Distributions of H-Bonding Aggregates in tert-Butyl Alcohol: The Pure Liquid and Its Alkane Mixtures

Intermolecular Hydrogen Bonding in Associated Fluids: The Case of Isopentyl Alcohol Dissolved in Carbon Tetrachloride

Fourier-transform infrared (FTIR) spectra of isopentyl-alcohol dissolved in carbon tetrachloride (CCl4) were recorded as a function of concentration and temperature. Dilute isopentyl alcohol/CCl4 solutions were prepared in alcohol at concentrations of 1, 0.5, 0.3, 0.2, 0.1, 0.05, 0.02, 0.01, 0.005, 0.001 and 0.0005 M. Infrared absorption measurements were taken within a temperature range of 17-67 °C below the boiling point of the solutions. Decomposition of the spectral features corresponding to associated and unassociated species was performed to quantitatively follow the effect of temperature and concentration on intermolecular hydrogen bonding (HB) in isopentyl alcohol. The spectral feature in the 3600-3650 cm-1 frequency range attributed to the free OH stretching band was studied in detail to determine changes based on concentration and temperature variations. Computational methodologies were applied to evaluate the energetics and vibrational properties of the species involved in the structure in the gaseous state where no interactions are present. The results are discussed in view of relevant structural models to gain quantitative information concerning the effect of concentration and temperature on intermolecular hydrogen bonding.

Infrared quantification of ethanol and 1-butanol hydrogen bonded hydroxyl distributions in cyclohexane

Quantifying the mid-range infrared hydroxyl stretch absorbance region has traditionally been a challenge due to the wavenumber dependence of the attenuation coefficient. Interpretation often assigns a single attenuation coefficient to each type of hydrogen-bonded aggregate. This work leverages a recently developed technique of scaling hydroxyl stretching absorbances in the mid-infrared region with a continuous attenuation coefficient function that produces integrated areas which directly correlate to hydroxyl concentrations. After scaling, the hydroxyl absorbance is fitted with five curves, of which four are dominant. These four curves represent unique hydroxyl configurations and translate to specific aggregate structures. The technique is applied to ethanol and 1-butanol. The resulting population distributions of hydrogen-bonded hydroxyl configurations are compared with the resummed thermodynamic perturbation theory (RTPT) model for linear chains as a function of concentration and temperature. The model is demonstrated to capture the critical features of the distributions.

Breaking the Structure of Liquid Hydrogenated Alcohols Using Perfluorinated tert-Butanol: A Multitechnique Approach (Infrared, Raman, and X-ray Scattering) Analyzed by DFT and Molecular Dynamics Calculations

The state of aggregation at room temperature of tert-butanol (TBH) and perfluoro tert-butanol (TBF) liquid mixtures is assessed by vibrational spectroscopy (Raman and infrared) and X-ray diffraction and analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. It is shown that larger clusters (mostly tetramers) of TBH are destroyed upon dilution with TBF. Small oligomers, monomers, and mainly heterodimers are present at the equimolar concentration. At variance with slightly interacting solvents, the signature of hetero-oligomers is shown by the appearance of a new broad band detected in the infrared region. The same spectral observation is detected for mixtures of other hydrogenated alcohols (methanol and 1-butanol). The new infrared feature is unaffected by dilution in a polar solvent (CDCl₃) in a high-concentration domain, allowing us to assign it to the signature of small hetero-oligomers. MD simulations are used to assess the nature of the species present in the mixture (monomers and small hetero-oligomers) and to follow the evolution of their population upon the dilution. Combining MD simulations with DFT calculations, the infrared spectral profile is successfully analyzed in equimolecular mixtures. This study shows that TBF is a structure breaker of hydrogen-bonded alcohol networks and that the TBF (donor)-TBH (acceptor) heterodimer is the dominant species in an extended range of concentration, centered in the vicinity of the equimolar fraction.

