3 research outputs found
Uptake of Epoxydiol Isomers Accounts for Half of the Particle-Phase Material Produced from Isoprene Photooxidation via the HO<sub>2</sub> Pathway
The
oxidation of isoprene is a globally significant source of secondary
organic material (SOM) of atmospheric particles. The relative importance
of different parallel pathways, however, remains inadequately understood
and quantified. SOM production from isoprene photooxidation was studied
under hydroperoxyl-dominant conditions for <5% relative humidity
and at 20 °C in the presence of highly acidic to completely neutralized
sulfate particles. Isoprene photooxidation was separated from SOM
production by using two continuously mixed flow reactors connected
in series and operated at steady state. Two online mass spectrometers
separately sampled the gas and particle phases in the reactor outflow.
The loss of specific gas-phase species as contributors to the production
of SOM was thereby quantified. The produced SOM mass concentration
was directly proportional to the loss of isoprene epoxydiol (IEPOX)
isomers from the gas phase. IEPOX isomers lost from the gas phase
accounted for (46 ± 11)% of the produced SOM mass concentration.
The IEPOX isomers comprised (59 ± 21)% (molecular count) of the
loss of monitored gas-phase species. The implication is that for the
investigated reaction conditions the SOM production pathways tied
to IEPOX isomers accounted for half of the SOM mass concentration
Assessment of DFT for Computing Sum Frequency Generation Spectra of an Epoxydiol and a Deuterated Isotopologue at Fused Silica/Vapor Interfaces
We
assess the capabilities of eight popular density functional
theory (DFT) functionals, in combination with several basis sets,
as applied to calculations of vibrational sum frequency generation
(SFG) spectra of the atmospherically relevant isoprene oxidation product <i>trans</i>-β-isoprene epoxydiol (IEPOX) and one of its
deuterated isotopologues at the fused silica/vapor interface. We use
sum of squared differences (SSD) and total absolute error (TAE) calculations
to estimate the performance of each functional/basis set combination
in producing SFG spectra that match experimentally obtained spectra
from <i>trans</i>-β-IEPOX and one of its isotopologues.
Our joined SSD/TAE analysis shows that while the twist angle of the
methyl <i>C</i><sub>3<i>v</i></sub> symmetry axis
of <i>trans</i>-β-IEPOX relative to the surface is
sensitive to the choice of DFT functional, the calculated tilt angle
relative to the surface normal is largely independent of the functional
and basis set. Moreover, we report that hybrid functionals such as
B3LYP, ωB97X-D, PBE0, and B97-1 in combination with a modest
basis set, such as 6-311GÂ(d,p), provides good agreement with experimental
data and much better performance than pure functionals such as PBE
and BP86. However, improving the quality of the basis set only improves
agreement with experimental data for calculations based on pure functionals.
A conformational analysis, based on comparisons of calculated and
experimental SFG spectra, suggests that <i>trans</i>-β-IEPOX
points all of its oxygen atoms toward the silica/vapor interface
Vibrational Mode Assignment of α‑Pinene by Isotope Editing: One Down, Seventy-One To Go
This study aims to reliably assign
the vibrational sum frequency
generation (SFG) spectrum of α-pinene at the vapor/solid interface
using a method involving deuteration of various methyl groups. The
synthesis of five deuterated isotopologues of α-pinene is presented
to determine the impact that removing contributions from methyl group
C–H oscillators has on its SFG response. 0.6 cm<sup>–1</sup> resolution SFG spectra of these isotopologues show varying degrees
of differences in the C–H stretching region when compared to
the SFG response of unlabeled α-pinene. The largest spectral
changes were observed for the isotopologue containing a fully deuterated
vinyl methyl group. Noticeable losses in signal intensities allow
us to reliably assign the 2860 cm<sup>–1</sup> peak to the
vinyl methyl symmetric stretch. Furthermore, upon removing the vinyl
methyl group entirely by synthesizing apopinene, the steric influence
of the unlabeled C<sub>9</sub>H<sub>14</sub> fragment on the SFG response
of α-pinene SFG can be readily observed. The work presented
here brings us one step closer to understanding the vibrational spectroscopy
of α-pinene