Integral elastic, vibrational-excitation, electronic-state excitation, ionization, and total cross sections for electron scattering from para-benzoquinone

We report absolute experimental integral cross sections (ICSs) for the electron impact excitation of 6 bands (Bands 0-V) of unresolved electronic-states in para-benzoquinone, for incident electron energies between 20 and 40 eV. Absolute vibrational-excitation ICSs, for 3 composite vibrational bands (Bands I-III), are also reported in that same energy range. In addition, ICSs calculated within our independent atom model (IAM) with screening corrected additivity rule (SCAR) formalism, extended to account for interference (I) terms that arise due to the multi-centre nature of the scattering problem, are also reported. The sum of those ICSs gives the IAM-SCAR+I total cross section (TCS) for electron-para-benzoquinone scattering. Where possible, those calculated IAM-SCAR+I ICSs are compared against corresponding results from the present measurements with an acceptable level of accord being obtained. Similarly, we also present results from our Schwinger multichannel method with pseudopotential (SMCPP) calculations. Here elastic ICSs and ICSs corresponding to the Bands 0-III of unresolved electronic-states are presented, with agreement between the SMCPP electronic-state ICSs and those from our measurements being in good qualitative accord. The energy range of our SMCPP computations is 16-50 eV. Using the binary-encounter-Bethe (BEB) approach, total ionization cross sections for this collision system were computed. Those total ionization cross sections were then added to our SMCPP ICS results, to derive SMCPP/BEB TCSs that are typically in very good accord with those from our IAM-SCAR+I approach. Published by AIP Publishing

The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview

On-the-fly dynamics simulation of dissociative electron attachment to chloroethane

International audienceSynopsis We present a novel theoretical approach for describing dissociative electron attachment. It is is based on classically propagating the nuclei on the potential surface of the resonant anion, which is described by bound state methods, while electron autodetachment is taken into account with an ad-hoc model built from scattering calculation results. As a first application, we considered the electron induced dissociation of chloroethane. Comparison to experimental data is very encouraging. The simulations reveal further details on the underlying dynamics of the dissociation

Precursor anion states in dissociative electron attachment to chlorophenol isomers

FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOWe report a theoretical study on low-energy (<10 eV) elastic electron scattering from chlorophenol isomers, namely, para-chlorophenol (pCP), meta-chlorophenol (mCP), and ortho-chlorophenol (oCP). The calculations were performed with the Schwinger multichannel method with pseudopotentials, and analysis of the computed integral cross sections and virtual orbitals revealed one sigma(CCl)*, one sigma(OH)*, and three pi* shape resonances. We show that electron capture into the two lower lying pi* orbitals initiates dissociative processes that lead to the elimination of the chloride ion, accounting for the two overlapping peaks where this fragment was observed. Despite the relatively small differences on the energetics of the pi* resonances, a major isomeric effect was found on their corresponding autodetachment lifetimes, which accounts for the observed increasing cross sections in the progression pCP < mCP < oCP. In particular, dissociation from the pi(1)* anion of pCP is largely suppressed because of the unfavorable mixing with the sigma(CCl)* state. We found the intramolecular hydrogen bond present in oCP to have the opposite effects of stabilizing the sigma(CCl)* resonance and destabilizing the sigma(OH)* resonance. We also suggest that the hydrogen abstraction observed in chlorophenols and phenol actually takes place by a mechanism in which the incoming electron is directly attached to the dissociative sigma(OH)* orbital. Published by AIP Publishing.We report a theoretical study on low-energy (<10 eV) elastic electron scattering from chlorophenol isomers, namely, para-chlorophenol (pCP), meta-chlorophenol (mCP), and ortho-chlorophenol (oCP). The calculations were performed with the Schwinger multichannel method with pseudopotentials, and analysis of the computed integral cross sections and virtual orbitals revealed one sigma(CCl)*, one sigma(OH)*, and three pi* shape resonances. We show that electron capture into the two lower lying pi* orbitals initiates dissociative processes that lead to the elimination of the chloride ion, accounting for the two overlapping peaks where this fragment was observed. Despite the relatively small differences on the energetics of the pi* resonances, a major isomeric effect was found on their corresponding autodetachment lifetimes, which accounts for the observed increasing cross sections in the progression pCP < mCP < oCP. In particular, dissociation from the pi(1)* anion of pCP is largely suppressed because of the unfavorable mixing with the sigma(CCl)* state. We found the intramolecular hydrogen bond present in oCP to have the opposite effects of stabilizing the sigma(CCl)* resonance and destabilizing the sigma(OH)* resonance. We also suggest that the hydrogen abstraction observed in chlorophenols and phenol actually takes place by a mechanism in which the incoming electron is directly attached to the dissociative sigma(OH)* orbital.145416FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO2015/23792-5Sem informaçãoF.K. acknowledges financial support from São Paulo Research Foundation (FAPESP) under Grant No. 2015/23792-5. M.T.doN.V. acknowledges financial support from FAPESP and also from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). This work employed computational resources from LCCA-USP and CENAPAD-SP

How does methylation suppress the electron-induced decomposition of 1-methyl-nitroimidazoles?

