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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
Author(s) -
Lee HwaYu,
Kislov Vadim V.,
Lin ShengHsien,
Mebel Alexander M.,
Neumark Daniel M.
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200390081
Subject(s) - chemistry , isomerization , ab initio , dissociation (chemistry) , branching fraction , potential energy surface , computational chemistry , propargyl , photodissociation , reaction rate constant , hydrogen atom abstraction , cyclobutene , diene , radical , branching (polymer chemistry) , bond cleavage , photochemistry , ab initio quantum chemistry methods , molecule , atomic physics , kinetics , organic chemistry , physics , ring (chemistry) , quantum mechanics , natural rubber , catalysis
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 CC 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 ( p1 ), buta‐1,2‐dien‐4‐yl ( p2 ), and but‐1‐yn‐3‐yl ( p3 ) isomers of C 4 H 5 . The calculated branching ratio of the CH 3 + C 3 H 3 /C 4 H 5 + 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 C 4 H 5 product undergoes secondary dissociation to C 4 H 4 + 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 CH 3 + C 3 H 3 , formed after the isomerization of buta‐1,3‐diene to buta‐1,2‐diene, but the contribution of the other radical channels, C 4 H 5 + H and C 2 H 3 + C 2 H 3 , as well as two molecular channels, C 2 H 2 + C 2 H 4 and C 4 H 4 + H 2 , significantly increases. The overall calculated C 4 H 5 + H/CH 3 + C 3 H 3 /C 2 H 3 + C 2 H 3 /C 4 H 4 + H 2 /C 2 H 2 + C 2 H 4 branching ratio is 24.0:49.6:4.6:6.1:15.2, which agrees with the experimental value of 20:50:8:2:2022 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 CH 3 + C 3 H 3 (23.8 %). The overall C 4 H 5 + H/CH 3 + C 3 H 3 /C 2 H 3 + C 2 H 3 /C 4 H 4 + H 2 /C 2 H 2 + C 2 H 4 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 C 4 H 6 molecule including buta‐1,2‐ and ‐1,3‐dienes, but‐2‐yne, 1‐methylcyclopropene, dimethylvinylidene, and cyclobutene have been also characterized in detail.