Indian Journal of Chemistry Vol. 49B, November 2010, pp. 1565-1570 Note Recombination of propargyl radicals to form benzene: A computational study Hari Ji Singh* & Nand Kishor Gour Department of Chemistry, DDU Gorakhpur University, Gorakhpur 273 009, India E-mail: hari_singh81@hotmail.com Received 1 May 2009; accepted (revised) 25 March 2010 Present study involves QCISD(T)/6-311G(d,p) level calcula- tions to analyze reaction pathways during the recombination of propargyl radicals leading to the formation of benzene. A new path on the C 3 H 3 recombination potential energy surface that connects 3,4-dimethylenecyclobutene (34DMCB) to benzene involving four-membered bicyclic compounds such as bicycle- [2.2.0]hexa-1(4),2-diene and bicyclo[2.2.0]hexa-2,5-diene have been determined. Geometries of all the species are optimized at HF/6-31G(d) level. All the stationary points along this path on the potential energy surface have been characterized. Single point energy calculation have been performed using QCISD(T) with 6-311G(d,p) basis set. Transition states are determined and characterized by the observation of only one imaginary frequency. Intrinsic Reaction Coordinate (IRC) calculation has also been performed in order to ascertain the existence of the transition states. Keywords: Propargyl radical, HF, IRC, PES, 3,4-dimethylene- cyclobutene One of the intriguing problems always faced by combustion chemists during the flame or high temperature pyrolytic studies of hydrocarbon fuels is to find an explanation for the formation of aromatics and soot from low molecular weight gaseous species such as C 2 H 2 , C 2 H 4 , etc. Propargyl radical C 3 H 3 (m/z 39) has been found abundantly in flames and it has also been detected during time-of-flight mass spectroscopic studies of many aliphatic fuels 1-4 . The latter studies also detected peak at m/z 78. Modeling calculations performed on the m/z 78 profile envisaged the species as benzene. Thus, the main question raised is how do these aliphatic fuels which breakup to smaller fragments mostly C 2 and C 3 species at high temperature say 1500-2000 K undergo cyclization and form benzene. C 3 H 3 is a resonantly stabilized free radical and is believed to play a critical role in the formation of aromatic hydrocarbons and soot 5-8 . The unpaired electron present in C 3 H 3 radical is delocalized and spread out over two or more sites. As a result of the delocalization of the unpaired electron, these free radicals are stabilized and normally form weaker bonds with stable molecules 9,10 . Such weakly bonded addition complexes are not easily stabilized by collisions at high temperature, nor do they readily undergo rearrangement. In such a situation such radicals are relatively non-reactive and can reach high concentrations in flames. High concentrations and rapid rates at which such radicals may react with another constituent make an important route for the formation of higher hydrocarbons in flames. There is a possibility of the formation of other C 3 H 3 isomers. Four such isomers viz., 2-propynyl (propargyl), 1-propynyl, cycloprop-1-enyl and cycloprop-2-enyl have been proposed to be thermally stable 11 . Additional isomers may also be formed but these have been ruled out, being kinetically unstable 11 . A recent computational study performed at a very high level utilizing electron correlation via second order Z-Averaged Perturbation Theory (ZAPT2) and primarily through coupled cluster theory by Wheeler et al. 11 reported the heat of formation of the different isomers of C 3 H 3 and the calculated isomerization energies. The results show that propargyl has the lowest energy and thus the most stable amongst the four C 3 H 3 isomers 11 . The recombination of propargyl radicals is proposed to be one of most important reactions occurring in hydrocarbon flames 5,6 . Depending on the reaction conditions, they may lead to the formation of a number of cyclic and chain hydrocarbons. It may be shown to exist in the following two resonating structural forms: C H H C C H C H H C C C H . . T H Out of these two resonating structures the structure marked as H is accepted to have a dominant one 12 . These stabilized free radicals can combine in three