scholarly journals Astrophysically motivated laboratory measurements of deuterium reacting with isotopologues of H

2019 ◽  
Vol 15 (S350) ◽  
pp. 114-115
Author(s):  
K. P. Bowen ◽  
P.-M. Hillenbrand ◽  
J. Liévin ◽  
X. Urbain ◽  
D. W. Savin
Keyword(s):  
Cross Sections ◽  
Zero Point ◽  
Rate Coefficients ◽  
Laboratory Measurements ◽  
Exchange Reactions ◽  
Point Energy ◽  
Chemical Tracers ◽  
Prestellar Cores ◽  
Order Of Magnitude ◽  
High Level

AbstractH2D+ and D2H+ are important chemical tracers of prestellar cores due to their pure rotational spectra that can be excited at the ~20 K temperature of these environments. The use of these molecules as probes of prestellar cores requires understanding the chemistry that forms and destroys these molecules. Of the eight key reactions that have been identified (Albertssonet al. 2013), five are thought to be well understood. The remaining three are the isotope exchange reactions of atomic D with H $${ + \over 3}$$ , H2D+, and D2H+. Semi-classical results differ from the classical Langevin calculations by an order of magnitude (Moyano et al. 2004). To resolve this discrepancy, we have carried out laboratory measurements for these three reactions. Absolute cross sections were measured using a dual-source, merged fast-beams apparatus for relative collision energies between ~10 meV to ~10 eV (Hillenbrand et al. 2019). A semi-empirical model was developed incorporating high level quantum mechanical ab initio calculations for the zero-point-energy-corrected potential energy barrier in order to generate thermal rate coefficients for astrochemical models. Based on our studies, we find that these three reactions proceed too slowly at prestellar core temperatures to play a significant role in the deuteration of H $${ + \over 3}$$ isotopologues.

scholarly journals Theoretical studies of carbon isotopic fractionation in reactions of C with C2: dynamics, kinetics, and isotopologue equilibria

2021 ◽  
Vol 647 ◽  
pp. A142
Author(s):  
C. M. R. Rocha ◽  
H. Linnartz
Keyword(s):  
Energy Content ◽  
Equilibrium Constants ◽  
Isotopic Fractionation ◽  
Zero Point ◽  
Classical Trajectory ◽  
Rate Coefficients ◽  
Kinetic Temperature ◽  
Positive Temperature ◽  
Exchange Reactions ◽  
Point Energy

Context. Our current understanding of interstellar carbon fractionation hinges on the interpretation of astrochemical kinetic models. Yet, the various reactions included carry large uncertainties in their (estimated) rate coefficients, notably those involving C with C2. Aims. We aim to supply theoretical thermal rate coefficients as a function of the temperature for the gas-phase isotope-exchange reactions 13C+12C2(X1Σg+,a3Πu)⇌13C12C(X1Σg+,a3Πu)+12C and 13C+13C12C(X1Σg+,a3Πu)⇌13C2(X1Σg+,a3Πu)+12C. Methods. By relying on the large masses of the atoms involved, we employ a variation of the quasi-classical trajectory method, with the previously obtained (mass-independent) potential energy surfaces of C3 dictating the forces between the colliding partners. Results. The calculated rate coefficients within the range of 25 ≤ T∕K ≤ 500 show a positive temperature dependence and are markedly different from previous theoretical estimates. While the forward reactions are fast and inherently exothermic owing to the lower zero-point energy content of the products, the reverse processes have temperature thresholds. For each reaction considered, analytic three-parameter Arrhenius-Kooij formulas are provided that readily interpolate and extrapolate the associated forward and backward rates. These forms can further be introduced in astrochemical networks. Apart from the proper kinetic attributes, we also provide equilibrium constants for these processes, confirming their prominence in the overall C fractionation chemistry. In this respect, the 13C+12C2(X1Σg+) and 13C+12C2(a3Πu) reactions are found to be particularly conspicuous, notably at the typical temperatures of dense molecular clouds. For these reactions and considering both equilibrium and time-dependent chemistry, theoretical 12C/13C ratios as a function of the gas kinetic temperature are also derived and shown to be consistent with available model chemistry and observational data on C2.

