Photolysis experiments, laser

Apart from the nuclear bromination observed (Section 2.15.13.1) in the attempted radical bromination of a side-chain methyl group leading to (396), which may or may not have involved radical intermediates, the only other reaction of interest in this section is a light-induced reduction of certain hydroxypyrido[3,4-f)]pyrazines or their 0x0 tautomers analogous to that well-known in the pteridine field (63JCS5156). Related one-electron reduction products of laser photolysis experiments with 1 -deazaflavins have been described (79MI21502). 

Novak and Windsor developed a very elegant and effective method for laser photolysis experiments, using the same laser to produce both pump and analyzing pulses. Their experimental set-up is shown in Fig. 9... 

In the laser photolysis experiments the aromatic compound (4-10" M) and the nucleophile (0 04 M ) in acetonitrile-water (1 1) were irradiated with the frequency doubled pulse (100 mj, 6 ns, 347 nm) of a ruby laser. Only time-dependent absorption changes were measured (double pulsed xenon flash lamp with 10 /is continuous output as light source) absorption spectra were constructed from these measurements at 12 or 25 nm intervals. 

We have also confirmed our previous results on the effect of methoxylation of phenolic hydroxyl groups within lignin. On the basis of our model studies, we suggest that the most likely explanation for this is that triplet carbonyl groups are quenched statically by hydroxyl groups within the lignin structure on timescales less than 20 ns thus reducing the amount of transient detected in our laser photolysis experiments. 

Optical flash or laser photolysis experiments in which the optical absorption spectrum of the system is recorded as a function of delay time after a short photolysis laser pulse could... 

Fig. 1. A schematic view of a pulsed laser photolysis experiment in which O (1Z ) atoms were generated by photolysis of O3 at 266 nm. The reaction...

Low temperature experiments have shown the formation of hypso intermediates from several species [99,103,105-107]. The study of early photoconversion processes in squid [108], which also involved the evaluation of the relative quantum yields among the four pigments (squid rhodopsin, squid batho-, hypso- and isorhodopsin) showed that hypsorhodopsin is a common intermediate of rhodopsin and isorhodopsin there is no direct conversion between rhodopsin and isorhodopsin bathorhodopsin is not converted directly to hypsorhodopsin and both rhodopsin and isorhodopsin convert more efficiently to bathorhodopsin than to hypsorhodopsin. While a temperature dependence of the relaxation processes from the excited state of rhodopsin, and an assumption that batho could be formed from one of the high vibrational levels of the ground state hypso have been invoked to explain these findings [108], the final clarification of this matter awaits results from subpicosecond laser photolysis experiments at liquid helium temperature. 

There is no direct evidence for participation of radical pairs in the reaction of catalase, but a radical pair is known to be formed through thermal or photochemical homolysis of alkylcob(ni)alamins (RCbl ) from ps-laser photolysis experiments. The structure of adenosyl-cob(ni)alamin (AdoCbP) and metylcob(III)alamin (MetCbl ) and the radical pair produced from RCbl are shown in Fig. 15-1. Vitamin B12 is a cofactor for many enzymatic reactions in its various forms. 

The third harmonic of a Nd YAG laser [355 nm, 5 ns FWHM (full width at half-maximum), 30 mj] was used as the excitation source in all reported laser photolysis experiments. 

The support by the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under grant No. FG-05-92-ER14310, is gratefully acknowledged. All pulse radiolysis experiments described here were performed in collaboration with John R. Miller at the Argonne National Laboratory linac facility. Renata Kobetic and Timothy R. Schatz are thanked for their work on the laser photolysis experiments. 

Excited state carbocations have been generated via two-step laser photolysis experiments. The first laser pulse results in ionic photodissociation of a carbon-heteroatom bond to generate the cation. The second pulse, tuned to the cation absorption, electronically excites the cation. For example, the singlet-excited... 

Interpretation of the results from the product studies and nanosecond laser photolysis experiments led to the rate constants for each step in the overall dimerization reaction summarized in Scheme 4. The addition step is quite rapid, taking place with a rate constant of 1.5 x 10 M" s" , but formation of the cyclobutane radical cation is reversible, with a calculated rate constant of 8 x 10 s" for cycloreversion to regenerate the 4-methoxystyrene radical cation and neutral 4-methoxystyrene. The two other processes available to the intermediate cyclobutane radical cation are rearrangement to the hexalriene radical cation with a first-order rate constant of 2.5 x 10 s". and electron transfer with neutral 4-methoxy- styrene with a rate constant of 1.5 X 10 M s to generate the neutral cyelobutane and to regenerate the 4-methoxystyrene radical cation. 

The limit of conventional, cryogenically cooled pulsed laser photolysis experiments is 80 K, and the technique suffers from the problem noted for flow tube experiments on ion + neutral reactions, viz. freezing out of reactants or precursors on the cold walls of the reaction cell or the pipes leading into the cell. The CRESU technique has been applied to neutral + neutral reactions by Smith and co-workers to overcome this problem. A diagram of the apparatus is shown in Fig. 3.3. Temperatures as low as 13 K have been obtained. An alternative approach is to introduce the gas mixture into the nozzle via a pulsed valve. This is less demanding on the pumping capacity, but produces less stable flows. It is employed in a number of laboratories. Mullen and Smith [55], for example, have studied NH - - NO at temperatures down to 53 K. 

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