Lineshapes in carbon 1s photoelectron spectra of methanol clusters

A general protocol for theoretical modeling of inner-shell photoelectron spectra of molecular clusters is presented and applied to C1s spectra of oligomers and medium-sized clusters of methanol. The protocol employs molecular dynamics for obtaining cluster geometries and a polarizable force field for computing site-specific chemical shifts in ionization energy and linewidth. Comparisons to spectra computed from first-principle theories are used to establish the accuracy of the proposed force field approach. The model is used to analyze the C1s photoelectron spectrum of medium-sized clusters in terms of surface and bulk contributions. By treating the surface-to-bulk ratio as an adjustable parameter, satisfactory fits are obtained to experimental C1s spectra of a beam of methanol clusters.     

Conformation and solvation structure for an isolated n-octadecane chain in water, methanol, and their mixtures

Configurational-bias Monte Carlo simulations in the isobaric-isothermal ensemble (T = 323 K and p = 10 atm) were carried out to probe structural properties of an isolated n-octadecane chain solvated in water, methanol, water-rich, or methanol-rich mixtures and, for comparison, of an isolated chain in the gas phase and for neat liquid n-octadecane. The united-atom version of the TraPPE (transferable potentials for phase equilibria) force field was used to represent n-octadecane and methanol and the TIP-4P model was used for water. In all six environments, broad conformational distributions are observed and the n-octadecane chains are found to predominantly adopt extended, but not all-trans conformations. In addition, a small fraction of more collapsed conformations in which the chain ends approach each other is observed for aqueous hydration, the water-rich solvent mixture and the gas phase, but the simulation data do not support a simple two-state picture with folded and unfolded basins of attraction. For chains in these three "poor" solvent environments, the dihedral angles near the center of the chain show an enhancement of the gauche population. The ensemble of water-solvated chains with end-to-end contacts is preferentially found in a U-shaped conformation rather than a more globular state. An analysis of the local solvation structures in the water-methanol mixtures shows, as expected, an enrichment of the methyl group of methanol near the methylene and methyl segments of the n-octadecane chain. Interestingly, these local bead fractions are enhanced by factors of 2.5 and 1.5 for methyl and methylene segments reflecting the more hydrophobic nature of the former segments.     

Compact bipolar plate-free direct methanol fuel cell stacks

Fuel cells with a PtAu/C anode and a Pr-doped Mn(2)O(3)/C cathode were stacked without using a bipolar plate, and their discharge properties were investigated in a methanol aqueous solution bubbled with air. A three-cell stack exhibited a stack voltage of 2330 mV and a power output of 21 mW.     

Acid-base equilibria of substituted pyridine N-oxides in methanol

Acid dissociation constants in methanol for eight substituted pyridine N-oxides having a wide range of acid-base properties, [quinoline N-oxide (bi-cyclic amine N-oxide) and pyridine (heterocyclic amine)] have been determined using the potentiometric titration method. A linear correlation between ourmethanol data and aqueous pK _a values from the literature has been found. As in polar aprotic solvents cationic homoconjugation phenomenon has been found to be present for sufficiently basic N-oxides. The tendency of substituted pyridine N-oxides towards cationic homoconjugation in methanol is weaker than in polar aprotic solvents and increases with increasing basicity of N-oxides. It has also been found that, in contrast to polar aprotic solvents, the cationic homoconjugation phenomenon in methanol is much more pronounced for heterocyclic amines than their N-oxides.     

