Infrared absorption of trans-1-chloromethylallyl and trans-1-methylallyl radicals produced in photochemical reactions of trans-1,3-butadiene and Cl-2 in solid para-hydrogen

Abstract

The reactions of chlorine and hydrogen atoms with trans-1,3-butadiene in solid para-hydrogen (p-H-2) were investigated with infrared (IR) absorption spectra. When a p-H-2 matrix containing Cl-2 and trans-1,3-butadiene was irradiated with ultraviolet light at 365 nm, intense lines at 650.3, 809.0, 962.2, 1240.6 cm(-1), and several weaker ones due to the trans-1-chloromethylallyl radical, center dot(CH2CHCH)CH2Cl, appeared. Observed wavenumbers and relative intensities agree with the anharmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++g(2d,2p) method. That the Cl atom adds primarily to the terminal carbon atom of trans-1,3-butadiene is in agreement with the path of minimum energy predicted theoretically, but in contrast to the reaction of Cl + propene in solid p-H-2 [J. Amicangelo and Y.-P. Lee, J. Phys. Chem. Lett. 1, 2956 (2010)] in which the addition of Cl to the central C atom is favored, likely through steric effects in a p-H-2 matrix. A second set of lines, intense at 781.6, 957.9, 1433.6, 2968.8, 3023.5, 3107.3 cm(-1), were observed when the UV-irradiated Cl-2/trans-1,3-butadiene/p-H-2 matrix was further irradiated with IR light from a SiC source. These lines are assigned to the trans-1-methylallyl radical, center dot(CH2CHCH)CH3, produced from reaction of 1,3-butadiene with a H atom resulted from the reaction of Cl atoms with solid p-H-2 exposed to IR radiation. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745075]