Infrared Spectra of the 1-Chloromethyl-1-methylallyl and 1-Chloromethyl-2-methylallyl Radicals Isolated in Solid para-Hydrogen.

J Phys Chem A

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan.

Published: November 2017

AI Article Synopsis

  • Researchers studied how chlorine atoms react with isoprene in solid para-hydrogen at very low temperatures (3.2 K) using infrared spectroscopy.
  • After introducing UV light, new infrared lines emerged, indicating the formation of two main types of chemical species (Sets A and B) from this reaction.
  • These species were identified as two specific radicals resulting from the addition of chlorine to different carbon atoms in isoprene, with one being slightly more stable than the other, while both being produced in roughly equal amounts.

Article Abstract

The reaction of chlorine atoms (Cl) with isoprene (2-methyl-1,3-butadiene, CH) in solid para-hydrogen (p-H) matrices at 3.2 K was studied using infrared (IR) spectroscopy. Mixtures of CH and Cl were codeposited in p-H at 3.2 K, followed by irradiation with ultraviolet light at 365 nm to induce the photodissociation of Cl and the subsequent reaction of the Cl atoms with CH. Upon 365 nm photolysis, a multitude of new lines appeared in the IR spectrum, and, based on the secondary photolysis behavior, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set A (intense lines at 1237.9, 807.8, and 605.6/608.2 cm, and several other weaker lines) and set B (intense lines at 942.4, 1257.7, 796.7/798.5, 667.9, and 569.7 cm, and several other weaker lines). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level for ·CH and the four possible isomers of the ·CHCl radicals, produced from the addition of the Cl atom to the four distinct sites of carbon atoms in CH, to determine the relative energetics and predict IR spectra for each radical. The newly observed lines of sets A and B are assigned to the 1-chloromethyl-2-methylallyl radical (addition to carbon 4) and the 1-chloromethyl-1-methylallyl radical (addition to carbon 1) according to comparison with predicted IR spectra of possible products. The 1-chloromethyl-2-methylallyl radical and 1-chloromethyl-1-methylallyl radicals were predicted to be the most stable, with the latter ∼8 kJ mol lower in energy than the former. The ratio of the 1-chloromethyl-1-methylallyl to the 1-chloromethyl-2-methylallyl radicals is estimated to be (1.2 ± 0.5):1.0, indicating that the two radicals are produced in approximately equal amounts. The exclusive production of the radicals involving the addition of the Cl atom to the two terminal carbons of isoprene is analogous to what was previously observed for the reaction of Cl atoms with trans-1,3-butadiene in solid p-H.

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http://dx.doi.org/10.1021/acs.jpca.7b07922DOI Listing

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Article Synopsis
  • Researchers studied how chlorine atoms react with isoprene in solid para-hydrogen at very low temperatures (3.2 K) using infrared spectroscopy.
  • After introducing UV light, new infrared lines emerged, indicating the formation of two main types of chemical species (Sets A and B) from this reaction.
  • These species were identified as two specific radicals resulting from the addition of chlorine to different carbon atoms in isoprene, with one being slightly more stable than the other, while both being produced in roughly equal amounts.
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