PET suppression of acridinedione dyes by urea derivatives in water and methanol

Spectroscopic investigations involving the interaction of acridinedione dyes with urea and its derivatives in water and methanol were carried out by absorption, steady-state fluorescence, and time-resolved fluorescence measurements. The hydrogen-bonding properties of urea and derivatives in aqueous solutions are found to be distinctly different from those observed in methanol. Urea, which can serve both as a hydrogen bond donor as well as an acceptor and has a unique hydrogen-bonding feature, helps in studying urea interaction with fluorophores in aqueous solutions, micelles, and alcohol. In our studies, we have used acridinedione dyes as the probe. We report that the hydrophobic interaction of urea with dye predominates by weakening of the hydrogen-bonding interaction of the solvent and urea derivatives with increase in the hydrophobicity of urea derivatives. In methanol, the hydrogen bonding between solvent and urea derivatives predominating over the hydrophobicity of the urea derivatives is observed. The presence of alkyl group substitution in the N-H moiety with a function of increasing concentration resulting in the creation of a more favorable hydrophobic environment to the dye molecule to reside in the hydrophobic shell phase rather than in the bulk aqueous phase is illustrated. The hydrophobic interaction of dye with urea in aqueous solution predominates because of the weakening of the hydrogen bonding of the solvent and urea derivatives, and the photoinduced electron transfer (PET) process is used as a marker to identify the hydrophobic interaction illustrated in our studies.     

Accurate prediction of absolute acidity constants in water with a polarizable force field: substituted phenols, methanol, and imidazole

Validity of a force field with explicit treatment of electrostatic polarization in a form of inducible point dipoles for computing acidity constants was tested by calculating absolute pK(a) values of substituted phenols, methanol, and imidazole in water with the molecular dynamics technique. The last two systems were selected as tyrosine and histidine side-chain analogues, respectively. The solvent was represented by an explicit polarizable water model. Similar calculations were also performed with a modified OPLS-AA nonpolarizable force field. The resulting pK(a) values were compared with available experimental data. While the nonpolarizable force field yields errors of about 5 units in the absolute pK(a) values for the phenols and methanol, the polarizable force field produces the acidity constant values within a ca. 0.8 units accuracy. For the case of imidazole, the fixed-charges force field was capable of reproducing the experimental value of pK(a) (6.4 versus the experimental 7.0 units), but only at a cost of dramatically underestimating dimerization energy for the imidazolium-water complex. At the same time, the polarizable force field yields an even more accurate result of pK(a) = 6.96 without any sacrifice of the accuracy in the dimerization energy. It has also been demonstrated that application of Ewald summation for the long-range electrostatics is important, and substitution of a simple cutoff procedure with Born correction for ions can lead to underestimation of absolute pK(a) values by more than 5 units. The accuracy of the absolute acidity constants computed with the polarizable force field is very encouraging and opens road for further tests on more diverse organic molecules sets, as well as on proteins.     

Determination of ionisation constants of organic bases in aqueous methanol solutions using capillary electrophoresis

The pKa of eight organic bases was determined in aqueous and aqueous methanol solutions of 0-70% (v/v) methanol using capillary electrophoresis. The bases investigated include compounds commonly used to test the activity of RP columns in HPLC. The variation of pKa with temperature in aqueous methanol solutions was also investigated and found to closely resemble temperature coefficients reported for bases in purely aqueous solutions. pKa values determined by CE were compared to those reported using NMR spectroscopy. The good agreement of the results is evidence that either technique is suitable to perform pKa measurements.     

Binary phase behavior and aggregation of dilute methanol in supercritical carbon dioxide: a Monte Carlo simulation study

Configurational-bias Monte Carlo simulations in the Gibbs and isobaric-isothermal ensembles using the transferable potentials for phase equilibria force field were carried out to investigate the thermophysical properties of mixtures containing supercritical carbon dioxide and methanol. The binary vapor-liquid coexistence curves were calculated at 333.15 and 353.15 K and are in excellent agreement with experimental measurements. The self-association of methanol in supercritical carbon dioxide was investigated over a range of temperatures and pressures near the mixture critical point. The temperature dependence of the equilibrium constants for the formation of hydrogen-bonded aggregates (from dimer to heptamer) allowed for the determination of the enthalpy of hydrogen bonding, DeltaHHB, in supercritical carbon dioxide with values for DeltaHHB of about 15 kJ mol(-1) falling within the range of previously proposed values. No strong pressure dependence was observed for the formation of aggregates. Apparently the decrease of the entropic penalty and of the enthalpic benefit upon increasing pressure or solvent density mostly cancel each other's effect on aggregate formation.     

