9 research outputs found
Ab Initio Computations and Active Thermochemical Tables Hand in Hand: Heats of Formation of Core Combustion Species
The fidelity of combustion simulations
is strongly dependent on
the accuracy of the underlying thermochemical properties for the core
combustion species that arise as intermediates and products in the
chemical conversion of most fuels. High level theoretical evaluations
are coupled with a wide-ranging implementation of the Active Thermochemical
Tables (ATcT) approach to obtain well-validated high fidelity predictions
for the 0 K heat of formation for a large set of core combustion species.
In particular, high level ab initio electronic structure based predictions
are obtained for a set of 348 C, N, O, and H containing species, which
corresponds to essentially all core combustion species with 34 or
fewer electrons. The theoretical analyses incorporate various high
level corrections to base CCSD(T)/cc-pVnZ analyses (n = T or Q) using
H<sub>2</sub>, CH<sub>4</sub>, H<sub>2</sub>O, and NH<sub>3</sub> as
references. Corrections for the complete-basis-set limit, higher-order
excitations, anharmonic zero-point energy, core–valence, relativistic,
and diagonal Born–Oppenheimer effects are ordered in decreasing
importance. Independent ATcT values are presented for a subset of
150 species. The accuracy of the theoretical predictions is explored
through (i) examination of the magnitude of the various corrections,
(ii) comparisons with other high level calculations, and (iii) through
comparison with the ATcT values. The estimated 2σ uncertainties
of the three methods devised here, ANL0, ANL0-F12, and ANL1, are in
the range of ±1.0–1.5 kJ/mol for single-reference and
moderately multireference species, for which the calculated higher
order excitations are 5 kJ/mol or less. In addition to providing valuable
references for combustion simulations, the subsequent inclusion of
the current theoretical results into the ATcT thermochemical network
is expected to significantly improve the thermochemical knowledge
base for less-well studied species
Resolving Some Paradoxes in the Thermal Decomposition Mechanism of Acetaldehyde
The
mechanism for the thermal decomposition of acetaldehyde has
been revisited with an analysis of literature kinetics experiments
using theoretical kinetics. The present modeling study was motivated
by recent observations, with very sensitive diagnostics, of some unexpected
products in high temperature microtubular reactor experiments on the
thermal decomposition of CH<sub>3</sub>CHO and its deuterated analogs,
CH<sub>3</sub>CDO, CD<sub>3</sub>CHO, and CD<sub>3</sub>CDO. The observations
of these products prompted the authors of these studies to suggest
that the enol tautomer, CH<sub>2</sub>CHOH (vinyl alcohol), is a primary
intermediate in the thermal decomposition of acetaldehyde. The present
modeling efforts on acetaldehyde decomposition incorporate a master
equation reanalysis of the CH<sub>3</sub>CHO potential energy surface
(PES). The lowest-energy process on this PES is an isomerization of
CH<sub>3</sub>CHO to CH<sub>2</sub>CHOH. However, the subsequent product
channels for CH<sub>2</sub>CHOH are substantially higher in energy,
and the only unimolecular process that can be thermally accessed is
a reisomerization to CH<sub>3</sub>CHO. The incorporation of these
new theoretical kinetics predictions into models for selected literature
experiments on CH<sub>3</sub>CHO thermal decomposition confirms our
earlier experiment and theory-based conclusions that the dominant
decomposition process in CH<sub>3</sub>CHO at high temperatures is
C–C bond fission with a minor contribution (∼10–20%)
from the roaming mechanism to form CH<sub>4</sub> and CO. The present
modeling efforts also incorporate a master-equation analysis of the
H + CH<sub>2</sub>CHOH potential energy surface. This bimolecular
reaction is the primary mechanism for removal of CH<sub>2</sub>CHOH,
which can accumulate to minor amounts at high temperatures, <i>T</i> > 1000 K, in most lab-scale experiments that use large
initial concentrations of CH<sub>3</sub>CHO. Our modeling efforts
indicate that the observation of ketene, water, and acetylene in the
recent microtubular experiments are primarily due to bimolecular reactions
of CH<sub>3</sub>CHO and CH<sub>2</sub>CHOH with H-atoms and have
no bearing on the unimolecular decomposition mechanism of CH<sub>3</sub>CHO. The present simulations also indicate that experiments using
these microtubular reactors when interpreted with the aid of high-level
theoretical calculations and kinetics modeling can offer insights
into the chemistry of elusive intermediates in the high-temperature
pyrolysis of organic molecules
Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room-Temperature Flow Reactor
Chirped-pulse Fourier
transform millimeter-wave spectroscopy is
a potentially powerful tool for studying chemical reaction dynamics
and kinetics. Branching ratios of multiple reaction products and intermediates
can be measured with unprecedented chemical specificity; molecular
isomers, conformers, and vibrational states have distinct rotational
spectra. Here we demonstrate chirped-pulse spectroscopy of vinyl cyanide
photoproducts in a flow tube reactor at ambient temperature of 295
K and pressures of 1–10 μbar. This <i>in situ</i> and time-resolved experiment illustrates the utility of this novel
approach to investigating chemical reaction dynamics and kinetics.
