Photophysics of 3-hydroxyflavone in supercritical CO2: a probe to study the microenvironment of SCF

The excitation of 3-hydroxyflavone (3HF) to its second excited singlet state (S2) gives rise to dual fluorescence in supercritical carbon dioxide. The ultraviolet fluorescence originated from the S2 state of 3HF is well separated from the green emission emanating from the tautomeric form, produced via the excited state intramolecular proton transfer. The relative intensity of the S2 to the tautomer fluorescence (S2/T) has been studied as a function of pressure and temperature. It is shown that this ratio reflects the microheterogeneity of the supercritical CO2, and confirms the value of fluorometric probes in disclosing the microscopic properties of supercritical fluids.http://www.sciencedirect.com/science/article/B6TFN-4BVP7G9-3/1/02dd61c567fe3e9c8d6ac86a01f79ce

New Halogenated Phenylbacteriochlorins and Their Efficiency in Singlet-Oxygen Sensitization

Halogenated phenylbacteriochlorins are synthesized with high yields in a two-step procedure. They have strong absorbances in the red and are very stable to air and light at room temperature. Flash photolysis measurements show that the triplet states of these bacteriochlorins have 30 μs lifetimes in deaerated toluene, that are quenched with diffusion-controlled rate constants by molecular oxygen. Time-resolved photoacoustic measurements, with nanosecond and nanocalorie resolution, show that these bacteriochlorins sensitize the formation of singlet oxygen with nearly unity quantum yield. However, singlet-oxygen phosphorescence measurements indicate that physical quenching occurs before the singlet-oxygen molecules diffuse into solution, and nearly half of the sensitized singlet states are lost

Understanding Chemical Reactivity: The Case for Atom, Proton and Methyl Transfers

The concept of ldquochemical reactivityrdquo assumes that atoms and molecules contain the necessary information to describe their evolution over time as they transform from reactants to products. This concept was useful in the past to rationalize reactivity trends and predict the behavior of new systems. Free-energy relationships have played a central role in this field. However, electronic effects often counter the energetic effects and give rise to ldquoanomaliesrdquo or separate correlations. We discuss a quantification of the concept of ldquochemical reactivityrdquo, emphasizing the role of molecular and electronic factors in Chemistry

Tunnelling in low-temperature hydrogen-atom and proton transfers

The reaction path of the interacting-state model with the Lippincott-Schroeder potential for hydrogen bonds, is used in transition-state theory calculations with the semiclassical correction for tunnelling (LS-ISM/scTST) to estimate proton and hydrogen-atom transfer rates at low temperatures. Down to 100 K, the semiclassical correction leads to semi-empirical rates and isotope effects that are in good agreement with the thermal tautomerism of porphine, and the excited-state tautomerisms of salicylideneanilines and 2-(2'-hydroxyphenyl)benzoxazole. For lower temperatures, the tunnelling corrections become extremely high and unreliable. It is shown that the permeability of an Eckart barrier fitted to the curvature of the LS-ISM reaction path leads to good estimates of these reaction rates down to 2 K.http://www.sciencedirect.com/science/article/B6TGS-4HM82TC-1/1/4f3417d1e9b73a409dac8ef57fa0d35

Absolute Rate Calculations. Proton Transfers in Solution

The reaction path of the intersecting-state model is used in transition-state theory with the semiclassical correction for tunneling (ISM/scTST) to calculate the rates of proton-transfer reactions from hydrogen-bond energies, reaction energies, electrophilicity indices, bond lengths, and vibration frequencies of the reactive bonds. ISM/scTST calculations do not involve adjustable parameters. The calculated proton-transfer rates are within 1 order of magnitude of the experimental ones at room temperature, and cover very diverse systems, such as deprotonations of nitroalkanes, ketones, HCN, carboxylic acids, and excited naphthols. The calculated temperature dependencies and kinetic isotope effects are also in good agreement with the experimental data. These calculations elucidate the roles of the reaction energy, electrophilicity, structural parameters, hydrogen bonds, tunneling, and solvent in the reactivity of acids and bases. The efficiency of the method makes it possible to run absolute rate calculations through the Internet

