Agnes Derecskei-Kovacs

Ab initio study of OH addition reaction to isoprene

Ab initio molecular orbital calculations have been employed to investigate the structures and energetics of the adduct isomers arising from the addition reaction of OH to isoprene. Several levels of ab initio theory were evaluated using a set of organic radical species to establish the appropriate level of approximation. The method of gradient corrected density functionals (NLDFT) in conjunction with moderate basis sets was found to yield satisfactory molecular geometries and vibrational frequencies. Single-point energy calculations were performed using various methods, including MP2, MP4, and CCSD(T). The most energetically favorable isomers are those with OH addition to the terminal carbon positions. At the CCSD(T)/6-311G** level of theory corrected with zero-point energy (ZPE), the isomers with OH additions to isoprene at C1 to C4 positions (i.e., isomers I–IV) are 34.8, 24.2, 22.4, and 32.3 kcal mol−1 more stable than the OH and isoprene, respectively. The activation energies against OH migration tran...

Theoretical study of isomeric branching in the isoprene–OH reaction: implications to final product yields in isoprene oxidation

Abstract The OH initiated oxidation of isoprene is important in tropospheric chemistry and has major implications for local and regional air quality. The reaction of isoprene with OH has been investigated by ab initio molecular orbital theory and unimolecular rate theory calculations. We report the energetics of the four possible adducts and the transition states associated with isomerization, which are energetically comparable to the dissociation of the adduct. We have employed CVTST and RRKM/ME theories based on the ab initio calculations to calculate the rates and branching ratios for the OH–isoprene reaction. The calculated fall-off behavior is consistent with recent experimental measurements.

Molecular modelling simulations to predict density and solubility parameters of ionic liquids

Molecular modelling simulations are one of the most convenient tools to predict solvent properties of ionic liquids, which are otherwise difficult to predict or measure by experimental means. Ionic liquids have emerged as effective and desirable alternative solvents during the last decade. Their use is greatly favoured because of their ‘green’ characteristics: low vapour pressure, non-flammability and chemical stability. An enormous variety of ionic liquids are potentially available and thus the design of task-specific ionic liquids is gaining importance. Traditional solvents are often evaluated based on their multi-component solubility parameters to assess their compatibility with different solutes. The analogous parameter values for ionic liquids are also useful, though their availability is often limited. Consequently, solvent design and pre-screening of candidates by molecular modelling tools are attractive alternatives. In this work, the density and two-component solubility parameter of a variety of ionic liquids were investigated using atomistic level molecular modelling and the commercially available Materials Studio® software package. The ionic liquids chosen consisted of ethyl, butyl and hexyl imidazolium cations in conjunction with four anions (Cl− , [(CN)2N] − , [CF3CO2]− and [Tf2N]− ). The goal was to develop appropriate computational protocols and carry out calculations, which are able to predict densities and two-component solubility parameters for these ionic liquids, as well as to evaluate the usefulness and accuracy of the currently available commercial molecular modelling software for these applications. It was found that the density of ionic liquids comprised of all possible combinations of these cations and anions could be calculated with satisfactory accuracy with the exception of systems containing the Cl− anion. A Hess cycle-based protocol was developed to correct cohesive energy densities and solubility parameters as calculated in Materials Studio to take into account ion pair interactions to successfully reproduce the experimental results. It was found that both the cation and the anion of the ionic liquid had a strong impact on the components of the solubility parameter.

The unimolecular dissociation of vinylcyanide: A theoretical investigation of a complex multichannel reaction

Ab initio molecular orbital calculations were performed toward the determination of the potential energy surface for the unimolecular ground-state dissociation of vinylcyanide. Reaction pathways for the three- and four-center elimination reactions of HCN and H2, as well as migration and radical elimination channels of H and CN, were examined. MP2 gradient geometry optimizations and QCISD(T) single point energy calculations were performed for all the relevant product species and transition states. The results are compared to the analogous unimolecular dissociation of vinylchloride which has been theoretically investigated by Morokuma and co-workers [J. Chem. Phys. 100, 8976 (1994)]. The unimolecular rates for all reaction channels have been calculated using Rice–Ramsperger–Kassel–Marcus (RRKM) theory employing ab initio transition state energies and MP2 vibrational frequencies. Our calculations indicate that the elimination of H2 and HCN preferentially proceed via three-center transition states. We also fi...

