Products of Criegee intermediate reactions with NO2::experimental measurements and tropospheric implications

The reactions of Criegee intermediates with NO2 have been proposed as a potentially significant source of the important nighttime oxidant NO3, particularly in urban environments where concentrations of ozone, alkenes and NOx are high. However, previous efforts to characterize the yield of NO3 from these reactions have been inconclusive, with many studies failing to detect NO3. In the present work, the reactions of formaldehyde oxide (CH2OO) and acetaldehyde oxide (CH3CHOO) with NO2 are revisited to further explore the product formation over a pressure range of 4–40 Torr. NO3 is not observed; however, temporally resolved and [NO2]-dependent signal is observed at the mass of the Criegee–NO2 adduct for both formaldehyde- and acetaldehyde-oxide systems, and the structure of this adduct is explored through ab initio calculations. The atmospheric implications of the title reaction are investigated through global modelling.</p

Good Computational Practice in the Assignment of Absolute Configurations by TDDFT Calculations of ECD Spectra

Quantum-mechanical calculations of chiroptical properties have rapidly become the most popular method for assigning absolute configurations (AC) of organic compounds, including natural products. Black-box time-dependent Density Functional Theory (TDDFT) calculations of electronic circular dichroism (ECD) spectra are nowadays readily accessible to nonexperts. However, an uncritical attitude may easily deliver a wrong answer. We present to the Chirality Forum a discussion on what can be called good computational practice in running TDDFT ECD calculations, highlighting the most crucial points with several examples from the recent literature

Electronic structure theory and multi-structural statistical thermodynamics for computational chemical kinetics.

University of Minnesota Ph.D. dissertation. August 2012. Major: Chemical Physics. Advisor: Donald G. Truhlar. 1 computer file (PDF); xiii, 366 pages.This thesis involves the development and application of methods for accurate computational thermochemistry. It consists of two parts. The first part focuses on the accuracy of the electronic structure methods. In particular, various augmentation schemes for one-electron basis sets are presented and tested for density functional theory (DFT) calculations and for wave function theory (WFT) calculations. The relationship between diffuse basis functions and basis set superposition error is discussed. For WFT, we also compare the efficiency of conventional one-electron basis-sets to that of newly developed explicitly correlated methods. Various ways of approaching the complete basis set limit of WFT calculations are explained, and recommendations are made for the best ways of achieving balance between the basis set size, higher-order correlation, and relativistic corrections. Applications of this work include computation of barrier heights, reaction and bond energies, electron affinities, ionization potentials, and noncovalent interactions. The second part of this thesis focuses on the problem of incorporating multistructural effects and anharmonicity effects in the torsional modes into partition function calculations, especially by using a new multi-structural torsion (MS-T) method. Applications of the MS-T method include partition functions of molecules and radicals important for combustion research. These partition functions are used to obtain thermodynamic functions that are the most reliable results available to date for these molecules. The multi-structural approach is also applied to two kinetics problems: • the hydrogen abstraction from carbon-3 of 1-butanol by hydroperoxyl radical • the 1,5-hydrogen shift isomerization of the 1-butoxyl radical In both cases multi-structural effects play an important role in the final results

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