The other method is based on preventing the further reaction of the intermediate, so that it accumulates until a high enough concentration is reached to allow detection [33] (see Fig.6.1). This method was devised independently by Pimentel and Porter in the early 1950s [34,35]. Pimentel was investigating the mechanism of combus- tion and was baffled by the low concentration of the intermediates, which prevented spectroscopic detection. “The intent of the method”, Pimentel commented “is to trap active molecules in a solid matrix of inert materials, crystalline or glassy. If the temperature is sufficiently low, the matrix will inhibit diffusion of the trapped molecules, thus holding the active molecules effectively immobile in a non reactive environment.” In this way, further reactions were prevented and a high enough concentration was reached for allowing detection by a normal instrument [35].
In a typical experiment, intermediates were produced in the gas phase and along with a flow of an inert gas were carried to a cold surface. Rapid solidification ensured that each host molecule would find itself confined in an isolated position and surrounded by a large excess of inert host molecules. In the sample obtained in this way, each substrate molecule was immobilized in a cavity surrounded by one or more layers of inert material and was thus “isolated” from the other substrate molecules in a “matrix” of the host gas, typically a rare gas at a few K (Scheme6.10).
The term has been used extensively for any method where intermediates pro- duced are prevented from diffusion by the rigid host molecules, such as glassy solvents obtained by fast freezing in liquid nitrogen of solvents that produces clear (and transparent in UV and visible range) glasses, such as ethanol, EPA, methylte- trahydrofuran, but also crystals, zeolites, polymers, boric acid glasses, or cryptands.
These conditions have been largely used for UV-visible spectra and EPR spectra measurements. Rare gases are transparent over the IR region and thus a favorite choice since this region is usually richer of structural information. Intermediates Scheme 6.10 Homolytic
cleavage detected in an inert matrix. Recombination of radicals may occur, but is less likely if a double fragmentation occurs liberating a stable molecule (A). Compare Fig.6.1
158 6 Photochemistry, a Powerful Science
may be generated in situ by irradiation, a method that has shown to be reliable, provided that the possible interference with the matrix is taken into account. Thus, rare gases are transparent and the spectra registered at 10 K or below show a more detailed pattern, but are very poor heat sinks. This makes thermalization slow and the incipient intermediate may be formed with excess energy and further react. On the other hand, recombination of the fragments that can’t escape from the cage may hide the occurring of a fragmentation reaction. As pointed out by Chapman, in these experiments “kT is a nuisance, and must be taken as low as possible” [36].
The convenient choice of the precursor may help to obtain the desired interme- diate; thus, carbon-centered radicals are more efficiently obtained from iodides and nitroso derivatives, since fragments such as I or NO survive some time even when in cage with an organic radical. Likewise, double fragmentation causing elimina- tion of a stable molecule, such as CO2or CO from acylperoxides or anhydrides, makes formation of radicals more effective. On the other hand, if the precursor is aggregated in the matrix, dimers may be formed. All of these factors may lead to a different course of the reaction with respect to solution. The ethyl radical has been obtained and characterized by irradiation of dipropionyl peroxide, thanks to the fact that interposed CO2molecules hindered fast recombination; see Scheme6.11and Fig.6.16. Otherwise, the best solution is producing radicals externally at a high temperature, where they freely move, and trap them afterward [37].
Fig. 6.16 IR spectrum of dipropionyl peroxide isolated in an argon matrix after 60 h irradiation with UV light with>20 nm. The bands different from those of the precursor are highlighted by an asterisk. Reprinted with permission from [37]
Scheme 6.11 Generation of dideuteroethyl radicals from propionyl peroxide in matrix [37]
6.3 This Is How It Happens: Blocking the Intermediates 159
An experiment in rare gas matrix at a temperature of a few K is by no means comparable to an experiment in a frozen solvent. Likewise, the IR spectrum is much richer of lines and thus gives much more structural information. This is apparent when the much better resolved spectrum of an intermediate, a nitrene, obtained by photolysis of azide26 [38] in an argon matrix at 12 K is compared to that in ethanol at 90 K and the wealth of data that are obtained from the IR spectrum of this species is considered. Furthermore, it easy to check for their correspondence to the calcu- lated spectrum. The secondary conversion of the first formed intermediate is also conveniently followed under these conditions (Figs.6.17and6.18) (Scheme6.12).
