Ab Initio MO Study of the Global Potential Energy Surface of C4H4 in Triplet Electronic State and the Reactions of C(3Pj) with C3H4 (Allene and Propyne) and C2(A3Πu) with C2H4(X1A1g+)

The global potential energy surface of C 4 H 4 in the lowest triplet electronic state have been studied at the G2M(RCC,MP2) level. Of 28 distinct isomers the most stable are aromatic cyclobutene q3 ( 3 A 1g, D 4 h ) and linear butyne c1 ( 3 E, D 2 d ), and 66 transition states for various isomerization and dissociation pathways have been found. The information about the global PES is applied to describe the potential energy surfaces for the C( 3 P j ) + H 2 CCCH 2, C( 3 P j ) + H 3 CCCH, and C 2 ( 3 Π u ) + C 2 H 4 reactions, recently studied experimentally in crossed molecular beams. The reaction of the carbon atom with allene is shown to occur by a barrierless addition of C to the C C bond to yield the three-member ring structure t1 and/or to the central carbon atom of allene to form the branching structure b1 which isomerizes to t1 with a low barrier. t1 undergoes ring opening to c1 with a barrier of 9.4 kcal/mol, and the latter emits a H atom to give the major reaction product n -C 4 H 3 with an exit barrier of 2.2 kcal/mol. The minor reaction product, i -C 4 H 3, is formed through a 1,2-H shift in c1 leading to c3, followed by the hydrogen loss. The reaction of the carbon atom with methylacetylene starts with a barrierless C addition to the β-C-atom of H 3 CCCH to form c6, to the α-C-atom to give b2 or b2 ‘, or to the acetylenic C⋮C bond to yield the cyclic isomers t4 or t4 ‘. c6, b2, and b2 ‘ are metastable and would rapidly rearrange to the linear isomer c5 or to the cyclic t4 and then to t4 ‘ with low barriers. The latter ring opens to c5 with a barrier of 14.7 kcal/mol. At high collision energies, the major reaction product n -C 4 H 3 is formed through the methyl hydrogen emission in c5 with an exit barrier of 5.8 kcal/mol. The second product i -C 4 H 3 can be reached via a H-shift from c5 to c4 and subsequent hydrogen elimination. Cyclic isomers C 4 H 3 p3 and p4 can originate from t4, t4 ‘, and t5 . The reaction of C 2 ( 3 Π u ) with ethylene proceeds through a C 2 addition to a carbon atom of C 2 H 4 to yield the chain isomer c2 with an entrance barrier of ∼4 kcal/mol. c2 rearranges to linear c1 in several steps via the four-member ring intermediate q1 or the three-membered ring structures t2 and t1 . c1 loses a H atom to yield n -C 4 H 3 or undergoes a hydrogen migration to c3 followed by a H emission producing n - or i -C 4 H 3 . Similarly to C( 3 P j ) + allene, n -C 4 H 3 is expected to be the major product of C 2 ( 3 Π u ) + C 2 H 4, while i -C 4 H 3 to give a minor contribution.