Effect of Matrix Electronic Characteristics on Trapping and Degradation of Organic Radical Cations in Solid Rare Gases:  A Case Study of Methylal Radical Cation

Abstract

The matrix effects on trapping and degradation of methylal radical cation generated in solid argon, krypton, and xenon doped with an electron scavenger at 16 K were investigated by EPR spectroscopy. A relatively weak characteristic signal from trapped methylal radical cations was recorded in an argon matrix. However, the most intense signals in this matrix result from the products of degradation of the primary cations, mainly methyl and methoxy radicals. A comparatively low g max value for methoxy radical (g = 2.032) was explained by formation of a π-complex [CH3O•...CH2O+CH3] upon fragmentation of the parent cation in solid argon. In the case of a xenon matrix, the main observed species is the CH3O•CHOCH3 radical, which appears to result from proton loss in the primary cation. Both deprotonation and fragmentation products were found in krypton. The matrix effects were attributed to the variations in ionization potentials and polarizability (or basicity) of matrix atoms. Fragmentation predominating in argon was explained by excess energy resulting from highly exothermic positive hole transfer. Deprotonation in xenon is favored by basicity of the matrix atoms. Two possible mechanisms were discussed for the latter case, i.e., thermodynamic effect (deprotonation to matrix) and kinetic effect (catalysis of intramolecular H shift by matrix atoms).

The radiation-induced chemistry in solid xenon matrices

“…This process may become favorable due to high acidity of the primary radical cations and appreciable polarizability and proton affinity for xenon (the latter value is as high as 5.2 eV [44]). An indication of similar-type process was also found in krypton, but not in argon [17]. A remarkable feature of deprotonation is site selectivity, which correlates with spin density distribution in the radical cations.…”

“…On the other hand, the matrix polarizability was found to be an important factor, which may control the reactivity of the radiation-induced ionic species [16,18]. As a whole, our studies on radiation-induced transformations of organic molecules in solid noble gas matrices revealed a diverse chemistry, which is essentially a matter of interplay between different electronic characteristics of the matrix used [16,17,20]. Among the noble gas media, xenon is often considered as the worst matrix host for spectroscopic studies of isolated species.…”

“…Experimental indications of occurrence of the high-energy ionic channels (particularly important in argon with the highest IP value) were reported in early studies [10,24,25]. Detailed investigations revealed a rather strong and complex effect of molecular structure on the relaxation of excited radical cations in a solid argon matrix [17,19,22]. On the other hand, the matrix polarizability was found to be an important factor, which may control the reactivity of the radiation-induced ionic species [16,18].…”

“…) with low IP values (e.g., ethers and aldehydes), even in the presence of electron scavengers [15,17,18]. Instead of this, the EPR spectra revealed high yields of the corresponding radicals (R .…”

“…However, the mechanistic aspects were not studied in detail. During the past fifteen years we performed systematic studies of the effects of high-energy radiation on organic molecules isolated in different noble-gas hosts using a combination of IR and EPR spectroscopy [13][14][15][16][17][18][19][20][21][22]. These experiments provided quantitative estimates for overall efficiency of the radiation-induced degradation of guest molecules in terms of radiation-chemical yields and made it possible to propose a general formal scheme of chemical transformations:…”