Radical cations matrix isolation

At the same time, studies of emission spectra of matrix isolated radical cations were undertaken by Miller and Bondybey at Bell Laboratories233 and by Maier and coworkers... 

In the early 1980s, one of the authors of this chapter began to study argon matrix isolation of radical cations235 by applying the radiolytic techniques elaborated by Hamill and Shida. A central factor was the addition of an electron scavenger to the matrix which was expected to increase the yield of radical cations and the selectivity of the method. For practical reasons, X-rays replaced y-rays as a radiolytic source and argon was chosen as a matrix material because of its substantial cross section for interaction with keV photons (which presumably effect resonant core ionization of Ar). Due to the temporal separation of the process of matrix isolation of the neutral molecules and their ionization, it was possible to obtain difference spectra which show exclusively the bands of the radical cations. 

The C2O2 problem has a long history.151 Many attempts to generate or matrix-isolate 120 using routine methods were not successful so far.152,153 Neutralization of the radical cation in the gas phase also failed.154,155... 

As seen from above, the mode of electron trapping in sc CO2 cannot be deduced from the results obtained in the gas phase or matrix isolation studies. It is not obvious whether the solvent radical anion should be similar to multimer cluster anions found in the gas phase, dimer cation(s) in solid matrices, or monomer CO2 anions in inert liquids. Such a situation is typical for other molecular liquids. 

He ( impact (PES) in the gas phase, but oxidation in solution is difficult. Alkane radical cations have become readily accessible only with the advent of matrix isolation techniques combined with ESR detection. 

The above technique, which is very easy to implement (all that is needed besides the usual matrix isolation equipment is a conventional X-ray source for crystallography), was used first to record UV-vis spectra and later IR spectra of numerous radical cations isolated in Ar matrices. The method seems to be quite generally apphcable to any substrate that can be volatilized without decomposition. 

In spite of all this research, systematic criteria for trapping of organic radical cations produced by positive hole transfer in solid argon are not yet fully understood. It would be useful to establish such criteria to evaluate the prospects of characterization of chemically important aliphatic radical cations under the conditions of classic matrix isolation experiments. 

It is interesting to note that the catalysts that show good selectivities at the higher temperatures generally do not contain easily reducible metal ions, such as V, Mo, or Sb. Many of the catalysts for the lower-temperatures operation, on the other hand, contain these reducible cations. In a study using a Li-Mg oxide, it was established that gas-phase ethyl radicals could be generated by reaction of ethane with the surface at about 600°C (17). These radicals could be trapped by matrix isolation and identified by electron spin resonance spectroscopy. 

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol 1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ab initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=S02) are best viewed as dihalocarbenc-SO complexes with a carbon-sulfur bond order of approximately zero.14 hi a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]+ ... 

The third mechanism of isomerization, photoinduced rearrangements of radical cations, has been pursued in a variety of systems. Matrix isolated radical cations have been noted to undergo some rigorous reorganizations as well as subtle ones. For example, the ring opening of cyclohexadiene to hexatriene radical cation and the interconversion of its different rotamers have been achieved by irradiation with UV or visible light [173-174]. 

N+ and N+, have been observed only as free gaseous or matrix-isolated ions or radicals. The only polynitrogen species synthesized on a macroscopic scale thus far is the pentanitrogen cation Nj by Christe and co-workers in 1999. 

In this section, the selected topics of photoelectron spectroscopic investigations on silicocene (Section IV.B.l), the charge delocalization in (I Si Si—Si(SiR3)3 (Section IVB.2) and matrix isolation of organosilicon radical cations (Section IVB.3) are presented. Table 3 will provide data on all other organosilicon molecules investigated by their ionization patterns. 

Application of pulse radiolysis to polymers and polymerization was motivated at first by the success of radiation-induced polymerization as a novel technique for polymer synthesis. It turned out that a variety of monomers could be polymerized by means of radiolysis, but only a little was known about the reaction mechanisms. Early studies were, therefore, devoted to searching for initiators of radiation-induced polymerization such as radicals, anions and cations derived from monomers or solvents. Transient absorption spectra of those reactive intermediates were assigned with the aid of matrix isolation technique. Thus the initiation mechanisms were successfully elucidated by this method. Propagating species also were searched for enthusiastically in some polymerization systems, but the results were rather negative, because of the low steady state concentration of the species of interest. 

A case where both effects are important is the ring opening of the cyclobutene radical cation l +.13 This reaction was studied experimentally by Bally and coworkers, who found that, in contrast to the thermal reaction, only the trans-1,3-butadiene radical cation, trans-2 +, is formed in a matrix isolation experiment.14,15 This surprising result can be explained by considering the electronic states of the species involved in the reaction, which are shown in Fig. 1. 

Figure 1 UV-visible absorption spectra of matrix-isolated p-dichlorobenzene and of its radical cation (a) spectrum of p-dichlorobenzene before photolysis (b) spectrum after 15 min photolysis at = 220-1000nm (c) spectrum after 15 min Ar resonance photolysis. The much increased yield of product upon vacuum UV photolysis can clearly be seen. (Reprinted with permission from Friedman et al., J. Phys. Chem., 1984, 88, 1944. 1984 American Chemical Society)...

The cation radical cyclodimerization of 2-butyne has been observed by ESR spectroscopy under matrix isolation conditions (Scheme 12) [34]. 

---