[Show abstract][Hide abstract] ABSTRACT: Quantum mechanical calculations are important tools for predicting the rates of elementary reactions, particularly for those involving hydrogen and at low temperatures where quantum effects become increasingly important. These approaches are computationally expensive, however, particularly when applied to complex polyatomic systems or processes characterized by deep potential wells. While several approximate techniques exist, many of these have issues with reliability. The ring-polymer molecular dynamics method was recently proposed as an accurate and efficient alternative. Here, we test this technique at low temperatures (300?50 K) by analyzing the behavior of the barrierless C(1D) + H2 reaction over the two lowest singlet potential energy surfaces. To validate the theory, rate coefficients were measured using a supersonic flow reactor down to 50 K. The experimental and theoretical rates are in excellent agreement, supporting the future application of this method for determining the kinetics and dynam
[Show abstract][Hide abstract] ABSTRACT: The ketenyl radical (HCCO) has recently been discovered in two cold dense
clouds with a non-negligible abundance of a few 1e-11 (compared to H2) (Agundez
et al. 2015). Until now, no chemical network has been able to reproduce this
observation. We propose here a chemical scheme that can reproduce HCCO
abundances together with HCO, H2CCO and CH3CHO in the dark clouds Lupus-1A and
L486. The main formation pathway for HCCO is the OH + CCH -> HCCO + H reaction
as suggested by Agundez et al. (2015) but with a much larger rate coefficient
than used in current models. Since this reaction has never been studied
experimentally or theoretically, this larger value is based on a comparison
with other similar systems.
Monthly Notices of the Royal Astronomical Society Letters 09/2015; 453(1). DOI:10.1093/mnrasl/slv097 · 5.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The mechanisms of the C + HNC, N + C2H, H + C2N and H + CNC reactions have been investigated using ab initio and DFT calculations. The long-range interactions and species involved in these systems on the triplet potential energy surface were optimized at the M06-2X/cc-pVQZ, MRCI+Q/aug-cc-pVTZ and CCSD(T)/aug-cc-pVQZ levels. The calculations show that these reactions proceed without a barrier in the entrance valley, and are mostly characterized by transition states which are submerged with respect to the reagent level. As a result, these processes are potentially very important for interstellar chemistry.
Chemical Physics Letters 07/2015; 635:174-179. DOI:10.1016/j.cplett.2015.05.060 · 1.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Context. A hot vortex formed in Saturn’s stratosphere following the 2010-2011 Northern Storm. Huge temperature increases have been measured in the vortex around the millibar level. Enhancements in hydrocarbon abundances has been observed at the millibar level in 2011-2012 inside this vortex.
Aims. We model the time-dependent photochemistry inside the vortex by accounting for the temperature variability over the period from January 2011 to March 2012 to assess whether photochemistry alone can explain the enhancements seen in the hydrocarbon abundances.
Methods. We use a 1D time-dependent photochemical model of Saturn and adapt it to the perturbed conditions of the vortex, after validating it in quiescent conditions.
Results. Our model predicts non-variability for ethane (C2H6) and acetylene (C2H2) and an increase in ethylene (C2H4) by a factor of 3, in the mbar region. Heavier hydrocarbons show a larger variability than the lighter ones. We are not able to reproduce the increase seen in C2H2 and we significantly underestimate the increase seen in C2H4.
Conclusions. Pure photochemistry does not explain the variability seen in the abundance of most hydrocarbons. This means that dynamics (eddy diffusion and/or advection) must have played a significant role in shaping the vertical profiles of the main hydrocarbons.
Astronomy and Astrophysics 06/2015; DOI:10.1051/0004-6361/201425444 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The rate of the gas-phase N(4S) + C2N( ) reaction has been measured in a continuous supersonic flow reactor down to 54 K through the relative-rate method using the N(4S) + OH(X2Π) H(2S) + NO(X2Π) reaction as a reference. The microwave discharge technique was employed to produce high concentrations of atomic nitrogen. Pulsed laser photolysis of precursor molecules Cl3C2N and H2O2 at 212 nm in-situ led to C2N and OH radical formation respectively. The rate constant is shown to be approximately independent of temperature, in contrast to previous studies of atom-radical reactions involving atomic nitrogen. While the reaction rate is faster than previously estimated, astrochemical simulations indicate that this reaction is probably only a minor source of CN radicals in dense interstellar clouds.
