32 research outputs found
A new Gaussian MCTDH program: implementation and validation on the levels of the water and glycine molecules
We report the main features of a new general implementation of the Gaussian Multi-Configuration Time-Dependent Hartree model. The code allows effective computations of time-dependent phenomena, including calculation of vibronic spectra (in one or more electronic states), relative state populations, etc. Moreover, by expressing the Dirac-Frenkel variational principle in terms of an effective Hamiltonian, we are able to provide a new reliable estimate of the representation error. After validating the code on simple one-dimensional systems, we analyze the harmonic and anharmonic vibrational spectra of water and glycine showing that reliable and converged energy levels can be obtained with reasonable computing resources. The data obtained on water and glycine are compared with results of previous calculations using the vibrational second-order perturbation theory method. Additional features and perspectives are also shortly discussed
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
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
Gas-phase formation of acetaldehyde: review and new theoretical computations
Among all the interstellar complex organic molecules (iCOMs), acetaldehyde is
one of the most widely detected species. The question of its formation route(s)
is, therefore, of a major interest regarding astrochemical models. In this
paper, we provide an extensive review of the gas-phase formation paths that
were, or are, reported in the literature and the major astrochemical databases.
Four different gas-phase formation routes stand out : (1) CHOCH + H
/ CHCHOH + e, (2) CH + O(P), (3) CHOH + CH and (4)
CHCHOH + OH / CHCHOH + O(P). Paths (2) and (3) were not studied
neither via laboratory or theoretical works in the low temperature and density
regime valid for the ISM. Thus, we carried out new accurate quantum chemistry
computations. A theoretical kinetics study at low temperatures (7-300 K),
adopting the RRKM scheme, was also performed. We confirm that reaction (2) is
efficient in forming acetaldehyde in the 7-300 temperature range (alpha = 1.21
x 10 cm s and beta = 0.16). On the contrary, our new
computations disprove the formation of acetaldehyde through reaction (3) (alpha
= 1.84, 0.67 x 10 cm s and beta = -0.07, -0.95). Path (1)
was showed to be inefficient too by recent computations, while path (4) was
formerly considered for glycolaldehyde formation, having acetaldehyde as a
by-product. In conclusions, of the four above paths only two, the (2) and (4),
are potentially efficient gas-phase reaction routes for the formation of
acetaldehyde and we encourage astrochemical modellers to only consider them.
Comparison with astronomical observations suggest that path (4) may actually
play the major role
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
Quantum mechanical study of the correlation of attack and recoil angles for the Cl + H2 reaction using the stereodirected and discrete variable representations on two potential energy surfaces
The zero total angular momentum (J = 0) S matrix elements, calculated using a time-dependent wave packet method for the Cl (2P) + H2 reaction on two different potential energy surfaces, have been matrix transformed to the stereodirected and Gauss-Legendre discrete variable representations. Although the results in the two representations are (as expected) quantitatively different with respect to the angular selectivity and-specificity of the reactive process, the qualitative similarity has allowed us to draw for the first time conclusions with respect to some characteristics of the potential energy surface