Measurements of ionization cross sections by molecular beam experiments: information content on the imaginary part of the optical potential

In this work, we present and analyze in detail new and recent ionization cross section and mass spectrum determinations, collected in the case of He*, Ne*-H2O, -H2S, and -NH3 ionizing collisions. These sets of data, obtained under the same experimental conditions, are relevant to identify differences in the autoionization stereodynamics of the three hydrogenated molecules and on the selective role of the imaginary part of the optical potential. We demonstrate that in these autoionization processes hydrogen and halogen bonds are competing because they are controlling both real and imaginary components of the optical potential that drives the complete reaction dynamics. In particular, we found that both components critically depend on the angular and radial approach between the reagent partners in determining the collision dynamics

Low-energy structures of benzene clusters with a novel accurate potential surface

11 pags.; 9 figs.; 2 tabs.The benzene-benzene (Bz-Bz) interaction is present in severalchemical systems and it is known to be crucial in understand-ing the specificity of important biological phenomena. In thiswork, we propose a novel Bz-Bz analytical potential energysurface which is fine-tuned on accurate ab initio calculations inorder to improve its reliability. Once the Bz-Bz interaction ismodeled, an analytical function for the energy of the Bznclus-ters may be obtained by summing up over all pair potentials.We apply an evolutionary algorithm (EA) to discover thelowest-energy structures of Bznclusters (for n52225), and theresults are compared with previous global optimization studieswhere different potential functions were employed. Besidesthe global minimum, the EA also gives the structures of otherlow-lying isomers ranked by the corresponding energy. Addi-tional ab initio calculations are carried out for the low-lyingisomers of Bz3and Bz4clusters, and the global minimum isconfirmed as the most stable structure for both sizes. Finally, adetailed analysis of the low-energy isomers of the n 5 13 and19 magic-number clusters is performed. The two lowest-energy Bz13isomers show S6and C3symmetry, respectively,which is compatible with the experimental results available inthe literature. The Bz19structures reported here are all non-symmetric, showing two central Bz molecules surrounded by12 nearest-neighbor monomers in the case of the five lowest-energy structures.VC2015 Wiley Periodicals, Inc.Contract grant sponsor: Coimbra Chemistry Centre; Contract grant number: UID/QUI/00313/2013; Contract grant sponsor: Spanish“Ministerio de Ciencia e Innovacion”; Contract grant number: FIS2013-48275-C2-1-P; Contract grant sponsor: Italian Ministry of University and Research (MIUR) for PRIN 2010-2011; Contract grant number: 2010ERFKXL_002. The authors are grateful for the provision of computational time inthe supercomputer resources hosted at Laboratorio de Computación Avançada, Universidade de Coimbra. Allocation of computing timeby CESGA (Spain) is also acknowledged.Peer reviewe

General treatment for stereo-dynamics of state-to-state chemi-ionization reactions

The microscopic evolution of elementary chemical reactions remains challenging to describe, as a plethora of parallel channels often determines reaction dynamics. Here the authors propose a theoretical approach to formulate the optical potential for Ne*(3P2,0) chemi-ionizations as a prototype gas-phase oxidation process

Graphdiyne based membranes: exceptional performances for helium separation applications

Graphdiyne is a novel two-dimensional material deriving from graphene that has been recently synthesized and featuring uniformly distributed sub-nanometer pores. We report accurate calculations showing that graphdiyne pores permit an almost unimpeded helium transport which can be used for its chemical and isotopic separation. Exceptionally high He/CH_4 selectivities are found which largely exceed the performance of the best membranes used to date for extraction from natural gas. Moreover, by exploiting slight differences in the tunneling probabilities of ^3He and ^4He, we also find promising results for the separation of the Fermionic isotope at low temperature

