-radical, (4), formed by loss of the hydroxyl hydrogen. 

Although it is known that free radicals add predominantly to the least substituted end of an olefinic double bond there is very little quantitative information on the relative rate of addition at the two positions in asymmetric olefins (Cadogan and Hey, 1954 Cvetanovid, 1963). The rotating cryostat has been used to examine this aspect for the case of the addition of hydrogen atoms to a variety of olefins deposited in a matrix of adamantane. The ratios of the rates of addition are given in Table 7, and for illustration the reaction with propylene is considered below. 

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