8 research outputs found
Production of and Dissociative Electron Attachment to the Simplest Criegee Intermediate in an Afterglow
The simplest Criegee intermediate,
CH<sub>2</sub>OO, has been produced
in a flowing afterglow using a novel technique. CH<sub>2</sub>I is
produced by dissociative electron attachment to CH<sub>2</sub>I<sub>2</sub>, leading to the established reaction CH<sub>2</sub>I + O<sub>2</sub> ā CH<sub>2</sub>OO + I. The presence of CH<sub>2</sub>OO is established by observation of dissociative electron attachment
to yield O<sup>ā</sup> using the variable electron and neutral
density attachment mass spectrometry (VENDAMS) technique. The measurements
establish the electron attachment rate coefficient of thermal electrons
at 300 K to CH<sub>2</sub>OO as 1.2 Ā± 0.3 Ć 10<sup>ā8</sup> cm<sup>3</sup> s<sup>ā1</sup>. Thermal electron attachment
is solely dissociative and is not a promising route to producing stable
CH<sub>2</sub>OO<sup>ā</sup>. The results open the possibility
of measuring ionāmolecule chemistry involving Criegee intermediates,
as well as the reactivity of other unstable radicals produced in an
analogous manner
Reactions of Fe<sup>+</sup> and FeO<sup>+</sup> with C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>H<sub>4</sub>, and C<sub>2</sub>H<sub>6</sub>: Temperature-Dependent Kinetics
We present the first temperature-dependent
rate constants and branching
ratios for the reactions of Fe<sup>+</sup> and FeO<sup>+</sup> with
C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>H<sub>4</sub>, and C<sub>2</sub>H<sub>6</sub> from 170 to 700 K. Fe<sup>+</sup> is observed
to react only by association with the three hydrocarbons, with temperature
dependencies of <i>T</i><sup>ā2</sup> to <i>T</i><sup>ā3</sup>. FeO<sup>+</sup> reacts with C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>4</sub> at the collision
rate over the temperature range, and their respective product branchings
show similar temperature dependences. In contrast, the reaction with
ethane is collisional at 170 K but varies as <i>T</i><sup>ā0.5</sup>, while the product branching remains essentially
flat with temperature. These variations in reactivity are discussed
in terms of the published reactive potential surfaces. The effectiveness
of Fe<sup>+</sup> as an oxygen-transfer catalyst toward the three
hydrocarbons is also discussed
Comment on āRole of (NO)<sub>2</sub> Dimer in Reactions of Fe<sup>+</sup> with NO and NO<sub>2</sub> Studied by ICP-SIFT Mass Spectrometryā
Comment on āRole of (NO)<sub>2</sub> Dimer
in Reactions of Fe<sup>+</sup> with NO and NO<sub>2</sub> Studied
by ICP-SIFT Mass Spectrometry
Temperature and Pressure Dependences of the Reactions of Fe<sup>+</sup> with Methyl Halides CH<sub>3</sub>X (X = Cl, Br, I): Experiments and Kinetic Modeling Results
The
pressure and temperature dependences of the reactions of Fe<sup>+</sup> with methyl halides CH<sub>3</sub>X (X = Cl, Br, I) in He
were measured in a selected ion flow tube over the ranges 0.4 to 1.2
Torr and 300ā600 K. FeX<sup>+</sup> was observed for all three
halides and FeCH<sub>3</sub><sup>+</sup> was observed for the CH<sub>3</sub>I reaction. FeCH<sub>3</sub>X<sup>+</sup> adducts (for all
X) were detected in all reactions. The results were interpreted assuming
two-state reactivity with spin-inversions between sextet and quartet
potentials. Kinetic modeling allowed for a quantitative representation
of the experiments and for extrapolation to conditions outside the
experimentally accessible range. The modeling required quantum-chemical
calculations of molecular parameters and detailed accounting of angular
momentum effects. The results show that the FeX<sup>+</sup> products
come via an insertion mechanism, while the FeCH<sub>3</sub><sup>+</sup> can be produced from either insertion or S<sub>N</sub>2 mechanisms,
but the latter we conclude is unlikely at thermal energies. A statistical
modeling cannot reproduce the competition between the bimolecular
pathways in the CH<sub>3</sub>I reaction, indicating that some more
direct process must be important
Kinetics of Cations with C<sub>2</sub> Hydrofluorocarbon Radicals
Reactions
of the cations Ar<sup>+</sup>, O<sub>2</sub><sup>+</sup>, CO<sub>2</sub><sup>+</sup>, and CF<sub>3</sub><sup>+</sup> with
the C<sub>2</sub> radicals C<sub>2</sub>H<sub>5</sub>, H<sub>2</sub>C<sub>2</sub>F<sub>3</sub>, C<sub>2</sub>F<sub>3</sub>, and C<sub>2</sub>F<sub>5</sub> were investigated using the variable electron
and neutral density attachment mass spectrometry technique in a flowing
afterglowāLangmuir probe apparatus at room temperature. Rate
coefficients for observed product channels for these 16 reactions
are reported as well as rate coefficients and product branching fractions
for the 16 reactions of the same cations with each of the stable neutrals
used as radical precursors (the species RI, where R is the radical
studied). Reactions with the stable neutrals proceed by charge transfer
at or near the collisional rate coefficient where energetically allowed;
where charge transfer is endothermic, bond-breaking/bond-making chemistry
occurs. While also efficient, reactions with the radicals are more
likely to occur at a smaller fraction of the collisional rate coefficient,
