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

The reaction of chlorine atoms (Cl) with isoprene (2-methyl-1,3-butadiene, C₅H₈) in solid para-hydrogen (p-H₂) matrices at 3.2 K was studied using infrared (IR) spectroscopy. Mixtures of C₅H₈ and Cl₂ were codeposited in p-H₂ at 3.2 K, followed by irradiation with ultraviolet light at 365 nm to induce the photodissociation of Cl₂ and the subsequent reaction of the Cl atoms with C₅H₈. 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^-1, 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^-1, and several other weaker lines). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level for ·C₅H₇ and the four possible isomers of the ·C₅H₈Cl radicals, produced from the addition of the Cl atom to the four distinct sites of carbon atoms in C₅H₈, 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^-1 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-H₂.