I. P. Hamilton

Mechanisms for the ozonolysis of ethene and propene: Reliability of quantum chemical predictions

Reactions of ozone with ethene and propene leading to primary ozonide (concerted and stepwise ozonolysis) or epoxide and singlet molecular oxygen (partial ozonolysis) are studied theoretically. The mechanism of concerted ozonolysis proceeds via a single transition structure which is a partial diradical. The transition structures and intermediates in the stepwise ozonolysis and partial ozonolysis mechanisms are singlet diradicals. Spin-restricted and unrestricted density functional methods are employed to calculate the structures of the closed-shell and diradical species. Although the partial diradicals exhibit moderate to pronounced instability in their RDFT and RHF solutions, RDFT is required to locate the transition structure for concerted ozonolysis. Spin projected fourth-order Moller–Plesset theory (PMP4) was used to correct the DFT energies. The calculated pre-exponential factors and activation energies for the concerted ozonolysis of ethene and propene are in good agreement with experimental values. However, the PMP4//DFT procedure incorrectly predicts the stepwise mechanism as the favored channel. UCCSD(T) predicts the concerted mechanism as the favored channel but significantly overestimates the activation energies. RCCSD(T) is found to be more accurate than UCCSD(T) for the calculation of the concerted mechanism but is not applicable to the diradical intermediates. The major difficulty in accurate prediction of the rate constant data for these reactions is the wide range of spin contamination for the reference UHF wave functions and UDFT solutions across the potential energy surface. The possibility of the partial ozonolysis mechanism being the source of epoxide observed in some experiments is discussed.

Insights into the Surface Complexation of Dimethylarsinic Acid on Iron (Oxyhydr)oxides from ATR-FTIR Studies and Quantum Chemical Calculations

The surface chemistry of methylated arsenicals with ubiquitous geosorbents and industrial catalysts is poorly understood. These arsenic compounds pose both a health and an environmental risk in addition to being a challenge to the energy industry. We report herein a detailed spectroscopic analysis of the surface structure of dimethylarsinic acid (DMA) adsorbed on hematite and goethite using attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Spectra of adsorbed DMA, DMA(ads), were collected in situ as a function of pH and ionic strength, using both H(2)O and D(2)O at 298 K in flow mode. Experimental data were complemented with DFT calculations of geometries and frequencies of hydrated DMA-iron oxide clusters. Results indicate the simultaneous formation of inner- and outer-sphere complexes with distinct spectral components. Desorption behavior of DMA due to chloride and phosphate was studied as a function of time from the decrease in the absorbance of apparent spectral features. The impact of our studies on the environmental fate of DMA in geochemical environments and the design of technologies to reduce arsenic content in fuels are discussed.

VALENCE SHELL STRUCTURES IN THE DISTRIBUTIONS OF THE LAPLACIAN OF THE ELECTRON DENSITY AND THE ONE-ELECTRON POTENTIAL FOR DIATOMIC MOLECULES

The valence shell charge concentration, VSCC, in an atom is defined by Bader as the outermost spherical region for which ∇2ρ(r)<0. We compare and contrast an alternate definition of the VSCC as the outermost spherical region for which ∇2ρ(r)/2ρ(r)<0. The quantity ∇2ρ(r)/2ρ(r), termed the one-electron potential (OEP), is implicit in the (exact) one-electron Schrodinger equation. In a homonuclear diatomic bound by shared interaction, the VSCCs of the atoms are merged. Provided that the merged VSCCs persist, the diatomic is enclosed by an outermost zero-valued surface encompassing the VSCC of the molecule. The outermost ∇2ρ(r)=0 surface is termed the reactive surface while the outermost OEP=0 surface is termed the molecular envelope. In cases where the VSCCs are not revealed in the atoms, the reactive surfaces and molecular envelopes are incomplete or absent in the diatomics. We show that in many diatomics the molecular envelope is present although the reactive surface is missing. In an ionic diatomic bound ...

Interacting electrons, spin statistics, and information theory

We consider a nearly (or quasi) uniform gas of interacting electrons for which spin statistics play a crucial role. A previously developed procedure, based on the extension of the Levy-Lieb constrained search principle and Monte Carlo sampling of electron configurations in space, allows us to approximate the form of the kinetic-energy functional. For a spinless electron gas, this procedure led to a correlation term, which had the form of the Shannon entropy, but the resulting kinetic-energy functional does not satisfy the Lieb-Thirring inequality, which is rigorous and one of the most general relations regarding the kinetic energy. In this paper, we show that when the fermionic character of the electrons is included via a statistical spin approach, our procedure leads to correlation terms, which also have the form of the Shannon entropy and the resulting kinetic-energy functional does satisfy the Lieb-Thirring inequality. In this way we further strengthen the connection between Shannon entropy and electron correlation and, more generally, between information theory and quantum mechanics.

Vibrational levels for the lowest‐lying triplet and singlet states of CH2 and NH+2

The 30 lowest vibrational energy levels are calculated for the X 3B1 and a 1A1 states of CH2 and NH+2 for various potential energy surfaces. We include a new surface for the NH+2 triplet state, with predicted fundamentals ν1=3059, ν2=845, and ν3=3360 (expt 3359.9) cm−1. Where possible, results are compared with vibrational levels calculated using two other methods and with experiment. At low energy, the differences between the calculated vibrational levels are due primarily to the choice of potential energy surface. Of secondary importance is the method used to calculate the vibrational energy levels. The differences resulting from the method used are most apparent for cases with a diffuse vibrational wave function or for cases where the bond distance along the minimum energy path depends strongly on the bending angle.