and bond-breaking/bond-making chemistry occurs even in some cases
where charge transfer is exothermic. It is noted that unlike radical
reactions with neutral species, which occur with rate coefficients
that are generally elevated compared to those of stable species, ionāradical
reactivity is generally decreased relative to that of reactions with
stable species. In particular, long-range charge transfer appears
more likely to be frustrated in the ionāradical systems
Determining Rate Constants and Mechanisms for Sequential Reactions of Fe<sup>+</sup> with Ozone at 500 K
We present rate constants
and product branching ratios for the
reactions of FeO<sub><i>x</i></sub><sup>+</sup> (<i>x</i> = 0ā4) with ozone at 500 K. Fe<sup>+</sup> is observed
to react with ozone at the collision rate to produce FeO<sup>+</sup> + O<sub>2</sub>. The FeO<sup>+</sup> in turn reacts with ozone at
the collision rate to yield both Fe<sup>+</sup> and FeO<sub>2</sub><sup>+</sup> product channels. Ions up to FeO<sub>4</sub><sup>+</sup> display similar reactivity patterns. Three-body clustering reactions
with O<sub>2</sub> prevent us from measuring accurate rate constants
at 300 K although the data do suggest that the efficiency is also
high. Therefore, it is probable that little to no temperature dependence
exists over this range. Implications of our measurements to the regulation
of atmospheric iron and ozone are discussed. Density functional calculations
on the reaction of Fe<sup>+</sup> with ozone show no substantial kinetic
barriers to make the FeO<sup>+</sup> + O<sub>2</sub> product channel,
which is consistent with the reactionās efficiency. While a
pathway to make FeO<sub>2</sub><sup>+</sup> + O is also found to be
barrierless, our experiments indicate no primary FeO<sub>2</sub><sup>+</sup> formation for this reaction
Activation of Methane by FeO<sup>+</sup>: Determining Reaction Pathways through Temperature-Dependent Kinetics and Statistical Modeling
The temperature dependences of the
rate constants and product branching
ratios for the reactions of FeO<sup>+</sup> with CH<sub>4</sub> and
CD<sub>4</sub> have been measured from 123 to 700 K. The 300 K rate
constants are 9.5 Ć 10<sup>ā11</sup> and 5.1 Ć 10<sup>ā11</sup> cm<sup>3</sup> s<sup>ā1</sup> for the CH<sub>4</sub> and CD<sub>4</sub> reactions, respectively. At low temperatures,
the Fe<sup>+</sup> + CH<sub>3</sub>OH/CD<sub>3</sub>OD product channel
dominates, while at higher temperatures, FeOH<sup>+</sup>/FeOD<sup>+</sup> + CH<sub>3</sub>/CD<sub>3</sub> becomes the majority channel.
The data were found to connect well with previous experiments at higher
translational energies. The kinetics were simulated using a statistical
adiabatic channel model (vibrations are adiabatic during approach
of the reactants), which reproduced the experimental data of both
reactions well over the extended temperature and energy ranges. Stationary
point energies along the reaction pathway determined by ab initio
calculations seemed to be only approximate and were allowed to vary
in the statistical model. The model shows a crossing from the ground-state
sextet surface to the excited quartet surface with large efficiency,
indicating that both states are involved. The reaction bottleneck
for the reaction is found to be the quartet barrier, for CH<sub>4</sub> modeled as ā22 kJ mol<sup>ā1</sup> relative to the
sextet reactants. Contrary to previous rationalizations, neither less
favorable spin-crossing at increased energies nor the opening of additional
reaction channels is needed to explain the temperature dependence
of the product branching fractions. It is found that a proper treatment
of state-specific rotations is crucial. The modeled energy for the
FeOH<sup>+</sup> + CH<sub>3</sub> channel (ā1 kJ mol<sup>ā1</sup>) agrees with the experimental thermochemical value, while the modeled
energy of the Fe<sup>+</sup> + CH<sub>3</sub>OH channel (ā10
kJ mol<sup>ā1</sup>) corresponds to the quartet iron product,
provided that spin-switching near the products is inefficient. Alternative
possibilities for spin switching during the reaction are considered.
The modeling provides unique insight into the reaction mechanisms
as well as energetic benchmarks for the reaction surface
Further Insight into the Reaction FeO<sup>+</sup> + H<sub>2</sub> ā Fe<sup>+</sup> + H<sub>2</sub>O: Temperature Dependent Kinetics, Isotope Effects, and Statistical Modeling
The reactions of FeO<sup>+</sup> with
H<sub>2</sub>, D<sub>2</sub>, and HD were studied in detail from 170
to 670 K by employing a
variable temperature selected ion flow tube apparatus. High level
electronic structure calculations were performed and compared to previous
theoretical treatments. Statistical modeling of the temperature and
isotope dependent rate constants was found to reproduce all data,
suggesting the reaction could be well explained by efficient crossing
from the sextet to quartet surface, with a rigid near thermoneutral
barrier accounting for both the inefficiency and strong negative temperature
dependence of the reactions over the measured range of thermal energies.
The modeling equally well reproduced earlier guided ion beam results
up to translational temperatures of about 4000 K