Interaction of Chiral Propylene Oxide (CH₃CHCH₂O) with Helium: Potential Energy Surface and Scattering Calculations

Abstract

The first chiral interstellar organic molecule, propylene oxide (CH 3 CHCH 2 O), was detected recently toward the galactic center. Accurate determination of its abundance relies on the knowledge of collisional cross sections. We investigate here the rotational excitation of propylene oxide induced by collisions with helium. The calculations are based on a three-dimensional CH 3 CHCH 2 O−He potential energy surface computed using the explicitly correlated coupled-cluster theory extended to the complete basis set limit [CCSD(T)-F12b/CBS]. The interaction energies are fitted using an interpolating moving least squares method, and this potential is refitted using a partial wave expansion based on spherical harmonics. Rotational cross sections are obtained at the quantum close-coupling level for a collision energy of 10 cm −1 . Convergence issues and collisional propensity rules are discussed.

Collisional excitation and non-LTE modelling of interstellar chiral propylene oxide

“…In the analysis of McGuire et al (2016) it was noted that the excitation temperature that fits their observations best was not well-constrained and other column densities for an excitation temperature range 5 K ≤ T ex ≤ 35 K could also produce a good fit. This was highlighted by some of us (Faure et al 2019) as a source of uncertainty in the derived abundance. Additionally, the rotational level populations of propylene oxide cannot be accurately described by a single excitation temperature, as it relies on the local thermodynamic equilibrium (LTE) assumption in which the detected levels follow a Boltzmann distribution.…”

“…In 2019, some of us generated the 3D potential energy surface (PES) of the CH 3 CHCH 2 O−He interacting system (Faure et al 2019). The first objective of this study is to use this PES to calculate rotational cross sections for the inelastic collision between propylene oxide and He at low collision energy and at the quantum level.…”

Inelastic scattering in isotopologues of O2–Ar: the effects of mass, symmetry, and density of states

“…The two dimensional (2D) PES was constructed using an automated interpolating moving least squares (IMLS) methodology, which has been used in several previous studies 24,[83][84][85][86][87] and has been recently released as a software package under the name AUTOSURF. 88 As usual, 89 a local fit was expanded about each data point and the final potential is obtained as the normalized weighted sum of the local fits.…”

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

“…Furthermore, to interpret the inferred temperatures (kinetic or radiative) and to determine the importance of infrared pumping, collisional data are essential. Detailed quantum mechanical calculations exist for CH 3 CHO [120] and are being calculated for NH 2 CHO, CH 3 OCH 3 , and CH 3 CHCH 2 O [170]. For larger molecules, such calculations are currently unfeasible, but at the temperatures of ∼100-300 K of star-forming regions, approximate recipes and extrapolations for their collision rates would be adequate.…”