jp9b01867_si_001.pdf (873.31 kB)
Ethyl + O2 in Helium Nanodroplets: Infrared Spectroscopy of the Ethylperoxy Radical
journal contribution
posted on 2019-04-04, 00:00 authored by Peter
R. Franke, Joseph T. Brice, Christopher P. Moradi, Henry F. Schaefer, Gary E. DouberlyHelium-solvated ethylperoxy
radicals (CH3CH2OO•) are formed
via the in situ reaction between 2A′ ethyl radical
and 3Σg– dioxygen. The
reactants are captured sequentially
through the droplet pick-up technique. Helium droplets are doped with
ethyl radical via pyrolysis of di-tert-amyl peroxide
or n-propylnitrite in an effusive, low-pressure source.
An infrared spectrum of ethylperoxy, in the CH stretching region,
is recorded with species-selective droplet beam depletion spectroscopy.
Spectral assignments are made via comparisons to second-order vibrational
perturbation theory with resonances (VPT2 + K) based on coupled-cluster
full quartic force fields. Cubic and quartic force constants, evaluated
using a small basis set, are transformed into the normal coordinate
system of the higher level quadratic force constants. This transformation
procedure eliminates the mismatch between normal modes, which is a
source of error whenever normal coordinate force constants from different
levels of theory are combined. The spectrum shows signatures of both
the C1 gauche and Cs trans rotamers
in an approximate 2:1 ratio; this is despite the prediction that the gauche rotamer lies 44 cm–1 lower on the
zero-Kelvin enthalpic potential surface for torsional interconversion.
Helium droplets are 0.4 K at equilibrium; therefore, in situ ethylperoxy
production is highly nonthermal.