The Journal of Physical Chemistry A 03/2015; 119(13). DOI:10.1021/acs.jpca.5b01259 · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Chemical models used to study the chemical composition of the gas and the
ices in the interstellar medium are based on a network of chemical reactions
and associated rate coefficients. These reactions and rate coefficients are
partially compiled from data in the literature, when available. We present in
this paper kida.uva.2014, a new updated version of the kida.uva public
gas-phase network first released in 2012. In addition to a description of the
many specific updates, we illustrate changes in the predicted abundances of
molecules for cold dense cloud conditions as compared with the results of the
previous version of our network, kida.uva.2011.
The Astrophysical Journal Supplement Series 03/2015; 217(2). DOI:10.1088/0067-0049/217/2/20 · 11.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The photochemistry of N2 and CH4 in the atmosphere of Titan leads to a very rich chemistry which is not well understood. The aim of our study is to improve our understanding of the production of nitrogen compounds and to predict the abundances of those with high molar mass with better accuracy. We have made a careful investigation of the neutral nitrogen photochemistry to improve current chemical schemes including the most abundant species and the most efficient reactions. We also studied the propagation of uncertainties on rate constants in our model and determined the key reactions from a global sensitivity analysis. Our photochemical model contains 124 species, 60 of which are nitrogen containing compounds, and 1141 reactions. Our results are in reasonable agreement with Cassini/INMS data in the higher atmosphere but our model overestimates the mole fractions of several nitriles in the lower stratosphere. New species such as CH3C3N and C3H7CN could be relatively abundant in Titan's atmosphere. Uncertainties on some nitrogen compounds are important and further studies of the key reactions that we have identified are needed to improve the predictivity of photochemical models. Meridional transport is expected to be an efficient process to govern the abundances of several nitriles in the lower stratosphere.
[Show abstract][Hide abstract] ABSTRACT: Context. The increased sensitivity and high spectral resolution of millimeter
telescopes allow the detection of an increasing number of isotopically
substituted molecules in the interstellar medium. The 14N/ 15N ratio is
difficult to measure directly for carbon containing molecules. Aims. We want to
check the underlying hypothesis that the 13C/ 12C ratio of nitriles and
isonitriles is equal to the elemental value via a chemical time dependent gas
phase chemical model. Methods. We have built a chemical network containing D,
13C and 15N molecular species after a careful check of the possible
fractionation reactions at work in the gas phase. Results. Model results
obtained for 2 different physical conditions corresponding respectively to a
moderately dense cloud in an early evolutionary stage and a dense depleted
pre-stellar core tend to show that ammonia and its singly deuterated form are
somewhat enriched in 15N, in agreement with observations. The 14N/ 15N ratio in
N2H+ is found to be close to the elemental value, in contrast to previous
models which obtain a significant enrichment, as we found that the
fractionation reaction between 15N and N2H+ has a barrier in the entrance
channel. The large values of the N2H+/15NNH+ and N2H+/ N15NH+ ratios derived in
L1544 cannot be reproduced in our model. Finally we find that nitriles and
isonitriles are in fact significantly depleted in 13C, questioning previous
interpretations of observed C15N, HC15N and H15NC abundances from 13C
Astronomy and Astrophysics 01/2015; 576. DOI:10.1051/0004-6361/201425113 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent observations have revealed the existence of complex organic molecules (COMs) in cold dense cores and pre-stellar cores.
The presence of these molecules in such cold conditions is not well understood and remains a matter of debate since the previously
proposed ‘warm-up’ scenario cannot explain these observations. In this paper, we study the effect of Eley–Rideal and complex
induced reaction mechanisms of gas-phase carbon atoms with the main ice components of dust grains on the formation of COMs
in cold and dense regions. Based on recent experiments, we use a low value for the chemical desorption efficiency (which was
previously invoked to explain the observed COM abundances). We show that our introduced mechanisms are efficient enough to
produce a large amount of COMs in the gas phase at temperatures as low as 10 K.
