Reactions of N+(3P) ions with H2and HD molecules at low temperatures (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 2000
The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral±neutral reactions thought to be important in the formation of HNC and HCN, respectively ± C 1 NH 2 3 HNC 1 H, and N 1 CH 2 3 HCN 1 HX We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds.
A crucial step in the gas-phase formation of ammonia in the interstellar medium is the reaction of 2 + with molecular hydrogen. Understanding the electronic structure of the participating species in this reaction and the evaluation of the rate coefficients at interstellar temperatures are, therefore, critical to gain new insight into the mechanisms of formation of interstellar ammonia. We present here the first theoretical results of the rate coefficients of this reaction as a function of temperatures relevant to the interstellar medium, computed using transition state theory. The results are in reasonable agreement with recent experimental results. This exothermic reaction features a tiny
An extensive ab initio study of a process of astrophysical interest: the N+(N)+CH3(CH3+) reaction
Chemical Physics Letters, 1999
The impact of the N N q CH CH ™ NH C q H process on the relative HNC and HCN space abundances is 3 3 2 investigated in order to test the astrophysical hypothesis stating that, in cold interstellar clouds, this reaction may contribute to the observed HNCrHCN abundance ratio greater than unity. Our extensive ab initio study of the lowest surfaces of both doublet and quartet multiplicities shows that the HNCH q entity is formed in its linear arrangement, the dissociative recombination of which is known to lead to an equal amount of HCN and HNC. Therefore it can be inferred that this reaction cannot account for an HNCrHCN abundance ratio larger than unity and that the above hypothesis has to be reconsidered.
Astronomy and Astrophysics, 1998
The hypothesis that the C + +NH 3 → CH 2 N + +H reaction contributes to an HNC/HCN abundance ratio greater than unity in dark interstellar clouds has been tested using ab initio quantum chemical techniques. The hypothesis is based on the argument that a significant fraction of the ion product is the metastable H 2 NC + isomer of C 2V geometry, rather than the linear HNCH + structure, and that the metastable isomer subsequently recombines with electrons to form HNC preferentially. Our extensive ab initio study of the ground and excited surfaces for the C + +N H 3 reaction shows, however, that this is most probably not the case. We find that the lowest energy path for reaction does lead initially to the formation of the metastable isomer in its ground singlet state, but that this product can then isomerize into the ground electronic state (1 Σ +) of the linear HCNH + form. Dynamics calculations show that the isomerization destroys 97-98% of the product H 2 NC + ion. We also follow excited potential energy surfaces which lead to the excited (3 B 2) electronic state of H 2 NC + , a state which does not interconvert to the linear ion. However, the potential energy surfaces exhibit barriers on the paths to formation of H 2 NC + (3 B 2). We conclude that the H 2 NC + isomer is a minor product of the C + +NH 3 → CH 2 N + + H reaction.
The hypothesis that the C+ +N H3 ! CH2N+ +H reaction contributes to an HNC/HCN abundance ratio greater than unity in dark interstellar clouds has been tested using ab initio quantum chemical techniques. The hypothesis is based on the argument that a significant fraction of the ion product is the metastable H2NC+ isomer of C2V geometry, rather than the linear HNCH+ structure, and that the metastable isomer subse- quently recombines with electrons to form HNC preferentially. Our extensive ab initio study of the ground and excited surfaces for the C+ +N H3 reaction shows, however, that this is most probably not the case. We find that the lowest energy path for reaction does lead initially to the formation of the metastable isomer in its ground singlet state, but that this product can then isomerize into the ground electronic state (1+) of the linear HCNH+ form. Dynamics calculations show that the isomer- ization destroys 97-98% of the product H2NC+ ion. We also follow excited potential...
Theoretical analysis of the rate constants for the interstellar reaction N+OH?NO+H
International Journal of Chemical Kinetics, 1995
The title reaction, a key elementary process involved in the chemistry of molecular clouds, has been theoretically studied over the 5 -600 K temperature range. Rate constants calculations have been carried out using the full version of the statistical adiabatic channel model in conjunction with a potential energy surface that has been derived from recent ab initio quantum chemical data. By using various switching functions, the influence of the attenuation of the bound-complex bending frequency upon N-OH bond elongation on the temperature dependence of the reaction was investigated. The rate constants exhibit a slightly positive temperature dependence with a calculated rate constant value at 300 K in very good agreement with the measured value. A comparison with the available experimental data between 250 and 515 K suggests that recrossing trajectories might occur with increasing importance as the temperature increases. However, the nonstatistical recrossing effects are expected to be of minor importance at interstellar temperatures such that the rate constants over the 5-200 K temperature range are given by k = 8.41 X T+0.30 cm3 molecule-' s-l. The rate constant calculated at 10 K is consistent with that derived in the astrochemical modeling of the L134N dark cloud. Rate constants for individual quantum states are also presented.
Review of important reactions for the nitrogen chemistry in the interstellar medium
Predictions of astrochemical models depend strongly on the reaction rate coefficients used in the simulations. We reviewed a number of key reactions for the chemistry of nitrogen-bearing species in the dense interstellar medium and proposed new reaction rate coefficients for those reactions. The details of the reviews are given in the form of a datasheet associated with each reaction. The new recommended rate coefficients are given with an uncertainty and a temperature range of validity and will be included in KIDA (http://kida.obs.u-bordeaux1.fr).
Journal of Chemical Physics, 2020
We report a large set of state-to-state rate constants for the H + HD reactive collision, using Quasi-Classical Trajectory (QCT) simulations on the accurate H 3 global potential energy surface of Mielke et al. [J. Chem. Phys. 116, 4142 (2002)]. High relative collision energies (up to ≈56 000 K) and high rovibrational levels of HD (up to ≈50 000 K), relevant to various non thermal equilibrium astrophysical media, are considered. We have validated the accuracy of our QCT calculations with a new efficient adaptation of the Multi Configuration Time Dependent Hartree (MCTDH) method to compute the reaction probability of a specific reactive channel. Our study has revealed that the high temperature regime favors the production of H 2 in its highly rovibrationnally excited states, which can de-excite radiatively (cooling the gas) or collisionally (heating the gas). Those new state-to-state QCT reaction rate constants represent a significant improvement in our understanding of the possible mechanisms leading to the destruction of HD by its collision with a H atom.
2015
The results of the laboratory study of reaction rate coefficients of several ion-molecule reactions with atomic and molecular hydrogen and molecular deuterium at low temperatures are presented in the thesis. The reaction rate coefficients of the N+ and H+ reaction with H2 were measured with respect to the nuclear spin configuration and rotational excitation of H2. The reactions of anions were a subject of the isotope exchange and isotope effect study. The measurements of the rate coefficients of H2O and D2O formation in the reaction of O– with H2 and D2, isotope exchange reactions OH– + D2 and OD– + H2, and associative detachment and charge transfer channels of D– + H interaction were performed. Experiments were carried out using an AB-22PT instrument with an ion trap. It has producing, guiding, trapping, and detecting systems for ions and a separate source of atomic H. The cooling system allowed to measure the temperature dependencies of the reaction rate coefficients at temperatur...