Solvation

Two ionic liquids (ILs), namely (S,S)-1-butyl-3-(2′-hydroxy-cyclohexyl)-3H-imidazol-1-ium tetrafluoroborate and (S,S)-1-butyl-3-(2′-acetyl-cyclohexyl)-3H-imidazol-1-ium tetrafluoroborate have been employed as stationary phases in capillary gas chromatography. These new phases exhibit a column efficiency of 1,600 and 2,100 plates m−1 for IL 1 and IL 2, respectively, a wide operating temperature range and good thermal stability (bleeding temperature of 250 °C for IL 1 and 160 °C for IL 2). Inverse gas chromatography (GC) analyses were used to study the solvation properties of these ILs through a linear solvation energy model. The application of these ILs as new GC stationary phases was studied. These stationary phases exhibited unique selectivity for many organic substances, such as alkanes, ketones, esters, and aromatic compounds. The efficient separation of several mixtures containing compounds of different polarities and the good separation of fatty acid methyl esters (FAMEs) and cis/trans isomers indicate that these ILs may be applicable as a new type of GC stationary phases.

The sodium cation plays an important role in several physiological processes. Understanding its solvation may help understanding ion selectivity in sodium channels that are pivotal for nerve impulses. This paper presents a thorough investigation of over 75 isomers of gas-phase Na+ (H2O) n=1−8 clusters, whose optimized structures, energies, and (harmonic) vibrational frequencies were computed quantum mechanically at the full MP2/6-31++G(d,p) level of theory. From these data, we have calculated the temperature effects on the cluster thermodynamic functions, and thus the equilibrium Boltzmann distribution for each n. For a selected number of isomers, we have corrected the calculations for basis set superposition error (BSSE) to obtain accurate clustering energies, in excellent agreement with experiment. The computed clusters are overwhelmingly 4-coordinated, as opposed to bulk liquid water, where sodium cations are believed to be mostly 5-or 6-coordinated. To explain this, we suggest the "cluster stability rules" , a set of coordination-number-dependent hydrogen-bond (HB) strengths that can be obtained using a single BSSE correction. Assuming additivity and transferability, these reproduce the relative stability of most of our computed isomers. These rules enable us to elucidate the trends in HB strengths, outlining the major determinants of cluster stability. For n = 4 and 5, we have also performed anharmonic vibrational calculations (VPT2) to compare with available photodissociation infrared spectra of these gas-phase clusters. The comparison suggests that the experiments actually monitor a mixture of predominantly 3-coordinated isomers, which is quite remote from the computed Boltzmann distribution, particularly at low temperatures. Surprisingly, for these experiments, water evaporation pathways can rationalize the non-equilibrium isomer distribution. The equilibrium isomer distribution is, in turn, rationalized by the entropy of internal rotations of "dangling" water molecules.

Work has been carried out over the last few years on the interaction of dolomite surfaces with common, environmentally significant species that are found in the same geographical locations as the mineral. The thesis has evolved from the desire to understand the molecular processes in this environmental problem. In addition to a number of results elucidating the behaviour of a number of pollutant species in contact with a dolomite surface, it also presents an evaluation of some of the oft-used computational methods applied to such systems. Initial work was carried out on dry surfaces, studying the substitution of native dolomite cations by six divalent, metal cations that are known to form end-member carbonate minerals: Ni, Mn, Zn, Fe, Cd and Co. It was found that, under vacuum, none of the straightforward surface substitutions was energetically favourable. Substitutions at both the edges and terraces of steps were found to be favourable, although with different ordering of cations. Addition of monolayer solvation at the perfect surface generated substitution ordering in accordance with experimental data, but still most were not favoured. Further, a new implementation of implicit surface solvation, COSMIC 1 , has been used in this work. It was found that this method provides a useful tool for investigating relative substitution energies at surfaces, and that, as expected from experiment, substitutions at Mg sites are favoured a reversal of the vacuum results. Finally, DFT calculations were used to model the absorption of a more complicated pollutant, the arsenate molecule on the dolomite surface. Initially, implicit and explicit solvation methods were used to model a molecule in solution, which allowed the molecule's behaviour when modelled using DFT to be determined. An ab initio solvation energy was calculated for the molecule, which favourably compared with that obtained using implicit solvation methods. In addition, the preferred adsorption configuration of the molecule at the carbonate surface was determined. It was shown that the use of DFT methods are worthwhile in understanding the subtleties of molecular scale surface-solution interactions.

We have measured X-ray absorption spectra for acetic acid molecules in an organic solvent in order to study the effect of hydrogen-bonding formation. The spectral shapes distorted by saturation effect were compensated by using a new correction procedure for X-ray absorption based on the changes in the concentration of the samples. A resonance peak shift was observed when the acetic acid molecules formed a cyclic dimer in hexane solution, which is similar to the behavior of cluster molecules in vacuum. This is the first report on the XAS spectral changes due to cyclic dimer formation in a solution.

Three dimensional implementations of liquid state theories offer an efficient alternative to computer simulations for the atomic-level description of aqueous solutions in complex environments. In this context, we present a (classical) molecular density functional theory (MDFT) of water that is derived from first principles and is based on two classical density fields, a scalar one, the particle density, and a vectorial one, the multipolar polarization density. Its implementation requires as input the partial charge distribution of a water molecule and three measurable bulk properties, namely the structure factor and the k-dependent longitudinal and transverse dielectric constants. It has to be complemented by a solute-solvent three-body term that reinforces tetrahedral order at short range. The approach is shown to provide the correct three-dimensional microscopic solvation profile around various molecular solutes, possibly possessing H-bonding sites, at a computer cost two-three or...