Gregory H. Jones

The superHEAT model chemistry is presented and used to explore the limits of accuracy in the computational prediction of the thermochemistry of first-row molecules. This model chemistry extends every component of the HEAT protocol to their effective computational limits, including rigorous treatments of CCSD basis-set completeness, post-CCSD(T) correlation, and vibrational zero-point energies. These extensive changes enable superHEAT to achieve mean-absolute deviations of only 9 cm–1 for the atomization energies of small, first-row molecules as benchmarked against the Active Thermochemical Tables. Combined with new statistical analysis, we determine a nominal 95% confidence interval for superHEAT of 25 cm–1, almost 1/16th of the chemical-accuracy proposed as a goal for the quantum chemical community in Pople’s Nobel Lecture. This represents a significant step in the development of computational methods that can provide ground thermochemical truth in the absence of experimental data.

We generated the aromatic resonance-stabilized vinylcyclopentadienyl radical, C₅H₄–CH=CH₂, in the gas phase via flash pyrolysis of meta-vinylanisole, CH₃O–C₆H₄–CH=CH₂. Double-imaging photoelectron photoion coincidence spectroscopy was used to measure the photoion mass-selected threshold photoelectron spectrum of the vinylcyclopentadienyl radical. Spectral assignments were enabled by independent, high-level ab initio coupled cluster calculations of adiabatic ionization energies originating from the ground electronic state of the doublet neutral, X̃ ²A″, to both the ground state of the cation, X̃^+ 1A′, and the lowest triplet state, ã^+ 3A′. The experimentally determined band origin energies are found to be (7.81 ± 0.02) eV (X̃^+ 1A′ ← X̃ ²A″) and (8.08 ± 0.02) eV (ã^+ 3A′ ← X̃ ²A″), in agreement with the respective calculated values of (7.83 ± 0.01) eV and (8.09 ± 0.01) eV. The combined experimental and theoretical characterization presented here provides a foundation to identify important resonance-stabilized radicals in complex gas-phase environments.

We extend the CCSDTQ implementation in CFOUR to UHF and ROHF references and demonstrate its efficiency. We apply it to basis set convergence of post-CCSDT(Q) corrections for the W4-08 thermochemical dataset. Convergence of (Q)$_\Lambda$--(Q) is relatively rapid. For difficult species (e.g., \ce{B2}, \ce{O3}), CCSDTQ--CCSDT(Q)$_\Lambda$ may converge more slowly than (5)$_\Lambda$, but the effects and and basis-set trends oppose each other. Consequently, a single-shot CCCSDTQ(5)$_\Lambda$-CCSDT(Q)$_\Lambda$ correction appears most efficient. For radicals with bifurcating UHF solutions, energetics of the `less spin-contaminated' solution are clearly more well-behaved. Our best computed adiabatic electron affinity of ozone is in excellent agreement with experiment.

The reaction of CH + N₂ forming H + NCN is a remarkable example of activation of the nitrogen triple bond and is an important source of prompt NO in combustion. The reaction pathway is complex and proceeds through two competing mechanisms: a cyclic addition channel initiated by c-HC(NN) and a chain-addition channel initiated by HCNN, both of which eventually form HNCN prior to dissociation to H + NCN. This work reinvestigates this reaction with composite coupled cluster protocols, including a novel spin-flip equation of motion coupled cluster scheme, combined with pragmatic two-dimensional master equation simulations of the resulting rate coefficients. These improved calculations predict the CH + N₂ rate coefficient between the two more recent previous theoretical results and reduce the uncertainties of the best theoretical models of this reaction to less than a factor of 1.3. Additionally, we provide a closer theoretical investigation of the simultaneous dependence of the CH + N₂ rate coefficient on pressure and temperature, and affirm that collisionally stabilized HNCN, another potential source of prompt NO, emerges as an appreciable product of this reaction under conditions relevant to automotive internal combustion engines and aircraft gas turbine engines.

We measured and analyzed the microwave (11-35 GHz) and millimeter-wave (85-750 GHz) rotational spectra of the normal isotopologue of cyclopropanone and nine additional isotopologues. The ability to perform experimental measurements is predicated on the synthesis of stable oligomers (or polymers) of cyclopropanone that release the monomeric form under vacuum to enable the investigation of cyclopropanone by gas-phase rotational spectroscopy. The spectral data provided in this work establish the foundation for radioastronomical searches for cyclopropanone. In particular, the new low-frequency microwave observations provide hyperfine-resolved rotational transitions that are necessary for radioastronomical observations at the low temperatures that characterize many cold interstellar molecular clouds. The extensive isotopic data set allows the first complete semiexperimental equilibrium (r_e^SE) structure of cyclopropanone. The 10 isotopologues used in this structure determination provide 30 moments of inertia. As a result, the values of all six independent structural parameters are highly converged, establishing a new experimental benchmark for the structure of this archetypal organic molecule. The highly precise and accurate r_e^SE structure was compared to a computed equilibrium (r_e) structure at the CCSD(T)/cc-pCV6Z level with additional corrections addressing finite basis set, higher-level electron correlation, and relativistic effects, as well as the diagonal Born-Oppenheimer correction. The computed r_e structure systematically deviates from the r_e^SE structure, despite the high-level methodology utilized. This finding is interpreted as revealing a subtle, but real, challenge for theoretical chemistry in predicting molecular structure at the level of accuracy that is now attainable experimentally.

We consider the thermochemical impact of post-CCSD(T) contributions to the total atomization energy (TAE, the sum of all bond energies) of first- and second-row molecules, and specifically their coupling with the subvalence correlation contribution. In particular, we find large contributions from (Q) when there are several neighboring second-row atoms. Otherwise, both higher-order triples T₃–(T) and connected quadruples (Q) are important in systems with strong static correlation. Reoptimization of the reference geometry for core–valence correlation increases the calculated TAE across the board, most pronouncedly for second-row compounds with neighboring second-row atoms. We present a first proposal for a “W5 theory” protocol and compare computed TAEs for the W4-08 benchmark with prior reference values. For some key second-row species, the new values represent nontrivial revisions. Our predicted TAE₀ values (TAE at 0 K) agree well with the ATcT (Active Thermochemical Tables) values, including for the very recent expansion of the ATcT network to boron, silicon, and sulfur compounds.