Collision excitation of sodium cyanide molecule by helium at low temperature

2019 ◽  
Vol 489 (3) ◽  
pp. 4322-4328
Author(s):  
C Gharbi ◽  
Y Ajili ◽  
D Ben Abdallah ◽  
M Mogren Al Mogren ◽  
M Hochlaf
Keyword(s):  
Electronic Structure ◽  
Cross Sections ◽  
Energy Surface ◽  
Three Dimensional ◽  
Sodium Cyanide ◽  
Rate Coefficients ◽  
Quantum Scattering ◽  
Stable Form ◽  
Excitation Cross Sections ◽  
Order Of Magnitude

ABSTRACT Cyanides/isocyanides are the most common metal-containing molecules in interstellar medium. In this work, quantum scattering calculations were carried out to determine the rotational (de-)excitation cross-sections of the most stable form of the sodium cyanide molecule, t-NaCN, in collision with the helium atom. Rate coefficients for the first 43 rotational levels (up to ${j_{{K_a}{K_c}}}$ = 63,3) of NaCN were determined for kinetic temperatures ranging from 1 to 30 K. Prior to that, we constructed a new three-dimensional potential energy surface (3D-PES) for the t-NaCN–He interacting system. These electronic structure computations are done at the CCSD(T)-F12/aug-cc-pVTZ level of theory. Computations show the dominance of Δj = ΔKc = −1 transitions, which is related to the dissymmetric shape of the t-NaCN–He 3D-PES. The NaCN–He rate coefficients are of the same order of magnitude (∼10−11 cm3.s−1) as those of other metal CN-containing molecules such as MgCN and AlCN in collision with He. This work is a contribution for understanding and modelling the abundances and chemistry of nitriles in astrophysical media.

THEORETICAL STUDY OF STEREODYNAMICS AND ISOTOPIC EFFECTS FOR THE REACTION C(3P) + OD(X2Π) → CO(X1Σ+) + D(2S)

2011 ◽  
Vol 10 (04) ◽  
pp. 447-469 ◽  
Author(s):  
RUNZE LIU ◽  
YIJUE DING ◽  
CHENGYUAN WEN ◽  
JINFENG LI ◽  
MINGWANG ZHONG ◽  
...  
Keyword(s):  
Angular Distribution ◽  
Cross Sections ◽  
Collision Energy ◽  
Theoretical Study ◽  
Zero Point ◽  
Point Energy ◽  
Mass Factor ◽  
Generalized Differential ◽  
Paper Product ◽  
Isotopic Effects

Theoretical study on stereodynamics for the title reaction as well as its isotopic effects has been studied via QCT calculations on the ground X2A′ state of ab initio potential energy surface according to the study by Zanchet et al. Four polarization-dependent generalized differential cross-sections PDDCSs ((2π/σ) (dσ00/dωt), (2π/σ)(dσ20/dωt)), (2π/σ)(dσ22+/dωt), (2π/σ)(dσ21-/dωt), and the distributions of P(θr) and P(φr) that denotes the correlations of k-j′ and k-k′-j′ are presented in this work. Product angular distribution and rotational polarization have been analyzed at different collision energies and compared with C+OH reaction. Product angular distribution shows strong forward scattering at low collision energy and becomes more symmetric with forward and backward scattering with the increasing collision energy. The alignment and orientation of product angular momentum presents a different behavior with collision energy, the former one increases monotonically with collision energy, whereas the latter one shows first decreasing and then increasing behavior, which have been analyzed in the present paper. Product rotational polarization for C+OD is weaker than that for C+OH , which is mainly due to the mass factor and zero point energy of C+OD .

scholarly journals 14N/15N ratio measurements in prestellar cores with N2H+: new evidence of 15N-antifractionation

2018 ◽  
Vol 617 ◽  
pp. A7 ◽  
Author(s):  
E. Redaelli ◽  
L. Bizzocchi ◽  
P. Caselli ◽  
J. Harju ◽  
A. Chacón-Tanarro ◽  
...  
Keyword(s):  
Core Sample ◽  
Rate Coefficients ◽  
Chemical Models ◽  
Prestellar Cores ◽  
Bona Fide ◽  
Molecular Tracer ◽  
Non Local ◽  
The One ◽  
Collisional Rate Coefficients ◽  
High Level

Context. The 15N fractionation has been observed to show large variations among astrophysical sources, depending both on the type of target and on the molecular tracer used. These variations cannot be reproduced by the current chemical models. Aims. Until now, the 14N/15N ratio in N2H+ has been accurately measured in only one prestellar source, L1544, where strong levels of fractionation, with depletion in 15N, are found (14N/15N ≈ 1000). In this paper, we extend the sample to three more bona fide prestellar cores, in order to understand if the antifractionation in N2H+ is a common feature of this kind of source. Methods. We observed N2H+, N15NH+, and 15NNH+ in L183, L429, and L694-2 with the IRAM 30 m telescope. We modelled the emission with a non-local radiative transfer code in order to obtain accurate estimates of the molecular column densities, including the one for the optically thick N2H+. We used the most recent collisional rate coefficients available, and with these we also re-analysed the L1544 spectra previously published. Results. The obtained isotopic ratios are in the range 580–770 and significantly differ with the value, predicted by the most recent chemical models, of ≈440, close to the protosolar value. Our prestellar core sample shows a high level of depletion of 15N in diazenylium, as previously found in L1544. A revision of the N chemical networks is needed in order to explain these results.

