63 research outputs found
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
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
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
The Stability of C6H6+2: The Implication for Polycyclic Aromatic Hydrocarbon Dications
The fragmentation pathways of C6H are studied using density functional theory. All of the transition states that were found, excluding the rearrangement to the C5v structure, are significantly above the six-membered ring structure. This is consistent with the soft ionization experiments that find mostly dication production instead of fragmentation. Since the C5v structure is probably of limited importance for multiple-ring polycyclic aromatic hydrocarbons, the computed barriers suggest that Coulomb explosion is not a significant channel under most astrophysical conditions, and therefore, the dications are long-lived and should be considered in modeling of the interstellar medium
The Peroxymonocarbonate anion HCO4- as an effective oxidant in the gas phase: A mass spectrometric and theoretical study on the reaction with SO2
The peroxymonocarbonate anion, HCO4-, the covalent adduct between the carbon dioxideand hydrogen peroxide anion, effectively reacts with SO2 in the gas phase following three oxidative routes. Mass spectrometric and electronic structure calculations show that sulphur dioxide is oxidised through a common intermediate to the hydrogen sulphate anion, sulphur trioxide, and sulphur trioxide anion as primary products through formal HO2-, oxygen atom, and oxygen ion transfers. The hydrogen sulphite anion is also formed as a secondary product from the oxygen atom transfer path. The uncommon nucleophilic behaviour of HCO4- is disclosed by the Lewis acidic properties of SO2, an amphiphilic molecule that forms intermediates with characteristic and diagnostic geometries with peroxymonocarbonate
Revised gas-phase formation network of methyl cyanide: the origin of methyl cyanide and methanol abundance correlation in hot corinos
Methyl cyanide (CHCN) 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 CHCN 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 CHCN using our astrochemical
model. We confirm that the radiative association of CH and HCN,
forming CHCNH, followed by the electron recombination of
CHCNH, is the most important CHCN 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 CHCN predicted abundances with those measured in cold (10
K) and warm (90 K) objects. Unexpectedly, we also found a chemical link
between methanol and methyl cyanide via the CH ion, which can explain
the observed correlation between the CHOH and CHCN abundances measured
in hot corinos.Comment: 24 pages, 19 figures, accepted in MNRA
An Experimental and Theoretical Investigation of 1-Butanol Pyrolysis
Bioalcohols are a promising family of biofuels. Among them, 1-butanol has a strong potential as a substitute for petrol. In this manuscript, we report on a theoretical and experimental characterization of 1-butanol thermal decomposition, a very important process in the 1-butanol combustion at high temperatures. Advantage has been taken of a flash pyrolysis experimental set-up with mass spectrometric detection, in which the brief residence time of the pyrolyzing mixture inside a short, resistively heated SiC tube allows the identification of the primary products of the decomposing species, limiting secondary processes. Dedicated electronic structure calculations of the relevant potential energy surface have also been performed and RRKM estimates of the rate coefficients and product branching ratios up to 2,000 K are provided. Both electronic structure and RRKM calculations are in line with previous determinations. According to the present study, the H2O elimination channel leading to 1-butene is more important than previously believed. In addition to that, we provide experimental evidence that butanal formation by H2 elimination is not a primary decomposition route. Finally, we have experimental evidence of a small yield of the CH3 elimination channel
The Italian National Project of Astrobiology-Life in Space-Origin, Presence, Persistence of Life in Space, from Molecules to Extremophiles
The \u2018\u2018Life in Space\u2019\u2019 project was funded in the wake of
the Italian Space Agency\u2019s proposal for the development
of a network of institutions and laboratories conceived to
implement Italian participation in space astrobiology experiments
Production and Characterization of Molecular Dications: Experimental and Theoretical Efforts
Molecular dications are doubly charged cations of importance in flames, plasma chemistry and physics and in the chemistry of the upper atmosphere of Planets. Furthermore, they are exotic species able to store a considerable amount of energy at a molecular level. This high energy content of several eV can be easily released as translational energy of the two fragment monocations generated by their Coulomb explosion. For such a reason, they were proposed as a new kind of alternative propellant. The present topic review paper reports on an overview of the main contributions made by the authors’ research groups in the generation and characterization of simple molecular dications during the last 40 years of coupling experimental and theoretical efforts
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