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Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed 'roaming,' has been described in the photodissociation of [H.sub.2]CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of C[H.sub.3]CHO to C[H.sub.4] + CO. The C[H.sub.4] product is found to have extreme vibrational excitation, with the vibrational distribution peaked at [approximately equal to] 95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to C[H.sub.4] + CO products is via a roaming pathway where a C[H.sub.3] fragment abstracts an H from HCO. The conventional saddle-point TS pathway to C[H.sub.4] + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the C[H.sub.3] roaming mechanism is attributed to the fact that the C[H.sub.3] + HCO radical asymptote and the TS saddle-point barrier to C[H.sub.4] + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as [H.sub.2]CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory. reaction dynamics | roaming mechanisms | photochemistry | quasiclassical trajectories | transition state

In natural photosynthesis, the two photosystems that operate in series to drive electron transport from water to carbon dioxide are quite similar in structure and function, but operate at widely different potentials. In both systems photochemistry begins by photo-oxidation of a chlorophyll a, but that in photosystem II (PS2) has a 0.7 eV higher midpoint potential than that in photosystem I (PS1), so their electronic structures must be very different. Using reaction centers from [sup.15]N-labeled spinach, these electronic structures are compared by their photochemically induced dynamic nuclear polarization (photo-CIDNP) in magic-angle spinning (MAS) NMR measurements. The results show that the electron spin distribution in PS1, apart from its known delocalization over 2 chlorophyll molecules, reveals no marked disturbance, whereas the pattern of electron spin density distribution in PS2 is inverted in the oxidized radical state. A model for the donor of PS2 is presented explaining the inversion of electron spin density based on a tilt of the axial histidine toward pyrrole ring IV causing [pi]-[pi] overlap of both aromatic systems. photosynthesis | photosystem I | solid-state NMR | electron transfer | redox potential

The photochemistry of isomeric methoxyphenyl chlorides and phosphates were analyzed in different solvents and in the presence of benzene and were found to involve the triplet state. Results revealed that a unitary mechanism is proposed for the photochemistry of the three chloroanisoles and of the corresponding phosphates in polar solvents and involves a heterolytic [S.sub.N]1 path.

A study was conducted about the reactivity of [super -]SC(NH)N[H.sub.2], MeCO[S.sup.-], and PhCO[S.sup.-] toward 1-naphthyl radicals in DMSO. The inhibition experiments of the photoinduced substitution reaction in the presence of radical scavengers and the global quantum yield higher than the unity are evidence of a radical chain mechanism for the substitution reaction by anions [super -]SC(NH)N[H.sub.2] and MeCO[S.sup.-].

During Proterozoic time, Earth experienced two intervals with one or more episodes of low-latitude glaciation, which are probable 'Snowball Earth' events. Although the severity of the historical glaciations is debated, theoretical 'hard Snowball' conditions are associated with the nearly complete shutdown of the hydrological cycle. We show here that, during such long and severe glacial intervals, a weak hydrological cycle coupled with photochemical reactions involving water vapor would give rise to the sustained production of hydrogen peroxide. The photochemical production of hydrogen peroxide has been proposed previously as the primary mechanism for oxidizing the surface of Mars. During a Snowball, hydrogen peroxide could be stored in the ice; it would then be released directly into the ocean and the atmosphere upon melting and could mediate global oxidation events in the aftermath of the Snowball, such as that recorded in the Fe and Mn oxides of the Kalahari Manganese Field, deposited after the Paleoproterozoic low-latitude Makganyene glaciation. Low levels of peroxides and molecular oxygen generated during Archean and earliest Proterozoic non-Snowball glacial intervals could have driven the evolution of oxygen-mediating and -using enzymes and thereby paved the way for the eventual appearance of oxygenic photosynthesis. atmospheric processes | Paleoproterozoic | photochemistry

