5 research outputs found

### 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 âminimally augmentedâ 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 interpolationî¸based on molecular-mechanics-guided interpolation
of electronic-structure Hessian data obtained by the M08 HX/jun-cc-pVTZ
electronic model chemistryî¸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 Carbonâ2 of 2âMethyl-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 250â2400 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 Carbonâ1 in <i>n</i>âButanol 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 250â2000 K. The M08-SO hybrid meta-GGA
density functional was validated against CCSDÂ(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 SecondâO<sub>2</sub> 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> = 10â2000 Torr and <i>T</i> = 400â700 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 O<sub>2</sub> addition and subsequent
decomposition to dihydrofuranyl hydroperoxide + HO<sub>2</sub> or
to Îł-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 + O<sub>2</sub> and QOOH + O<sub>2</sub> 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