Product excitation, interpretation

The formation of the products is interpreted in terms of excitation of the carbonyl, y-proton abstraction, and clockwise and anticlockwise rotation around the C1-C2 bond in the 1,4-biradical. On irradiation of 2,2-dimethyl derivatives, small amounts of epoxycyclobutanol and benzoic acid are formed (the latter originating from opening, facilitated by the geminal dimethyl group) (Eq. 351). 

However, in studies conducted at about the same time on benzosilacyclobutenes, in which carbonyl compounds were inserted into the silacyclobutene ring (in yields which varied from 25-80%), the products were interpreted as arising from attack of an excited carbonyl species directly on the ring. The mechanism implies formation of a diradical intermediate, and explains the formation of minor isomeric products35 (equation 21). 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

A remaining point to interpret is the complete retention of stereochemistry in the cyclobutane products. Models that have previously been proposed for excited diene triplets 80>, 15 and 16, have a full bond between atom 2 and 3 of the diene, and cannot adequately represent the triplet dienes formed in this reaction. A great deal of double bond character at the terminal bonds is required to prevent rotation. This... 

The rate constant for formation of excited-state S02 was measured by Halstead and Thrush to be 1.7 X 10 12 exp(-2100/T) cm3 molec1 s 1 [28-31], while the overall rate constant for the loss of SO is 3.6 x 10 12 exp(-1100/T) cm3 molec-1 s 1 [11], Thus, at room temperature, S02 constitutes —1.6% of the product channels. This figure can only be considered approximate, however, since mixing of states within the singlet manifold affects the interpretation of the rate constant for formation of the excited state [32],... 

In pentane, the distribution of 1,3-insertion product 25 to 1,2-Me shift product 26 is 91 9. Upon addition of 2-methyl-1-butene, the yield of 25 smoothly decreases (to 19% with 4 M alkene), but the yield of 26 is unaffected 1 Moreover, correlation of addn/l,3-CH insertion (to 25) for 18 is nicely linear. The simplest interpretation is that 25 comes directly from carbene 18, whereas the 1,2-Me shift product 26 comes from the excited diazirine.27 Interestingly, thermolysis of 24 at 79°C produces 90% of 25 and 10% of 26, but now the yields of both products smoothly decrease in the presence of an alkene. In thermolysis the (electronically) excited diazirine is unavailable, both 25 and 26 stem from the carbene, and their formation is suppressed by the alkene s interception of the carbene. A pyridine ylide kinetic study gave the 1,3-CH insertion rate constant (18 - 25) as 9.3 x 10s s"1.27-47... 

The results and discussion section is divided into two parts. The first part deals with direct laser flash photolysis of the MDI-PUE polymer and appropriate small molecule models. The transient spectra generated by direct excitation of the polyurethane are interpreted by consideration of the primary photochemical reactions of the carbamate moiety. The second part describes results obtained by production of a radical transient species which is capable of abstracting labile hydrogens from the polyurethane. This latter procedure represents an alternative method for production of the transient species which were obtained by direct excitation. 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

However, coUisional deactivation in solution is so effective that no vibration-ally excited species is present. The reaction of photochemicaUy generated methylene with 2-methylpropene-l-)- C yields, 2-methyl-butene, which is formed by allylic insertion. In the liquid phase 2 % of the rearranged product labeled in the 3-position are formed, whereas in the gas phase 8% of this olefin can be isolated. This can be interpreted as follows 4% of 2-methyl-butene in solution and 16% of 2-methyl-butene in the gas phase are formed by an abstraction-recombination mechanism involving triplet methylene 96). 

With ESR spectroscopy, open-shell species can be observed and characterized as long as their total spin differs from zero. With variable-temperature ESR spectroscopy, it is possible to deduce whether the observed multiplicity is a thermally populated excited state or is the ground state [69]. From such experiments, the T-S splittings of a variety of biscarbene and bisnitrenes have been determined. ESR spectroscopy is very sensitive to paramagnetic species, and because it does not see any singlet impurities or by-products, it is relatively easy to pick out the desired signals. At the same time, analysis of ESR spectra is not trivial and special simulations are required for their interpretation. 

Progress in photochemistry could only be made following progress in spectroscopy and, in particular, the interpretation of spectra in at least semiquantitative terms, but history has shown that this was not enough. The arrival of new methods of analysis which permit determination of small amounts of products, the development of flash photolysis, nuclear magnetic resonance, and electron spin resonances which can yield valuable information about the natures of intermediate excited states, as well as of atoms and radicals, all have permitted the photochemist to approach the truly fundamental problem of photochemistry What is the detailed history of a molecule which absorbs radiation ... 

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