Monthly Notices of the Royal Astronomical Society 12/2014; 447(4). DOI:10.1093/mnras/stu2709 · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The coupled photochemistry of N2, CH4 and oxygen species in the atmosphere of Titan leads to a very rich chemistry which is not well understood. The aim of our study is to improve our understanding of the production of hydrocarbons and nitrogen compounds and to understand the origin of oxygen compounds in the atmosphere. As a first step, we have made a detailed investigation of neutral photochemistry of these species. New species, reactions and photolysis processes have been added from an extensive bibliography and theoretical works. We have also investigated the possible input of sulphur species in the atmosphere. Through the use of a Monte Carlo-based uncertainty propagation study and global sensitivity analysis, we have identified the key reactions that should be studied in priority to improve photochemical models of Titan's atmosphere.
Colloque du Programme National de Planétologie, Institut de Physique du Globe de Paris; 10/2014
[Show abstract][Hide abstract] ABSTRACT: We studied the hypothesis that micrometeorites and Enceladus' plume activity could carry sulfur-bearing species into the upper atmosphere of Titan, in a similar manner to oxygen-bearing species.
We have developed a detailed photochemical model of sulfur compounds in the atmosphere of Titan that couples hydrocarbon, nitrogen, oxygen and sulfur chemistries.
Photochemical processes produce mainly CS and H2CS in the upper atmosphere of Titan and C3S, H2S and CH3SH in the lower atmosphere. Mole fractions of these compounds depend significantly on the source of sulfur species.
A possible future detection of CS (or the determination of a low upper limit) could be used to discriminate between the two scenarios for the origin of sulfur species, which then could help to discriminate the various scenarios for the origin of H2O, CO and CO2 in the stratosphere of Titan.
Astronomy and Astrophysics 09/2014; 572. DOI:10.1051/0004-6361/201424703 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rate constants for the N((4)S) + C2((1)Σg(+)) reaction have been measured in a continuous supersonic flow reactor over the range 57 K ≤ T ≤ 296 K by the relative rate technique employing the N((4)S) + OH(X(2)Π) → H((2)S) + NO(X(2)Π) reaction as a reference. Excess concentrations of atomic nitrogen were produced by the microwave discharge method and C2 and OH radicals were created by the in situ pulsed laser photolysis of precursor molecules C2Br4 and H2O2 respectively. In parallel, quantum dynamics calculations were performed based on an accurate global potential energy surfaces for the three lowest lying quartet states of the C2N molecule. The 1(4)A'' potential energy surface is barrierless, having two deep potential wells corresponding to the NCC and CNC intermediates. Both the experimental and theoretical work show that the rate constant decreases to low temperature, although the experimentally measured values fall more rapidly than the theoretical ones except at the lowest temperatures. Astrochemical simulations indicate that this reaction could be the dominant source of CN in dense interstellar clouds.
[Show abstract][Hide abstract] ABSTRACT: We review the reactions involving HCN and HNC in dark molecular clouds to
elucidate new chemical sources and sinks of these isomers. We find that the
most important reactions for the HCN-HNC system are Dissociative Recombination
(DR) reactions of HCNH+ (HCNH+ + e-), the ionic CN + H3+, HCN + C+, HCN and HNC
reactions with H+/He+/H3+/H3O+/HCO+, the N + CH2 reaction and two new
reactions: H + CCN and C + HNC. We test the effect of the new rate constants
and branching ratios on the predictions of gas-grain chemical models for dark
cloud conditions. The rapid C + HNC reaction keeps the HCN/HNC ratio
significantly above one as long as the carbon atom abundance remains high.
However, the reaction of HCN with H3+ followed by DR of HCNH+ acts to isomerize
HCN into HNC when carbon atoms and CO are depleted leading to a HCN/HNC ratio
close to or slightly greater than 1. This agrees well with observations in
TMC-1 and L134N taking into consideration the overestimation of HNC abundances
through the use of the same rotational excitation rate constants for HNC as for
HCN in many radiative transfer models.
