7 research outputs found

### Active Thermochemical Tables: Water and Water Dimer

A new partition function for water
dimer in the temperature range 200–500 K was developed by exploiting
the equations of state for real water vapor, liquid water, and ice,
and demonstrated to be significantly more accurate than any proposed
so far in the literature. The new partition function allows the Active
Thermochemical Tables (ATcT) approach to be applied on the available
experimental and theoretical data relating to water dimer thermochemistry,
leading to accurate water dimer enthalpies of formation of −499.115
± 0.052 kJ mol<sup>–1</sup> at 298.15 K and −491.075
± 0.080 kJ mol<sup>–1</sup> at 0 K. With the current ATcT
enthalpy of formation of the water monomer, −241.831 ±
0.026 kJ mol<sup>–1</sup> at 298.15 K (−238.928 kJ mol<sup>–1</sup> at 0 K), this leads to the dimer bond dissociation
enthalpy at 298.15 K of 15.454 ± 0.074 kJ mol<sup>–1</sup> and a 0 K bond dissociation energy of 13.220 ± 0.096 kJ mol<sup>–1</sup> (1105 ± 8 cm<sup>–1</sup>), the latter
being in perfect agreement with recent experimental and theoretical
determinations. The new partition function of water dimer allows the
extraction and tabulation of heat capacity, entropy, enthalpy increment,
reduced Gibbs energy, enthalpy of formation, and Gibbs energy of formation.
Newly developed tabulations of analogous thermochemical properties
for gas-phase water monomer and for water in condensed phases are
also given, allowing the computations of accurate equilibria between
the dimer and monomer in the 200–500 K range of temperatures

### 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

### Configuration Space Integration for Adsorbate Partition Functions: The Effect of Anharmonicity on the Thermophysical Properties of CO–Pt(111) and CH<sub>3</sub>OH–Cu(111)

A method for computing anharmonic thermophysical properties
for
adsorbates on metal surfaces has been extended to include libration,
or frustrated rotation. Classical phase space integration is used
with Monte Carlo sampling of the configuration space to obtain the
partition function of CO on Pt(111) and CH3OH on Cu(111).
A minima-preserving neural network potential energy surrogate is used
within the integration routines. Direct state counting using discrete
variable representation is used to benchmark the results. We find
that the phase space integration approach is in excellent agreement
with the direct state counting results. Comparison with standard models
such as the harmonic oscillator indicates that anharmonicity contributes
significantly to the thermodynamic properties of CH3OH
on Cu(111). We find that there is also a considerable difference between
the harmonic oscillator and phase space integration for CO on Pt(111),
although the discrepancy can largely be attributed to the presence
of multiple binding sites within the unit cell. We demonstrate that
a multisite harmonic oscillator model might be sufficient for CO–Pt(111).
A more thorough description of the potential energy surface, which
can be achieved with phase space integration, is necessary for weakly
bound adsorbates such as CH3OH. The thermophysical properties
were used to calculate free energies of adsorption on the respective
metals, and subsequently the equilibrium constants and Langmuir isotherms
in relevant temperature ranges. The results show that the choice of
model to obtain partition functions greatly affects the resulting
surface coverages in kinetic models

### 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

### Unimolecular Reaction of Methyl Isocyanide to Acetonitrile: A High-Level Theoretical Study

A combination of
high-level coupled-cluster calculations and two-dimensional
master equation approaches based on semiclassical transition state
theory is used to reinvestigate the classic prototype unimolecular
isomerization of methyl isocyanide (CH<sub>3</sub>NC) to acetonitrile
(CH<sub>3</sub>CN). The activation energy, reaction enthalpy, and
fundamental vibrational frequencies calculated from first-principles
agree well with experimental results. In addition, the calculated
thermal rate constants adequately reproduce those of experiment over
a large range of temperature and pressure in the falloff region, where
experimental results are available, and are generally consistent with
statistical chemical kinetics theory (such as Rice–Ramsperger–Kassel–Marcus
(RRKM) and transition state theory (TST))

### Thermal Dissociation and Roaming Isomerization of Nitromethane: Experiment and Theory

The
thermal decomposition of nitromethane provides a classic example
of the competition between roaming mediated isomerization and simple
bond fission. A recent theoretical analysis suggests that as the pressure
is increased from 2 to 200 Torr the product distribution undergoes
a sharp transition from roaming dominated to bond-fission dominated.
Laser schlieren densitometry is used to explore the variation in the
effect of roaming on the density gradients for CH<sub>3</sub>NO<sub>2</sub> decomposition in a shock tube for pressures of 30, 60, and
120 Torr at temperatures ranging from 1200 to 1860 K. A complementary
theoretical analysis provides a novel exploration of the effects of
roaming on the thermal decomposition kinetics. The analysis focuses
on the roaming dynamics in a reduced dimensional space consisting
of the rigid-body motions of the CH<sub>3</sub> and NO<sub>2</sub> radicals. A high-level reduced-dimensionality potential energy surface
is developed from fits to large-scale multireference ab initio calculations.
Rigid body trajectory simulations coupled with master equation kinetics
calculations provide high-level a priori predictions for the thermal
branching between roaming and dissociation. A statistical model provides
a qualitative/semiquantitative interpretation of the results. Modeling
efforts explore the relation between the predicted roaming branching
and the observed gradients. Overall, the experiments are found to
be fairly consistent with the theoretically proposed branching ratio,
but they are also consistent with a no-roaming scenario and the underlying
reasons are discussed. The theoretical predictions are also compared
with prior theoretical predictions, with a related statistical model,
and with the extant experimental data for the decomposition of CH<sub>3</sub>NO<sub>2</sub>, and for the reaction of CH<sub>3</sub> with
NO<sub>2</sub>

### 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