Following 193 nm photodissociation of CH<sub>2</sub>CHCN, we observe
rotational relaxation of energized HCN, HNC, and HCCCN photoproducts
with 10 μs time resolution and sample the vibrational population
distribution of HCCCN. The experimental branching ratio HCN/HCCCN
is compared with a model based on RRKM theory using high-level ab
initio calculations, which were in turn validated by comparisons to
Active Thermochemical Tables enthalpies
Unconventional Peroxy Chemistry in Alcohol Oxidation: The Water Elimination Pathway
Predictive simulation for designing efficient engines
requires
detailed modeling of combustion chemistry, for which the possibility
of unknown pathways is a continual concern. Here, we characterize
a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH)
radicals derived from alcohols. The corresponding saddle-point structure
involves the interaction of radical and zwitterionic electronic states.
This interaction presents extreme difficulties for electronic structure
characterizations, but we demonstrate that these properties of this
saddle point can be well captured by M06-2X and CCSD(T) methods. Experimental
evidence for the existence and relevance of this pathway is shown
in recently reported data on the low-temperature oxidation of isopentanol
and isobutanol. In these systems, water elimination is a major pathway,
and is likely ubiquitous in low-temperature alcohol oxidation. These
findings will substantially alter current alcohol oxidation mechanisms.
Moreover, the methods described will be useful for the more general
phenomenon of interacting radical and zwitterionic states
Temperature and Pressure-Dependent Rate Coefficients for the Reaction of Vinyl Radical with Molecular Oxygen
State-of-the-art
calculations of the C<sub>2</sub>H<sub>3</sub>O<sub>2</sub> potential
energy surface are presented. A new method
is described for computing the interaction potential for R + O<sub>2</sub> reactions. The method, which combines accurate determination
of the quartet potential along the doublet minimum energy path with
multireference calculations of the doublet/quartet splitting, decreases
the uncertainty in the doublet potential and thence the rate constants
by more than a factor of 2. The temperature- and pressure-dependent
rate coefficients are computed using variable reaction coordinate
transition-state theory, variational transition-state theory, and
conventional transition-state theory, as implemented in a new RRKM/ME
code. The main bimolecular product channels are CH<sub>2</sub>O +
HCO at lower temperatures and CH<sub>2</sub>CHO + O at higher temperatures.
Above 10 atm, the collisional stabilization of CH<sub>2</sub>CHOO
directly competes with these two product channels. CH<sub>2</sub>CHOO
decomposes primarily to CH<sub>2</sub>O + HCO. The next two most significant
bimolecular products are OCHCHO + H and <sup>3</sup>CHCHO + OH, and
not C<sub>2</sub>H<sub>2</sub> + HO<sub>2</sub>. C<sub>2</sub>H<sub>3</sub> + O<sub>2</sub> will be predominantly chain branching above
1700 K. Uncertainty analysis is presented for the two most important
transition states. The uncertainties in these two barrier heights
result in a significant uncertainty in the temperature at which CH<sub>2</sub>CHO + O overtakes all other product channels
Temperature and Pressure-Dependent Rate Coefficients for the Reaction of Vinyl Radical with Molecular Oxygen
State-of-the-art
calculations of the C<sub>2</sub>H<sub>3</sub>O<sub>2</sub> potential
energy surface are presented. A new method
is described for computing the interaction potential for R + O<sub>2</sub> reactions. The method, which combines accurate determination
of the quartet potential along the doublet minimum energy path with
multireference calculations of the doublet/quartet splitting, decreases
the uncertainty in the doublet potential and thence the rate constants
by more than a factor of 2. The temperature- and pressure-dependent
rate coefficients are computed using variable reaction coordinate
transition-state theory, variational transition-state theory, and
conventional transition-state theory, as implemented in a new RRKM/ME
code. The main bimolecular product channels are CH<sub>2</sub>O +
HCO at lower temperatures and CH<sub>2</sub>CHO + O at higher temperatures.
