Mechanistic Insight and Intersystem Crossing Dynamics of the C( 3 P) + H 2 CO/D 2 CO Reaction.

The reaction of atomic carbon, C( 3 P), with H 2 CO 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 CH 2 + CO and H + HCCO are the two major product channels for the title reaction. The insertion mechanism of the C( 3 P) + H 2 CO 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 H 2 CO molecule. Our dynamics calculations predict a new product channel (H + HCCO(X 2 A'')) 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 CH 2 + CO channel. To understand the kinetic isotope effects on the reaction dynamics, we have extended our study for the C( 3 P) + D 2 CO 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.