1,721,036 research outputs found
Simulation of the single-vibronic-level emission spectra of HAsO and DAsO
No full textThe single-vibronic-level (SVL) emission spectra of HAsO and DAsO have been simulated by electronic structure/Franck-Condon factor calculations to confirm the spectral molecular carrier and to investigate the electronic states involved. Various multi-reference (MR) methods, namely, NEVPT2 (n-electron valence state second order perturbation theory), RSPT2-F12 (explicitly correlated Rayleigh-Schrodinger second order perturbation theory), and MRCI-F12 (explicitly correlated multi-reference configuration interaction) were employed to compute the geometries and relative electronic energies for the X˜1A? and A˜1A?? states of HAsO. These are the highest level calculations on these states yet reported. The MRCI-F12 method gives computed T0 (adiabatic transition energy including zero-point energy correction) values, which agree well with the available experimental T0 value much better than previously computed values and values computed with other MR methods in this work. In addition, the potential energy surfaces of the X˜1A? and A˜1A?? states of HAsO were computed using the MRCI-F12 method. Franck-Condon factors between the two states, which include anharmonicity and Duschinsky rotation, were then computed and used to simulate the recently reported SVL emission spectra of HAsO and DAsO [R. Grimminger and D. J. Clouthier, J. Chem. Phys. 135, 184308 (2011)]. Our simulated SVL emission spectra confirm the assignments of the molecular carrier, the electronic states involved, and the vibrational structures observed in the SVL emission spectra but suggest a loss of intensity in the reported experimental spectra at the low emission energy region almost certainly due to a loss of responsivity near the cutoff region (?800 nm) of the detector used. Computed and experimentally derived re (equilibrium) and/or r0 {the (0,0,0) vibrational level} geometries of the two states of HAsO are discussed
A theoretical study of the atmospherically important radical-radical reaction BrO + HO2; the product channel O2(1-Deltag) + HOBr is formed with the highest rate
No full textA theoretical study has been made of the BrO + HO2 reaction, a radical–radical reaction which contributes to ozone depletion in the atmosphere via production of HOBr. Reaction enthalpies, activation energies and mechanisms have been determined for five reaction channels. Also rate coefficients have been calculated, in the atmospherically important temperature range 200–400 K, for the two channels with the lowest activation energies, both of which produce HOBr: (R1a) HOBr(X1A?) + O2(X3??g) and (R1b) HOBr(X1A?) + O2(a1?g). The other channels considered are: (R2) BrO + HO2 ? HBr + O3, (R3) BrO + HO2 ? OBrO + OH and (R4) BrO + HO2 ? BrOO + OH. For all channels, geometry optimization and frequency calculations were carried out at the M06-2X/AVDZ level, while relative energies of the stationary points on the reaction surface were improved at a higher level (BD(TQ)/CBS or CCSD(T)/CBS). The computed standard reaction enthalpies (?HRX298K) for channels (R1a), (R1b), (R2), (R3) and (R4) are ?47.5, ?25.0, ?4.3, 14.9 and 5.9 kcal mol?1, and the corresponding computed activation energies (?E) are 2.53, ?3.07, 11.83, 35.0 and 37.81 kcal mol?1. These values differ significantly from those obtained in earlier work by Kaltsoyannis and Rowley (Phys. Chem. Chem. Phys., 2002, 4, 419–427), particularly for channel (R1b), and reasons for this are discussed. In particular, the importance of obtaining an open-shell singlet wavefunction, rather than a closed-shell singlet wavefunction, for the transition state of this channel is emphasized. Rate coefficient calculations from computed potential energy surfaces were made for BrO + HO2 for the first time. Although channel (R1a) is the most exothermic, channel (R1b) has the lowest barrier height, which is negative (at ?3.07 kcal mol?1). Most rate coefficient calculations were therefore made for (R1b). A two transition state model has been used, involving an outer and an inner transition state. The inner transition state was found to be the major bottleneck of the reaction with the outer transition state having essentially no effect on the overall rate coefficient (k) in the temperature range considered. Studying the entropy, enthalpy and free energy of activation of this channel as a function of temperature shows that the main contributor to the magnitude of ln k at a selected temperature is the entropy term (?S#/kB) whereas the temperature dependence of ln k is determined mainly by the enthalpy term (??H#/kBT). This compares with reactions with positive barrier heights where the enthalpy term makes a bigger contribution to ln k. Comparison of the computed rate coefficients with available experimental values shows that the computed values have a negative temperature dependence, as observed experimentally, but are too low by approximately an order of magnitude at any selected temperature in the range 200–400 K
The atmospherically important reaction of hydroxyl radicals with methyl nitrate: a theoretical study involving the calculation of reaction mechanisms, enthalpies, activation energies, and rate coefficients
