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Gas-phase formation of Complex Organic Models molecules in interstellar medium: computational investigations
No full text[excerpt form the abstract:] In the field of astro- and prebiotic chemistry, the building blocks of life, which are molecules composed of more than 6 atoms, are called Complex Organic Molecules (COMs). Their appearances on the early inorganic Earth is therefore one of the major issues faced by researchers interested in the origin of life. In this thesis, split into three parts, the main purpose is to show how different COMs are formed in interstellar medium (ISM), using computational chemistry.
The first part focuses mainly on preliminary studies aiming at evaluating the appropriate level of theory to use to perform studies of formation reactions. First, a comprehensive benchmark of C≡N stretching vibrations computed at harmonic and anharmonic levels is reported with the goal of proposing and validating a reliable computational strategy to get accurate results for this puzzling vibrational mode, involved in biological molcules, without any ad hoc scaling factor
CYANOMETHANIMINE ISOMERS IN COLD INTERSTELLAR CLOUDS: INSIGHTS FROM ELECTRONIC STRUCTURE AND KINETIC CALCULATIONS
Full text linkNew insights into the formation of interstellar cyanomethanimine, a species of great relevance in prebiotic chemistry, are provided by electronic structure and kinetic calculations for the reaction CN + CH2=NH. This reaction is a facile formation route of Z,E-C-cyanomethanimine, even under the extreme conditions of density and temperature typical of cold interstellar clouds. E-C-cyanomethanimine has been recently identified in Sgr B2(N) in the Green Bank Telescope (GBT) PRIMOS survey by P. Zaleski et al. and no efficient formation routes have been envisaged so far. The rate coefficient expression for the reaction channel leading to the observed isomer E-C-cyanomethanimine is 3.15 × 10-10 × (T/300)0.152 × e(−0.0948/T). According to the present study, the more stable Z-C-cyanomethanimine isomer is formed with a slightly larger yield (4.59 × 10−10 × (T/300)0.153 × e(−0.0871/T). As
the detection of E-isomer is favored due to its larger dipole moment, the missing detection of the Z-isomer can be due to the sensitivity limit of the GBT PRIMOS survey and the detection of the Z-isomer should be attempted with more sensitive instrumentation. The CN + CH2 = NH reaction can also play a role in the chemistry of the upper atmosphere of Titan where the cyanomethanimine products can contribute to the buildup of the observed nitrogenrich organic aerosols that cover the moon
Accurate Infrared (IR) Spectra for Molecules Containing the C≡N Moiety by Anharmonic Computations with the Double Hybrid B2PLYP Density Functional
No full textHerein, we report a comprehensive benchmark of C≡N stretching vibrations computed at harmonic and anharmonic levels with the aim of proposing and validating a reliable computational strategy to get accurate results for this puzzling vibrational mode without any ad hoc scaling factor. Anharmonic calculations employing second-order vibrational perturbation theory provide very good results when performed using the B2PLYP double-hybrid functional, in conjunction with an extended basis set and supplemented by semiempirical dispersion contributions. For larger systems, B2PLYP harmonic frequencies, together with B3LYP anharmonic corrections, offer a very good compromise between accuracy and computational cost without the need of any empirical scaling factor
Gas phase formation of the prebiotic molecule formamide: insights from new quantum computations
No full textNew insights into the formation of interstellar formamide, a species of great
relevance in prebiotic chemistry, are provided by electronic structure and
kinetic calculations for the reaction NH2 + H2CO -> NH2CHO + H. Contrarily to
what previously suggested, this reaction is essentially barrierless and can,
therefore, occur under the low temperature conditions of interstellar objects
thus providing a facile formation route of formamide. The rate coefficient
parameters for the reaction channel leading to NH2CHO + H have been calculated
to be A = 2.6x10^-12 cm^3 s^-1, beta = -2.1 and gamma = 26.9 K in the range
of temperatures 10-300 K. Including these new kinetic data in a refined
astrochemical model, we show that the proposed mechanism can well reproduce the
abundances of formamide observed in two very different interstellar objects:
the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular
shock L1157-B2. Therefore, the major conclusion of this Letter is that there is
no need to invoke grain-surface chemistry to explain the presence of formamide
provided that its precursors, NH2 and H2CO, are available in the gas-phase
