Spectral kinetic characteristics of the photoisomerization products of naphthylmethylideneiminospironaphthopyran induced by photolysis at different wavelengths

The spectral kinetic characteristics of intermediates generated by photolysis with light at the wavelengths 337 and 430 or 470 nm of the photobifunctional compound (PBC), 1,3-dihydro-5-(2-hydroxy-1-naphthylmethylideneimino)-1,3,3-trimethylspiro[2H-indole-2,3-[3H]-naphtho[2,1-b]pyran], whose molecule combines the spironaphthopyran and hydroxyazomethine fragments, and also parameters of model compounds, viz., naphthylmethylideneimine and spironaphthopyran, were studied in methanol and toluene. The relative quantum yields of formation of different intermediates of PBC were measured relatively to model compounds, namely, trans-keto isomer formed due to cis-trans-isomerization and prototropic equiliration in the azomethine fragment, and in the merocyanine form generated by spiro bond opening. It was found that the photolysis of the PBC with light at the wavelengths ?? = 430 or 470 nm nearly no produces the merocyanine form, whereas the relative yield of the trans-keto tautomer is ??0.6. For PBC photolysis at ?? = 337 nm, the yield of the merocyanine form is ??0.2 and the yield of the trans-keto isomer decreases substantially (??0.2). The solvent nature affects the kinetic behavior of the system. The consistency of the isomerization and proton transfer processes is discussed.     

Triplet states of humic acids studied by laser flash photolysis using different excitation wavelengths

The spectra and kinetics of short-lived intermediates formed from aqueous (0.1 N NaOH) solutions of the natural mixture of humic and fulvic acids (HFA) were studied by laser flash photolysis using excitation wavelengths of 337, 390, 470, and 520 nm. Laser photolysis of HFA with light of 520 and 470 nm results in the formation of triplet excited states (T_HFA) characterized by the broad absorption spectrum with a maximum near 630 nm and lifetimes of 0.15 ms in deoxygenated solutions. The formation of two types of T_HFA with lifetimes of 0.1 and 2 ms and absorption spectra with maxima at 570 nm is observed under photolysis with light of 337 and 390 nm. The estimation of quantum yields of T_HFA gives 1 and 0.3% under photolysis with excitation wavelengths of 337 and 520 nm, respectively. The rate constants of T_HFA quenching by molecular oxygen are equal to (7—8)·10⁸ L mol^–1 s^–1.     

Characterization of the transient species generated by the photoionization of Berberine: A laser flash photolysis study

Using 266 nm laser flash photolysis it has been demonstrated that Berberine (BBR) in aqueous solution is ionized via a mono-photonic process giving a hydrated electron, anion radical that formed by hydrated electron react with steady state of BBR, and neutral radical that formed from rapid deprotonation of the radical cation of BBR. The quantum yield of photoionization is determined to be 0.03 at room temperature with KI solution used as a reference. Furthermore utilizing pH changing method and the SO₄⁻ radical oxidation method, the assignment of radical cation of BBR was further confirmed, the pK_a value of it was calculated, and the related set up rate constant was also determined.     

Picosecond transient absorption spectral and kinetic study on benzophenone microcrystals by diffuse reflectance laser photolysis method

The absorption spectrum and formation dynamics of triplet benzophenone in microcrystals were measured for the first time by analyzing diffuse reflected spectra of the picosecond continuum. The absorption spectral shape is similar to that of 2-propanol solution, and no appreciable difference was observed in their rise curves.     

Study of spectral and kinetic characteristics of products of photolysis of a trifunctional compound with light of different wavelengths

Spectral and kinetic characteristics of intermediate products of photolysis of a novel phototrifunctional compound (PTC), whose molecule combines spironaphthoxazine, hydroxyazomethine, and azobenzene fragments, have been studied by microsecond flash photolysis with UV and visible light in toluene and methanol solutions. Three products of PTC photolysis have been detected. It has been found that the efficiency of the photoprocesses substantially depends on the solvent and the wavelength of excitation light.     

Characterization of transient species in laser photolysis of aromatic amino acids using acetone as photosensitizer

The photochemical processes of aromatic amino acids were investigated in aqueous solution using acetone as photosensitizer by KrF (248 nm) laser flash photolysis. Laser-induced transient species were characterized according to kinetic analysis and quenching experiments. The intermediates recorded were assigned to the excited triplet state of tryptophan, the radicals of tryptophan and tyrosine. The excited triplet state of tryptophan produced via a triplet-triplet excitation transfer and the radicals arising from electron transfer reaction has been identified. Neither electron transfer nor energy transfer between triplet acetone and phenylalanine can occur in photolysis of phenylalanine aqueous solution which contains acetone. Furthermore, triplet acetone-induced radical transformation: Trp/N-Tyr→Trp-Tyr/O was observed directly in photolysis of dipeptide (Trp-Tyr) aqueous solution containing acetone, and the transformation resulting from intramolecular electron transfer was suggested.     

