Efficient Diffuse Basis Sets for Density Functional Theory

Eliminating all but the <i>s</i> and <i>p</i> diffuse functions on the non-hydrogenic atoms and all diffuse functions on the hydrogen atoms from the aug-cc-pV(<i>x</i>+d)Z basis sets of Dunning and co-workers, where <i>x</i> = D, T, Q, ..., yields the previously proposed “minimally augmented” basis sets, called maug-cc-pV(<i>x</i>+d)Z. Here, we present extensive and systematic tests of these basis sets for density functional calculations of chemical reaction barrier heights, hydrogen bond energies, electron affinities, ionization potentials, and atomization energies. The tests show that the maug-cc-pV(<i>x</i>+d)Z basis sets are as accurate as the aug-cc-pV(<i>x</i>+d)Z ones for density functional calculations, but the computational cost savings are a factor of about two to seven

Kinetics of the Hydrogen Abstraction from Carbon-3 of 1-Butanol by Hydroperoxyl Radical: Multi-Structural Variational Transition-State Calculations of a Reaction with 262 Conformations of the Transition State

We estimated rate constants for the hydrogen abstraction from carbon-3 of 1-butanol by hydroperoxyl radical, a critically important reaction in the combustion of biofuel. We employed the recently developed multi-structural variational transition-state theory (MS-VTST), which utilizes a multifaceted dividing surface that allows us to include the contributions of multiple structures for reacting species and transition states. First, multiconfigurational Shepard interpolationbased on molecular-mechanics-guided interpolation of electronic-structure Hessian data obtained by the M08 HX/jun-cc-pVTZ electronic model chemistrywas used to obtain the portion of the potential energy surface needed for single-structure variational transition-state theory rate constants including multidimensional tunneling; then, the M08-HX/MG3S electronic model chemistry was used to calculate multi-structural torsional anharmonicity factors to complete the MS-VTST rate constant calculations. The lowest-energy structures of the transition state have strongly bent hydrogen bonds. Our results indicate that neglect of multi-structural anharmonicity would lead to errors of factors of 0.3, 46, and 171 at 200, 1000, and 2400 K for this reaction

Multistructural Variational Transition State Theory: Kinetics of the Hydrogen Abstraction from Carbon‑2 of 2‑Methyl-1-propanol by Hydroperoxyl Radical Including All Structures and Torsional Anharmonicity

We calculated the forward and reverse rate constants of the hydrogen abstraction reaction from carbon-2 of 2-methyl-1-propanol by hydroperoxyl radical over the temperature range 250–2400 K by using multistructural canonical variational transition state theory (MS-CVT) including both multiple-structure and torsional potential anharmonicity effects by the multistructural torsional anharmonicity (MS-T) method. In these calculations, multidimensional tunneling (MT) probabilities used to compute the tunneling transmission coefficients were evaluated by the small-curvature tunneling (SCT) approximation. Comparison with the rate constants obtained by the single-structural harmonic oscillator (SS-HO) approximation shows that multistructural anharmonicity increases the forward rate constants for all temperatures, but the reverse rate constants are reduced for temperatures lower than 430 K and increased for higher temperatures. The neglect of multistructural torsional anharmonicity would lead to errors of factors of 1.5, 8.8, and 13 at 300, 1000, and 2400 K, respectively, for the forward reaction, and would lead to errors of factors of 0.76, 3.0, and 6.0, respectively, at these temperatures for the reverse reaction

Biofuel Combustion. Energetics and Kinetics of Hydrogen Abstraction from Carbon‑1 in <i>n</i>‑Butanol by the Hydroperoxyl Radical Calculated by Coupled Cluster and Density Functional Theories and Multistructural Variational Transition-State Theory with Multidimensional Tunneling

Multistructural canonical variational transition-state theory with small-curvature multidimensional tunneling (MS-CVT/SCT) is employed to calculate thermal rate constants for hydrogen-atom abstraction from carbon-1 of <i>n</i>-butanol by the hydroperoxyl radical over the temperature range 250–2000 K. The M08-SO hybrid meta-GGA density functional was validated against CCSD­(T)-F12a explicitly correlated wave function calculations with the jul-cc-pVTZ basis set. It was then used to compute the properties of all stationary points and the energies and Hessians of a few nonstationary points along the reaction path, which were then used to generate a potential energy surface by the multiconfiguration Shepard interpolation (MCSI) method. The internal rotations in the transition state for this reaction (like those in the reactant alcohol) are strongly coupled to each other and generate multiple stable conformations, which make important contributions to the partition functions. It is shown that neglecting to account for the multiple-structure effects and torsional potential anharmonicity effects that arise from the torsional modes would lead to order-of-magnitude errors in the calculated rate constants at temperatures of interest in combustion

Pressure-Dependent Competition among Reaction Pathways from First- and Second‑O₂ Additions in the Low-Temperature Oxidation of Tetrahydrofuran

We report a combined experimental and quantum chemistry study of the initial reactions in low-temperature oxidation of tetrahydrofuran (THF). Using synchrotron-based time-resolved VUV photoionization mass spectrometry, we probe numerous transient intermediates and products at <i>P</i> = 10–2000 Torr and <i>T</i> = 400–700 K. A key reaction sequence, revealed by our experiments, is the conversion of THF-yl peroxy to hydroperoxy-THF-yl radicals (QOOH), followed by a second O₂ addition and subsequent decomposition to dihydrofuranyl hydroperoxide + HO₂ or to γ-butyrolactone hydroperoxide + OH. The competition between these two pathways affects the degree of radical chain-branching and is likely of central importance in modeling the autoignition of THF. We interpret our data with the aid of quantum chemical calculations of the THF-yl + O₂ and QOOH + O₂ potential energy surfaces. On the basis of our results, we propose a simplified THF oxidation mechanism below 700 K, which involves the competition among unimolecular decomposition and oxidation pathways of QOOH