A two-color laser photolysis method for determining reaction rates of short-lived intermediates by product analysis: application to the o-quinodimethane problem

A time-delayed, two-color pulse laser photolysis technique was used for a kinetic study of short-lived transient species through product analysis, the determination of the rate constant of the cycloaddition of o-quinodimethane (1) and maleic anhydride (2) in room-temperature solutions. o-Quinodimethane (1) was generated from 1,2-bis[(phenylseleno)methyl]benzene (3) by the irradiation of a pulse of a KrF excimer laser (248 nm) in the presence of excess 2, and a successive pulse of a XeCl excimer laser (308 nm) was irradiated to the reaction mixture after varied delay times from 0 to 0.1 s for the decomposition of the remaining 1 to quench the cycloaddition reaction. The rate constant of the cycloaddition of 1 and 2 was 2.1 x 10(5) M(-1) s(-1), which was obtained by the analysis of the delay-time dependence of the product yields.     

Determination of selenium by the ring oven technique

Summary A simple semi-quantitative fluorometric method for selenium analysis has been developed which employs the reagent 2,3-diaminonaphthalene and the ring oven technique. By this procedure selenium can be determined to 10 ppb; the optimum working range is 20–80 ppb. The concentration of selenium is ascertained by visual observation of the rings under long wavelength UV light.

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Zusammenfassung Eine halbquantitative Methode zur fluorimetrischen Selenbestimmung mit dem Ringofen und mit 2,3-Diaminonaphthalin als Reagens wurde ausgearbeitet. 10 ppb Se sind bestimmbar; das beste Arbeitsbereich liegt zwischen 20 und 80 ppb. Die Selenmengen werden durch visuelle Beurteilung der Ringe im UV ermittelt.

     

Selective, room-temperature transformation of methane to C1 oxygenates by deep UV photolysis over zeolites

Methane can directly be transformed into liquid C(1) oxygenated products with selectivities above 95% at 13% conversion by deep UV photocatalysis, in the presence of H(2)O and air. Pure silica zeolites, and more specifically, beta zeolite with a large number of internal silanol groups is active and selective, while amorphous silica with no micropores is much less efficient. Irradiation produces the homolytic cleavage of surface hydroxyl groups, leading to silyloxyl radicals that will generate methyl radicals from methane. The selectivity arises from the occurrence of the reaction in a confined space restricting the mobility of the radical intermediates that will be mostly attached to the solid surface. Energy consumption of the process is in the order of 7.2 Gcal × mol(-1) that compares very favorably with the energy required for transforming methane to synthesis gas (15.96 Gcal × mol(-1)).     

Higher triplet excited states of benzophenones and bimolecular triplet energy transfer measured by using nanosecond-picosecond two-color/two-laser flash photolysis

The lifetimes of benzophenone in the higher triplet excited state (BP(T(n))) and several BP derivatives in the T(n) states were measured directly to be tau(T(n))=37+/-7 ps and 20-33 ps, respectively, by using the nanosecond-picosecond (ns-ps) two-color/two-laser flash photolysis method. Based on the direct measurements of tau(T(n)) of BP(T(n)), the triplet energy transfer (TET) from BP(T(n)) to quenchers (Q), such as carbon tetrachloride (CCl4), benzene (Bz), and p-dichlorbenzene (DCB), was investigated. The fast TET from BP(T(n)) to Q can be attributed to the lifetime-dependent quenching process, according to the Ware theoretical model of the bimolecular energy transfer reaction. The contribution of the lifetime-dependent term on k(TET) was 27, 60, and 86% for CCl4, Bz, and DCB as the Q of BP(T(n)), respectively, indicating that the TET from BP(T(n)) to Q is influenced not only by tau(T(n)), but also by the size of Q.     

Microdetermination of caffeine using the ring oven technique

A rapid and highly selective method for the microdetermination of caffeine of special interest for air pollution studies is presented. With the ring oven technique and with solutions of acetylacetone in sodium hydroxide and p-dimethylaminobenzaldehyde in hydrochloric acid, as little as 0.5 μg caffeine on the ring can be determined with an average error of 3%. Compounds containing purine bases which would be likely to give the same color reaction as caffeine do not interfere. Of the organic air pollutants which might be collected from the atmosphere during thesampling for caffeine, benzo(α)pyrene and formic acid do not interfere. Interferences from formaldehyde and urea are easily eliminated.     

