Cooperative effects on the structure and stability of (ethanol)3–water, (methanol)3–water heterotetramers and (ethanol)4, (methanol)4 tetramers

Chirped pulse Fourier-transform microwave spectroscopy of alcohol and water tetramers

In an effort to build towards quantitative models of alcohol:water microaggregation in liquid mixtures, the present works characterizes the energy landscape and structures of pure ethanol and mixed ethanol:water tetramers using Chirped Pulse Fourier-transform Microwave spectroscopy. Many conformers of each type of tetramer are available, and those with sufficiently strong dipole moments are experimentally examined. This analysis considers, but does not explicitly fit, the splitting of rotational states due to internal rotation of the methyl groups present, as well as utilizes isotopic substitution experiments to verify the conformer variations observed. Implications of the listed results include a suggestion of the stability of micro-aggregated structures as opposed to homogeneously mixed clusters, informing future work on characterization of larger clusters and any potential modeling of the hydrogen bond network at play.

Looking for the Azeotrope: A Computational Study of (Ethanol)6-Water, (Methanol)6-Water, (Ethanol)7, and (Methanol)7 Heptamers

Considering that a molecular-level understanding of the azeotropic ethanol-water system can contribute to the search of new methodologies and/or modifications of industrial separation methods, this study tries to provide some clues to understand why azeotropes should be expected for ethanol, but not for methanol. Our exploration of the potential energy surface of (ethanol)₆-water heteroheptamers, carried out at the B3LYP-D3/6-311++G(d,p) level, shows these heteroclusters to exhibit a cyclic structure where the cooperativity effects between the OH···O HBs is a fundamental ingredient. An analysis of this cooperativity clearly indicates that ethanol-water systems will exhibit a similarly high stability as the heterocluster size approaches the azeotrope. However, a similar behavior should not be expected for the methanol-containing analogues. A comparison between (ethanol)₇, (ethanol)₆-water, (methanol)₇, and (methanol)₆-water shows the ethanol-containing systems to be significantly more stable than the methanol-containing analogues. This result is probably due to the fact that the OH···O HBs are weaker than those found between ethanol molecules. However, our atoms in molecule (AIM) and noncovalent interaction (NCI) analyses unambiguously show that important contributors to the enhanced stability of the ethanol-containing clusters are the secondary van der Waals interactions between ethyl groups, which are not observed between methyl groups. Hence, while the formation of stable azeotropes is expected for the case of ethanol, for the methanol-containing analogues, the relative stability of the clusters is significantly smaller, and its formation is accompanied by an increase of the free energy.

Molecular Mechanism for Azeotrope Formation in Ethanol/Benzene Binary Mixtures through Gibbs Ensemble Monte Carlo Simulation

Azeotropes have been studied for decades due to the challenges they impose on separation processes but fundamental understanding at the molecular level remains limited. Although molecular simulation has demonstrated its capability of predicting mixture vapor-liquid equilibrium (VLE) behaviors, including azeotropes, its potential for mechanistic investigation has not been fully exploited. In this study, we use the united atom transferable potentials for phase equilibria (TraPPE-UA) force field to model the ethanol/benzene mixture, which displays a positive azeotrope. Gibbs ensemble Monte Carlo (GEMC) simulation is performed to predict the VLE phase diagram, including an azeotrope point. The results accurately agree with experimental measurements. We argue that the molecular mechanism of azeotrope formation cannot be fully understood by studying the mixture liquid-state stability at the azeotrope point alone. Rather, azeotrope occurrence is only a reflection of the changing relative volatility between the two components over a much wider composition range. A thermodynamic criterion is thus proposed on the basis of the comparison of partial excess Gibbs energy between the components. In the ethanol/benzene system, molecular energetics shows that with increasing ethanol mole fraction, its volatility initially decreases but later plateaus, while benzene volatility is initially nearly constant and only starts to decrease when its mole fraction is low. Analysis of the mixture liquid structure, including a detailed investigation of ethanol hydrogen-bonding configurations at different composition levels, reveals the underlying molecular mechanism for the changing volatilities responsible for the azeotrope.

Using the C-O stretch to unravel the nature of hydrogen bonding in low-temperature solid methanol-water condensates

Transmission infrared spectroscopy has been used in a systematic laboratory study to investigate hydrogen bonding in binary mixtures of CH3OH and H2O, vapour deposited at 30 K, as a function of CH3OH/H2O mixing ratio, R. Strong intermolecular interactions are evident between CH3OH and H2O with infrared band profiles of the binary ices differing from that of the pure components and changing significantly with R. Consistent evidence from the O-H and C-H band profiles and detailed analysis of the C-O stretch band reveal two different hydrogen bonding structural regimes below and above R = 0.6-0.7. The vapour deposited solid mixtures were found to exhibit behaviour similar to that of liquids with evidence of inhomogeneity and higher coordination number of hydrogen bonds that are concentration dependent. The C-O stretch band is found to consist of three components around 1039 cm(-1) ('blue'), 1027 cm(-1) ('middle') and 1011 cm(-1) ('red'). The 'blue' and 'middle' components corresponding to environments with CH3OH dominating as a proton donor (PD) and proton acceptor (PA) respectively reveal preferential bonding of CH3OH as a PA and H2O as a PD in the mixtures. The 'red' component is only present in the presence of H2O and has been assigned to the involvement of both lone pairs of electrons on the oxygen atom of CH3OH as a PA to two PD H2O atoms. Cooperative effects are evident with concurrent blue-shifts in the C-H stretching modes of CH3OH below R = 0.6 indicating CH3 group participation in hydrogen bonding.

