An approach towards understanding the structure of complex molecular systems: the case of lower aliphatic alcohols

Pressure-Induced Aggregation of Associating Liquids as a Driving Force Enhancing Hydrogen Bond Cooperativity

The behavior of hydrogen bonds under extreme pressure is still not well understood. Until now, the shift of the stretching vibration band of the X-H group (X = the donor atom) in infrared spectra has been attributed to the variation in the length of the covalent X-H bond. Herein, we combined infrared spectroscopy and X-ray diffraction experimental studies of two H-bonded liquid hexane derivatives, i.e., 2-ethyl-1-hexanol and 2-ethyl-1-hexylamine, in diamond anvil cells at pressures up to the GPa level, with molecular dynamics simulations covering similar thermodynamic conditions. Our findings revealed that the observed changes in the X-H stretching vibration bands under compression are not primarily due to H-bond shortening resulting from increased density but mainly due to cooperative enhancement of H-bonds caused by intensified molecular clustering. This sheds new light on the nature of H-bond interactions and the structure of liquid molecular systems under compression.

Dynamics of Confined Short-Chain alkanol in MCM-41 by Dielectric Spectroscopy: Effects of matrix and system Treatments and Filling Factor

The dynamics of n-propanol confined in regular MCM-41 matrix with the pore size Dpore = 40 Å, under various matrix conditioning and sample confining conditions, using broadband dielectric spectroscopy (BDS), is reported. First, various drying procedures with the capacitor filling under air or N2 influence the BDS spectra of the empty MCM-41 and the confined n-PrOH/MCM-41 systems, but have a little effect on the maximum relaxation time of the main process. Finally, various filling factors of n-PrOH medium in the optimally treated MCM-41 system lead to unimodal or bimodal spectra interpreted in terms of the two distinct dynamic phases in the confined states.

The hydrogen-bond collective dynamics in liquid methanol

The relatively simple molecular structure of hydrogen-bonded (HB) systems is often belied by their exceptionally complex thermodynamic and microscopic behaviour. For this reason, after a thorough experimental, computational and theoretical scrutiny, the dynamics of molecules in HB systems still eludes a comprehensive understanding. Aiming at shedding some insight into this topic, we jointly used neutron Brillouin scattering and molecular dynamics simulations to probe the dynamics of a prototypical hydrogen-bonded alcohol, liquid methanol. The comparison with the most thoroughly investigated HB system, liquid water, pinpoints common behaviours of their THz microscopic dynamics, thereby providing additional information on the role of HB dynamics in these two systems. This study demonstrates that the dynamic behaviour of methanol is much richer than what so far known, and prompts us to establish striking analogies with the features of liquid and supercooled water. In particular, based on the strong differences between the structural properties of the two systems, our results suggest that the assignment of some dynamical properties to the tetrahedral character of water structure should be questioned. We finally highlight the similarities between the characteristic decay times of the time correlation function, as obtained from our data and the mean lifetime of hydrogen bond known in literature.

Investigation of the structure of ethanol-water mixtures by molecular dynamics simulation I: analyses concerning the hydrogen-bonded pairs

Series of molecular dynamics simulations for ethanol-water mixtures with 20-80 mol % ethanol content, pure ethanol, and water were performed. In each mixture, for ethanol the OPLS force field was used, combined with three different water force fields, the SPC/E, the TIP4P-2005, and the SWM4-DP. Water potential models were distinguished on the basis of deviations between calculated and measured total scattering X-ray structure factors aided by ethanol-water pair binding energy comparison. No single water force field could provide the best agreement with experimental data at all concentrations: at the ethanol content of 80% the SWM-DP, for 60 mol % the SWM4-DP and the TIP4P-2005, whereas for the 40 and 20 mol % mixtures TIP4P-2005 water force field provided the closest match. Coordination numbers and hydrogen bonds/molecule values were calculated, revealing that the oxygen-oxygen first coordination numbers strongly overestimate the average number of hydrogen bonds/molecule. The center-of-molecule distributions indicate that the ethanol-ethanol first coordination sphere expands with increasing water concentration while the size of the first water-water coordination sphere does not change. Various two and three-dimensional distributions were calculated that reveal the differences between simulations with different water force fields. Detailed conformational analyses of the hydrogen-bonded pairs were performed; drawings of the characteristic molecular arrangements are provided.

Liquid 1-propanol studied by neutron scattering, near-infrared, and dielectric spectroscopy

Liquid monohydroxy alcohols exhibit unusual dynamics related to their hydrogen bonding induced structures. The connection between structure and dynamics is studied for liquid 1-propanol using quasi-elastic neutron scattering, combining time-of-flight and neutron spin-echo techniques, with a focus on the dynamics at length scales corresponding to the main peak and the pre-peak of the structure factor. At the main peak, the structural relaxation times are probed. These correspond well to mechanical relaxation times calculated from literature data. At the pre-peak, corresponding to length scales related to H-bonded structures, the relaxation times are almost an order of magnitude longer. According to previous work [C. Gainaru, R. Meier, S. Schildmann, C. Lederle, W. Hiller, E. Rössler, and R. Böhmer, Phys. Rev. Lett. 105, 258303 (2010)] this time scale difference is connected to the average size of H-bonded clusters. The relation between the relaxation times from neutron scattering and those determined from dielectric spectroscopy is discussed on the basis of broad-band permittivity data of 1-propanol. Moreover, in 1-propanol the dielectric relaxation strength as well as the near-infrared absorbance reveal anomalous behavior below ambient temperature. A corresponding feature could not be found in the polyalcohols propylene glycol and glycerol.

Supramolecular x-ray signature of susceptibility amplification in hydrogen-bonded liquids

Mixing two nonconducting hydrogen-bonded liquids, each exhibiting a low dielectric relaxation strength, can result in a highly electrically absorbing fluid. This susceptibility amplification effect is demonstrated for mixtures of monohydroxy alcohols. Whereas in the pure liquids a tendency to form ringlike low-dipole moment clusters prevails, in the mixtures such supramolecular structures are disfavored leading to an up to tenfold enhancement of the dielectric loss. The compositional evolution of density and mean cluster-cluster separation is traced using x-ray scattering and indicates deviations from ideal mixing with decreased C-C but simultaneously increased O-O correlation lengths. Thus, the variation in the supramolecular absorption strength could be tracked using a static scattering technique. These observations are in harmony with volume exclusion and ring open effects that predict an optimized susceptibility amplification for mixtures in which the two components occupy equal volume fractions as experimentally observed.