Monthly Notices of the Royal Astronomical Society 06/2014; 443(1). DOI:10.1093/mnras/stu1089 · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rate constants for the C(3P) + CH3OH reaction have been measured in a
continuous supersonic flow reactor over the range 50 K to 296 K. C(3P) was
created by the in-situ pulsed laser photolysis of CBr4, a multiphoton process
which also produced some C(1D), allowing us to investigate simultaneously the
low temperature kinetics of the C(1D) + CH3OH reaction. C(1D) atoms were
followed by an indirect chemiluminescent tracer method in the presence of
excess CH3OH. C(3P) atoms were detected by the same chemiluminescence technique
and also by direct vacuum ultra-violet laser induced fluorescence (VUV LIF).
Secondary measurements of product H(2S) atom formation have been undertaken
allowing absolute H atom yields to be obtained by comparison with a suitable
reference reaction. In parallel, statistical calculations have been performed
based on ab-initio calculations of the complexes, adducts and transition states
(TSs) relevant to the title reaction. By comparison with the experimental H
atom yields, the preferred reaction pathways could be determined, placing
important constraints on the statistical calculations. The experimental and
theoretical work are in excellent agreement, predicting a negative temperature
dependence of the rate constant increasing from 2.2 x 10-11 cm3 molecule-1 s-1
at 296 K to 20.0 x 10-11 cm3 molecule-1 s-1 at 50 K. CH3 and HCO are found to
be the major products under our experimental conditions. As this reaction is
not considered in current astrochemical networks, its influence on interstellar
methanol abundances is tested using a model of dense interstellar clouds.
[Show abstract][Hide abstract] ABSTRACT: Analysis of recent detections of water by Herschel/HIFI-PACS and Cassini/CIRS suggest for a steep gradient of the water profile in the lower stratosphere of Titan's atmosphere (Cottini2012, Moreno2012). This result provides a good opportunity to better understand the origin of oxygen compounds. However, the current photochemical models use an incomplete oxygen chemical scheme. In the present work, we improve the photochemistry of oxygen and introduce in particular a coupling between hydrocarbon, oxygen and nitrogen chemistries. Through the use of several different scenarios, we show that some oxygen compound abundances are sensitive to the nature of oxygen atoms (O+, OH and H2O) and the source of the flux (micrometeorites ablation or Enceladus' plume activity). Our model also predicts the presence of new and as yet undetected compounds such as NO (nitric oxide), HNO (nitrosyl hydride), HNCO (isocyanic acid) and N2O (nitrous oxide). Their future putative detection will give valuable constraints to discriminate between the different hypotheses for the nature and the source of oxygen compounds in the atmosphere of Titan. Through the use of a Monte Carlo-based uncertainty propagation study and global sensitivity analysis, we identify the key reactions that should be studied in priority to improve coupled photochemical models of Titan's atmosphere.
[Show abstract][Hide abstract] ABSTRACT: We review the reactions between carbon chain molecules and radicals, namely
Cn, CnH, CnH2, C2n+1O, CnN, HC2n+1N, with C, N and O atoms. Rate constants and
branching ratios for these processes have been re-evaluated using experimental
and theoretical literature data. In total 8 new species have been introduced,
41 new reactions have been proposed and 122 rate coefficients from
kida.uva.2011 (Wakelam et al. 2012) have been modified. We test the effect of
the new rate constants and branching ratios on the predictions of gas-grain
chemical models for dark cloud conditions using two different C/O elemental
ratios. We show that the new rate constants produce large differences in the
predicted abundances of carbon chains since the formation of long chains is
less effective. The general agreement between the model predictions and
observed abundances in the dark cloud TMC-1 (CP) is improved by the new network
and we find that C/O ratios of 0.7 and 0.95 both produce a similar agreement
for different times. The general agreement for L134N (N) is not significantly
changed. The current work specifically highlights the importance of O + CnH and
N + CnH reactions. As there are very few experimental or theoretical data for
the rate constants of these reactions we highlight the need for experimental
studies of the O + CnH and N + CnH reactions, particularly at low temperature.
Monthly Notices of the Royal Astronomical Society 11/2013; 437(1). DOI:10.1093/mnras/stt1956 · 5.11 Impact Factor