Low temperature matrices, alkenes

Alkenes and conjugated dienes can react with NO under the action of visible light in low-temperature matrices [73-77]. The chemical reaction has been observed upon irradiation of ethylene - NO pairs in solid Ar at 12 K with continuous-wave dye... 

This chapter covers photochemical studies of alkenes and cycloalkenes isolated in very low temperature matrices, typically solidified noble gases or nitrogen at temperatures of 10-20 K. Under the conditions of such matrix isolation, trapped species are prevented from diffusing and are therefore prevented from undergoing bimolecular reactions. As a result, extremely reactive species can be stabihzed and investigated by a variety of normal spectroscopic methods, of which IR spectroscopy usually provides the most useful structural information. A brief description of the matrix-isolation technique and an account of its applications in several areas of organic photochemistry are given in a later chapter. 

A series of experiments using alkene matrices clearly suggests that in a rigid matrix at low temperatures, triplet states of arylcarbenes undergo abstraction of... 

The organometallic chemistry of aluminum is dominated by the chemistry of aluminum(lll), but lower oxidation state compounds are now accessible. The first examples of this class of compounds are carbonyl complexes such as Al(CO), A1(C0)2, and Al3(CO), which were generated upon exposure of aluminum atoms to CO in matrix-isolation experiments near 20 K. The number, relative intensities, and frequency of the carbonyl stretches in the IR spectra, along with isotopic labeling and EPR studies were used to verify these compositions. These complexes exhibit vco values of 1868, 1985 and 1904, and 1715 cm , respectively, indicative of Al- CO 7t backbonding. The carbonyl species are unstable at higher temperatures and no stable carbonyl complex of aluminum, in any oxidation state, has been reported. The monomeric aluminum-alkene adducts A1( -C2H4) and k rf-CeHe) were similarly identified in inert matrices at low temperature. No room-stable alkene complexes of aluminum have been reported. 

A number of alkene radical cations have been generated in matrices at low temperature and have also been studied by ESR, CIDNP, and electrochemical methods. However, until recently very little absolute kinetic data have been available for the reactions of these important reactive intermediates in solution under conditions comparable to those used in mechanistic or synthetic studies. In a few cases, competitive kinetic techniques have been used to estimate rates for nucleophilic additions or radical cation/alkene cycloaddition reactions. In addition, pulse radiolysis has been used to provide rate constants for some of the radical cation chemistry relevant to the pho-topolymerization of styrenes. More recently, wc and others have used laser flash photolysis to generate and characterize a variety of alkene radical cations. This method has been extensively applied to the study of other reactive intermediates such as radicals, carbenes, and carbenium ions and is particularly well-suited for kinetic measurements of species that have lifetimes in the tens of nanoseconds range and up and that have at least moderate extinction coeffleients in the UV-visible region. 

As noted above, most kinetic studies of alkene radical cations in solution have focused on aryl-substituted systems since they have convenient optical propenies and have been extensively studied by other techniques. Radical cations are frequently identified on the basis of their characteristic UV-visible absorptions and the comparison of their spectra to those obtained in matrices at low temperature." However, a number of other diagnostic tests are also commonly employed to identify these intermediates. For example, their kinetic behavior as a function of solvent nucleophilicity or added nucleophiles is analogous to that of other electrophilic species. Thus, reaction with nucleophiles such as azide and halide ions provides support for the assignment of a transient to a radical cation, although it will not serve to eliminate a carbocation intermediate. More useful in the latter respect is the method of generation of the transient since PET does not in general lead to the formation of carbocations. Quenching of the observed transient with a more easily oxidized... 

Photolysis of [Fe(CO)4(alkene)], where alkene = a,)8-unsaturated esters, in a variety of matrices at low temperatures produces CO and three types of [Fe(CO)3(alkene)] complexes. The bonding modes for the a,)8-unsaturated ester... 

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