An ab initio/RRKM study of product branching ratios in the photodissociation of buta-1,2- and -1,3-dienes and but-2-yne at 193 nm

Abstract

Ab initio G2M(MP2)//B3LYP/6-311G** calculations have been performed to investigate the reaction mechanism of photodissociation of buta-1,2- and -1,3-dienes and but-2-yne after their internal conversion into the vibrationally hot ground electronic state. The detailed study of the potential-energy surface was followed by microcanonical RRKM calculations of energy-dependent rate constants for individual reaction steps (at 193 nm photoexcitation and under collision-free conditions) and by solution of kinetic equations aimed at predicting the product branching ratios. For buta-1,2-diene, the major dissociation channels are found to be the single C-C bond cleavage to form the methyl and propargyl radicals and loss of hydrogen atoms from various positions to produce the but-2-yn-1-yl (pl), buta-1,2-dien-4-yl (p2), and but-1-yn-3-yl (p3) isomers of C4H5. The calculated branching ratio of the CH3 + C2H3/C4H5 + H products, 87.9:5.9, is in a good agreement with the recent experimental value of 96:4 (ref. [21]) taking into account that a significant amount of the C4H5 product undergoes secondary dissociation to C4H4 + H. The isomerization of buta-1,2-diene to buta-1,3-diene or but-2-yne appears to be slower than its one-step decomposition and plays only a minor role. On the other hand, the buta-1,3-diene → buta-1,2-diene, buta-1,3-diene → but-2-yne, and buta-1,3-diene → cyclobutene rearrangements are significant in the dissociation of buta-1,3-diene, which is shown to be a more complex process. The major reaction products are still CH3 + C3H3, formed after the isomerization of buta-1,3-diene to buta-1,2-diene, but the contribution of the other radical channels, C4H5 + H and + C2H3, as well as two molecular channels, C2H2 + C2H4 and C4H4 + H2, significantly increases. The overall calculated C4H5 + H/CH3 + C3H3/C2H3 + C2H3/C2H3 C4H4 + H2/C2H2 + C2H4 branching ratio is 24.0:49.6:4.6:6.1:15.2, which agrees with the experimental value of 20:50:8:2:20[22] within 5% margins. For but-2-yne, the one-step decomposition pathways, which include mostly H atom loss to produce p1 and, to a minor extent, molecular hydrogen elimination to yield methylethynylcarbene, play an approximately even role with that of the channels that involve the isomerization of but-2-yne to buta-1,2- or -1,3-dienes. p1 + H are the most important reaction products, with a branching ratio of 56.6%, followed by CH3 + C3H3 (23.8 %). The overall C4H5 + H/CH3 + C3H3/C2H3 + C2H3/C4H4 + H2/C2H2 + C2H4 branching ratio is predicted as 62.0:23.8:2.5:5.7:5.6. Contrary to buta-1,2- and -1,3-dienes, photodissociation of but-2-yne is expected to produce more hydrogen atoms than methyl radicals. The isomerization mechanisms between various isomers of the C4H6 molecule including buta-1,2- and -1,3-dienes, but-2-yne, 1-methylcyclopropene, dimethyl-vinylidene, and cyclobutene have been also characterized in detail.