W4 theory for computational thermochemistry: in pursuit of confident sub-kJ/mol predictions

In an attempt to improve on our earlier W3 theory [J. Chem. Phys. {\bf 120}, 4129 (2004)] we consider such refinements as more accurate estimates for the contribution of connected quadruple excitations ($\hat{T}_4$), inclusion of connected quintuple excitations ($\hat{T}_5$), diagonal Born-Oppenheimer corrections (DBOC), and improved basis set extrapolation procedures. Revised experimental data for validation purposes were obtained from the latest version of the ATcT (Active Thermochemical Tables) Thermochemical Network. We found that the CCSDTQ$-$CCSDT(Q) difference converges quite rapidly with the basis set, and that the formula 1.10[CCSDT(Q)/cc-pVTZ+CCSDTQ/cc-pVDZ$-$CCSDT(Q)/cc-pVDZ] offers a very reliable as well as fairly cost-effective estimate of the basis set limit $\hat{T}_4$ contribution. The largest $\hat{T}_5$ contribution found in the present work is on the order of 0.5 kcal/mol (for ozone). DBOC corrections are significant at the 0.1 kcal/mol level in hydride systems. . Based on the accumulated experience, a new computational thermochemistry protocol for first-and second-row main-group systems, to be known as W4 theory, is proposed. Our W4 atomization energies for a number of key species are in excellent agreement (better than 0.1 kcal/mol on average, 95% confidence intervals narrower than 1 kJ/mol) with the latest experimental data obtained from Active Thermochemical Tables. A simple {\em a priori} estimate for the importance of post-CCSD(T) correlation contributions (and hence a pessimistic estimate for the error in a W2-type calculation) is proposed.Comment: J. Chem. Phys., in press; electronic supporting information available at http://theochem.weizmann.ac.il/web/papers/w4.htm

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^–1 at 298.15 K and −491.075 ± 0.080 kJ mol^–1 at 0 K. With the current ATcT enthalpy of formation of the water monomer, −241.831 ± 0.026 kJ mol^–1 at 298.15 K (−238.928 kJ mol^–1 at 0 K), this leads to the dimer bond dissociation enthalpy at 298.15 K of 15.454 ± 0.074 kJ mol^–1 and a 0 K bond dissociation energy of 13.220 ± 0.096 kJ mol^–1 (1105 ± 8 cm^–1), 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

A STUDY OF THE IONIZATION THRESHOLDS OF METHYL AND METHYLENE BY PHOTOIONIZATION MASS SPECTROMETRY

Author Institution: Chemistry Division, Argonne National LaboratoryThe $CH_{3}$ and $CH_{2}$ radicals were produced in situ at room temperature by hydrogen abstraction from methane precursor. A model of the observed ionization onset of $CH_{3}$, which includes rotational ionization, yields an ionization energy congruous with previously reported zero-electron kinetic energy (ZEKE) measurement. The ionization threshold of methylene, which was obtained for the first time, also exhibits rotational autoionization effects. The adiabatic ionization energy of $CH_{2}$ extracted from the photoionization spectrum is $10.393\pm 0.011eV$. In related experiments, the appearance energies of the $CH_{3}^{+}$ fragment from $CH_{4}$ and $CH_{2}^{+}$ from $CH_{3}$ were determined accurately by fitting as $AP_{0}(CH_{3}^{+}/CH_{4}) = 14.322 \pm 0.003$ eV and $AP_{0}(CH_{2}^{+}/CH_{3}) = 15.120 \pm 0.006$ eV. The combination of these measurements with the adiabatic ionization energies of the radicals also yields the best current values of the first two sequential bond dissociation energies of methane: $D_{0}(H-CH_{3})= 4.484 \pm 0.003eV = 103.40 \pm 0.07$ kcal/mol $(104.96 \pm 0.07$ kcal/mol at 298 K) and $D_{0}(H-CH_{2}) = 4.727 \pm 0.012 eV = 109.0 \pm 0.3$ kcal/mol $(110.4 \pm 0.3$ kcal/mol at 298 K). This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38

