Cryogenic matrices

In all of these applications, the emphasis to date has been on the use of lasers to study chemically and physically well characterized systems, that is, simple molecules in the gas phase, or in ordered phases such as molecular crystals, or in cryogenic matrices. There are exceptions to this statement, but the basic fact is that the great strides in chemical applications of lasers have been made by the chemical physics and analytical chemistry communities and largely ignored by inorganic, organic, and biological chemists. 

Methylchlorocarbene exhibits a small KIE that increases with increasing temperature QMT is considered important in its 1,2-H shift.60 63 The KIEs of 54-F and 54-C1 are larger, and that of 54-F decreases with increasing temperature. QMT has not been invoked for these rearrangements, at least at the temperatures studied. However, it is possible that QMT could intervene at lower temperatures (e.g., <200 K, as calculated for MeCCl63) or in cryogenic matrices.58,59... 

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

The preparation of single isomers for methanol dimer, trimer, and presumably tetramer [16] in a supersonic jet expansion contrasts the structural diversity that can be prepared and manipulated in cryogenic matrices [34]. It underscores the ability of supersonic jet expansions to funnel all intermolecular isomers down to the global minimum, if there are no major barriers to overcome on the way. 

The greater stability of simple ketones relative to their enol tautomers is reversed on formation of the corresponding radical cations (88a) (88b). In appropriate cases, ionization of the ketone to its cation is followed by spontaneous hydrogen transfer to give the enol radical cation. 1,5-Hydrogen transfer via a six-membered-ring transition state is a common route. Characterization of such mechanisms has been reviewed for a variety of such reactions in cryogenic matrices, where many of the processes that compete in solution are suppressed. ... 

Exposure of several methyl-substituted derivatives to y-radiolysis at 77 K in cryogenic matrices gave rise to a family of radical cations of the same structure type, some of which had been previously identified on the basis of CIDNP results. We begin with a discussion of the CIDNP investigations, since they preceded the ESR studies of all species but the prototype. The first CIDNP results attributed to a cyclopropane radical cation were observed during the photoreaction between 1,4-dicyanonaphthalene and cis-l,2-diphenylcyclopropane. However, the nature of the cyclopropane radical cation was characterized by CIDNP effects observed during the reaction of chloranil with cis- and /rans-l,2-diphenylcyclo-propane. ... 

Photolysis of phenyl azide (32) produces singlet phenylnitrene (33s), but what happens next depends on temperature and phase. In the gas-phase 33s isomerizes to cyanocyclopentadiene 31, in solution at ambient temperature, it isomerizes to ke-tenimine 30 and in cryogenic matrices, singlet phenylnitrene isomerizes to triplet phenylnitrene (33t). 

The first spectroscopic evidence for a derivative of 13 was presented by Bucher et al. in 1992, who were able to isolate 2,4-didehydrophenol (19) in cryogenic matrices (Scheme 16.8). Irradiation of matrix isolated quinone diazide (20) with light of wavelength A, = 432 10 nm yields carbene (21), which is... 

Fluoro-o-benzyne (41) has also been isolated in cryogenic matrices by photolysis of the corresponding phthalic anhydride. The C=C stretching vibration of 41 has been observed at 1866 cm Compared to the parent, the triple bond length is only modestly increased according to DFT calculations (Fig. 16.2). Note, however, that the observed frequency shifts upon fluorine substitution in 4 are not reproduced at the B3LYP or BLYP level of theory (Fig. 16.2). Thus, the performance of different DFT functionals to account for the properties of o-arynes still remains a matter of debate,and casts some doubts on the results of certain DFT studies. 

So far, most of this discussion applies to the generation and study of radical cations. How about radical anions These species are more difficult to generate in isolated form in cryogenic matrices because in solvents that do not significantly stabilize charged species (such as noble gases), isolated radical anions are prone to suffer detachment of the loosely bound extra electron, either spontanously or under the impact of low-energy radiation. This situation contrasts with that encountered in solvents such as methyltetrahydrofuran, which can be routinely... 

Of course, matrix isolation studies are not limited to the classes of reactive intermediates discussed in the above five sections. There are many other types of highly reactive species that can be stabilized (and were often observed for the first time) in cryogenic matrices. Only a comprehensive review could do justice to all those efforts, but this effort is beyond the scope of this chapter. So, we will limit ourselves to a few typical cases. 

There is rather little direct experimental evidence for the intermediacy of BZ. Cyclic ketenimine K has been detected by matrix IR spectroscopy, benzazirine BZ has not. However, fluorinated and naphthalenic derivatives of BZ have been generated as persistent species in cryogenic matrices and characterized. Parent benzazirine BZ has been intercepted with ethanethiol, and certain derivatives of BZ have been trapped with... 

Rather recently, the electronic structure of spiro[4.4]nonatetraene (31), as well as those of its radical anion and radical cation, respectively, have been probed by a variety of spectroscopic methods (electron transmission and energy loss in the gas phase, electronic absorption in cryogenic matrices) [24]. The extension of this concept of spiroconjugation to higher analogues of spiro[4.4]nonatetraene 31 would lead to the hydrocarbons 32 and 33. However, no attempts to prepare these compounds have yet been reported. 

Schriver, L., Schriver, A, and Perchard, J. P, Spectroscopic evidence for proton transfer within the bimolecular complex HI—NH trapped in cryogenic matrices, J. Am. Chem. Soc. 105, 3843-3848 (1983). 

Many other branched alkanes undergo electron-transfer reactions in cryogenic matrices. Although the ESR spectra of the resulting radical cations reveal interesting structural features [60-62], detailed discussion would exceed the scope of this review. 

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