Above 10 atm, the collisional stabilization of CH<sub>2</sub>CHOO
directly competes with these two product channels. CH<sub>2</sub>CHOO
decomposes primarily to CH<sub>2</sub>O + HCO. The next two most significant
bimolecular products are OCHCHO + H and <sup>3</sup>CHCHO + OH, and
not C<sub>2</sub>H<sub>2</sub> + HO<sub>2</sub>. C<sub>2</sub>H<sub>3</sub> + O<sub>2</sub> will be predominantly chain branching above
1700 K. Uncertainty analysis is presented for the two most important
transition states. The uncertainties in these two barrier heights
result in a significant uncertainty in the temperature at which CH<sub>2</sub>CHO + O overtakes all other product channels
Rate Constant and Branching Fraction for the NH<sub>2</sub> + NO<sub>2</sub> Reaction
The NH<sub>2</sub> + NO<sub>2</sub> reaction has been studied experimentally
and theoretically. On the basis of laser photolysis/LIF experiments,
the total rate constant was determined over the temperature range
295–625 K as <i>k</i><sub>1,exp</sub>(<i>T</i>) = 9.5 × 10<sup>–7</sup>(<i>T</i>/K)<sup>−2.05</sup> exp(−404 K/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. This value is in the upper
range of data reported for this temperature range. The reactions on
the NH<sub>2</sub> + NO<sub>2</sub> potential energy surface were
studied using high level ab initio transition state theory (TST) based
master equation methods, yielding a rate constant of <i>k</i><sub>1,theory</sub>(<i>T</i>) = 7.5 × 10<sup>–12</sup>(<i>T</i>/K)<sup>−0.172</sup> exp(687 K/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, in good agreement with the experimental value in the overlapping
temperature range. The two entrance channel adducts H<sub>2</sub>NNO<sub>2</sub> and H<sub>2</sub>NONO lead to formation of N<sub>2</sub>O
+ H<sub>2</sub>O (R1a) and H<sub>2</sub>NO + NO (R1b), respectively.
The pathways through H<sub>2</sub>NNO<sub>2</sub> and H<sub>2</sub>NONO are essentially unconnected, even though roaming may facilitate
a small flux between the adducts. High- and low-pressure limit rate
coefficients for the various product channels of NH<sub>2</sub> +
NO<sub>2</sub> are determined from the ab initio TST-based master
equation calculations for the temperature range 300–2000 K.
The theoretical predictions are in good agreement with the measured
overall rate constant but tend to overestimate the branching ratio
defined as β = <i>k</i><sub>1a</sub>/(<i>k</i><sub>1a</sub> + <i>k</i><sub>1b</sub>) at lower temperatures.
Modest adjustments of the attractive potentials for the reaction yield
values of <i>k</i><sub>1a</sub> = 4.3 × 10<sup>–6</sup>(<i>T</i>/K)<sup>−2.191</sup> exp(−229 K/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> and <i>k</i><sub>1b</sub> = 1.5 × 10<sup>–12</sup>(<i>T</i>/K)<sup>0.032</sup> exp(761 K/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, in good agreement with experiment, and we recommend these rate
coefficients for use in modeling
Theoretical Determination of the Rate Coefficient for the HO<sub>2 </sub>+ HO<sub>2</sub> → H<sub>2</sub>O<sub>2</sub><i>+</i>O<sub>2</sub> Reaction: Adiabatic Treatment of Anharmonic Torsional Effects
The HO<sub>2</sub> + HO<sub>2</sub> → H<sub>2</sub>O<sub>2</sub> + O<sub>2</sub> chemical reaction is studied using
statistical
rate theory in conjunction with high level ab initio electronic structure
calculations. A new theoretical rate coefficient is generated that is appropriate for both high and low temperature
regimes. The transition state region for the ground triplet potential
energy surface is characterized using the CASPT2/CBS/aug-cc-pVTZ method
with 14 active electrons and 10 active orbitals. The reaction is found
to proceed through an intermediate complex bound by approximately
9.79 kcal/mol. There is no potential barrier in the entrance channel,
although the free energy barrier was determined using a large Monte
Carlo sampling of the HO<sub>2</sub> orientations. The inner (tight)
transition state lies below the entrance threshold. It is found that
this inner transition state exhibits two saddle points corresponding
to torsional conformations of the complex. A unified treatment based
on vibrational adiabatic theory is presented that permits the reaction
to occur on an equal footing for any value of the torsional angle.
