INFRARED SPECTRA OF PROPENE IN HELIUM NANODROPLETS AND SOLID PARA-HYDROGEN

We report the infrared spectra of propene in the C–H stretching region measured in helium nanodroplets (HENDI) at 0.4 K and solid \textit{para}-hydrogen (\textit{p}-\chem{H_2}) matrices at 3.2 K, in order to probe the effects of the matrix host environments on the experimental spectra. Propene is an ideal test molecule to study these matrix effects, due to the many anharmonic resonance polyads present in the C–H stretching region of the spectrum. We observe a 4 – 5 cm$^{-1}$ on average red-shift of the bands in \textit{p}-\chem{H_2} relative to HENDI. Moreover, the choice of matrix environment influences the positions and intensity ratios of transitions within each resonance polyad, leading to qualitatively different spectra. To better understand the nuances involved, simulations were performed that capture the important resonance interactions in a VPT2+K effective Hamiltonian. Certain elements of the Hamiltonian were adjusted to model the impact that different matrix environments have on the anharmonic couplings. In addition, propene reacted with hydrogen atoms \textit{via} electron bombardment of a \textit{p}-\chem{H_2} matrix during sample deposition, producing propyl radicals. \textit{i}-Propyl radicals were produced in greater proportion than \textit{n}-propyl radicals, indicating that for hydrogen addition to the double bond, the rate of addition to the terminal carbon (\textit{i}-propyl) is faster than the rate of addition to the center carbon (\textit{n}-propyl). Because the barriers for addition are approximately 700 cm$^{-1}$ – 1500 cm$^{-1}$ (1000 K – 2000 K), the only available mechanism for reaction in the \textit{p}-\chem{H_2} matrix (3.2 K) is tunneling. \textit{Ab initio} calculations were used to compute the tunneling probabilities for the formation of the \textit{n}-propyl and \textit{i}-propyl radicals. The rate of addition to the terminal carbon (\textit{i}-propyl) was calculated to be faster, in agreement with experiment