Infrared spectroscopy of the n-propyl and i-propyl radicals in solid para-hydrogen

Abstract

We report infrared spectra of the n- and i-propyl radicals isolated in solid para-hydrogen (p-H2). Propyl radicals were produced via 248 nm irradiation of matrices formed by co-deposition of p-H2 and either 1-iodopropane (forming n-propyl) or 2-iodopropane (forming i-propyl). Secondary photolysis was used to group spectral bands arising from the same species. In addition to those observed previously in either argon matrix or helium droplet spectra, we assign approximately 20 previously unreported bands. The assignment of features to propyl radicals is made via comparisons to VPT2+K anharmonic frequency computations. In separate measurements, propyl radicals were generated via the reaction between propene and hydrogen atoms produced within the p-H2 matrix. Because the barriers are between 700 and 1500 cm−1, the H + propene reaction is necessarily under tunneling control when carried out in solid p-H2 at 3.3 K. i-Propyl radicals were produced in ∼5–20 times greater abundance than n-propyl radicals, indicating that H atom addition to the terminal carbon is faster than addition to the central carbon. The Wentzl-Kramers-Brillouin approximation is applied to one-dimensional abinitio potentials to estimate the tunneling time constants for formation of n- and i-propyl. This model predicts the rate of addition to the terminal carbon (forming i-propyl) to be 17 times faster, in qualitative agreement with the experimental branching ratio.