Lawrence J. Dunne

Analytical potential energy surface and quasi-classical dynamics for the reaction LiH(X, 1Σ+) + H(2S) → li(2s) + H2(X, 1Σ+g)

Abstract A potential energy surface has been derived using ab initio and spectroscopic data for the lowest 2 A ′ state of LiH 2 . Quasi-classical trajectories have been carried for the astrophysically important reaction LiH ( X , 1 Σ + )+ H ( 2 S ) → Li ( 2 S )+ H 2 ( X , 1 Σ + g ) . Between 150 and 4000 K the rate is 2–6×10 −10 – 6×10 −10 cm 3 molecule −1 s −1 and a satisfactory analytical fit to this is 8.4×10 −13 T exp(−0.0004 T ) while below 150 K the rate goes as 4.36×10 −11 T 0.28 cm 3 molecule s −1 which is close to that expected from the long-range behaviour of the electronic energy. The reaction is exothermic by about 2 eV which is disposed of largely in high vibrational states of H 2 ( X , 1 Σ + g ).

Statistical mechanical lattice model of the dual-peak electrocaloric effect in ferroelectric relaxors and the role of pressure

Despite considerable effort, the microscopic origin of the electrocaloric (EC) effect in ferroelectric relaxors is still intensely discussed. Ferroelectric relaxors typically display a dual-peak EC effect, whose origin is uncertain. Here we present an exact statistical mechanical matrix treatment of a lattice model of polar nanoregions forming in a neutral background and use this approach to study the characteristics of the EC effect in ferroelectric relaxors under varying electric field and pressure. The dual peaks seen in the EC properties of ferroelectric relaxors are due to the formation and ordering of polar nanoregions. The model predicts significant enhancement of the EC temperature rise with pressure which may have some contribution to the giant EC effect.

Mechanism of photocatalytic oxidation of 3,4-dichlorophenol on TiO2 semiconductor surfaces

Abstract In this paper, the results of a detailed study of the kinetics of photocatalytic oxidation of 3,4-dichlorophenol at various concentrations in oxygenated solutions on TiO2 are presented. Electron–hole recombination is often suggested to be decreased by increasing the molecular oxygen concentration; but our study indicates that this is not the primary rate enhancing property of the dissolved oxygen. It is argued here that a hydroxyl radical is ejected from the catalyst surface by photo-excitation onto a repulsive excited electronic state leading to a translationally hot OH( 2 π ) radical. In the presence of molecular oxygen, a simple hydroxyl addition to the dichlorophenol occurs. In the absence of an adsorbed oxygen molecule, the electron transfer to the aromatic ring from a Ti3+ site causes partial dechlorination of the dichlorophenol. Subsequently, hydroxyl radical addition to the aromatic ring may occur. Hence, we find that dissolved molecular oxygen has two important roles in the photo-catalytic oxidation of 3,4-dichlorophenol on the semiconducting TiO2 surfaces. One of these is as a H-atom acceptor required in direct hydroxyl radical addition to the phenyl ring while the other is as an electron transfer inhibitor when adsorbed at defective Ti3+ sites. A theoretical model of the kinetics is proposed which is able to account semi-quantitatively for the overall features of the reaction state space. Significantly, monitoring of the intermediate species produced by these two routes shows that the relative yields can be inverted by changing the dissolved oxygen concentration which significantly is accord with the theoretical predictions.

Classical trajectories for non-adiabatic molecular collisions

Abstract New dynamical equations of motion are proposed for the classical trajectories of molecular collisions to include non-adiabatic transitions between two electronic surfaces. These equations lead to trajectories on the pure adiabatic surfaces in the asymptotic regions of reactants and products.

Quasi-classical dynamics on the ground state surface of H2O

Quasi-classical trajectories have been carried out on a 2-valued surface for the singlet ground state of H2O. The calculated rate constants for both O(1 D) + H2(1Σg +) →OH(2Π) + H(2 S) and D(2 S) + OH(2Π) →H(2 S) + OD(2Π) are in good agreement with experimental values. For D(2 S) + OH(2Π) collisions above 2 eV, O(1 D) can be produced but the probability for this is low.