State-of-the-Art Thermochemical and Kinetic Computations for Astrochemical Complex Organic Molecules: Formamide Formation in Cold Interstellar Clouds as a Case Study
No full textWe describe an integrated
computational strategy aimed at providing
reliable thermochemical and kinetic information on the formation processes
of astrochemical complex organic molecules. The approach involves
state-of-the-art quantum-mechanical computations, second-order vibrational
perturbation theory, and kinetic models based on capture and transition
state theory together with the master equation approach. Notably,
tunneling, quantum reflection, and leading anharmonic contributions
are accounted for in our model. Formamide has been selected as a case
study in view of its interest as a precursor in the abiotic amino
acid synthesis. After validation of the level of theory chosen for
describing the potential energy surface, we have investigated several
pathways of the OH + CH2NH and NH2 + H2CO reaction channels. Our results show that both reaction channels
are essentially barrierless (in the sense that all relevant transition
states lie below or only marginally above the reactants) and once
tunneling is taken into the proper account indicate that the reaction
can occur under the low temperature conditions of interstellar environments
Noncovalent Interactions and Internal Dynamics in Pyridine-Ammonia: A Combined Quantum-Chemical and Microwave Spectroscopy Study
Full text linkThe 1:1 complex of ammonia with pyridine is characterized by using state-of-the-art quantum-chemical computations combined with pulsed-jet Fourier-transform microwave spectroscopy. The computed potential energy landscape indicates the formation of a stable σ-type complex, which is confirmed experimentally: analysis of the rotational spectrum shows the presence of only one 1:1 pyridine-ammonia adduct. Each rotational transition is split into several components owing to the internal rotation of NH3 around its C3 axis and to the hyperfine structure of both (14) N quadrupolar nuclei, thus providing unequivocal proof that the two molecules form a σ-type complex involving both a N-H⋅⋅⋅N and a C-H⋅⋅⋅N hydrogen bond. The dissociation energy (BSSE- and ZPE-corrected) is estimated to be 11.5 kJ mol(-1) . This work represents the first application of an accurate yet efficient computational scheme, designed for the investigation of small biomolecules, to a molecular cluster
Reassessment of the Thermodynamic, Kinetic, and Spectroscopic Features of Cyanomethanimine Derivatives: A Full Anharmonic Perturbative Treatment
No full textHerein we report a full thermodynamic
and vibrational investigation
of C-cyanomethanimine isomers rooted into the Density Functional Theory
(DFT) and the second-order vibrational perturbation theory (VPT2).
We show that an anharmonic treatment affects dramatically the vibrational
behavior of the molecules, especially thanks to the inclusion of interaction
terms between the various modes. Furthermore, the equilibrium constant
between the isomers, as well as the rate constant, have been obtained
at both harmonic and anharmonic levels showing, as expected, slight
but non-negligible differences. To support our investigation, dispersion
effects have been employed
The Borderline between Reactivity and Pre-reactivity of Binary Mixtures of Gaseous Carboxylic Acids and Alcohols
No full textBy mixing primary and secondary alcohols with carboxylic acids just before the supersonic expansion within pulsed Fourier transform microwave experiments, only the rotational spectrum of the ester was observed. However, when formic acid was mixed with tertiary alcohols, adducts were formed and their rotational spectra could be easily measured. Quantum mechanical calculations were performed to interpret the experimental evidence
On the competition between weak O H⋯F and C H⋯F hydrogen bonds, in cooperation with C H⋯O contacts, in the difluoromethane – tert -butyl alcohol cluster
Full text linkThe 1:1 complex of tert-butyl alcohol with difluoromethane has been characterized by means of a joint experimental-computational investigation. Its rotational spectrum has been recorded by using a pulsed-jet Fourier-Transform microwave spectrometer. The experimental work has been guided and supported by accurate quantum-chemical calculations. In particular, the computed potential energy landscape pointed out the formation of three stable isomers. However, the very low interconversion barriers explain why only one isomer, showing one O. H⋯F and two C. H⋯O weak hydrogen bonds, has been experimentally characterized. The effect of the H. →. tert-butyl-group substitution has been analyzed from the comparison to the difluoromethane-water adduct
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