PHOTOIONIZATION MASS SPECTROMETRIC MEASUREMENTS OF COMBUSTION-RELATED TRANSIENT AND METASTABLE SPECIES

Author Institution: Chemistry Division, Argonne National LaboratoryPhotoionization mass spectrometry is a versatile tool providing a wealth of spectroscopic, structural and dynamical details as well as information needed to successfully detect and monitor various species in real-life processes. It can also provide, through the use of the positive ion cycle, reliable and accurate thermochemical quantities, such as bond dissociation energies and enthalpies of formation. In the simplest variant, thermochemistry can be deduced from two correlated studies: the first aims to determine the ionization potential of a transient species and the second measures a related fragment appearance potential from a stable parent molecule. While the former involves challenges entailed in measuring ephemeral species, the latter involves the understanding of the shape of the fragmentation threshold resulting from the underlying unimolecular decomposition process. The fitting procedures developed to aid the extraction of thermodynamically significant values for fragmentation thresholds will be exemplified using several recent determinations of combustion-related bond dissociation energies. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No. W-31-109-ENG-38

PHOTOIONIZATION OF HOCO RADICAL: A NEW UPPER LIMIT TO THE ADIABATIC IONIZATION ENERGY AND LOWER LIMIT TO THE ENTHALPY OF FORMATION

Author Institution: Chemistry Division, Argonne National LaboratoryHOCO radical is important in combustion and atmospheric chemistry. A recent photoionization investigation provides a new value for the adiabatic ionization energy of $EI(t-HOCO) \leq 8.195 \pm 0.022 eV$. Through the positive ion thermochemical cycle, this translates into a lower limit to the enthalpy of formation, $\Delta H^{0}_{f0}(t-HOCO) \geq -45.8 \pm 0.7 kcal/mol (\geq -46.5 \pm 0.7 kcal/mol at 298 K)$, placing t-HOCO only $3.5 \pm 0.7 kcal/mol$ below the $CO_{2}+H$ asymptote. The photoionization spectrum of HOCO corroborates the previous finding of a progression in the double C=O bond stretch of the ion of ${\sim} 2300 cm^{-1}$, suggests the presence of the single C-O bond stretch of $\sim 1200-1300 cm^{-1}$, and provides indirect evidence for the excitation of an even lower frequency, such as the OCO bend. In addition, the data tentatively suggest an ionization onset as low as $8.06 \pm 0.03 eV$. While it is not quite clear whether the latter should correspond to the cis or trans isomer, it may indicate that the enthalpy of formation of HOCO is even higher. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No. W-31-109-ENG-38

Benchmark atomization energy of ethane: Importance of accurate zero-point vibrational energies and diagonal BornOppenheimer corrections for a 'simple' organic molecule

Abstract A benchmark calculation of the atomization energy of the 'simple' organic molecule C 2 H 6 (ethane) has been carried out by means of W4 theory. While the molecule is straightforward in terms of one-particle and n-particle basis set convergence, its large zero-point vibrational energy (and anharmonic correction thereto) and nontrivial diagonal Born-Oppenheimer correction (DBOC) represent interesting challenges. For the W4 set of molecules and C 2 H 6 , we show that DBOCs to the total atomization energy are systematically overestimated at the SCF level, and that the correlation correction converges very rapidly with the basis set. Thus, even at the CISD/cc-pVDZ level, useful correlation corrections to the DBOC are obtained. When applying such a correction, overall agreement with experiment was only marginally improved, but a more significant improvement is seen when hydrogen-containing systems are considered in isolation. We conclude that for closed-shell organic molecules, the greatest obstacles to highly accurate computational thermochemistry may not lie in the solution of the clamped-nuclei Schrö dinger equation, but rather in the zero-point vibrational energy and the diagonal Born-Oppenheimer correction

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. The new protocol was shown to be in good agreement with the Active Thermochemical Tables database for the four species: the methyl peroxy radical, methoxyoxoniumylidene (methyl peroxy cation), methyl peroxy anion, and methyl hydroperoxide. Updating the Active Thermochemical Tables to include those results yielded significantly improved accuracy for the formation enthalpies of those species. The derived protocol was then used to predict formation enthalpies for the larger ethyl peroxy family of species