Dynamics of the C(3P) + Ethylene Reaction: A Trajectory Surface Hopping Study

Mechanistic Insight and Intersystem Crossing Dynamics of the C(3P) + H2CO/D2CO Reaction

The reaction of atomic carbon, C(³P), with H₂CO has been investigated using the direct dynamics trajectory surface hopping (DDTSH) method with Tully's fewest switches algorithm. The lowest lying ground triplet and single states are considered for the dynamics study at a reagent collision energy of 8.0 kcal/mol. From the trajectory calculations, we observed that CH₂ + CO and H + HCCO are the two major product channels for the title reaction. The insertion mechanism of the C(³P) + H₂CO reaction is rather complex and is followed by three distinct intermediates with no entrance channel barrier to the reaction on the B3LYP/6-31G(d,p) potential energy surfaces. The triplet insertion complexes are formed by three different approaches; "Sideways", "End-on" and "Head-on" attack of the triplet carbon atom toward H₂CO molecule. Our dynamics calculations predict a new product channel (H + HCCO(X ²A'')) with a contribution of ∼46% of the overall products formation via ketocarbene intermediate through "Head-on" approach. Despite the weak spin-orbit coupling (SOC) interactions, intersystem crossing (ISC) via a ketocarbene intermediate has a small but significant contribution, about 2.3%, for the CH₂ + CO channel. To understand the kinetic isotope effects on the reaction dynamics, we have extended our study for the C(³P) + D₂CO reaction. It is seen that isotopic substitution of both the H atoms has a small reduction in the extent of ISC dynamics for the carbene formation. Our results, certainly, reveal the importance of the ketocarbene intermediate and the H + HCCO products channel as one of the major product formation channels in the title reaction, which was not reported earlier.

Discovery of the propargyl radical (CH2CCH) in TMC-1: one of the most abundant radicals ever found and a key species for cyclization to benzene in cold dark clouds

We present the first identification in interstellar space of the propargyl radical (CH2CCH). This species was observed in the cold dark cloud TMC-1 using the Yebes 40m telescope. The six strongest hyperfine components of the 20,2-10,1 rotational transition, lying at 37.46 GHz, were detected with signal-to-noise ratios in the range 4.6-12.3 σ. We derive a column density of 8.7 × 1013 cm-2 for CH2CCH, which translates to a fractional abundance relative to H2 of 8.7 × 10-9. This radical has a similar abundance to methyl acetylene, with an abundance ratio CH2CCH/CH3CCH close to one. The propargyl radical is thus one of the most abundant radicals detected in TMC-1, and it is probably the most abundant organic radical with a certain chemical complexity ever found in a cold dark cloud. We constructed a gas-phase chemical model and find calculated abundances that agree with, or fall two orders of magnitude below, the observed value depending on the poorly constrained low-temperature reactivity of CH2CCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + C2H4 reaction and by the dissociative recombination of C3Hn + ions with n = 4-6. The propargyl radical is believed to control the synthesis of the first aromatic ring in combustion processes, and it probably plays a key role in the synthesis of large organic molecules and cyclization processes to benzene in cold dark clouds.