Interaction of NaI with solid water and methanol

The interaction of NaI with amorphous solid water (ASW) and methanol (MeOH) has been investigated with metastable impact electron spectroscopy (MIES), UPS(HeI), and temperature programmed desorption (TPD). We have studied the electron emission from the ionization of the highest-lying states of H(2)O, CH(3)OH, and of 5pI. We have prepared NaI layers on ASW (MeOH) films at about 105 K and annealed them up to about 200 K. Surface segregation of iodide is observed in ASW, as predicted for NaI aqueous solutions. On the other hand, surface segregation is not observed in MeOH, again as predicted for the interaction of NaI with liquid methanol. Electronic properties (ionization potentials, optical band gaps) and water binding energies are reported and are analyzed on the basis of available DFT results for hydrated NaI clusters.     

Solvation of perfluorooctane and octane in water, methanol, acetonitrile, and aqueous mixtures of methanol and acetonitrile

Molecular dynamics simulations are used to examine the local solvation structure of single octane and perfluorooctane molecules in liquid water, methanol, acetonitrile, and aqueous mixtures of methanol and acetonitrile. The motivation is to obtain baseline information about the solvation of perfluorooctane by liquids used as the mobile phase in liquid chromatography and how it differs from the solvation of octane. While octane is uniformly solvated by both water and the second component, perfluorooctane is solvated by methanol and acetonitrile with the exclusion of water from the first solvation layer when the solvent is a mixture.     

Computational study of ion distributions at the air/liquid methanol interface

Molecular dynamic simulations with polarizable potentials were performed to systematically investigate the distribution of NaCl, NaBr, NaI, and SrCl(2) at the air/liquid methanol interface. The density profiles indicated that there is no substantial enhancement of anions at the interface for the NaX systems, in contrast to what was observed at the air/aqueous interface. The surfactant-like shape of the larger more polarizable halide anions, which is part of the reason they are driven to air/aqueous interfaces, was compensated by the surfactant nature of methanol itself. These halide anions had on average an induced dipole of moderate magnitude in bulk methanol. As a consequence, methanol hydroxy groups donated hydrogen bonds to anions where the negatively charged side of the anion induced dipole pointed, and methyl groups interacted with anions where the positively charged side of the anion-induced dipole pointed. Furthermore, salts were found to disrupt the surface structure of methanol. For the neat air/liquid methanol interface, there is relative enhancement of methyl groups at the outer edge of the air/liquid methanol interface in comparison with hydroxy groups, but with the addition of NaX this enhancement was reduced somewhat. Finally, with the additional of salts to methanol, the computed surface potentials decreased, which is in contrast to what is observed in corresponding aqueous systems, where the surface potential increases with the addition of salts. Both of these trends have been indirectly observed with experiments. The surface potential trends were found to be due to the greater propensity of anions for the air/water interface that is not present at the air/liquid methanol interface.     

A nonadditive methanol force field: bulk liquid and liquid-vapor interfacial properties via molecular dynamics simulations using a fluctuating charge model

We study the bulk and interfacial properties of methanol via molecular dynamics simulations using a CHARMM (Chemistry at HARvard Molecular Mechanics) fluctuating charge force field. We discuss the parametrization of the electrostatic model as part of the ongoing CHARMM development for polarizable protein force fields. The bulk liquid properties are in agreement with available experimental data and competitive with existing fixed-charge and polarizable force fields. The liquid density and vaporization enthalpy are determined to be 0.809 g/cm3 and 8.9 kcal/mol compared to the experimental values of 0.787 g/cm3 and 8.94 kcal/mol, respectively. The liquid structure as indicated by radial distribution functions is in keeping with the most recent neutron diffraction results; the force field shows a slightly more ordered liquid, necessarily arising from the enhanced condensed phase electrostatics (as evidenced by an induced liquid phase dipole moment of 0.7 D), although the average coordination with two neighboring molecules is consistent with the experimental diffraction study as well as with recent density functional molecular dynamics calculations. The predicted surface tension of 19.66+/-1.03 dyn/cm is slightly lower than the experimental value of 22.6 dyn/cm, but still competitive with classical force fields. The interface demonstrates the preferential molecular orientation of molecules as observed via nonlinear optical spectroscopic methods. Finally, via canonical molecular dynamics simulations, we assess the model's ability to reproduce the vapor-liquid equilibrium from 298 to 423 K, the simulation data then used to obtain estimates of the model's critical temperature and density. The model predicts a critical temperature of 470.1 K and critical density of 0.312 g/cm3 compared to the experimental values of 512.65 K and 0.279 g/cm3, respectively. The model underestimates the critical temperature by 8% and overestimates the critical density by 10%, and in this sense is roughly equivalent to the underlying fixed-charge CHARMM22 force field.     

