Daniel P. Schofield

Understanding the sensor response of metal-decorated carbon nanotubes

We have explored the room temperature response of metal nanoparticle decorated single-walled carbon nanotubes (NP-SWNTs) using a combination of electrical transport, optical spectroscopy, and electronic structure calculations. We have found that upon the electrochemical growth of Au NPs on SWNTs, there is a transfer of electron density from the SWNT to the NP species, and that adsorption of CO molecules on the NP surface is accompanied by transfer of electronic density back into the SWNT. Moreover, the electronic structure calculations indicate dramatic variations in the charge density at the NP-SWNT interface, which supports our previous observation that interfacial potential barriers dominate the electrical behavior of NP-SWNT systems.

How the Shape of an H-Bonded Network Controls Proton-Coupled Water Activation in HONO Formation

Its the Network Numerous reactions of small molecules and ions in the atmosphere take place in the confines of watery aerosols. Relph et al. (p. 308; see the Perspective by Siefermann and Abel) explored the specific influence of a water clusters geometry on the transformation of solvated nitrosonium (NO+) to nitrous acid (HONO). The reaction involves (O)N–O(H) bond formation with one water molecule, concomitant with proton transfer to additional, surrounding water molecules. Vibrational spectroscopy and theoretical simulations suggest that certain arrangements of the surrounding water network are much more effective than others in accommodating this charge transfer, and thus facilitating the reaction. Vibrational spectroscopy uncovers the role of a surrounding water network in the mediating reaction of a solvated ion. Many chemical reactions in atmospheric aerosols and bulk aqueous environments are influenced by the surrounding solvation shell, but the precise molecular interactions underlying such effects have rarely been elucidated. We exploited recent advances in isomer-specific cluster vibrational spectroscopy to explore the fundamental relation between the hydrogen (H)–bonding arrangement of a set of ion-solvating water molecules and the chemical activity of this ensemble. We find that the extent to which the nitrosonium ion (NO+)and water form nitrous acid (HONO) and a hydrated proton cluster in the critical trihydrate depends sensitively on the geometrical arrangement of the water molecules in the network. Theoretical analysis of these data details the role of the water network in promoting charge delocalization.

Calculated OH-stretching and HOH-bending vibrational transitions in the water dimer

We have calculated the fundamental and overtone OH-stretching and HOH-bending vibrational spectrum of the water dimer up to 20 000 cm−1. The calculated frequencies and intensities of the transitions up to 8000 cm−1 agree well with previously recorded matrix isolation spectra. We compare our calculated spectrum of the water dimer to the observed water monomer spectrum, and suggest regions in which spectral features due to the water dimer are most likely to be observed in the laboratory and our atmosphere.

Comparison of models with distributed polarizable sites for describing water clusters

In this work we examine three water models, including one developed by us, with distributed polarizable sites. These models are assessed by comparison with the results of MP2 calculations. The use of distributed polarizable sites is found to be especially important for describing the 3-body interaction energies of water clusters. It is also shown that all-atom repulsion terms are necessary to accurately describe cluster geometries.

High-level ab initio studies of the electronic excited states of the hydroxyl radical and water-hydroxyl complex

The lowest-energy electronic transitions in the hydroxyl radical and the hydrogen bound complex H(2)O.HO have been studied using ab initio methods. We have used the complete active-space self-consistent field and multireference configuration interaction (MRCI) methods to calculate vertical excitation energies and oscillator strengths. At the MRCI level the lowest-lying (2)Sigma(+)<--(2)Pi electronic transition is redshifted by about 2500 cm(-1) upon formation of the H(2)O.HO complex. We propose that this transition could be used to identify the complex in the gas phase, which in turn could be used to examine the role of H(2)O.HO in atmospheric reactions.

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

Resonant two-photon ionization (R2PI), resonant ion-dip infrared (RIDIR), and UV-UV hole-burning spectroscopies have been employed to obtain conformation-specific infrared and ultraviolet spectra under supersonic expansion conditions for O-(2-acetamidoethyl)-N-acetyltyramine (OANAT), a doubly substituted aromatic in which amide-containing alkyl and alkoxy side chains are located in para positions on a phenyl ring. For comparison, three single-chain analogs were also studied: (i) N-phenethyl-acetamide (NPEA), (ii) N-(p-methoxyphenethyl-acetamide) (NMPEA), and (iii) N-(2-phenoxyethyl)-acetamide (NPOEA). Six conformations of OANAT have been resolved, with S(0)-S(1) origins ranging from 34,536 to 35,711 cm(-1), denoted A-F, respectively. RIDIR spectra show that conformers A-C each possess an intense, broadened amide NH stretch fundamental shifted below 3400 cm(-1), indicative of the presence of an interchain H bond, while conformers D-F have both amide NH stretch fundamentals in the 3480-3495 cm(-1) region, consistent with independent-chain structures with two free NH groups. NPEA has a single conformer with S(0)-S(1) origin at 37,618 cm(-1). NMPEA has three conformers, two that dominate the R2P1 spectrum, with origin transitions between 35,580 and 35,632 cm(-1). Four conformations, one dominate and three minor, of NPOEA have been resolved with origins between 35,654 and 36,423 cm(-1). To aid the making of conformational assignments, the geometries of low-lying structures of all four molecules have been optimized and the associated harmonic vibrational frequencies calculated using density functional theory (DFT) and RIMP2 methods. The S(0)-S(1) adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the S(0)-S(1) energy separation in OANAT and NPOEA primarily arises from different orientations of the chain attached to the phenoxy group. Using the results of the single-chain analogs, tentative assignments have been made for the observed conformers of OANAT. The RIMP2 calculations predict that interchain H-bonded conformers of OANAT are 25-30 kJ/mol more stable than the extended-chain structures. However, the free energies of the interchain H-bonded and extended structures calculated at the preexpansion temperature (450 K) differ by less than 10 kJ/mol, and the number of extended structures far outweighs the number of H-bonded conformers. This entropy-driven effect explains the presence of the independent-chain conformers in the expansion, and cautions future studies that rely solely on relative energies of conformers in considering possible assignments.