Destruction of dimethyl ether and methyl formate by collisions with He+^+

To correctly model the abundances of interstellar complex organic molecules (iCOMS) in different environments, both formation and destruction routes should be appropriately accounted for. While several scenarios have been explored for the formation of iCOMs via grain and gas-phase processes, much less work has been devoted to understanding the relevant destruction pathways, with special reference to (dissociative) charge exchange or proton transfer reactions with abundant atomic and molecular ions such as He$^+$, H$_3^+$ and HCO$^+$. By using a combined experimental and theoretical methodology we provide new values for the rate coefficients and branching ratios (BRs) of the reactions of He$^+$ ions with two important iCOMs, namely dimethyl ether (DME) and methyl formate (MF). We also review the destruction routes of DME and MF by other two abundant ions, namely H$_3^+$ and HCO$^+$. Based on our recent laboratory measurements of cross sections and BRs for the DME/MF + He$^+$ reactions over a wide collision energy range, we extend our theoretical insights on the selectivity of the microscopic dynamics to calculate the rate coefficients $k(T)$ in the temperature range from 10 to 298 K. We implement these new and revised kinetic data in a general model of cold and warm gas, simulating environments where DME and MF have been detected. Due to stereodynamical effects present at low collision energies, the rate coefficients, BRs and temperature dependences here proposed differ substantially from those reported in KIDA and UDfA, two of the most widely used astrochemical databases. These revised rates impact the predicted abundances of DME and MF, with variations up to 40% in cold gases and physical conditions similar to those present in prestellar coresComment: accepted for publication in Astronomy and Astrophysics (manuscript no. AA/2018/34585), 10 pages, 3 figure

A Novel Intermolecular Potential to Describe the Interaction Between the Azide Anion and Carbon Nanotubes

47 P.International audienceIn this contribution we propose a novel and accurate intermolecular potential that can be used for the simulation of the azide anion confined inside carbon nanotubes of arbitrary size. The peculiarity of our approach is to include an explicit term, modeling the induction attractive contributions from the negatively charged azide ion, that can be generalized to other ions confined in carbon nanotubes of different size and length. Through a series of accurate DLPNO-CCSD(T) calculations, we show that this potential reproduces the ab initio interaction energy to within a few kcal/mol. The potential is implemented in a molecular dynamics program, with which we carried out illustrative simulations to demonstrate the effectiveness of our approach. At last, the guidelines provided by this investigation can be applied to build up force fields for many neutral/ionic molecular species confined within carbon nanotubes; a crucial requirement to carry out molecular dynamics simulations under a variety of conditions

Enhanced flexibility of the O2 + N2 interaction and Its effect on collisional vibrational energy exchange

12 págs.; 8 figs.; 1 app. This article is part of the Piergiorgio Casavecchia and Antonio Lagana Festschrift special issue.Prompted by a comparison of measured and computed rate coefficients of Vibration-to-Vibration and Vibration-to-Translation energy transfer in O2 + N2 nonreactive collisions, extended semiclassical calculations of the related cross sections were performed to rationalize the role played by attractive and repulsive components of the interaction on two different potential energy surfaces. By exploiting the distributed concurrent scheme of the Grid Empowered Molecular Simulator we extended the computational work to quasiclassical techniques, investigated in this way more in detail the underlying microscopic mechanisms, singled out the interaction components facilitating the energy transfer, improved the formulation of the potential, and performed additional calculations that confirmed the effectiveness of the improvement introduced.The authors acknowledge financial support from the Phys4entry FP7/2007-2013 project (Contract No. 242311), ARPA Umbria, INSTM, the EGI-Inspire project (Contract No. 261323), MIUR PRIN 2008 (2008KJX4SN 003) and 2010/2011 (2010ERFKXL_002), the ESA-ESTEC Contract No. 21790/ 08/NL/HE and the Spanish CTQ2012-37404 and FIS2013- 48275-C2-1-P projects. Computations have been supported by the use of Grid resources and services provided by the European Grid Infrastructure (EGI) and the Italian Grid Infrastructure (IGI) through the COMPCHEM Virtual Organization. Thanks are also due to the COST CMST European Cooperative Project CHEMGRID (Action D37) EGI Inspire.Peer reviewe