Polarizable empirical force field for the primary and secondary alcohol series based on the classical Drude model

A polarizable empirical force field based on the classical Drude oscillator has been developed for the aliphatic alcohol series. The model is optimized with emphasis on condensed-phase properties and is validated against a variety of experimental data. Transferability of the developed parameters is emphasized by the use of a single electrostatic model for the hydroxyl group throughout the alcohol series. Aliphatic moiety parameters were transferred from the polarizable alkane parameter set, with only the Lennard-Jones parameters on the carbon in methanol optimized. The developed model yields good agreement with pure solvent properties with the exception of the heats of vaporization of 1-propanol and 1-butanol, which are underestimated by approximately 6%; special LJ parameters for the oxygen in these two molecules that correct for this limitation are presented. Accurate treatment of the free energies of aqueous solvation required the use of atom-type specific O(alcohol)-O(water) LJ interaction terms, with specific terms used for the primary and secondary alcohols. With respect to gas phase properties the polarizable model overestimates experimental dipole moments and quantum mechanical interaction energies with water by approximately 10 and 8 %, respectively, a significant improvement over 44 and 46 % overestimations of the corresponding properties in the CHARMM22 fixed-charge additive model. Comparison of structural properties of the polarizable and additive models for the pure solvents and in aqueous solution shows significant differences indicating atomic details of intermolecular interactions to be sensitive to the applied force field. The polarizable model predicts pure solvent and aqueous phase dipole moment distributions for ethanol centered at 2.4 and 2.7 D, respectively, a significant increase over the gas phase value of 1.8 D, whereas in a solvent of lower polarity, benzene, a value of 1.9 is obtained. The ability of the polarizable model to yield changes in dipole moment as well as the reproduction of a range of condensed phase properties indicates its utility in the study of the properties of alcohols in a variety of condensed phase environments as well as representing an important step in the development of a comprehensive force field for biological molecules.     

Liquid-vapor equilibrium of dilute aqueous solutions of methanol

Numerous sets of vapor pressure-composition data for methanol in dilute aqueous solutions at 25°C and 35°C have been obtained with an automated vapor pressure apparatus developed in this laboratory. Activity coefficients of both components and values of the pairwise free energy of interaction of methanol molecules have been inferred from the results. The osmotic second virial coefficient of methanol has also been calculated and compared with literature values of this quantity.     

Quantum chemical investigation of the adsorption of methanol on a cluster model of faujasite

The adsorption of methanol in zeolites of the faujasite type with sodium and calcium counter ions is studied with quantum chemical methods. The zeolite is represented with a cluster model allowing calculations at the Møller-Plesset as well as the DFT level of theory. An adsorption energy of −62.4 kJ/mol is calculated at the MP2/TZVP//BP86/TZVP level of theory for one methanol molecule at one site II sodium cation. Insight into the adsorption process is obtained with a three-body decomposition which reveals a strong destabilisation of the adsorption strength by large, positive three-body terms, which is important for force field development.     

Capillary zone electrophoresis in methanol: migration behavior and background electrolytes

Nonaqueous (NA) solutions are often used as background electrolytes (BGEs) and NA solvents are added to aqueous BGEs as organic modifiers in capillary zone electrophoresis (CZE), in order to optimize the separations. This can be tricky, however, because the pH* and pK* concepts may be totally different in NA solvents, whereas often less knowledge is available concerning phenomena, such as system zones, applying NA solvents. In this paper, the concepts of pH* and pK* are considered for methanol as a solvent and pK* values are determined for several components in mixtures of water and methanol. With a mathematical model, adapted for calculations in methanol, parameters are calculated describing the fronting or tailing character of peaks and the question of peaks or dips, and the existence of system zones is discussed for pure methanol as a solvent. These aspects are experimentally verified, applying BGEs useful for the separation of cationic species in the indirect UV mode. It can be concluded that the mathematical model developed for aqueous BGEs is applicable to BGEs in methanol, too, and that the behavior of BGEs in methanol is comparable with that in water concerning the fronting or tailing character of peaks and the question of peaks and dips, although the mobilities and pK values can change significantly.     