(More) About biphenyl first excited triplet state energy

Biphenyl photophysics was extensively studied in the past and much attention was given to the measurement of its first excited triplet state energy. Phosphorescence and the corresponding T1 <-- S0 absorption give triplet energies that differ by 10 kcal mol-1. We revisited biphenyl photophysics using photoacoustic calorimetry (PAC), a technique that directly measures the thermochemistry and kinetics of short-lived transients in solution. PAC measurements give the relaxed biphenyl triplet energy of 67.8 ± 0.6 kcal mol-1; 2.3 kcal mol-1 higher than the spectroscopic value. The direct measurement of the biphenyl relaxed triplet energy reported in this work should resolve the continued misuse of its spectroscopic energy, in particular in the growing field of polyphenylene polymers.http://www.sciencedirect.com/science/article/B6TGY-4JTXCSD-B/1/ca745f27500ae8609414c7d3a5d2e5d

Hydrogen-atom abstractions: a semi-empirical approach to reaction energetics, bond lengths and bond-orders

We propose the use of the Intersecting-State Model (ISM) to estimate activation barriers and reactive bond distances for reactions involving the transfer of hydrogen atoms. The method is used in a variety of systems with transition states of the (H)C–H–C(H), N–H–C(H), O–H–C(H), S–H–C(H), Si–H–C, Si–H–Si, Sn–H–C and Ge–H–C types. Hydrogen abstractions by halogen atoms are also investigated. Results are compared with available experimental, semi-empirical or ab initio data. Other transition state types (such as O–H–O) which cannot be properly rationalized in the light of an elementary bond-breaking/bond-forming process are also analyzed.Junta Nacional de Investigação Científica; PRAXIS/2/2.1/QUI/390/94

Fluorescence from the second excited singlet state of 3-hydroxyflavone in supercritical CO2

Fluorescence from the second excited singlet state (S2) of 3-hydroxyflavone (3HF) has been observed for the first time in supercritical carbon dioxide (sc-CO2) environment. Steady-state experiments reveal that the intramolecular proton transfer is less effective from the S2 state of 3HF compared to that from the S1 state.http://www.sciencedirect.com/science/article/B6TFN-4BVP7G9-5/1/fe6fe6a663b47e1a622241a93ef55a8

Absolute Rate Calculations for Atom Abstractions by Radicals: Energetic, Structural and Electronic Factors

We calculate transition-state energies of atom-transfer reactions from reaction energies, electrophilicity indices, bond lengths, and vibration frequencies of the reactive bonds. Our calculations do not involve adjustable parameters and uncover new patterns of reactivity. The generality of our model is demonstrated comparing the vibrationally adiabatic barriers obtained for 100 hydrogen-atom transfers with the corresponding experimental activation energies, after correction for the heat capacities of reactants and transition state. The rates of half of these reactions are calculated using the Transition-State Theory with the vibrationally adiabatic path of the Intersecting-State Model and the semiclassical correction for tunneling (ISM/scTST). The calculated rates are within an order of magnitude of the experimental ones at room temperature. The temperature dependencies and kinetic isotope effects of selected systems are also in good agreement with the available experimental data. Our model elucidates the roles of the reaction energy, electrophilicity, structural parameters, and tunneling in the reactivity of these systems and can be applied to make quantitative predictions for new systems

Photon momentum transfer at water/air interfaces under total internal reflection

The transfer of photon momentum at the water/air interface is important for optical manipulation of minute particles and is at the heart of the Minkowski–Abraham controversy.Weuse photoacoustic (PA) detection of ultrasound waves generated when pulsed laser light meets the water/air interface at 3.9 °C(zero thermal expansion), to distinguish momentum transfer from thermoelastic effects. The PA waves dependence on the angle of incidence reveals that momentum transfer maximizes at the critical angle. Momentum transfer is most efficient when the photons travel in water and remain in water after total reflection at the interface, rather than when they cross the interface between dielectric media