One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.     

Epoxide formation by ring closure of the cinnamyloxy radical

[formula: see text] The cinnamyloxy and oxiranyl benzyl radicals were generated by photolysis of alkyl 4-nitrobenzenesulfenates. The yet unprecedented epoxide ring formation from a primary alkoxy radical was observed. Experimental evidence supports the fact that the mode of ring opening of the oxiranyl carbinyl radical system is thermodynamically driven. B3LYP/6-31G* calculations indicate that the closed form of the radical is approximately 5 kcal/mol more stable than the open one.     

Determination of carbaryl in foods by solid-phase room-temperature phosphorimetry

A phosphorimetric solid phase assay is proposed for the determination of the pesticide carbaryl (CBL) at room temperature. CBL was spotted on filter paper together with Tl(I) as heavy metal, and dried for 3 min, after which the diffuse transmitted phosphorescence was measured using two quartz plates to avoid the quenching effect produced by atmospheric oxygen. The linear dynamic range was 0.5–4.0 μg/mL and the detection and quantification limits were 0.09 and 0.31 μg/mL, respectively. The precision of the method, expressed as relative standard deviation, was 2.3% for a sample containing 2.0 μg/mL of CBL. The method was applied to the determination of CBL residues in cereals, potatoes and waters, obtaining recoveries ranging between 92 and 105%.     

Determination of carbaryl in foods by solid-phase room-temperature phosphorimetry

A phosphorimetric solid phase assay is proposed for the determination of the pesticide carbaryl (CBL) at room temperature. CBL was spotted on filter paper together with Tl(I) as heavy metal, and dried for 3 min, after which the diffuse transmitted phosphorescence was measured using two quartz plates to avoid the quenching effect produced by atmospheric oxygen. The linear dynamic range was 0.5–4.0 μg/mL and the detection and quantification limits were 0.09 and 0.31 μg/mL, respectively. The precision of the method, expressed as relative standard deviation, was 2.3% for a sample containing 2.0 μg/mL of CBL. The method was applied to the determination of CBL residues in cereals, potatoes and waters, obtaining recoveries ranging between 92 and 105%. Received: 10 October 1997 / Revised: 2 February 1998 / Accepted: 7 February 1998     

The mechanism of ring contraction in the photolysis of o-quinonediazides

Conclusions In the photochemical rearrangement of 1,2-naphthoquinonediazide-2-(1-¹³C) in water with added acetic acid, indene-3-carboxylic acid if formed, which contains the label only in the carboxylic acid group.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 943–945, April, 1976.     

Determination of iodine by atomic-absorption spectrometry using the platinum-loop technique

The use of a platinum-loop atomizer and an electrodeless discharge lamp has been applied to the atomic-absorption determination of mercury at the 183.0-nm resonance line. Oxygen was flushed from the light-path with argon, giving adequate readiation transmission characteristics and a sensitivity of 9 ppm of iodine for 1% absorption. The method is prone to both cationic and anionic interferences, but the use of cation-exchange and solvent-extraction procedures overcomes some of the problems.     

Nonequilibrium nuclear polarization in the photolysis of aniline solutions

Conclusions Negative polarization of the protons of aniline and other compounds is observed upon the photoexcitation of aniline solutions. This effect is a consequence of optical nuclear polarization in short-lived donor-acceptor complexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1890–1892, August, 1987.     

Probing the primary event in the photocycle of photoactive yellow protein using photochemical hole-burning technique