scholarly journals The Quest to Uncover the Nature of Benzonitrile Anion

2019 ◽  
Author(s):  
Sahil Gulania ◽  
Thomas-C. Jagau ◽  
Andrei Sanov ◽  
Anna I. Krylov
Keyword(s):  
Cross Sections ◽  
Zero Point ◽  
Bound State ◽  
Coupled Cluster ◽  
Equilibrium Geometry ◽  
Electronic Structure Methods ◽  
Condon Factors ◽  
Vibrational Energies ◽  
High Level ◽  
Franck Condon

<div><div><div><p>Anionic states of benzonitrile are investigated by high-level electronic structure methods. The calculations using equation-of-motion coupled-cluster theory for electron-attached states confirm earlier conclusions drawn from the photodetachment experiments that the ground state of the anion is the valence <sup>2</sup>B<sub>1</sub> state, while the dipole bound state lies adiabatically ~0.1 eV above. Inclusion of triple excitations and zero-point vibrational energies is important for recovering relative state ordering. The computed Franck–Condon factors and photodetachment cross-sections further confirm that the observed photodetachment spectrum originates from the valence anion. The valence anion is electronically bound at its equilibrium geometry, but is metastable at the equilibrium geometry of the neutral. The dipole-bound state, which is the only bound anionic state at the neutral geometry, may serve as a gateway state for capturing the electron. Thus, the emerging mechanistic picture entails electron capture via dipole bound state, followed by non-adiabatic relaxation forming valence anion.</p></div></div></div>

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

2019 ◽  
Vol 625 ◽  
pp. A72 ◽  
Author(s):  
Daniela Ascenzi ◽  
Andrea Cernuto ◽  
Nadia Balucani ◽  
Paolo Tosi ◽  
Cecilia Ceccarelli ◽  
...  
Keyword(s):  
Dimethyl Ether ◽  
Cross Sections ◽  
Methyl Formate ◽  
Molecular Ions ◽  
Rate Coefficients ◽  
Langevin Model ◽  
Chemical Systems ◽  
Prestellar Cores ◽  
Temperature Dependences ◽  
Proton Transfer Reactions

Context. 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+, H3+ and HCO+. Aims. 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 H3+ and HCO+. Methods. Based on our recent laboratory measurements of cross sections and BRs for the DME/MF + He+ reactions over a wide collision energy, we extended 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 implemented these new and revised kinetic data in a general model of cold and warm gas, simulating environments where DME and MF have been detected. Results. 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 cores. Conclusions. This work demonstrates that the accuracy of astrochemical models can be improved by a thorough characterisation of the destruction routes of iCOMs. The details of the chemical systems can, indeed, strongly affect their efficiency and significant deviations with respect to the commonly used Langevin model estimates are possible.

scholarly journals The Quest to Uncover the Nature of Benzonitrile Anion

2019 ◽  
Author(s):  
Sahil Gulania ◽  
Thomas-C. Jagau ◽  
Andrei Sanov ◽  
Anna I. Krylov
Keyword(s):  
Cross Sections ◽  
Zero Point ◽  
Bound State ◽  
Coupled Cluster ◽  
Equilibrium Geometry ◽  
Electronic Structure Methods ◽  
Condon Factors ◽  
Vibrational Energies ◽  
High Level ◽  
Franck Condon

<div><div><div><p>Anionic states of benzonitrile are investigated by high-level electronic structure methods. The calculations using equation-of-motion coupled-cluster theory for electron-attached states confirm earlier conclusions drawn from the photodetachment experiments that the ground state of the anion is the valence <sup>2</sup>B<sub>1</sub> state, while the dipole bound state lies adiabatically ~0.1 eV above. Inclusion of triple excitations and zero-point vibrational energies is important for recovering relative state ordering. The computed Franck–Condon factors and photodetachment cross-sections further confirm that the observed photodetachment spectrum originates from the valence anion. The valence anion is electronically bound at its equilibrium geometry, but is metastable at the equilibrium geometry of the neutral. The dipole-bound state, which is the only bound anionic state at the neutral geometry, may serve as a gateway state for capturing the electron. Thus, the emerging mechanistic picture entails electron capture via dipole bound state, followed by non-adiabatic relaxation forming valence anion.</p></div></div></div>

scholarly journals Does glycosyl transfer involve an oxacarbenium intermediate? Computational simulation of the lifetime of the methoxymethyl cation in water

2011 ◽  
Vol 83 (8) ◽  
pp. 1507-1514 ◽  
Author(s):  
Ian H. Williams ◽  
J. Javier Ruiz Pernía ◽  
Iñaki Tuñón
Keyword(s):  
Free Energy ◽  
Computational Simulation ◽  
Zero Point ◽  
Md Simulations ◽  
Quantum Corrections ◽  
Water Molecules ◽  
Point Energy ◽  
Classical Estimate ◽  
High Level ◽  
Energy Surfaces

