124 research outputs found
The (impossible?) formation of acetaldehyde on the grain surfaces: insights from quantum chemical calculations
Complex Organic Molecules (COMs) have been detected in the interstellar
medium (ISM). However, it is not clear whether their synthesis occurs on the
icy surfaces of interstellar grains or via a series of gas-phase reactions. As
a test case of the COMs synthesis in the ISM, we present new quantum chemical
calculations on the formation of acetaldehyde (CH3CHO) from the coupling of the
HCO and CH3 radicals, both in gas phase and on water ice surfaces. The binding
energies of HCO and CH3 on the amorphous water ice were also computed (2333 and
734 K, respectively). Results indicate that, in gas phase, the products could
be either CH3CHO, CH4 + CO, or CH3OCH, depending on the relative orientation of
the two radicals. However, on the amorphous water ice, only the CH4 + CO
product is possible due to the geometrical constraints imposed by the water ice
surface. Therefore, acetaldehyde cannot be synthesized by the CH3 + HCO
coupling on the icy grains. We discuss the implications of these results and
other cases, such as ethylene glycol and dimethyl ether, in which similar
situations can occur, suggesting that formation of these molecules on the grain
surfaces might be unlikely
Reactions of oxygen atoms with aliphatic and aromatic hydrocarbons by crossed beam experiments
A theoretical investigation of the reaction between the amidogen, NH, and the ethyl, C2H5, radicals: a possible gas-phase formation route of interstellar and planetary ethanimine
The reaction between the amidogen, NH, radical and the ethyl, C2H5, radical
has been investigated by performing electronic structure calculations of the
underlying doublet potential energy surface. Rate coefficients and product
branching ratios have also been estimated by combining capture and RRKM
calculations. According to our results, the reaction is very fast, close to the
gas-kinetics limit. However, the main product channel, with a yield of ca.
86-88% in the range of temperatures investigated, is the one leading to
methanimine and the methyl radical. The channels leading to the two E-, Z-
stereoisomers of ethanimine account only for ca. 5-7% each. The resulting ratio
[E-CH3CHNH]/[Z-CH3CHNH] is ca. 1.2, that is a value rather lower than that
determined in the Green Bank Telescope PRIMOS radio astronomy survey spectra of
Sagittarius B2 North (ca. 3). Considering that ice chemistry would produce
essentially only the most stable isomer, a possible conclusion is that the
observed [E-CH3CHNH]/[Z-CH3CHNH] ratio is compatible with a combination of
gas-phase and grain chemistry. More observational and laboratory data are
needed to definitely address this issue
Investigating the Efficiency of Explosion Chemistry as a Source of Complex Organic Molecules in TMC-1
Many species of complex organic molecules (COMs) have been observed in
several astrophysical environments but it is not clear how they are produced,
particularly in cold, quiescent regions. One process that has been proposed as
a means to enhance the chemical complexity of the gas phase in such regions is
the explosion of the ice mantles of dust grains. In this process, a build up of
chemical energy in the ice is released, sublimating the ices and producing a
short lived phase of high density, high temperature gas. The gas-grain chemical
code UCLCHEM has been modified to treat these explosions in order to model the
observed abundances of COMs towards the TMC-1 region. It is found that, based
on our current understanding of the explosion mechanism and chemical pathways,
the inclusion of explosions in chemical models is not warranted at this time.
Explosions are not shown to improve the model's match to the observed
abundances of simple species in TMC-1. Further, neither the inclusion of
surface diffusion chemistry, nor explosions, results in the production of COMs
with observationally inferred abundances.Comment: Accepted for publication in Ap
Can Formamide Be Formed on Interstellar Ice? An Atomistic Perspective
Interstellar formamide (NH2CHO) has recently attracted significant attention
due to its potential role as a molecular building block in the formation of
precursor biomolecules relevant for the origin of life. Its formation, whether
on the surfaces of the interstellar grains or in the gas phase, is currently
debated. The present article presents new theoretical quantum chemical
computations on possible NH2CHO formation routes in water-rich amorphous ices,
simulated by a 33-H2O-molecule cluster. We have considered three possible
routes. The first one refers to a scenario used in several current
astrochemical models, that is, the radical-radical association reaction between
NH2 and HCO. Our calculations show that formamide can indeed be formed, but in
competition with formation of NH3 and CO through a direct H transfer process.