The quantum tunneling is also reformulated based on this new approach.
The rate coefficient obtained is in good agreement with low temperature
experimental results but is significantly lower than the results of
shock tube experiments for high temperatures
Electronic States of the Quasilinear Molecule Propargylene (HCCCH) from Negative Ion Photoelectron Spectroscopy
We
use gas-phase negative ion photoelectron spectroscopy to study
the quasilinear carbene propargylene, HCCCH, and its isotopologue
DCCCD. Photodetachment from HCCCH<sup>–</sup> affords the <i>X̃</i>(<sup>3</sup>B) ground state of HCCCH and its <i>ã</i>(<sup>1</sup>A), <i>b̃</i> (<sup>1</sup>B), <i>d̃</i>(<sup>1</sup>A<sub>2</sub>),
and <i>B̃</i>(<sup>3</sup>A<sub>2</sub>) excited states.
Extended, negatively anharmonic vibrational progressions in the <i>X̃</i>(<sup>3</sup>B) ground state and the open-shell
singlet <i>b̃</i> (<sup>1</sup>B) state arise from
the change in geometry between the anion and the neutral states and
complicate the assignment of the origin peak. The geometry change
arising from electron photodetachment results in excitation of the
ν<sub>4</sub> symmetric CCH bending mode, with a measured fundamental
frequency of 363 ± 57 cm<sup>–1</sup> in the <i>X̃</i>(<sup>3</sup>B) state. Our calculated harmonic frequency for this
mode is 359 cm<sup>–1</sup>. The Franck–Condon envelope
of this progression cannot be reproduced within the harmonic approximation.
The spectra of the <i>ã</i>(<sup>1</sup>A), <i>d̃</i>(<sup>1</sup>A<sub>2</sub>), and <i>B̃</i>(<sup>3</sup>A<sub>2</sub>) states are each characterized by a short
vibrational progression and a prominent origin peak, establishing
that the geometries of the anion and these neutral states are similar.
Through comparison of the HCCCH<sup>–</sup> and DCCCD<sup>–</sup> photoelectron spectra, we measure the electron affinity of HCCCH
to be 1.156 ± <sub>0.095</sub><sup>0.010</sup> eV, with a singlet–triplet splitting between the <i>X̃</i>(<sup>3</sup>B) and the <i>ã</i>(<sup>1</sup>A) states of Δ<i>E</i><sub>ST</sub> =
0.500 ± <sub>0.01</sub><sup>0.10</sup> eV (11.5 ± <sub>0.2</sub><sup>2.3</sup> kcal/mol). Experimental term energies of the higher excited
states are <i>T</i><sub>0</sub> [<i>b̃</i>(<sup>1</sup>B)] = 0.94 ± <sub>0.20</sub><sup>0.22</sup>eV, <i>T</i><sub>0</sub> [<i>d̃</i>(<sup>1</sup>A<sub>2</sub>)] = 3.30 ± <sub>0.02</sub><sup>0.10</sup>eV, <i>T</i><sub>0</sub> [<i>B̃</i>(<sup>3</sup>A<sub>2</sub>)] = 3.58 ± <sub>0.02</sub><sup>0.10</sup>eV. The photoelectron angular distributions
show significant π character in all the frontier molecular orbitals,
with additional σ character in orbitals that create the <i>X̃</i>(<sup>3</sup>B) and <i>b̃</i>(<sup>1</sup>B) states upon electron detachment. These results are consistent
with a quasilinear, nonplanar, doubly allylic structure of <i>X̃</i>(<sup>3</sup>B) HCCCH with both diradical and carbene
character