Capillary zone electrophoresis of basic analytes in methanol as non-aqueous solvent mobility and ionisation constant

The electrophoretically relevant properties of monoacidic 21 bases (including common drugs) containing aliphatic or aromatic amino groups were determined in methanol as solvent. These properties are the actual mobilities (that of the fully ionised weak bases), and their pKa values. Actual mobilities were measured in acidic methanolic solutions containing perchloric acid. The ionisation constants of the amines were derived from the dependence of the ionic mobilities on the pH of the background electrolyte solution. The pH scale in methanol was established from acids with known conventional pK*a values in this solvent used as buffers, avoiding thus further adjustment with a pH sensitive electrode that might bias the scale. Actual mobilities in methanol were found larger than in water, and do not correlate well with the solvent's viscosity. The pK*a values of the cation acids, HB-, the corresponding form of the base, B, are higher in methanol, whereas a less pronounced shift was found than for neutral acids of type HA. The mean increase (compared to pure aqueous solution) for aliphatic ammonium type analytes is 1.8, for substituted anilinium 1.1, and for aromatic ammonium from pyridinium type 0.5 units. The interpretation of this shift was undertaken with the concept of the medium effect on the particles involved in the acid-base equilibrium: the proton, the molecular base, B, and the cation HB+.     

Flow methodology for methanol determination in biodiesel exploiting membrane-based extraction

A methodology based in flow analysis and membrane-based extraction has been applied to the determination of methanol in biodiesel samples. A hydrophilic membrane was used to perform the liquid-liquid extraction in the system with the organic sample fed to the donor side of the membrane and the methanol transfer to an aqueous acceptor buffer solution. The quantification of the methanol was then achieved in aqueous solution by the combined use of immobilised alcohol oxidase (AOD), soluble peroxidase and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The optimization of parameters such as the type of membrane, the groove volume and configuration of the membrane unit, the appropriate organic solvent, sample injection volume, as well as immobilised packed AOD reactor was performed. Two dynamic analytical working ranges were achieved, up to 0.015% and up to 0.200% (m/m) methanol concentrations, just by changing the volume of acceptor aqueous solution. Detection limits of 0.0002% (m/m) and 0.007% (m/m) methanol were estimated, respectively. The decision limit (CCα) and the detection capacity (CCβ) were 0.206 and 0.211% (m/m), respectively. The developed methodology showed good precision, with a relative standard deviation (R.S.D.) <5.0% (n = 10). Biodiesel samples from different sources were then directly analyzed without any sample pre-treatment. Statistical evaluation showed good compliance, for a 95% confidence level, between the results obtained with the flow system and those furnished by the gas chromatography reference method. The proposed methodology turns out to be more environmental friendly and cost-effective than the reference method.     

Investigating the innate selectivity issues of methane to methanol: consideration of an aqueous environment

The higher reactivity of the methanol product over the methane reactant for the direct oxidation of methane to methanol is explored. C–H activation, C–O coupling, and C–OH coupling are investigated as key steps in the selective oxidation of methane using DFT. These elementary steps are initially considered in the gas phase for a variety of fcc (111) pristine metal surfaces. Methanol is found to be consistently more reactive for both C–H activation and subsequent oxidation steps. With an aqueous environment being understood experimentally to have a profound effect on the selectivity of this process, these steps are also considered in the aqueous phase by ab initio molecular dynamics calculations. The water solvent is modelled explicity, with each water molecule given the same level of theory as the metal surface and surface species. Free energy profiles for these steps are generated by umbrella sampling. It is found that an aqueous environment has a considerable effect on the kinetics of the elementary steps yet has little effect on the methane/methanol selectivity-conversion limit. Despite this, we find that the aqueous phase promotes the C–OH pathway for methanol formation, which could enhance the selectivity for methanol formation over that of other oxygenates.

Consideration of the selectivity of methane to methanol in the aqueous phase with AIMD.