Effects of molecular shape on alcohol aggregation and water hydrogen bond network behavior in butanol isomer solutions

Despite butanol isomers such as n-butanol, sec-butanol, isobutanol and tert-butanol having the same chemical formula, their liquid-liquid phase diagrams are distinct. That is, tert-butanol is miscible in water at all concentrations, while the other three butanol isomers are partially miscible under ambient conditions. The molecular shape of tert-butanol is close to globular and differs from the other three butanol molecules with a relatively long carbon chain. By performing molecular dynamics simulations and graph theoretical analysis of the four water-butanol isomer mixtures at varying concentrations, we show how distinct butanol aggregates are formed which depend upon the molecular shape and affect the water H-bond network structure and phase diagram in the binary liquid. The three butanol isomers of n-butanol, sec-butanol and isobutanol at concentrated solutions form chain-like alcohol aggregates, but tert-butanol forms small aggregates due to the distinct packing behavior caused by its globular molecular shape. By employing the graph theoretical analysis such as the degree distribution and the eigenvalue spectrum from the adjacency matrix in the graphical representation of the alcohol H-bond network, we show that the tert-butanol aggregates have a different morphological structure from that of the other three butanol isomers in aqueous solution. The graph theoretically distinct butanol aggregates are categorized into two groups, water-compatible and water-incompatible, depending upon the interaction between the alcohol and water molecules. Based upon our observations, we propose that the water-incompatible networks of n-butanol, sec-butanol and isobutanol aggregates do not change the water structure significantly, forming two separate liquid phases that are alcohol-rich and water-rich. However, the water-compatible network of tert-butanol aggregates has a considerable interaction with the water molecules and causes significant disruption of the water H-bond network, forming a homogeneous solution. Understanding the alcohol aggregation behavior and water structure in butanol-water mixtures provides a critical clue in appreciating fundamental issues such as miscibility and phase separation in aqueous solution systems.

Theoretical and Experimental Study of the Excess Thermodynamic Properties of Highly Nonideal Liquid Mixtures of Butanol Isomers + DBE

Binary alcohol + ether liquid mixtures are of significant importance as potential biofuels or additives for internal combustion engines and attract considerable fundamental interest as model systems containing one strongly H-bonded self-associating component (alcohol) and one that is unable to do so (ether), but that can interact strongly as a H-bond acceptor. In this context, the excess thermodynamic properties of these mixtures, specifically the excess molar enthalpies and volumes (H^E and V^E), have been extensively measured. Butanol isomer + di-n-butyl ether (DBE) mixtures received significant attention because of interesting differences in their V^E, changing from negative (1- and isobutanol) to positive (2- and tert-butanol) with increasing alkyl group branching. With the aim of shedding light on the differences in alcohol self-association and cross-species H-bonding, considered responsible for the observed differences, we studied representative 1- and 2-butanol + DBE mixtures by molecular dynamics simulations and experimental excess property measurements. The simulations reveal marked differences in the self-association of the two isomers and, while supporting the existing interpretations of the H^E and V^E in a general sense, our results suggest, for the first time, that subtle changes in H-bonded topologies may contribute significantly to the anomalous volumetric properties of these mixtures.

Is the Fourier Transform Infrared Free-OH Band of t-Butanol Only from Free OHs? Case Studies on the Binary Systems of the Alcohol with CCl4 and CHCl3

Attenuated total reflection-Fourier transform infrared spectroscopy and quantum chemical calculations were performed on tert-butyl alcohol (t-BuOH) and its binary solutions with CCl₄ and CHCl₃. The study was focused on the free-OH stretching bands. Two resolution-enhancing methods, excess spectroscopy and two-dimensional correlation spectroscopy, were employed to examine the structural heterogeneity and search for the detailed contributors to the free-OH bands. Unexpectedly, CCl₄ was found not to be an inert solvent and, similar to CHCl₃, formed hydrogen/halogen bonds (H-/X-bond) with t-BuOH. It was observed that the free-OH band in the t-BuOH-CHCl₃ system is larger and more red-shifted than that in the t-BuOH-CCl₄ system, indicating the stronger intermolecular interactions in the former system. Furthermore, in the t-BuOH-CHCl₃ system, the H-bonds are stronger than the X-bonds, while in the t-BuOH-CCl₄ system, both interactions are similar in strength. To assign the free-OH bands, it was found that they are not only from the free OH of the t-BuOH monomer, but they are also contributed by the quasi-free OH with the oxygen bonded to H or Cl and even the weakly H-bonded OH of t-BuOH molecules. Finally, all the identified species increased simultaneously via cosolvent addition, suggestive of the destabilization of the highly associated t-BuOH clusters.

Simple and complex disorder in binary mixtures with benzene as a common solvent

Substituting benzene for water in computer simulations of binary mixtures, allows one to study the various forms of disorder, without the complications often encountered in aqueous mixtures.