Efficiency of Collisional O2 + N2 Vibrational Energy Exchange

10 pags.; 6 figs.; 5 tabs. In press.By following the scheme of the Grid Empowered Molecular Simulator (GEMS), a new O2 + N2 intermolecular potential, built on ab initio calculations and experimental (scattering and second virial coefficient) data, has been coupled with an appropriate intramolecular one. On the resulting potential energy surface detailed rate coefficients for collision induced vibrational energy exchanges have been computed using a semiclassical method. A cross comparison of the computed rate coefficients with the outcomes of previous semiclassical calculations and kinetic experiments has provided a foundation for characterizing the main features of the vibrational energy transfer processes of the title system as well as a critical reading of the trajectory outcomes and kinetic data. On the implemented procedures massive trajectory runs for the proper interval of initial conditions have singled out structures of the vibrational distributions useful to formulate scaling relationships for complex molecular simulations.The authors acknowledge financial support from the Phys4- entry FP7/2007-2013 project (Contract 242311), ARPA Umbria, INSTM, the EGI-Inspire project (Contract 261323), MIUR PRIN 2008 (2008KJX4SN 003) and 2010/2011 (2010ERFKXL_002), the ESA-ESTEC contract 21790/08/ NL/HE, and the Spanish CTQ2012-37404 and FIS2013- 48275-C2-1-P projects. Computations have been supported by the use of Grid resources and services provided by the European Grid Infrastructure (EGI) and the Italian Grid Infrastructure (IGI) through the COMPCHEM Virtual Organization. Thanks are also due to the COST CMST European Cooperative Project CHEMGRID (Action D37) EGI Inspire.Peer reviewe

Spin precession in the Schwarzschild spacetime: circular orbits

We study the behavior of nonzero rest mass spinning test particles moving along circular orbits in the Schwarzschild spacetime in the case in which the components of the spin tensor are allowed to vary along the orbit, generalizing some previous work.Comment: To appear on Classical and Quantum Gravity, 200

Revised gas-phase formation network of methyl cyanide: the origin of methyl cyanide and methanol abundance correlation in hot corinos

Methyl cyanide (CH$_3$CN) is one of the most abundant and widely spread interstellar complex organic molecules (iCOMs). Several studies found that, in hot corinos, methyl cyanide and methanol abundances are correlated suggesting a chemical link, often interpreted as a synthesis of them on the interstellar grain surfaces. In this article, we present a revised network of the reactions forming methyl cyanide in the gas-phase. We carried out an exhaustive review of the gas-phase CH$_3$CN formation routes, propose two new reactions and performed new quantum mechanics computations of several reactions. We found that 13 of the 15 reactions reported in the databases KIDA and UDfA have incorrect products and/or rate constants. The new corrected reaction network contains 10 reactions leading to methyl cyanide. We tested the relative importance of those reactions in forming CH$_3$CN using our astrochemical model. We confirm that the radiative association of CH${_3}{^+}$ and HCN, forming CH$_{3}$CNH$^{+}$, followed by the electron recombination of CH$_{3}$CNH$^{+}$, is the most important CH$_3$CN formation route in both cold and warm environments, notwithstanding that we significantly corrected the rate constants and products of both reactions. The two newly proposed reactions play an important role in warm environments. Finally, we found a very good agreement between the CH$_3$CN predicted abundances with those measured in cold ($\sim$10 K) and warm ($\sim$90 K) objects. Unexpectedly, we also found a chemical link between methanol and methyl cyanide via the CH$_{3}^{+}$ ion, which can explain the observed correlation between the CH$_3$OH and CH$_3$CN abundances measured in hot corinos.Comment: 24 pages, 19 figures, accepted in MNRA