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Puerto Serrano, Spain

Kirkby O.M.,University College London | Sala M.,Laboratory Interdisciplinaire Carnot de Bourgogne | Balerdi G.,Unidad University | De Nalda R.,Institute Quimica Fisica Rocasolano | And 3 more authors.
Physical Chemistry Chemical Physics

Femtosecond time-resolved photoelectron spectroscopy experiments have been used to compare the electronic relaxation dynamics of aniline and d7-aniline following photoexcitation in the range 272-238 nm. Together with the results of recent theoretical investigations of the potential energy landscape [M. Sala, O. M. Kirkby, S. Guérin and H. H. Fielding, Phys. Chem. Chem. Phys., 2014, 16, 3122], these experiments allow us to resolve a number of unanswered questions surrounding the nonradiative relaxation mechanism. We find that tunnelling does not play a role in the electronic relaxation dynamics, which is surprising given that tunnelling plays an important role in the electronic relaxation of isoelectronic phenol and in pyrrole. We confirm the existence of two time constants associated with dynamics on the 11πσ∗ surface that we attribute to relaxation through a conical intersection between the 11πσ∗ and 11ππ∗ states and motion on the 11πσ∗ surface. We also present what we believe is the first report of an experimental signature of a 3-state conical intersection involving the 21ππ∗, 11πσ∗ and 11ππ∗ states. This journal is © the Owner Societies. Source

Garcia E.,CSIC - Institute of Ceramics and Glass | Gancedo J.R.,Institute Quimica Fisica Rocasolano | Gracia M.,Institute Quimica Fisica Rocasolano
Materials Characterization

A combination of 57Fe-Mössbauer spectroscopy and X-ray Powder Diffraction analysis has been employed to study modifications in chemical and mechanical stability occurring in a cordierite burner aged under combustion conditions which simulate the working of domestic boilers. Mössbauer study shows that Fe is distributed into the structural sites of the cordierite lattice as Fe2+ and Fe3+ ions located mostly at octahedral sites. Ferric oxide impurities, mainly hematite, are also present in the starting cordierite material accounting for ≅40% of the total iron phases. From Mössbauer and X-ray diffraction data it can be deduced that, under the combustion conditions used, new crystalline phases were formed, some of the substitutional Fe3+ ions existing in the cordierite lattice were reduced to Fe2+, and ferric oxides underwent a sintering process which results in hematite with higher particle size. All these findings were detected in the burner zone located in the proximity of the flame and were related to possible chemical reactions which might explain the observed deterioration of the burner material. © 2010 Elsevier Inc. Source

Velez E.,University of Medellin | Ruiz P.,National University of Colombia | Quijano J.,National University of Colombia | Notario R.,Institute Quimica Fisica Rocasolano
International Journal of Chemical Kinetics

The gas-phase elimination reaction of ethyl (5-cyanomethyl-1,3,4-thiadiazol-2-yl)carbamate has been studied computationally at the MP2/6–31++G(2d,p) level of theory. The values of the activation parameters and rate constants for the thermal decomposition were evaluated over a temperature range from 405.0 to 458.0 K. The temperature dependence of the rate constants was used to deduce the modified Arrhenius expression: log k405–458 K = (9.01 ± 0.49) + (1.32 ± 0.16) log T – (6946 ± 30) 1/T, which is in good agreement with the expression obtained from experimental data. The results confirm that the mechanism is a cis-concerted elimination that occurs in two steps: The first one corresponds to the formation of ethylene and an intermediate, 5-(cyanomethyl)-1,3,4-thiadiazol-2-yl-carbamic acid, via a six-membered cyclic transition state, and the second one is the decarboxylation of this intermediate via a four-membered cyclic transition step, leading to carbon dioxide and the corresponding 1,3,4-thiadiazole derivative (5-amino-1,3,4-thiadiazole-2-acetonitrile). The connectivity of transition states with their respective minima was verified through intrinsic reaction coordinate calculations, and the progress of the reaction was followed by means of Wiberg bond indices, resulting that both transition states have an “early” character, nearer to the reactants than to the products. © 2015 Wiley Periodicals, Inc. Source

