A recently developed
model chemistry (jun-Cheap) has been slightly
modified and proposed as an effective, reliable, and parameter-free
scheme for the computation of accurate reaction rates with special
reference to astrochemical and atmospheric processes. Benchmarks with
different sets of state-of-the-art energy barriers spanning a wide
range of values show that, in the absence of strong multireference
contributions, the proposed model outperforms the most well-known
model chemistries, reaching a subchemical accuracy without any empirical
parameter and with affordable computer times. Some test cases show
that geometries, energy barriers, zero point energies, and thermal
contributions computed at this level can be used in the framework
of the master equation approach based on the ab initio transition-state
theory for obtaining accurate reaction rates.
A recently
developed model chemistry (denoted as junChS [Alessandrini,
S.; et al. J. Chem. Theory Comput.
2020,
16, 988–1006]) has been extended to the
employment of explicitly correlated (F12) methods. This led us to
propose a family of effective, reliable, and parameter-free schemes
for the computation of accurate interaction energies of molecular
complexes ruled by noncovalent interactions. A thorough benchmark
based on a wide range of interactions showed that the so-called junChS-F12
model, which employs cost-effective revDSD-PBEP86-D3(BJ) reference
geometries, has an improved performance with respect to its conventional
counterpart and outperforms well-known model chemistries. Without
employing any empirical parameter and at an affordable computational
cost, junChS-F12 reaches subchemical accuracy. Accurate characterizations
of molecular complexes are usually limited to energetics. To take
a step forward, the conventional and F12 composite schemes developed
for interaction energies have been extended to structural determinations.
A benchmark study demonstrated that the most effective option is to
add MP2-F12 core–valence correlation corrections to fc-CCSD(T)-F12/jun-cc-pVTZ
geometries without the need of recovering the basis set superposition
error and the extrapolation to the complete basis set.
The atmospheric reaction of H 2 S with Cl has been re-investigated in order to check if, as previously suggested, only explicit dynamical computations can lead to an accurate evaluation of the reaction rate because of strong recrossing effects and the breakdown of the variational extension of transition state theory. For this reason, the corresponding potential energy surface has been thoroughly investigated, thus leading to an accurate characterization of all stationary points, whose energetics has been computed at the state of the art. To this end, coupled-cluster theory including up to quadruple excitations has been employed, together with the extrapolation to the complete basis set limit and also incorporating core-valence correlation, spin-orbit, and scalar relativistic effects as well as diagonal Born-Oppenheimer corrections. This highly accurate 1
A gas-phase formation route is proposed for the recently detected propargylimine molecule. In analogy to other imines, such as cyanomethanimine, the addition of a reactive radical (C2H in the present case) to methanimine (CH2NH) leads to reaction channels open also in the harsh conditions of the interstellar medium. Three possible isomers can be formed in the CH
2
NH + C2H reaction: Z- and E-propargylimine (Z-,E-PGIM) as well as N-ethynyl-methanimine (N-EMIM). For both PGIM species, the computed global rate coefficient is nearly constant in the 20–300 K temperature range, and of the order of 2–3 × 10−10 cm3 molecule−1 s−1, while that for N-EMIM is about two orders of magnitude smaller. Assuming equal destruction rates for the two isomers, these results imply an abundance ratio for PGIM of [Z]/[E] ∼ 1.5, which is only slightly underestimated with respect to the observational datum.
Despite the fact that the majority of current models assume that interstellar complex organic molecules (iCOMs) are formed on dust–grain surfaces, there is some evidence that neutral gas-phase reactions play an important role. In this paper, we investigate the reaction occurring in the gas phase between methylamine (CH3NH2) and the cyano (CN) radical, for which only fragmentary and/or inaccurate results have been reported to date. This case study allows us to point out the pivotal importance of employing quantum-chemical calculations at the state of the art. Since the two major products of the CH3NH2 + CN reaction, namely the CH3NH and CH2NH2 radicals, have not been spectroscopically characterized yet, some effort has been made for filling this gap.
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