Understanding the azeotropic diethyl carbonate-water mixture by structural and energetic characterization of DEC(H2O)(n) heteroclusters

Diethyl carbonate (DEC) is an oxygenated fuel additive. During its synthesis through a promising green process, a DEC-water azeotrope is formed, which decreases DEC production efficiency in the gas phase. Molecular information about this system is scarce but could be of benefit in understanding (and potentially improving) the synthetic process. Therefore, we report a detailed computational study of the conformers of DEC, and their microsolvation with up to four water molecules, with the goal of understanding the observed 1:3 DEC:H2O molar ratio. The most stable DEC conformers (with mutual energy differences < 1.5 kcal mol(-1)) contribute to the energetic and structural properties of the complexes. An exhaustive stochastic exploration of each potential energy surface of DEC-(H2O)n, (where n = 1, 2, 3, 4) heteroclusters discovered 3, 8, 7, and 4 heterodimers, heterotrimers, heterotetramers, and heteropentamers, respectively, at the MP2/6-311++G(d,p) level of theory. DEC conformers and energies of the most stable structures at each heterocluster size were refined using CCSD(T)/6-311++G(d,p). Energy decomposition, electron density topology, and cooperative effects analyses were carried out to determine the relationship between the geometrical features of the heteroclusters and the non-covalent interaction types responsible for their stabilization. Our findings show that electrostatic and exchange energies are responsible for heterocluster stabilization, and also suggest a mutual weakening among hydrogen bonds when more than three water molecules are present. All described results are complementary and suggest a structural and energetic explanation at the molecular level for the experimental molar ratio of 1:3 (DEC:H2O) for the DEC-water azeotrope.

Investigation into the structural composition of hydroalcoholic solutions as basis for the development of multiple suppression pulse sequences for NMR measurement of alcoholic beverages

An eight-fold suppression pulse sequence was recently developed to improve sensitivity in (1) H NMR measurements of alcoholic beverages [Magn. Res. Chem. 2011 (49): 734-739]. To ensure that only one combined hydroxyl peak from water and ethanol appears in the spectrum, adjustment to a certain range of ethanol concentrations was required. To explain this observation, the structure of water-ethanol solutions was studied. Hydroalcoholic solutions showed extreme behavior at 25% vol, 46% vol, and 83% vol ethanol according to (1) H NMR experiments. Near-infrared spectroscopy confirmed the occurrence of four significant compounds ('individual' ethanol and water structures as well as two water-ethanol complexes of defined composition - 1 : 1 and 1 : 3). The successful multiple suppression can be achieved for every kind of alcoholic beverage with different alcoholic strengths, when the final ethanol concentration is adjusted to a range between 25% vol and 46% vol (e.g. using dilution or pure ethanol addition). In this optimum region, an individual ethanol peak was not detected, because the 'individual' water structure and the 1 : 1 ethanol-water complex predominate. The nature of molecular association in ethanol-water solutions is essential to elucidate NMR method development for measurement of alcoholic beverages. The presented approach can be used to optimize other NMR suppression protocols for binary water-organic solvent mixtures, where hydrogen bonding plays a dominant role.

Determination of size of molecular clusters of ethanol by means of NMR diffusometry and hydrodynamic calculations

The microscopic structure of ethanol in the liquid state is characterized as a dynamic equilibrium of hydrogen-bonded clusters of different sizes and topologies. We have developed a novel method for determination of the average size of the clusters that combines the measurement of diffusion coefficient by means of NMR diffusometry technique and hydrodynamic simulations. The approach includes the use of HydroNMR [J. Garcı̀a de la Torre, M. L. Huertas, and B. Carrasco, J. Magn. Reson. 147, 2000, 138] for small molecules, which is attained here by the calibration procedure using a dilute solution of tetramethylsilane. It is thus possible to correlate the experimentally determined diffusion coefficient of ethanol with calculated diffusion coefficients of the modeled clusters of different sizes. We found that average size of the clusters in 0.16 M solution of ethanol in n-hexane corresponds to the monomer above 300 K and to the pentamer/hexamer below 240 K. The clusters in the case of 0.44 M solution are generally slightly larger, from the average size corresponding to the dimer at 320 K and the hexamer at 210 K.

A combined Raman- and infrared jet study of mixed methanol-water and ethanol-water clusters

The vibrational dynamics of vacuum-isolated hydrogen-bonded complexes between water and the two simplest alcohols is characterized at low temperatures by Raman and FTIR spectroscopy. Conformational preferences during adaptive aggregation, relative donor/acceptor strengths, weak secondary hydrogen bonding, tunneling processes in acceptor lone pair switching, and thermodynamic anomalies are elucidated. The ground state tunneling splitting of the methanol-water dimer is predicted to be larger than 2.5 cm(-1). Two types of alcohol-water trimers are identified from the spectra. It is shown that methanol and ethanol are better hydrogen bond donors than water, but even more so better hydrogen bond acceptors. As a consequence, hydrogen bond induced red shifts of OH modes behave non-linearly as a function of composition and the resulting cluster excess quantities correspond nicely to bulk excess enthalpies at room temperature. The effects of weak C-H···O hydrogen bonds are quantified in the case of mixed ethanol-water dimers.