Magnetic resonance raman optical activity of ferrocytochrome e at different laser wavelengths

Magnetic field induced resonance Raman circular intensity differential spectra are reported for the two argon ion laser wavelengths (514.5 and 501.7 nm) that fall within the β electronic absorption band of ferrocytochrome c. The signs and magnitudes of the effects are completely different in the two sets of spectra, and might be related to the slope of the corresponding magnetic circular dichroism band envelope at the excitation wavelength.     

Direct detection of a transient oxenium ion in water generated by laser flash photolysis

Laser flash photolysis of the quinol ester 2b in O2-saturated aqueous phosphate buffer at pH 7.1 with excitation at 266 nm generates a transient intermediate with lambda(max) 460 nm that decays in a first-order manner with an aqueous solution lifetime of (170 +/- 10) ns at 22 degrees C. This intermediate is not affected by O2, but reacts rapidly with N3- with an apparently diffusion-limited rate constant of (6.6 +/- 0.2) x 10(9) M-1 s-1. Steady state photolysis of 2b yields the quinol 3b as a major reaction product with a yield of ca. 30-35% after correction for photolytic decomposition of 3b. This is the same product that is quantitatively produced by hydrolysis of 2b in the dark. Photolysis of 2b in the presence of 40 mM N3- completely suppresses the yield of 3b The photolytic intermediate is identified as the aryloxenium ion 1b, that was previously indirectly detected by N3--trapping during the hydrolysis of 2b, based on the chemical behavior of the transient and the quantitative agreement of the N3-/solvent selectivity ratio, kaz/ks, measured directly during the flash photolysis experiment, and indirectly by the azide clock procedure during the hydrolysis reaction. Other, as of yet unidentified, transients are produced during the photolysis reaction. A strong transient absorbance band observed at 360 nm decays in a biphasic manner with two first-order rate constants, neither of which are affected by O2 or N3-. The lifetimes of the two intermediates of ca. 12 and 75 mus are considerably longer than that of 1b. Another very short-lived species can be detected at early reaction times (相似文献   

Studies of LA-ICP-MS on quartz glasses at different wavelengths of a Nd:YAG laser

The capability of LA-ICP-MS for determination of trace impurities in transparent quartz glasses was investigated. Due to low or completely lacking absorption of laser radiation, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) proves difficult on transparent solids, and in particular the quantification of measurement results is problematic in these circumstances. Quartz glass reference materials of various compositions were studied by using a Nd:YAG laser system with focused laser radiation of wavelengths of 1064 nm, 532 nm and 266 nm, and an ICP-QMS (Elan 6000, Perkin Elmer). The influence of ICP and laser ablation conditions in the analysis of quartz glasses of different compositions was investigated, with the laser power density in the region of interaction between laser radiation and solid surface determining the ablation process. The trace element concentration was determined via calibration curves recorded with the aid of quartz glass reference materials. Under optimized measuring conditions the correlation coefficients of the calibration curves are in the range of 0.9–1. The relative sensitivity factors of the trace elements determined in the quartz glass matrix are 0.1–10 for most of the trace elements studied by LA-ICP-MS. The detection limits of the trace elements in quartz glass are in the low ng/g to pg/g range.     

Studies of LA-ICP-MS on quartz glasses at different wavelengths of a Nd:YAG laser

The capability of LA-ICP-MS for determination of trace impurities in transparent quartz glasses was investigated. Due to low or completely lacking absorption of laser radiation, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) proves difficult on transparent solids, and in particular the quantification of measurement results is problematic in these circumstances. Quartz glass reference materials of various compositions were studied by using a Nd:YAG laser system with focused laser radiation of wavelengths of 1064 nm, 532 nm and 266 nm, and an ICP-QMS (Elan 6000, Perkin Elmer). The influence of ICP and laser ablation conditions in the analysis of quartz glasses of different compositions was investigated, with the laser power density in the region of interaction between laser radiation and solid surface determining the ablation process. The trace element concentration was determined via calibration curves recorded with the aid of quartz glass reference materials. Under optimized measuring conditions the correlation coefficients of the calibration curves are in the range of 0.9-1. The relative sensitivity factors of the trace elements determined in the quartz glass matrix are 0.1-10 for most of the trace elements studied by LA-ICP-MS. The detection limits of the trace elements in quartz glass are in the low ng/g to pg/g range.     