International audienceThe efficient decomposition of nitroimidazoles (NIs) by low energy electrons is believed to underlie their radiosensitizing properties. Recent dissociative electron attachment (DEA) measurements showed that methylation at the N1 site unexpectedly suppresses the electron-induced reactions in 4(5)-NI. We report theoretical results that provide a clear interpretation of that astounding finding. Around 1.5 eV, DEA reactions into several fragments are initiated by a pi* resonance, not considered in previous studies. The autoionization lifetime of this anion state, which limits the predissociation dynamics, is considerably shorter in the methylated species, thereby suppressing the DEA signals. On the other hand, the lifetime of the pi* resonance located around 3 eV is less affected by methylation, which explains why DEA is still observed at these energies. Our results demonstrate how even a simple methylation can significantly modify the probabilities for DEA reactions, which may be significant for NI-based cancer therapy. Published by AIP Publishing

How does methylation suppress the electron-induced decomposition of 1-methyl-nitroimidazoles?

FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQThe efficient decomposition of nitroimidazoles (NIs) by low energy electrons is believed to underlie their radiosensitizing properties. Recent dissociative electron attachment (DEA) measurements showed that methylation at the N1 site unexpectedly suppresses the electron-induced reactions in 4(5)-NI. We report theoretical results that provide a clear interpretation of that astounding finding. Around 1.5 eV, DEA reactions into several fragments are initiated by a pi* resonance, not considered in previous studies. The autoionization lifetime of this anion state, which limits the predissociation dynamics, is considerably shorter in the methylated species, thereby suppressing the DEA signals. On the other hand, the lifetime of the pi* resonance located around 3 eV is less affected by methylation, which explains why DEA is still observed at these energies. Our results demonstrate how even a simple methylation can significantly modify the probabilities for DEA reactions, which may be significant for NI-based cancer therapy.1471617FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ2014/10012-92015/23792-5305672/2014-2F.K. acknowledges financial support from Sao Paulo Research Foundation (FAPESP), under Grant No. 2015/23792-5. M.T. do N.V. acknowledges financial support from FAPESP (Grant No. 2014/10012-9) and also from the Brazilian National Council for Scientific and Technological Development (CNPq), Grant No. 305672/2014-2. This work used computational resources from LCCA-USP

On-the-fly dynamics simulations of transient anions

WOS:000505596000033A novel theoretical framework for describing the dynamics of transient anions is presented. An ensemble of classical trajectories is propagated on-the-fly, where resonance energies are computed with bound state techniques, and resonance widths are modeled with a combination of bound state and scattering calculations. The methodology was benchmarked against quantum dynamics results for model potential energy curves, and excellent agreement was attained. As a first application, we considered the electron induced dissociation of chloroethane. We found that electron attachment readily stretches the C-Cl bond, which stabilizes the transient anion within similar to 10 fs and leads to the release of fast chloride ions. Both magnitude and shape of the computed dissociative electron attachment cross sections are very similar to the available experimental data, even though we found the results to be very sensitive on the accuracy of the underlying methods. These encouraging results place the proposed methodology as a promising approach for studies on transient anions' dynamics of medium sized molecules

Transient anion spectra of the potential radiosensitizers 5-cyanateuracil and 5-thiocyanateuracil

FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQWe report on the low energy anion spectra of 5-cyanateuracil (5-OCNU) and 5-thiocyanateouracil (5-SCNU), which have been the suggested potential radiosensitizers for use in cancer therapy [L. Chomicz et al., J. Phys. Chem. Lett. 4, 2853-2857 (2013)]. Employing bound state and scattering calculations, we obtained, for both molecules, a dipole bound state, a pi* valence bound state, and four pi* resonances, besides a sigma*(SCN) resonance for 5-SCNU. The cyanate and thiocyanate sub-stituents give rise to additional long-lived pi* resonances, compared to 5-halouracil radiosensitizers. From the reaction thresholds and the expected vibronic couplings among the anion states, efficient production of SCN and CN anions and radical fragments should be observed in dissociative electron attachment measurements for 5-SCNU. The corresponding dissociation processes in 5-OCNU are expected to be less effective in view of the lack of a long-lived sigma*(OCN) shape resonance and the little sigma* admixture into the pi* resonances located on the cyanate group. The present results thus indicate 5-SCNU as a more promising radiosensitizer at sub-excitation energies.1472118FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ2014/10012-92015/17273-52015/23792-5305672/2014-

Shape resonance spectra of uracil, 5-fluorouracil, and 5-chlorouracil.

We report on the shape resonance spectra of uracil, 5-fluorouracil, and 5-chlorouracil, as obtained\ud from fixed-nuclei elastic scattering calculations performed with the Schwinger multichannel method\ud with pseudopotentials. Our results are in good agreement with the available electron transmission\ud spectroscopy data, and support the existence of three π* resonances in uracil and 5-fluorouracil.\ud As expected, the anion states are more stable in the substituted molecules than in uracil. Since the\ud stabilization is stronger in 5-chlorouracil, the lowest π* resonance in this system becomes a bound\ud anion state. The present results also support the existence of a low-lying σ\ud ∗\ud CCl shape resonance in 5-\ud chlorouracil. Exploratory calculations performed at selected C–Cl bond lengths suggest that the σ\ud ∗\ud CCl\ud resonance could couple to the two lowest π* states, giving rise to a very rich dissociation dynamics.\ud These facts would be compatible with the complex branching of the dissociative electron attachment\ud cross sections, even though we cannot discuss any details of the vibration dynamics based only on\ud the present fixed-nuclei results.FAPESPCNP