Infrared laser spectroscopy of the n-propyl and i-propyl radicals in helium droplets: significant bend-stretch coupling revealed in the CH stretch region

Abstract

The n-propyl and i-propyl radicals were generated in the gas phase via pyrolysis of n-butyl nitrite (CH$_{3}$(CH$_{2}$)$_{3}$ONO) and i-butyl nitrite (CH$_{3}$CH(CH$_{3}$)CH$_{2}$ONO) precursors, respectively. Nascent radicals were promptly solvated by a beam of He nanodroplets, and the infrared spectra of the radicals were recorded in the C-H stretching region. In addition to three vibrations of n-propyl previously measured in an Ar matrix, we observe many unreported bands between 2800 and 3150 \wn, which we attribute to propyl radicals. The C-H stretching modes observed above 2960 \wn for both radicals are in excellent agreement with anharmonic frequencies computed using VPT2. Between 2800 and 2960 \wn, however, the spectra of n-propyl and i-propyl radicals become quite congested and difficult to assign due to the presence of multiple anharmonic resonances. Computations employing a local mode Hamiltonian reveal the origin of the spectral congestion to be strong coupling between the high frequency C-H stretching modes and the lower frequency bending/scissoring motions. The only significant local coupling is between stretches and bends on the same CH$_{2}$/CH$_{3}$ group.