Ab initio study of C 4 H 3 potential energy surface and reaction of ground‐state carbon atom with propargyl radical

The potential energy surface for the reaction of the ground‐state carbon atom [C( 3 P j )] with the propargyl radical [HCCCH 2 (X 2 B 1 )] is investigated using the G2M(RCC,MP2) method. Numerous local minima and transition states for various isomerization and dissociation pathways of doublet C 4 H 3 are studied. The results show that C( 3 P j ) attacks the π system of the propargyl radical at the acetylenic carbon atom and yields the n ‐C 4 H 3 ( 2 A′) isomer i3 after an 1,2‐H atom shift. This intermediate either splits a hydrogen atom and produces singlet diacetylene, [HCCCCH ( p1 )+H] or undergoes (to a minor amount) a 1,2‐H migration to i ‐C 4 H 3 ( 2 A′) i5 , which in turn dissociates to p1 plus an H atom. Alternatively, atomic carbon adds to the triple CC bond of the propargyl radical to form a three‐member ring C 4 H 3 isomer i1 , which ring opens to i3 . Diacetylene is concluded to be a nearly exclusive product of the C( 3 P j )+HCCCH 2 reaction. At the internal energy of 10.0 kcal/mol above the reactant level, Rice–Ramsperger–Kassel–Marcus calculations show about 91.7% of HCCCCH comes from fragmentation of i3 and 8.3% from i5 . The other possible minor channels are identified as HCCCC+H 2 and C 2 H+HCCH. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1522–1535, 2001