C2H+H2CO: a new route for formaldehyde removal

The sulphur saga in TMC-1: Discovery of HCSCN and HCSCCH

We report the detection, for the first time in space, of cyano thioformaldehyde (HCSCN) and propynethial (HCSCCH) towards the starless core TMC-1. Cyano thioformaldehyde presents a series of prominent a- and b-type lines, which are the strongest previously unassigned features in our Q-band line survey of TMC-1. Remarkably, HCSCN is four times more abundant than cyano formaldehyde (HCOCN). On the other hand, HCSCCH is five times less abundant than propynal (HCOCCH). Surprisingly, we find an abundance ratio HCSCCH/HCSCN of ∼ 0.25, in contrast with most other ethynyl-cyanide pairs of molecules for which the CCH-bearing species is more abundant than the CN-bearing one. We discuss the formation of these molecules in terms of neutral-neutral reactions of S atoms with CH2CCH and CH2CN radicals as well as of CCH and CN radicals with H2CS. The calculated abundances for the sulphur-bearing species are, however, significantly below the observed values, which points to an underestimation of the abundance of atomic sulphur in the model or to missing formation reactions, such as ion-neutral reactions.

Ab initio spectroscopic characterization of the radical CH3OCH2 at low temperatures

Spectroscopic and structural properties of methoxymethyl radical (CH₃OCH₂, RDME) are determined using explicitly correlated ab initio methods. This radical of astrophysical and atmospheric relevance has not been fully characterized at low temperatures, which has delayed astrophysical research. We provide rovibrational parameters, excitations to the low energy electronic states, torsional and inversion barriers, and low vibrational energy levels. In the electronic ground state (X²A), which appears "clean" from nonadiabatic effects, the minimum energy structure is an asymmetric geometry whose rotational constants and dipole moment have been determined to be A₀ = 46 718.67 MHz, B₀ = 10 748.42 MHz, and C₀ = 9272.51 MHz, and 1.432D (μ_A = 0.695D, µ_B = 1.215D, µ_C = 0.302D), respectively. A variational procedure has been applied to determine torsion-inversion energy levels. Each level splits into 3 subcomponents (A₁/A₂ and E) corresponding to the three methyl torsion minima. Although the potential energy surface presents 12 minima, at low temperatures, the infrared band shapes correspond to a surface with only three minima because the top of the inversion V^α barrier at α = 0° (109 cm^-1) stands below the zero point vibrational energy and the CH₂ torsional barrier is relatively high (∼2000 cm^-1). The methyl torsion barrier was computed to be ∼500 cm^-1 and produces a splitting of 0.01 cm^-1 of the ground vibrational state.

The interstellar chemistry of H2C3O isomers

We present the detection of two H2C3O isomers, propynal and cyclopropenone, toward various starless cores and molecular clouds, together with upper limits for the third isomer propadienone. We review the processes controlling the abundances of H2C3O isomers in interstellar media showing that the reactions involved are gas-phase ones. We show that the abundances of these species are controlled by kinetic rather than thermodynamic effects.

DOES P-YLIDIC RADICAL FEATURE THE YLIDIC OR RADICAL CHARACTER TOWARD FORMALDEHYDE?

Theoretical investigations are preformed on the model reaction of PH 3 CH + H 2 CO to unravel reactivity of PH 3 CH , so-called Ylide-like radical, at G3MP2 and CCSD(T)/aug-cc-pVDZ//MP2/6-311++G(d,p) level. The results indicate that P -Ylidic radical behaves differently from the P -Ylide or usual unsaturated radical toward formaldehyde. For the entrance channels, [2+2] cycloaddition to form oxaphosphetane radical INT1 is more favorable, which is an analogical process to Witting reaction of P -Ylide. So PH 3 CH shows P -Ylide character in entrance channel, which has found the support from the dual one-center frontier–orbital interaction based on Fukui's frontier–orbital theory. However, due to the radical character of INT1, it can competitively give P2 PH 3 CHCHO + H and INT2, but not Witting-type products of alkene and phosphine oxide. At last, INT2 almost can barrierlessly fragmentate to Corey-type products of ethene oxide radical and phosphine. The present study shows that PH 3 CH , Ylide-like radical, has duplicate reactivity of both P-Ylide and radical, and title reaction presents a completely new reaction mechanism, compared with previous well-known H -abstract or addition–elimination mechanism for radical- H 2 CO reactions.