High level ab initio studies of the excited states of sulfuric acid and sulfur trioxide

We have calculated the vertical excitation energies and oscillator strengths of the lowest energy electronic transitions in sulfuric acid (H2SO4) and sulfur trioxide (SO3) with a range of ab initio methods. We have found that the highest level calculations with the complete active space self-consistent field and multireference configuration interaction (MRCI) methods predict transition energies much lower than those previously calculated with the simpler configuration interaction-singles method. The MRCI calculated electronic transitions for SO3 are in good agreement with the experimental results, whereas electronic transitions in vapor phase H2SO4 have yet to be observed. Our MRCI results suggest that the lowest lying electronic excitation in H2SO4 occurs around 144 nm and that the cross section in the actinic region is very small.

Vibrational spectroscopy of the water-nitrate complex in the O-H stretching region.

The vibrational spectroscopy of the nitrate-water isotopologues is studied in the O-H and O-D stretching regions using temperature-dependent infrared multiple photon dissociation spectroscopy combined with calculations of the anharmonic spectra. At a temperature of 15 K a series of discrete peaks is observed in the IRMPD spectra of NO3(-)·H2O, NO3(-)·HDO, and NO3(-)·D2O. This structure is considerably more complex than predicted by harmonic calculations. A signal is only observed in the hydrogen-bonded O-H (O-D) stretching region, characteristic of a double hydrogen-bond donor binding motif. With increasing temperature, the peaks broaden, leading to a quasi-continuous absorption from 3150 to 3600 cm(-1) (2300-2700 cm(-1)) for NO3(-)·H2O (NO3(-)·D2O) and, above 100 K, an additional band in the free O-H (O-D) stretching region, suggesting the population of complexes containing only a single hydrogen bond at higher internal energies. Vibrational configuration interaction calculations confirm that much of the structure observed in the IRMPD spectra derives from progressions in the water rocking mode resulting from strong cubic coupling between the O-H (O-D) stretch and water rock degrees of freedom. The spectra of both NO3(-)·H2O and NO3(-)·D2O display a strong peak that does not derive from the water rock progression but results instead from a Fermi resonance between the O-H (O-D) stretch and H-O-H (D-O-D) bend overtone. Additional insight into the nature of the O-H (O-D) stretch and water rocking coupling in these complexes is provided by an effective Hamiltonian that allows for the cubic coupling between the O-H stretch and water rock degrees of freedom.

Molecular dynamics simulations of bromine clathrate hydrates.

A polarizable force field that explicitly includes contributions from exchange repulsion, dispersion, charge penetration, and multipole electrostatics was developed to describe the interaction between bromine and water. This force field was combined with a polarizable force field for water and used in molecular dynamics simulations to calculate the relative energetics of three bromine clathrate hydrates. The simulations predict the tetragonal structure (Allen, K. W.; Jeffrey, G. A. J. Chem. Phys. 1963, 38, 2304) to be the most stable, with the CS-I and CS-II cubic structures being less stable. Although the CS-II species is not the most stable energetically, we argue that it could be formed under conditions of low bromine concentration, in agreement with recent measurements (Goldschleger, I. U.; Kerenskaya, G.; Janda, K. C.; Apkarian, V. A. J. Phys. Chem. A 2008, 112, 787) that provide evidence for three different bromine hydrate crystal types.

Chapter 8 – Atmospheric Photolysis of Sulfuric Acid

Abstract We describe theoretical methods for the calculation of vibrational and electronic transitions in sulfuric acid, from which absorption cross sections can be obtained in the infrared through to the vacuum ultraviolet region. In the absence of experimental cross sections these calculations provide invaluable input for the assessment of the atmospheric photolysis of sulfuric acid. The vibrational model is based on a local mode model that includes the OH-stretching and SOH-bending vibrations, while the electronic transitions are calculated with coupled cluster response theory. These approaches are sufficient to describe the dominant vibrational transitions in the near infrared and visible regions, the lowest lying electronic transitions in the ultraviolet region and the higher energy electronic transitions in the region of Lyman- α radiation. We highlight the influence quantum mechanical calculations have had in the recent discussion of the atmospheric photolysis of sulfuric acid, and show that theoretical calculations can provide absorption cross sections of an accuracy that is useful in atmospheric science.