2D free-energy surfaces for transfer of the methoxymethyl cation between two water molecules are constructed from molecular dynamics (MD) simulations in which these atoms are treated quantum-mechanically within a box of 1030 classical solvent water molecules at 300 K. This provides a simple model for glycosyl transfer in water. The AM1/TIP3P surfaces with 2D-spline corrections at either MPWB1K/6-31+G(d,p) or MP2/6-31+G(d,p) contain a shallow free-energy well corresponding to an oxacarbenium ion intermediate in a DN*AN mechanism. MD analysis at three temperatures leads to a classical estimate of the lifetime of the methoxymethyl cation in water; when quantum corrections for vibrational zero-point energy are included, the lifetime is estimated to be about 1 ps, in agreement with the best experimental estimate. This suggests that computational simulation, with appropriate high-level correction, is a reliable tool to obtain detailed and reliable mechanistic descriptions for glycosidases. In view of the importance of developing improved anti-influenza drugs, simulations of sialidases that considered both sialyl oxacarbenium ion and covalent sialyl-enzyme as possible intermediates could provide particular insight.

scholarly journals Isotopic fractionation in interstellar molecules

2017 ◽  
Vol 13 (S332) ◽  
pp. 163-174 ◽  
Author(s):  
Kenji Furuya
Keyword(s):  
Isotopic Exchange ◽  
Isotopic Fractionation ◽  
Mass Star ◽  
Exchange Reactions ◽  
Interstellar Molecules ◽  
Star Forming ◽  
Chemical Tracers ◽  
Star Forming Regions ◽  
Prestellar Cores ◽  
Low Mass

AbstractThe level of isotopic fractionation in molecules provides insights into their formation environments and how they formed. In this article, we review hydrogen and nitrogen isotopic fractionation in low-mass star-forming regions. Interstellar molecules are significantly enriched in deuterium. The importance of the nuclear spin states of light species on deuterium fractionation and the usefulness of singly and doubly deuterated molecules as chemical tracers are discussed. Observations have revealed that molecules in prestellar cores are enriched in or depleted in15N depending on molecules. Compared with deuterium fractionation chemistry, our understanding of15N fractionation chemistry is not well established. We briefly discuss potential15N fractionation routes, i.e., isotopic-exchange reactions and isotopic selective photodissociation of N2. In addition, the selective freeze-out of15N atoms onto dust grains around the transition between N atoms and N2is discussed as a potential mechanism that causes the depletion of15N in the gas phase.

scholarly journals Non-LTE modelling of cyanoacetylene: evidence for isomer-specific excitation

2020 ◽  
Vol 501 (2) ◽  
pp. 1911-1919
Author(s):  
Cheikh T Bop ◽  
François Lique ◽  
Alexandre Faure ◽  
Ernesto Quintas-Sánchez ◽  
Richard Dawes
Keyword(s):  
Radiative Transfer ◽  
Cross Sections ◽  
Emission Spectra ◽  
Abundance Ratio ◽  
Rate Coefficients ◽  
Close Coupling ◽  
Circumstellar Gas ◽  
Order Of Magnitude ◽  
Rotational Transitions ◽  
The Impact

ABSTRACT Cyanoacetylene molecules are widespread in the interstellar medium (ISM) and several of its isomers have been detected in cold molecular clouds and circumstellar gas. Accurate estimates of the abundance ratio of cyanoacetylene isomers may provide deep insight into their environment. Such knowledge requires rigorous modelling of the emission spectra based on non-local thermodynamic equilibrium (LTE) radiative transfer calculations. To this end, we computed excitation cross-sections of HC2NC and HNC3 induced by collision with para- and ortho-H2, using a quantum mechanical close-coupling method. Then, by thermally averaging these data, we derived rate coefficients for the first 31 low-lying rotational levels of each isomer for temperatures up to 80 K. For the para-H2 collider, the propensity rules are in favour of rotational transitions involving Δj1 = 2 for both isomers, while for the ortho-H2 collider, Δj1 = 2 and Δj1 = 1 rotational transitions are favoured for HC2NC and HNC3, respectively. A comparison of rate coefficients for the HC3N isomers shows differences up to an order of magnitude, especially at low temperatures. Finally, we performed non-LTE radiative transfer calculations to assess the impact of such variations in the analysis of observations. Our simulation suggests that the lack of collisional data specific to each isomer could lead to errors up to a factor of 2–3 in the excitation temperatures. We expect that these data could help in better understanding the cyanoacetylene chemistry and constraining the nitrogen chemistry in the ISM.

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