Photochemical hole-burning spectroscopy was used to study the excited-state electronic structure of the 4-hydroxycinnamyl chromophore in photoactive yellow protein (PYP). This system is known to undergo a trans-to-cis isomerization process on a femtosecond-to-picosecond time scale, similar to membrane-bound rhodopsins, and is characterized by a broad featureless absorbance at 446 nm. Resolved vibronic structure was observed for the hole-burned spectra obtained when PYP in phosphate buffer at pH 7 was frozen at low temperature and irradiated with narrow bandwidth laser light at 431 nm. The approximate homogeneous width of 752 cm-1 could be calculated from the deconvolution of the hole-burned spectra leading to an estimated dephasing time of approximately 14 fs for the PYP excited-state structure. The resolved vibronic structure also enabled us to obtain an estimated change in the C=C stretching frequency, from 1663 cm-1 in the ground state to approximately 1429 cm-1 upon photoexcitation. The results obtained allowed us to speculate about the excited-state structure of PYP. We discuss the data for PYP in relation to the excited-state model proposed for the photosynthetic membrane protein bacteriorhodopsin, and use it to explain the primary event in the function of photoactive biological protein systems. Photoexcitation was also carried out at 475 nm. The vibronic structure obtained was quite different both in terms of the frequencies and Franck-Condon envelope. The origin of this spectrum was tentatively assigned.     

Determination of clodronate in aqueous solutions by HPLC using postcolumn derivatisation

A method for the determination of clodronate at ppm levels in aqueous solutions by HPLC is presented. Clodronate is first separated by anion exchange chromatography using a solution of nitric acid and sodium nitrate as eluent. Th-EDTA-XO mixed ligand complex is used as a postcolumn reagent for clodronate. Clodronate reacts quantitatively with this complex in slightly acid solutions, and a change in the absorbance of Th-EDTA-XO is used as a measure for the clodronate concentration. The detection limit is about 3 ppm, and the linear dynamic range of the method extends up to about 200 ppm. The RSD values varied between 5 and 16%. The effect of pH on appearance and retention of different forms of clodronate is studied. Different forms of clodronate can be separated and detected in the used anion-exchange system.     

Direct photolysis of chlorophenols in aqueous solutions by UV radiation from excilamps

Direct photolysis of 2- and 4-chlorophenol and 2,4-dichlorophenol in aqueous solutions by UV radiation from XeBr (282 nm) and KrCl (222 nm) excilamps at various pH values was subjected to a comparative study.     

Ultrafast bidirectional dihydroazulene/vinylheptafulvene (DHA/VHF) molecular switches: photochemical ring closure of vinylheptafulvene proven by a two-pulse experiment

The photochromic couple dihydroazulene/vinylheptafulvene (DHA/VHF) is an established system for molecular switching. The photoinduced ring opening of the thermally stable dihydroazulene proceeds in the picosecond time regime with subsequent rapid relaxation to the electronic ground state. So far it was not possible to reverse the reaction photochemically. It was always found that the back reaction proceeds thermally on a longer time scale. In the case of the cyanophenyl-DHA derivative utilized in the present study, this is accounted to s-cis/s-trans isomerization of the primarily formed s-cis VHF. Since this particular isomerization takes much longer than nanoseconds, we now successfully applied a second visible femtosecond pulse subsequent to the initial UV pulse (25 ps delay) achieving ring closure of the primarily formed s-cis VHF. The now bidirectional photoswitching was monitored by the changes in the cw spectrum. As a result, the DHA/VHF system is found to be a multifold switchable system by itself: it is both a very fast photoreversible switch and a photochemical/thermal switch with a thermal lock mode.     

The role of the beta-ionone ring in the photochemical reaction of rhodopsin

Time-dependent density functional theory (TDDFT) calculations on the photoabsorption process of the 11-cis retinal protonated Schiff base (PSB) chromophore show that the Franck-Condon relaxation of the first excited state of the chromophore involves a torsional twist motion of the beta-ionone ring relative to the conjugated retinyl chain. For the ground state, the beta-ionone ring and the retinyl chain of the free retinal PSB chromophore form a -40 degrees dihedral angle as compared to -94 degrees for the first excited state. The double bonds of the retinal are shorter for the fully optimized structure of the excited state than for the ground state suggesting a higher cis-trans isomerization barrier for the excited state than for the ground state. According to the present TDDFT calculations, the excitation of the retinal PSB chromophore does not primarily lead to a reaction along the cis-trans torsional coordinate at the C11-C12 bond. The activation of the isomerization center seems to occur at a later stage of the photo reaction. The results obtained at the TDDFT level are supported by second-order M?ller-Plesset (MP2) and approximate singles and doubles-coupled cluster (CC2) calculations on retinal chromophore models; the MP2 and CC2 calculations yield for them qualitatively the same ground state and excited-state structures as obtained in the density functional theory and TDDFT calculations.