A kinetic study of chlorine radical reactions with ketones by laser photolysis technique

Rate coefficients for reactions between Cl radicals and four ketones were determined at 294 ± 1 K with a relative rate method using a laser photolysis technique. The experiments were conducted in synthetic air in a flow system at atmospheric pressure. A mixture of Cl₂/ClONO₂ was photolyzed and the formation of NO₃ through the reaction Cl + ClONO₂ → Cl₂ + NO₃ was measured with and without ketones in the reaction mixture. The NO₃ radical concentration was measured by optical absorption using a diode laser as the light source. The rate coefficients for the Cl-ketone reactions could then be evaluated. The following rate coefficients were obtained (in units of cm³ molecule⁻¹ s⁻¹): cyclohexanone (7.00 ± 1.15) × 10⁻¹¹; cyclopentanone (4.76 ± 0.33) × 10⁻¹¹; acetone (1.69 ± 0.32) × 10⁻¹²; and 2,3-butanedione (7.62 ± 1.66) × 10⁻¹³. The accuracy of the method employed was tested by using the well-studied reaction between Cl and methane and a rate coefficient of (9.37 ± 1.04) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ was obtained, which is in good agreement with previous work. The errors are at the 95% confidence level. The results in this work indicate that a carbonyl group in a ketone lowers the reactivity towards α-hydrogen abstraction by Cl radicals, compared to the corresponding Cl-alkane reactions. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 195–201, 1997.     

Vibrattonal excitation of molecules by collisions with “hot” hydrogen atoms from laser photolysis

Excimer laser (ArF) photolysis of diatomic and triatomic hydrides produces hydrogen atoms with translational energies in excess of 15000 cm^?1 per atom. Subsequent collisions of these “hot” atoms with CO₂ and N₂O produces vibrationally excited molecules which can be detected by their characteristic infrared emission.     

A theoretical study of the isotope separation of Li2 using different laser wavelengths for the excitation and ionization processes

Cross sections of the sequential two-photon ionization of Li₂ are evaluated starting from ab initio CI wavefunctions and using different laser wavelengths for the excitation and ionization processes. In agreement with experimental results and previous calculations, a substantial isotope separation is found for the three isotopes of Li₂, and a wide spectrum of wavelengths useful for effective isotope enrichment is suggested. Even in with an overcrowded spectrum, the fundamental role of Franck-Condon factors and thermal populations in defining the characteristic resonances of the fractionation process is confirmed. This result suggests a practical way to predict transitions useful for isotope enrichment.     

Transient species of fullerenes studied with pulse radiolysis and laser photolysis

The excited triplet of C60 or C70 is generated via either direct excitation by laser light or energy transfer from excited states of solvent to C60 and C70. The cation radical of C60 is produced either via hole transfer from cation radical of CCl4 to C60 or via electron transfer from excited triplet of C60 to CCl4. C60 and C70 could be added to trichloromethyl radical to produce adduct radicals with different mechanisms.     

On the long-lived transient absorption observed in nanosecond laser photolysis studies of two polymethine cyanine dyes

It is shown that the long-lived transient absorption which is observed when solutions of cryptocyanine and DDI (1, 1′-diethyl-2, 2′-dicarbocyanine iodide) in methanol and other alcohols are exposed to nanosecond ruby laser pulses arises from a photoproduct whose formation requires consecutive absorption of two photons.     

ANN-MATOPT hybrid algorithm: determination of kinetic and non-kinetic parameters in different reaction mechanisms

In this work we have applied the computational methodology based on Artificial Neural Networks (ANN) to the kinetic study of distinct reaction mechanisms to determine different types of parameters. Moreover, the problems of ambiguity or equivalence are analyzed in the set of parameters to determine in different kinetic systems when these parameters are from different natures. The ambiguity in the set of parameters show the possibility of existence of two possible set of parameter values that fit the experimental data. The deterministic analysis is applied to know beforehand if this problem occurs when rate constants of the different stages of the mechanism and the molar absorption coefficients of the species participating in the reaction are obtained together. Through the deterministic analysis we will analyze if a system is identifiable (unique solution or finite number of solutions) or if it is non-identifiable if it possesses infinite solutions. The determination of parameters of different nature can also present problems due to the different magnitude order, so we must analyze in each case the necessity to apply a second method to improve the values obtained through ANN. If necessary, an optimization mathematical method for improving the values of the parameters obtained with ANN will be used. The complete process, ANN and mathematical optimizations constitutes a hybrid algorithm ANN-MATOPT. The procedure will be applied first for the treatment of synthetic data with the purpose of checking the applicability of the method and after, it will be used in the case of experimental kinetic data.

     

Mutual interactions of the methyl and methylperoxy radicals studied by flash photolysis and kinetic spectroscopy

The decadic extinction coefficient of the methyl radical at 216.4 nm and the rate constant for mutual combination were redetermined as: . The application of the Beer–Lambert law to these measurements was justified experimentally. The absorption spectrum of the methylperoxy radical was characterized as a weak, broad, structureless band, having a maximum at 240 nm with ?(240) = 1.55 × 10³ l./mol cm. The mutual interaction of methylperoxy radicals leads to the generation of methoxy and hydroperoxy radicals as a consequence of the nonterminating interaction . Each derivative radical may consume a significant fraction of the methylperoxy radicals, and either of these cross interactions may be made predominant by a suitable choice of oxygen pressure. The mutual interaction was studied under both conditions. The overall mechanism was analyzed by a precise computational method, and the rate constant of the total mutual interaction was estimated as .