Through-space 13C–19F coupling can reveal conformations of modified BODIPY dyes†

The fact that only compounds 1a, 1b and 2a in the series 1– 3 show long-range 13C–19F coupling can be used to draw conclusions regarding the structures of these molecules.

Correlation of electrochemistry, nucleophilicity and density functional calculations of the cis-dithiolate (bme*-daco)Ni

Upon the sequential stoichiometric addition of methyl iodide to acetonitrile solutions of the square planar nickel (II) complex of N , N ′-bis(2-mercapto-2-methylpropyl)-1,5-diazacyclooctane, (bme*-daco)Ni, mono- and bis-methylated thioether compounds were isolated. Both are crystallographically and electrochemically characterized and compared to analogues of N , N ′-bis(2-mercaptoethyl)-1,5-diazacyclooctane, (bme-daco)Ni. Density functional theory calculations carried out at the B3LYP level using the LANL2DZ basis set with modified exponents compared HOMO/LUMO energies for the dithiolates, the mono- S -methylated and bis- S -methylated complexes. The presence of the methyl groups on the α-carbon next to the sulfur creates steric encumbrance about the thiolate sulfurs, but there was found no significant differences in the frontier molecular orbitals. The more positive shifts of the Ni(II/I) reduction potentials are reflected by changes in LUMO energies upon S -methylation.

Crystal structures of heptakis(2,6-di-O-tert-butyldimethylsilyl)cyclomaltoheptaose, heptakis(2-O-methyl-3,6-di-O-tert-butyldimethylsilyl)cyclomaltoheptaose and heptakis(2-O-methyl)cyclomaltoheptaose

Crystal structures of heptakis(2,6-di-O-tert-butyldimethylsilyl)cyclomaltoheptaose, heptakis(2-O-methyl-3,6-di-O-tert-butyldimethylsilyl)cyclomaltohep taose and heptakis(2-O-methyl)cyclomaltoheptaose were determined from X-ray diffraction patterns obtained for single crystals of the title compounds grown from ethyl acetate and ethanol, respectively, as solvent. The crystal structures prove conclusively that quantitative migration of the tert-butyldimethylsilyl group from the 2-O- to the 3-O-position [D. Icheln, B. Gehrcke, Y. Piprek, P. Mischnick, W.A. Konig, M.A. Dessoy, A.F. Morel, Carbohydr. Res., 280 (1996) 237-250] was achieved during methylation of heptakis(2,6-di-O-tert-butyldimethylsilyl)cyclomaltoheptaose by iodomethane-sodium hydride.

New Volatile Strontium and Barium Imidazolate Complexes for the Deposition of Group 2 Metal Oxides

We report the synthesis, characterization, and experimental density function theory-derived properties of new volatile strontium and barium imidazolate complexes, which under atomic layer deposition conditions using ozone as a reagent can deposit crystalline strontium oxide at 375 °C.

Atomic-level molecular modeling of the nonionic surfactant Triton X-100: The OPE9 component in vacuum and water

The commercially available nonionic surfactant Triton X-100 is a mixture of polyoxyethylene tert-octylphenyl ethers (OPEn) with an average of n = 9.5 oxyethylene (OE) units in the molecules, and the population maximum at n = 9. Thus, the OPEn = 9 component was chosen to be studied by atomic level molecular modeling, using second-generation force fields. The 1,000 conformers generated via random sampling of torsional angles around single bonds yielded 11 clusters based on geometrical similarity. Representatives of geometrically distinctly different clusters with significant populations were chosen from a narrow energy range around the most probable energy to be analyzed for interaction with water. The effect of water on the conformation of the OE chain was found to be modest, similar to the situation that had been reported earlier for the anionic surfactant Aerosol-OT (AOT). The number of bound water molecules is strongly dependent on the conformation of the OE chain and is affected by electrostatic as well as steric effects. Unlike the case of AOT, for which the length of the hydrophobic tail was found to govern the size of reverse micelles in CCl4, the size of reverse micelles of OPEn = 9 cannot be predicted from the dimensions of the hydrophilic tail.

Simulation of silicate structures in their aqueous solutions

Simulated infrared spectra of ions derived from sodium monosilicate are compared with experimental spectra of aqueous sodium monosilicate solutions. It is demonstrated that models can only approximate the experimental IR spectra, if the calculations explicitly include a hydrate shell surrounding these ions. Applying the continuum solvent model alone does not change the qualitative behaviour of vibrational spectra.