a
b
c
1a
28
5a
5a
327
327
327 × 10
× 5
× 5
200 250 300 350 400 450 500 550 nm
26
327
Fig. 6.17 (a) Stepwise 254-nm photolysis of azide26(bluespectrum) in EtOH at 90 K. The visible band of triplet nitrene327(redspectrum) is enlarged for better visibility. (b) Photolysis of nitrene327(redspectrum) at>450 nm. (c) Difference spectra showing the formation (by 254 nm photolysis of azide26,redspectrum) and the bleaching (by>515-nm irradiation,bluespectrum) of nitrene327in an Ar matrix at 12 K. Reprinted with permission from [38]
160 6 Photochemistry, a Powerful Science
Certainly, the knowledge of the chemistry of radicals involved in combustion has made a great advancement in this time and Pimentel idea that this method was useful for finding evidence about processes occurring under real-world conditions has been found correct. Thus, the benzyl radical29(see Sect.6.3for flash photolysis) has been characterized in great detail. This has been generated by flash vacuum pyrolysis of 1,2-diphenylethane at 620C. Trapping in argon containing 1 % oxygen at 25 K, where the matrix becomes softer and bimolecular reactions are possible, generated the benzylperoxy radical 30. The matrix was then frozen at 3 K for registering the spectrum. Safe assignments of the latter radical, which is formed in two conformers, the main one with the peroxy group pointing toward—and the less abundant one far from—the aromatic ring, are based on the photolysis of both protio and deuterated bibenzyl by reaction with16O2and18O2. The changes occurring on isotopic substitu- tion were as expected. The conformers interconvert even at 3 K (see Scheme 6.13, Fig.6.19).
Scheme 6.12 Photolysis of pyrazolylphenylazide 26 gives triplet nitrene 327 that further rearranges to pyrazolobenzopyrazole28
700 900 1100 1300 1500cm-13400 3500
327 28 (calc)
(calc) b
a
*
* *
Fig. 6.18 IR difference spectra for (a) the formation of nitrene (327) by (254-nm photolysis of azide26,redspectrum) and (b) the>515-nm bleaching (bluespectrum) of nitrene327by further irradiation in an Ar matrix at 12 K. The bands that increase in (a) and decrease in (b) (dashed lines) are correlated with those of the simulated spectrum of nitrene327at the bottom (B3LYP/6-31G*
calculation of most stable conformer). The bands that increase in spectrum (b) are correlated with those of the simulated spectrum of pyrazolobenzopyrazole28shown at the top (dotted lines).
Reprinted with permission from [38]
6.3 This Is How It Happens: Blocking the Intermediates 161
Scheme 6.13 Reaction of benzyl radical (29) with oxygen in matrix
Fig. 6.19 (a) Part of the IR spectrum obtained after flash vacuum pyrolysis of bibenzyl and topoisomers at 620C, followed by cooling the products in argon at 25 K. The spectrum was recorded after cooling to 3 K and shows bands assigned to benzylperoxy radical PhCH2OO. (b–e) Difference IR spectra of the four topoisomers of the radical showing changes induced by cooling the matrix from 25 to 3 K. The first spectrum was taken immediately. After cooling at 3 K, the second one was registered approximately after 15 min. The bands pointing downward are of decreasing intensity, and those upward increasing at 3 K: (b) h216O2; (c) h218O2; (d) d216O2; (e) d218O2. Bands labeled A are assigned to the main conformer, B to the minor one. Reprinted with permission from [39]
162 6 Photochemistry, a Powerful Science
The photochemistry of the thus formed benzylperoxy radical was then studied in the matrix. Irradiation at 365 nm caused a formal 1,3 hydrogen migration followed by cleavage of the peroxyl bond and prolonged irradiation finally yielded phenyl radical31, CO, and water. This result shows that the benzyl radical is transformed via a series of exothermic steps into even more reactive radicals, such as OH and phenyl radicals [39] (see Scheme6.13, Fig.6.20).
Notice further that in situ generation of the intermediates is limited to com- pounds sufficiently volatile (though special techniques are available for nonvolatile compounds). In alternative, products can be generated externally in the gas phase, provided that, as mentioned, they live long enough to survive the transfer. This is typically applied with metals that are trapped by complexing agents.
Metal atomþA Hð 2O, N2, CO2. . .Þ !M-A ð6:2Þ Summing up in a sentence, matrix isolation gives more structure information based on spectroscopic identification of products, but under conditions that are very Fig. 6.20 Difference IR spectra in the O–H and C¼O stretching regions showing the photochem- istry of benzylperoxy radical matrix isolated in argon. Bands pointing upward appeared and bands pointing downward disappeared during the irradiation. (a) 10 min irradiation at 365 nm at 3 K. (b) Same matrix as in (a) warmed at 25 K. (c) Reference spectrum of benzaldehyde, matrix isolated in 1 % H2O-doped argon. The difference spectra, taken at 3 K and after warming at 30 K, show the formation of the 4-H2O complex. (d) Same matrix as (b) after additional 10 min irradiation at 320 nm [39]
6.3 This Is How It Happens: Blocking the Intermediates 163
far from the usual room temperature solution adopted for preparative photochem- istry, to which flash photolysis gets closer (once that differences due to the high concentration of excited states are taken into account), but with less structure information (Table6.1).
As discussed by Bally [33], matrix photochemistry has obtained excellent results, provided that back reaction is not overwhelming. This is a problem with radicals, much less for carbenes and nitrenes when formed by elimination reactions from diazo compounds and respectively azides. In the latter case, N2 is eliminated with no significant alteration of the matrix. There are exceptions, however, as is the case of parent carbene, the identification of which has long been hindered by the efficient recombination occurring, and finally demonstrated by14N–15N exchange [42].