Zoogman P.,Harvard - Smithsonian Center for Astrophysics | Liu X.,Harvard - Smithsonian Center for Astrophysics | Suleiman R.M.,Harvard - Smithsonian Center for Astrophysics | Pennington W.F.,NASA | And 49 more authors.
Journal of Quantitative Spectroscopy and Radiative Transfer

TEMPO was selected in 2012 by NASA as the first Earth Venture Instrument, for launch between 2018 and 2021. It will measure atmospheric pollution for greater North America from space using ultraviolet and visible spectroscopy. TEMPO observes from Mexico City, Cuba, and the Bahamas to the Canadian oil sands, and from the Atlantic to the Pacific, hourly and at high spatial resolution (~2.1. km N/S×4.4. km E/W at 36.5°N, 100°W). TEMPO provides a tropospheric measurement suite that includes the key elements of tropospheric air pollution chemistry, as well as contributing to carbon cycle knowledge. Measurements are made hourly from geostationary (GEO) orbit, to capture the high variability present in the diurnal cycle of emissions and chemistry that are unobservable from current low-Earth orbit (LEO) satellites that measure once per day. The small product spatial footprint resolves pollution sources at sub-urban scale. Together, this temporal and spatial resolution improves emission inventories, monitors population exposure, and enables effective emission-control strategies.TEMPO takes advantage of a commercial GEO host spacecraft to provide a modest cost mission that measures the spectra required to retrieve ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), formaldehyde (H2CO), glyoxal (C2H2O2), bromine monoxide (BrO), IO (iodine monoxide), water vapor, aerosols, cloud parameters, ultraviolet radiation, and foliage properties. TEMPO thus measures the major elements, directly or by proxy, in the tropospheric O3 chemistry cycle. Multi-spectral observations provide sensitivity to O3 in the lowermost troposphere, substantially reducing uncertainty in air quality predictions. TEMPO quantifies and tracks the evolution of aerosol loading. It provides these near-real-time air quality products that will be made publicly available. TEMPO will launch at a prime time to be the North American component of the global geostationary constellation of pollution monitoring together with the European Sentinel-4 (S4) and Korean Geostationary Environment Monitoring Spectrometer (GEMS) instruments. © 2016 Elsevier Ltd. Source

Becerra R.,Institute Quimica Fisica Rocasolano | Cannady J.P.,Dow Corning | Pfrang C.,University of Reading | Walsh R.,University of Reading
Journal of Physical Chemistry A

Time-resolved kinetics studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, were performed to obtain rate coefficients for its bimolecular reaction with 2,5-dihydrofuran (2,5-DHF). The reaction was studied in the gas phase over the pressure range of 1-100 Torr in SF6 bath gas, at five temperatures in the range of 296-598 K. The reaction showed pressure dependences characteristic of a third body assisted association. The second-order rate coefficients obtained by Rice-Ramsperger-Kassel-Marcus (RRKM)-assisted extrapolation to the high-pressure limit at each temperature fitted the following Arrhenius equation where the error limits are single standard deviations: log(k/cm3 molecule-1 s-1) = (-9.96 ± 0.08) + (3.38 ± 0.62 kJ mol-1)/RT ln 10. End-product analysis revealed no GC-identifiable product. Quantum chemical (ab initio) calculations indicate that reaction of SiH2 with 2,5-DHF can occur at both the double bond (to form a silirane) and the O atom (to form a donor-acceptor, zwitterionic complex) via barrierless processes. Further possible reaction steps were explored, of which the only viable one appears to be decomposition of the O-complex to give 1,3-butadiene + silanone, although isomerization of the silirane cannot be completely ruled out. The potential energy surface for SiH2 + 2,5-DHF is consistent with that of SiH2 with Me2O, and with that of SiH2 with cis-but-2-ene, the simplest reference reactions. RRKM calculations incorporating reaction at both π- and O atom sites, can be made to fit the experimental rate coefficient pressure dependence curves at 296-476 K, giving values for k∞(π) and k∞(O) that indicate the latter is larger in magnitude at all temperatures, in contrast to values from individual model reactions. This unexpected result suggests that, in 2,5-DHF with its two different reaction sites, the O atom exerts the more pronounced electrophilic attraction on the approaching silylene. Arrhenius parameters for the individual pathways were obtained. The lack of a fit at 598 K is consistent with decomposition of the O-complex to give 1,3-butadiene + silanone. © 2015 American Chemical Society. Source

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