Unifying thermochemistry concepts in computational heterogeneous catalysis.

Thermophysical properties of adsorbates and gas-phase species define the free energy landscape of heterogeneously catalyzed processes and are pivotal for an atomistic understanding of the catalyst performance. These thermophysical properties, such as the free energy or the enthalpy, are typically derived from density functional theory (DFT) calculations. Enthalpies are species-interdependent properties that are only meaningful when referenced to other species.

Accurate and reliable thermochemistry by data analysis of complex thermochemical networks using Active Thermochemical Tables: the case of glycine thermochemistry.

Active Thermochemical Tables (ATcT) were successfully used to resolve the existing inconsistencies related to the thermochemistry of glycine, based on statistically analyzing and solving a thermochemical network that includes >3350 chemical species interconnected by nearly 35 000 thermochemically-relevant determinations from experiment and high-level theory. The current ATcT results for the 298.15 K enthalpies of formation are -394.

Methanediol from cloud-processed formaldehyde is only a minor source of atmospheric formic acid.

Atmospheric formic acid is severely underpredicted by models. A recent study proposed that this discrepancy can be resolved by abundant formic acid production from the reaction (1) between hydroxyl radical and methanediol derived from in-cloud formaldehyde processing and provided a chamber-experiment-derived rate constant, = 7.5 × 10 cm s.

Pynta─An Automated Workflow for Calculation of Surface and Gas-Surface Kinetics.

Many important industrial processes rely on heterogeneous catalytic systems. However, given all possible catalysts and conditions of interest, it is impractical to optimize most systems experimentally. Automatically generated microkinetic models can be used to efficiently consider many catalysts and conditions.

Active Thermochemical Tables: Enthalpies of Formation of Bromo- and Iodo-Methanes, Ethenes and Ethynes.

The thermochemistry of halocarbon species containing iodine and bromine is examined through an extensive interplay between new Feller-Peterson-Dixon (FPD) style composite methods and a detailed analysis of all available experimental and theoretical determinations using the thermochemical network that underlies the Active Thermochemical Tables (ATcT). From the computational viewpoint, a slower convergence of the components of composite thermochemistry methods is observed relative to species that solely contain first row elements, leading to a higher computational expense for achieving comparable levels of accuracy. Potential systematic sources of computational uncertainty are investigated, and, not surprisingly, spin-orbit coupling is found to be a critical component, particularly for iodine containing molecular species.

Sub 20 cm computational prediction of the CH bond energy - a case of systematic error in computational thermochemistry.

The bond dissociation energy of methylidyne, (CH), is studied using an improved version of the High-Accuracy Extrapolated Thermochemistry (HEAT) approach as well as the Feller-Peterson-Dixon (FPD) model chemistry. These calculations, which include basis sets up to nonuple (aug-cc-pCV9Z) quality, are expected to be capable of providing results substantially more accurate than the 1 kJ mol level that is characteristic of standard high-accuracy protocols for computational thermochemistry. The calculated 0 K CH bond energy (27 954 ± 15 cm for HEAT and 27 956 ± 15 cm for FPD), along with equivalent treatments of the CH ionization energy and the CH dissociation energy (85 829 ± 15 cm and 32 946 ± 15 cm, respectively), were compared to the existing benchmarks from Active Thermochemical Tables (ATcT), uncovering an unexpected difference for (CH).

Mechanism, thermochemistry, and kinetics of the reversible reactions: CH + H ⇌ CH + H ⇌ CH.

High-level coupled cluster theory, in conjunction with Active Thermochemical Tables (ATcT) and ,-resolved master equation calculations, was used in a study of the title reactions, which play an important role in the combustion of hydrocarbons. In the set of radical/radical reactions leading to soot formation in flames, the addition of H-atoms to alkenes is likely a common reaction, triggering the isomerization of complex hydrocarbons to aromatics. The heats of formation of CH, CH, and CH are established to be 301.

Elaborated thermochemical treatment of HF, CO, N, and HO: Insight into HEAT and its extensions.

Empirical, highly accurate non-relativistic electronic total atomization energies (eTAEs) are established by combining experimental or computationally converged treatments of the nuclear motion and relativistic contributions with the total atomization energies of HF, CO, N, and HO obtained from the Active Thermochemical Tables. These eTAEs, which have estimated (2σ) uncertainties of less than 10 cm (0.12 kJ mol), form the basis for an analysis of high-level ab initio quantum chemical calculations that aim at reproducing these eTAEs for the title molecules.

Substitution Reactions in the Pyrolysis of Acetone Revealed through a Modeling, Experiment, Theory Paradigm.

The development of high-fidelity mechanisms for chemically reactive systems is a challenging process that requires the compilation of rate descriptions for a large and somewhat ill-defined set of reactions. The present unified combination of modeling, experiment, and theory provides a paradigm for improving such mechanism development efforts. Here we combine broadband rotational spectroscopy with detailed chemical modeling based on rate constants obtained from automated ab initio transition state theory-based master equation calculations and high-level thermochemical parametrizations.

Spectroscopic and theoretical studies of UN and UN.

The low-energy electronic states of UN and UN have been examined using high-level electronic structure calculations and two-color photoionization techniques. The experimental measurements provided an accurate ionization energy for UN (IE = 50 802 ± 5 cm). Spectra for UN yielded ro-vibrational constants and established that the ground state has the electronic angular momentum projection Ω = 4.

Health and public health approach to ending child abuse and neglect.

The 30 anniversary of the United Nations Convention on the Rights of the Child provides an opportunity to reflect on whether the approaches to date in dealing with child abuse and neglect (CAN) have been successful. Initial responsibility in most countries to address CAN has been given to Child Protective Services Agencies. Recently, there have been calls for CPS to take a Public Health Approach in their practice.