No full textA theoretical study, involving the calculation of reaction enthalpies and activation energies, mechanisms and rate coefficients, has been made of the reaction of hydroxyl radicals with methyl nitrate, an important process for methyl nitrate removal in the earth's atmosphere. Four reaction channels were considered:- formation of H2O + CH2ONO2, CH3OOH + NO2, CH3OH + NO3, and CH3O + HNO3 . For all channels, geometry optimization and frequency calculations were carried out at the M06-2X/6-31+G** level, while relative energies were improved at the UCCSD(T*)-F12/CBS level. The major channel is found to be the H abstraction channel, to give the products H2O + CH2ONO2. The reaction enthalpy (ΔH298KRX) of this channel is computed as -17.90 kcal.mol-1. Although the other reaction channels are also exothermic, their reaction barriers are high (> 24 kcal.mol-1) and therefore these reactions do not contribute to the overall rate coefficient in the temperature range considered (200-400 K). Pathways via three transition states have been identified for the H abstraction channel. Rate coefficients were calculated for these pathways at various levels of variational transition state theory (VTST) including tunneling. The results obtained are used to distinguish between two sets of experimental rate coefficients, measured in the temperature range 200-400K, one of which is approximately an order of magnitude greater than the other. This comparison, as well as the temperature dependence of the computed rate coefficients, shows that the lower experimental values are favoured. The implications of the results to atmospheric chemistry are discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A theoretical investigation of the atmospherically important reaction between chlorine atoms and formic acid: Determination of the reaction mechanism and calculation of the rate coefficient at different temperatures
No full textThe Cl + HCOOH reaction is important in the atmosphere, as the chlorine (Cl) atom is an important oxidant, especially in the marine boundary layer, and formic acid (HCOOH) is one of the most abundant organic acids in the troposphere. The reaction surfaces of the two H abstraction channels were computed by second-order unrestricted Møller–Plesset perturbation theory (UMP2) and density functional theory (DFT) calculations. Relative electronic energies were improved to the RCCSD(T)/CBS and UCCSD(T)-F12/CBS levels. The barrier of the C–H hydrogen abstraction channel was found to be lower by about 10 kcal mol-1. Rate coefficients (k) of this channel were calculated at different temperatures at various variational transition state theory (VTST) levels including tunnelling. For single-level direct dynamics VTST calculations, the computed k (2.5 × 10-13 cm3 molecule-1 s-1) using the BMK (Boese and Martin meta hybrid) functional at the highest level (ICVT/SCT) agrees the best with experimental values at 298 K (1.8 and 2.0 × 10-13 cm3 molecule-1 s-1). For dual-level direct dynamics calculations (RCCSD(T)/CBS//MP2 MEP), an adjusted barrier height of 3.1 kcal mol-1 is required to match the ICVT/SCT k with the experimental values. The computed rate coefficients of the Cl + HCOOH reaction is reported for the first time with a temperature range of 200–1500 K. The implications of the results obtained to atmospheric chemistry are discussed
A theoretical study of the mechanism of the atmospherically relevant reaction of chlorine atoms with methyl nitrate, and calculation of the reaction rate coefficients at temperatures relevant to the atmosphere
No full textThe reaction between atomic chlorine (Cl) and methyl nitrate (CH3ONO2) is significant in the atmosphere, as Cl is a key oxidant, especially in the marine boundary layer, and alkyl nitrates are important nitrogen-containing organic compounds, which are temporary reservoirs of the reactive nitrogen oxides NO, NO2 and NO3 (NOx). Four reaction channels HCl + CH2ONO2, CH3OCl + NO2, CH3Cl + NO3 and CH3O + ClNO2 were considered. The major channel is found to be the H abstraction channel, to give the products HCl + CH2ONO2. For all channels, geometry optimization and frequency calculations were carried out at the M06-2X/6-31+G** level, while relative electronic energies were improved to the UCCSD(T*)-F12/CBS level. The reaction barrier (Image ID:c4cp06007e-t1.gif) and reaction enthalpy (?HRX298K) of the H abstraction channel were computed to be 0.61 and ?2.30 kcal mol?1, respectively, at the UCCSD(T*)-F12/CBS//M06-2X/6-31+G** level. Reaction barriers (Image ID:c4cp06007e-t2.gif) for the other channels are more positive and these pathways do not contribute to the overall reaction rate coefficient in the temperature range considered (200–400 K). Rate coefficients were calculated for the H-abstraction channel at various levels of variational transition state theory (VTST) including tunnelling. Recommended ICVT/SCT rate coefficients in the temperature range 200–400 K are presented for the first time for this reaction. The values obtained in the 200–300 K region are particularly important as they will be valuable for atmospheric modelling calculations involving reactions with methyl nitrate. The implications of the results to atmospheric chemistry are discussed. Also, the enthalpies of formation, ?Hf,298K, of CH3ONO2 and CH2ONO2 were computed to be ?29.7 and 19.3 kcal mol?1, respectively, at the UCCSD(T*)-F12/CBS level
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