The final outcome of the NH2 + HCO reactivity depends on the relative
orientation of the two radicals on the ice surface. We then analyzed two other
possibilities, suggested here for the first time: reaction of either HCN or CN
with water molecules of the ice mantle. The reaction with HCN has been found to
be characterized by large energy barriers and, therefore, cannot occur under
the interstellar ice conditions. On the contrary, the reaction with the CN
radical can occur, possibly leading through multiple steps to the formation of
NH2CHO. For this reaction, water molecules of the ice act as catalytic active
sites since they help the H transfers involved in the process, thus reducing
the energy barriers (compared to the gas-phase analogous reaction).
Additionally, we apply a statistical model to estimate the reaction rate
coefficient when considering the cluster of 33-H2O-molecules as an isolated
moiety with respect to the surrounding environment, i.e., the rest of the ice
Interstellar dimethyl ether gas-phase formation: a quantum chemistry and kinetics study
Dimethyl ether is one of the most abundant interstellar complex organic
molecules. Yet its formation route remains elusive. In this work, we have
performed electronic structure and kinetics calculations to derive the rate
coefficients for two ion-molecule reactions recently proposed as a gas-phase
formation route of dimethyl ether in interstellar objects, namely CHOH +
CHOH (CH)OH + HO followed by
(CH)OH + NH CHOCH + NH. A
comparison with previous experimental rate coefficients for the reaction
CHOH + CHOH sustains the accuracy of the present calculations and
allow a more reliable extrapolation at the low temperatures of interest in
interstellar objects (10-100 K). The rate coefficient for the reaction
(CH)OH + NH is, instead, provided for the first time ever. The
rate coefficients derived in this work essentially confirm the prediction by
Taquet et al. (2016) concerning dimethyl ether formation in hot cores/corinos.
Nevertheless, this formation route cannot be efficient in cold objects (like
prestellar cores) where dimethyl ether is also detected, because ammonia has a
very low abundance in those environments
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 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 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 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
The genealogical tree of ethanol: gas-phase formation of glycolaldehyde, acetic acid and formic acid
Despite the harsh conditions of the interstellar medium, chemistry thrives in
it, especially in star forming regions where several interstellar complex
organic molecules (iCOMs) have been detected. Yet, how these species are
synthesised is a mystery. The majority of current models claim that this
happens on interstellar grain surfaces. Nevertheless, evidence is mounting that
neutral gas-phase chemistry plays an important role. In this article, we
propose a new scheme for the gas-phase synthesis of glycolaldehyde, a species
with a prebiotic potential and for which no gas-phase formation route was
previously known. In the proposed scheme, the ancestor is ethanol and the
glycolaldehyde sister species are acetic acid (another iCOM with unknown
gas-phase formation routes) and formic acid. For the reactions of the new
scheme with no available data, we have performed electronic structure and
kinetics calculations deriving rate coefficients and branching ratios.
Furthermore, after a careful review of the chemistry literature, we revised the
available chemical networks, adding and correcting several reactions related to
glycolaldehyde, acetic acid and formic acid. The new chemical network has been
used in an astrochemical model to predict the abundance of glycolaldehyde,
acetic acid and formic acid. The predicted abundance of glycolaldehyde depends
on the ethanol abundance in the gas phase and is in excellent agreement with
the measured one in hot corinos and shock sites. Our new model overpredicts the
abundance of acetic acid and formic acid by about a factor of ten, which might
imply a yet incomplete reaction network
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