We examined the response of bacteria and proto- and metazooplankton to photomodified dissolved organic matter (DOM). Sterile filtered water from a eutrophic and a humic lake, that was either exposed to artificial ultraviolet (UV) radiation or kept dark, was added to semicontinuous laboratory microcosms that lasted for 7 weeks. Bacterial production responded positively to photochemical modification of DOM regardless of lake type. Final heterotrophic biomass (bacteria+proto+metazooplankton) was 47 [+ or -] 5 and 37 [+ or -] 5 [micro]g carbon (C) [L.sup.-1] in microcosms with UV-exposed and unaltered eutrophic water DOM and 15 [+ or -] 4 and 11 [+ or -] 2 [micro]g C [L.sup.-1] in microcosms with UV-exposed and unaltered humic water DOM, respectively. For the eutrophic water, there were no significant differences in proto- or metazooplankton biomasses between microcosms receiving UV-exposed or nonexposed DOM. Differences between eutrophic water microcosms were not significant when flagellates, ciliates, cladocerans, and copepods were examined separately. In microcosms with UV-exposed humic water, biomasses of heterotrophic flagellates, rotifers, nauplii, and cladocerans were higher than in those with nonexposed DOM. Higher final metazooplankton biomass following addition of UV-exposed humic water indicates that photochemically modified DOM can be effectively transferred through the microbial loop.

Compact flow reactors are constructed and optimized to perform continuous organic photochemistry on a large scale. The reactors are constructed from commercially available or customized immersion well equipment combined with UV-transparent, solvent-resistant fluoropolymer (FEP) tubing.

A simple and efficient photo switching method to modulate binding of [alpha]-chymotrypsin to a surface-attached inhibitor is demonstrated.

A significant amount of Se-Ph bond cleavage is found in addition to the normal alkyl-SePh bond cleavage, where the ratio is dependent on the wavelength of light, solvent viscosity and the structure of alkyl groups. The synthetic efficiency of the reactions of organoselenium compounds are enhanced by selecting suitable reaction conditions.

Kelvin probe force microscopy is used for the cross-sectional potential imaging of a working organic thin-film transistor (OTFT). It is shown that the experiments have succeeded in visualizing an induced charge or its related features.

The disintegration that usually accompanies photodimerization can be prevented by controlling the nanoscale morphology of organic molecular crystals. The ability of the organic nanorods to survive the crystal-to-crystal transition is due to their nanoscale dimensions, which maximize the surface-to-volume ratio and provide a way to relieve the crystal strain induced by the photoreaction.

A combined ultrafast time-resolved study on photophysical process of two -hydroxyphenacyl (pHP) caged compound pHP diethyl phosphate (HPDP) and pHP acetate (HPA) in neat acetonitrile (MeCN) and H(sub 2)O/MeCN mixed solvent is reported. The strong dependence of the deprotection on the leaving group ability suggests a heterolysis cleavage mechanism may be involved in the deprotection mechanism.

An investigation of photonic control of photoinduced electron transfer, based on a dimethyldihydropyrene (DHP) porphyrin (P) fullerene (C60) molecular traid, is presented. The DHP form of the triad porphyrin excitation leads to formation of a DHP-P(super .+)-C60- state with a quantum yield of ninety-eight percent.

The study conducted to develop a five-step, atom-efficient synthesis of the Galanthan tetracyclic skeleton, is presented. Several approaches to B-ring construction using radical and photochemical cyclizations are discussed.

The S0, T1, and S1 potential energy surfaces for the HCOOH dissociation and isomerization processes are depicted with different ab initio methods. The wavelength-dependent mechanism for the HCOOH photodissociation is elucidated through the computed potential energy surfaces and the surface crossing points.

Results demonstrate that physiologically useful DNA-protein adducts can be produced via the oxidation of guanine by the flash-quench technique. Data indicate that the method is versatile and can be extended to cross-linking proteins that are only transiently associated with DNA. The cross-linking yield depends on protein and the photosensitizer.

Four strategies for asymmetric induction in organic photochemistry that derive their success from the use of a confined reaction cavity is explored. Examination of the zeolite interior suggests that the most likely factor responsible for the change in diastereomeric excess(de) or enantiomeric excess(ee) is the difference in conformational preference for the reactant molecules.

An in-depth characterization of optical properties of an organic laser dye, C28H20N2O2B2F4H2O, is presented. For evaluating laser and optical properties of organic dyes, both absorption and scattering must be considered.

Studies done on photochemical decomposition of 3,3-dimethyldiazirine (DMD) in pentane by using high-level ab initio calculations in conjunction with the 6-31G and cc-pvdz basis sets are presented. The calculated potential energy barrier height for dimethylcarbene to propene isomerization agrees well with the observed activation energy and no evidence is found for rearrangement in the first singlet excited state of DMD.

A dynamic supramolecular photochirogenesis using Bovine serum albumin (BSA), employing 2-anthracenecarboxylate (AC) as a prochiral substrate is proposed. The complexion behavior of AC by BSA, and subsequently performed enantiodifferentiating photoclodimerization is examined.