Thermodynamics of Dimethylarsinic Acid and Arsenate Interactions with Hydrated Iron-(Oxyhydr)oxide Clusters: DFT Calculations

Dimethylarsinic Acid (DMA) belongs to an important class of organoarsenical compounds commonly detected in arsenic speciation studies of environmental samples and pyrolysis products of fossil fuels. Transformation of DMA under certain conditions leads to the formation of other forms of arsenic, which could be more toxic than DMA to biota, and more efficient in deactivating catalysts used in petrochemical refining. Published surface sensitive X-ray and infrared spectroscopic work suggested that DMA simultaneously forms inner- and outer-sphere complexes with iron-(oxyhydr)oxides. Computational work on the complexation of arsenicals with various surfaces of environmental and industrial interest provides useful information that aids in the interpretation of experimental spectroscopic data as well as predictions of thermodynamic favorability of surface interactions. We report herein Gibbs free energies of adsorption, ΔG(ads), for various ligand exchange reactions between hydrated complexes of DMA and Fe-(oxyhydr)oxide clusters calculated using density functional theory (DFT) at the B3LYP/6-311+G(d,p) level. Calculations using arsenate were also performed for comparison. Calculated As-(O,Fe) distances and stretching frequencies of As-O bonds are also reported for comparison with experimental spectroscopic data. Gibbs free energies of desorption, ΔG(des), due to reactions with phosphorus species at pH 7 are reported as well. Our results indicate that the formation of both inner- and outer-sphere DMA complexes is thermodynamically favorable, with the former having a more negative ΔG(ads). Values of ΔG(des) indicate that desorption favorability of DMA complexes increases in this order: bidentate < mondentate < outersphere. The significance of our results for the overall surface complexation mechanism of DMA is discussed.

Geometries and vibrational frequencies for calcium and strontium radical salts of H, F, Cl, Br, I, OH, SH, CN, NC, CCH, NNN, NCO, NCS, NH2, CH3, OCH3, SCH3, HCOO, CH3COO, HCONH and HCONCH3

Abstract Geometries and vibrational frequencies for the ground states of a variety of metal–ligand alkaline earth radical salts of calcium and strontium are computed using the B3LYP density functional method with 6-31++G ∗∗ and DZVP basis sets. Ligands considered are both monodentate (H, F, Cl, Br, I, OH, SH, CN, NC, CCH, NNN, NCO, NCS, NH2, CH3, OCH3, SCH3) and bidentate (HCOO, CH3COO, HCONH, HCONCH3) species that have been studied in the gas phase.

Pulsed corona discharge for oxidation of gaseous elemental mercury

Positive pulsed corona discharge has been applied for the oxidation of gaseous elemental mercury (Hg{sup O} from a simulated flue gas. The oxidation of Hg{sup 0} to Hg{sup O} and HgCl{sub 2} can significantly enhance the mercury removal from flue gas. At a gas condition of O{sub 2} (10%), H{sub 2}O (3%), and N{sub 2} (balance), Hg{sup 0} oxidation efficiency of 84% was achieved at an input energy density of 45 J/l. The presence of NO, however, hinders Hg{sup 0} oxidation due to the preferential reaction of NO with O and O{sub 3}. On the contrary, SO{sub 2} shows little effect on Hg{sup 0} oxidation due to its preferential reaction with OH. It has been also observed that the HCl in gas stream can be dissociated to Cl and Cl{sub 2} and can induce additional Hg{sup 0} oxidation to HgCl{sub 2}.

Ab Initio Vibrational Levels For HO2 and Vibrational Splittings for Hydrogen Atom Transfer

We calculate vibrational levels and wave functions for HO2 using the recently reported ab initio potential energy points of Walch and Duchovic [J. Chem. Phys. 94, 7068 (1991)] as fit by Dateo (unpublished). There is intramolecular hydrogen atom transfer when the hydrogen atom tunnels through a T‐shaped saddle point separating the two equivalent equilibrium geometries, and correspondingly, the vibrational levels are split. We focus on vibrational levels and wave functions with significant splitting. The first three vibrational levels with splitting greater than 2 cm−1 are (1,5,0), (0,7,1), and (0,8,0), where v2 is the O–O–H bend quantum number. We discuss the dynamics of hydrogen atom transfer; in particular, the O–O distances at which hydrogen atom transfer is most probable for these vibrational levels.

Ab initio calculations on the ground and excited states of BeOH and MgOH

Ab initio configuration interaction calculations have been carried out on the ground and excited electronic states of the BeOH and MgOH molecules as well as of the cations BeOH+ and MgOH+, for linear and bent geometries. The excited states of the above molecules have not previously been calculated and experimental information exists only for the A–X system in MgOH and MgOD. The present results show that in the excited states the molecules MgOH and BeOH have similar M–O (M=Mg,Be) stretching and bending potentials. In general, the stretching potentials are rather complicated, showing a number of avoided crossings. Furthermore, most of the excited states show minima at R near Rmin of the corresponding cations BeOH+ and MgOH+, indicating Rydberg contributions to the molecular excited states. The first excited state in both BeOH and MgOH is 2 2A′, which along with 1 2A″, forms the 1 2Π state of linear geometries and which in both systems has minimum energy at a bent geometry with bond angle near 115°. In MgOH,...