Microwave dielectric loss in oxides: Theory and experiment

We present a model that provides a description of the microwave dielectric loss in oxides. The dielectric loss (tan δ) in single crystal and polycrystalline MgO and Al2O3 is measured over the temperature range 70–300 K. We are able to model the dielectric loss in terms of a two-phonon difference model. There are two key parameters in this model: The third derivative, φ3, of the lattice potential and the linewidth, γ, of the thermal phonons. In polycrystalline samples, rather than considering the different mechanisms of extrinsic loss, it is assumed that the main effect of extrinsic factors is a modification of the linewidth of the thermal phonons. By varying γ(T), it is shown that the model can describe the loss in both single crystals and polycrystallines materials. In single crystal and polycrystalline MgO, we use γ as a fitting parameter. In single crystal and polycrystalline Al2O3, we obtain γ(T) by Raman spectroscopy. The theory gives the right order of magnitude of the measured loss.

The role of the [Btilde]-[Xtilde] conical intersection in the photodissociation of water

Non-adiabatic quasi-classical trajectories have been carried out on the [Btilde]-[Xtilde] 1 A′ two-valued surface of H2O using initial conditions which are appropriate to photodissociation via the [Btilde] state. Two models have been used. One of these involves surface hopping between adiabatic surfaces and the other integrates the electronic density matrix in a diabatic basis along a classical trajectory. The results of the two are in broad agreement. The majority of OH fragments are formed in the OH(2Π) state which is consistent with experimental findings. Approximately 10 per cent of trajectories lead to O(1D) + H2. The majority of OH(2Σ+) is produced in high rotational states of v = 0, which is also confirmed by experiment. However, the calculations predict large populations of high vibrational states of OH(2Π) which disagrees with experiment. For this reason it is likely that OH(2Π) is mainly produced by the alternative [Btilde]-A non-adiabatic process.

Reactive solvent extraction of amino acids with cationic extractants

A mixture of amino acids (arginine, phenylalanine, alanine, glycine and aspartic acid) in solution was extracted by four acidic extractants (dinonylnaphthalenesulphonic acid (DNNSA), di(2-ethylhexyl)monothiophosphoric acid (D2EHPA(S)), di(2-ethylhexyl)phosphoric acid (D2EHPA) and Versatic 10) in toluene. The extractive capacity of the organic phase for the amino acids, using DNNSA, D2EHPA(S) and D2EHPA was found to decrease in the order arginine>phenylalanine>alanine>glycine>aspartic acid, although at low pH values phenylalanine>arginine occurred for D2EHPA(S) and D2EHPA. Separation factors derived for pairs of amino acids were in the range 2·0 (glycine–aspartic acid) to 20·1 (alanine–glycine). The extractive and loading capability of the extractants for the amino acids was found to decrease in the order DNNSA>D2EHPA(S)>D2EHPA>Versatic 10, which follows the reverse order of their respective acid dissociation constants. DNNSA was shown to be a promising extractant for the extraction and fractionation of amino acids.

Microscopic interpretation of sign reversal in the electrocaloric effect in a ferroelectric PbMg1/3Nb2/3O3-30PbTiO3 single crystal

With increasing temperature, PbMg1/3Nb2/3O3-30PbTiO3 (PMN-30PT) crystals change from pseudo-rhombohedral to tetragonal to cubic phases. In addition to the usual positive electrocaloric effect (ECE), a negative ECE, whose origin is uncertain, is observed. Here, these two types of the ECE contributions in PbMg1/3Nb2/3O3-30PbTiO3 crystals are modelled theoretically using a one dimensional statistical mechanical lattice model, which is solved by an exact matrix method. The quasi one-dimensional model reproduces the trends in the experimental behaviour and attributes the electrocaloric sign reversal to free energy changes induced by the electric field.

Exact statistical mechanical one-dimensional lattice model of alkane binary mixture adsorption in zeolites and comparision with Monte-Carlo simulations

Adsorption isotherms obtained by Monte-Carlo simulation for ethane–methane mixtures in the zeolite silicalite show a highly unusual structure with a shoulder at low coverage and adsorption preference reversal at high pressures whose interpretation is uncertain. To understand this behaviour an exact calculation of the statistical mechanics of a lattice model of alkane mixture adsorption in zeolites has been undertaken. The lattice model reproduces all of the novel features of the Monte-Carlo isotherms, allowing us to infer that these unusual characteristics arise from a combination of molecular reorientation of ethane molecules, and displacement of ethane by methane at higher pressures.