Matrix photochemistry

There may be several reasons for the difference between gas phase and matrix photochemistry, and we outline one possible explanation. Even at 355 nm (XeF laser), a uv photon has more energy (equivalent to 335 kJ mol-1) than is needed to break one M—CO bond (89,90). In a matrix, the isolated Fe(CO)5 molecule is in intimate contact with the matrix material, and any excess energy can be rapidly lost to the matrix. In the gas phase, collisions are the principal pathway for loss of this excess energy. Under the conditions used in the gas phase photolysis, the mean time between collisions was relatively long and the excess energy could not... 

C5Me5) was stable up to room temperature (108,109). A surprising feature of the matrix photochemistry (6,7) was that only the trans isomer of [CpFe(CO)2]2 was photolyzed, and it may be that this was the consequence of concerted CO loss from the trans isomer and isomerization to form the triply bridged product (6). 

Photochemical Routes. Finally, two sets of experiments from Rest s laboratory which demonstrate the subtle implications of quite complex matrix photochemistry experiments. The first (67) involves (n -C5H5)Co(CO)2 and can be summarised... 

The matrix photochemistry of 2n24 and 2o92 is completely analogous to that of 2a. The primary irreversible loss of nitrogen from 2 produces carbenes 1 in photostationary equilibria with cyclopropenes 3 (Scheme 15). The relative amounts of 1 and 3 formed in the matrix depends very much on the wavelength used for the irradiation. Both carbenes In and lo were chemically identified by oxygen trapping. UV irradiation (248 nm) of In produces a mixture of indeneketene 21, CO, and indenylidene 22 (Scheme 14). 

The matrix photochemistry of 2v proved to be fairly complicated.108 The primary product of the photolysis of 2v is carbene lv, which was identified by ESR spectroscopy. Under the conditions of matrix isolation the carbene showed the expected reactivity towards molecular oxygen (formation of carbonyl oxide 7v) and carbon monoxide (formation of ketene lOv) (Scheme 22). In contrast to the oxocyclohexadienylidenes (la and derivatives) carbene lv slowly reacted with CO2 to give an a-lactone with the characteristic C=0 stretching vibration at 1896 cm-1. The latter reaction indicates that lv is — as expected — more nucleophilic than la. 

Group VIB, oxyanions, redox reactions, kinetics and mechanism, 40 269-274 Group VIB carbonyls infrared spectra, 19 85 matrix photochemistry, 19 84-88 substituted, matrix photochemistry, 19 89,... 

Platz, M.S., Leyva. E., Haider, K. (1991), Selected Topics in the Matrix Photochemistry of Nitrenes, Carbenes, and Excited Triplet States, Org. Photochem. (Padwa, A., Ed.) 11, 367. 

The matrix photochemistry of binary metal carbonyls has been extensively investigated. In all cases the first observable consequence of UV/VIS irradiation is expulsion of a CO group. In many cases interaction of the coordinatively unsaturated product with the matrix molecules (even with such "inert" matrices as the... 

Gordon, C. M., Kiszka, M., Dunkin, I. R., Kerr, W. J., Scott, J. S., Gebicki, J. Elucidating the mechanism of the photochemical Pauson-Khand reaction matrix photochemistry of (phenylacetylene)hexacarbonyldicobalt. J. Organomet. Chem. 1998, 554,147-154. 

Photfrfysis of a-Diazo Carbonyl Compounds - Some recent advances in the matrix photochemistry of diazoketones, including some heterocyclic species, have been reviewed. Flash photolysis of 10-diazo-9(10//)-phenanthrenone (35) in aqueous solution led to the detection of two transient species on the pathway to the final product, fluorene-9-carboxylic acid. These were identified, from solvent isotope effects and the nature of the observed acid-base catalysis, as fluorenylideneketene (36, X = CO) and the enol of fluorene-9-carboxylic acid (36, X = C(0H)2), formed by hydration of the ketene. In related studies, fluorenylideneketene was found to react with amines to give ylides as intermediates on the route to the amide final products. The product distribution from the photochemical reactions of 2-diazo-3-oxo-5,10,15,20-tetraphenylchlorins with alcohols strongly depends on the central metal ion of the irradiated diazoketones. ... 

Based on detailed mechanistic experiments, quantum yields of secondary product distribution, labelling studies, matrix photochemistry and added reagents, a mechanism involving a photoinduced, concerted -elimination was proposed. Thus, in the primary photochemical event -hydride elimination reaction occurs with formation of a quite reactive thorium hydride and olefin ... 

Firstly in this section, we deal with the chemistry of mononuclear carbonyls and substituted carbonyls. The matrix photochemistry of Fe(C0)5 in 02/Ar matrices has been reported and the experimental findings are backed up by a theoretical discussion. It appears that Fe02 and Fe03 are the eventual photoproducts. The mechanism of Fe(CO>5 photolysis on Pt surfaces has been investigated and Fe(C0)5 decomposition in shock waves has also been studied . The SFg sensitized IR-photodecomposition of the same compounds has been investigated. The reaction of Fe C0)5 with high-pressure N2 in poly(ethene) films, and the subsequent reaction of Fe(CO)4(N2) with H2 has been investigated . 

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 by exchange [42]. 

Dunkin IR (2004) Matrix photochemistry. In Horspool W, Lenci F (eds) Organic photochemistry and photobiology, 2nd edn. CRC, Orlando, Chapter 14... 

Turner JJ, Burdett JK, Perutz RN et al (1977) Matrix photochemistry of metal-carbonyls. Pure Appl Chem 49 271-285... 

In subsequent studies, emphasis was given to studies of the matrix photochemistry of tertiary alkyl azides. Dunkin and Thomson studied the photochemistry of tert-butyl azide (6) in an N2 matrix at 12 K. Using IR spectroscopy they detected the formation of only one product - imine 7. 

Scheme 11.6 Matrix photochemistry of tert-butyl azide ...

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