An Automated Thermochemistry Protocol Based on Explicitly Correlated Coupled-Cluster Theory: The Methyl and Ethyl Peroxy Families.

An automated computational thermochemistry protocol based on explicitly correlated coupled-cluster theory was designed to produce highly accurate enthalpies of formation and atomization energies for small- to medium-sized molecular species (3-12 atoms). Each potential source of error was carefully examined, and the sizes of contributions to the total atomization enthalpies were used to generate uncertainty estimates. The protocol was first used to generate total atomization enthalpies for a family of four molecular species exhibiting a variety of charges, multiplicities, and electronic ground states.

High-accuracy extrapolated ab initio thermochemistry. IV. A modified recipe for computational efficiency.

A number of economical modifications to the high-accuracy extrapolated ab initio thermochemistry (HEAT) model chemistry are evaluated. The two resulting schemes, designated as mHEAT and mHEAT+, are designed for efficient and pragmatic evaluation of molecular energies in systems somewhat larger than can be practically studied by the unapproximated HEAT scheme. It is found that mHEAT+ produces heats of formation with nearly subchemical (±1 kJ/mol) accuracy at a substantially reduced cost relative to the full scheme.

Active Thermochemical Tables: The Partition Function of Hydroxymethyl (CHOH) Revisited.

The best currently available set of temperature-dependent nonrigid rotor anharmonic oscillator (NRRAO) thermochemical and thermophysical properties of hydroxymethyl radical is presented. The underlying partition function relies on a critically evaluated complement of accurate experimental and theoretical data and is constructed using a two-pronged strategy that combines contributions from large amplitude motions obtained from direct counts, with contributions from the other internal modes of motion obtained from analytic NRRAO expressions. The contributions from the two strongly coupled large-amplitude motions of CHOH, OH torsion and CH wag, are based on energy levels obtained by solving the appropriate two-dimensional projection of a fully dimensional potential energy surface that was recently obtained at the CCSD(T)/cc-pVTZ level of theory.

Enthalpy of Formation of CHO (Oxalic Acid) from High-Level Calculations and the Active Thermochemical Tables Approach.

High-level coupled cluster calculations obtained with the Feller-Peterson-Dixon (FPD) approach and new data from the most recent version of the Active Thermochemical Tables (ATcT) are used to reassess the enthalpy of formation of gas-phase CHO (oxalic acid). The theoretical value was further calibrated by comparing FPD and ATcT gas-phase enthalpies of formation for HCO (formaldehyde) and the two low-lying conformations of CHO ( syn and anti acetic acid). The FPD approach produces a theoretical enthalpy of formation of gas-phase oxalic acid of -732.

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 (CHNC) to acetonitrile (CHCN). 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)).

Active Thermochemical Tables: The Adiabatic Ionization Energy of Hydrogen Peroxide.

The adiabatic ionization energy of hydrogen peroxide (HOOH) is investigated, both by means of theoretical calculations and theoretically assisted reanalysis of previous experimental data. Values obtained by three different approaches: 10.638 ± 0.

Enthalpy of Formation of NH (Hydrazine) Revisited.

In order to address the accuracy of the long-standing experimental enthalpy of formation of gas-phase hydrazine, fully confirmed in earlier versions of Active Thermochemical Tables (ATcT), the provenance of that value is re-examined in light of new high-end calculations of the Feller-Peterson-Dixon (FPD) variety. An overly optimistic determination of the vaporization enthalpy of hydrazine, which created an unrealistically strong connection between the gas phase thermochemistry and the calorimetric results defining the thermochemistry of liquid hydrazine, was identified as the probable culprit. The new enthalpy of formation of gas-phase hydrazine, based on balancing all available knowledge, was determined to be 111.

Thermal Decomposition of Potential Ester Biofuels. Part I: Methyl Acetate and Methyl Butanoate.

Two methyl esters were examined as models for the pyrolysis of biofuels. Dilute samples (0.06-0.

A vacuum ultraviolet laser pulsed field ionization-photoion study of methane (CH): determination of the appearance energy of methylium from methane with unprecedented precision and the resulting impact on the bond dissociation energies of CH and CH.

We report on the successful implementation of a high-resolution vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) detection method for the study of unimolecular dissociation of quantum-state- or energy-selected molecular ions. As a test case, we have determined the 0 K appearance energy (AE) for the formation of methylium, CH, from methane, CH, as AE(CH/CH) = 14.32271 ± 0.

A combined photoelectron spectroscopy and relativistic ab initio studies of the electronic structures of UFO and UFO(-).

The observation of the gaseous UFO(-) anion is reported, which is investigated using photoelectron spectroscopy and relativisitic ab initio calculations. Two strong photoelectron bands are observed at low binding energies due to electron detachment from the U-7sσ orbital. Numerous weak detachment bands are also observed due to the strongly correlated U-5f electrons.

Theoretical spectroscopy study of the low-lying electronic states of UX and UX(+), X = F and Cl.

Spectroscopic constants (Te, re, B0, ωe, and ωexe) have been calculated for the low-lying electronic states of UF, UF(+), UCl, and UCl(+) using complete active space 2nd-order perturbation theory (CASPT2), with a series of correlation consistent basis sets. The latter included those based on both pseudopotential (PP) and all-electron Douglas-Kroll-Hess Hamiltonians for the U atom. Spin orbit (SO) effects were included a posteriori using the state interacting method using both PP and Breit Pauli (BP) operators, as well as from exact two-component methods for U(+) and UF(+).