C–H···O Interaction in Methanol–Water Solution Revealed from Raman Spectroscopy and Theoretical Calculations

Soft confinement of water in aliphatic alcohols: MIR/NIR spectroscopic and DFT studies

Here, we present the first examination of the state of water under a soft confinement in eight aliphatic alcohols including cyclopentanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-decanol, 2-octanol and 3-octanol. Due to relatively large size of the aliphatic part, water has limited solubility in all studied alcohols. Water content in saturated solutions was determined by Karl Fischer titration and correlated with the spectroscopic data. This way, we determined the molar absorptivity of the ν2+ν3 combination mode. The effect of addition of water and temperature variation was monitored by ATR-IR and NIR spectroscopy. Analysis of the experimental results was guided by DFT calculations, which provided the structures, harmonic MIR spectra and binding energies of selected alcohol-water complexes. Our studies demonstrated that the state of water in alcohols is related to its solubility, which depends on structure of solvent molecules. The solubility of water in 1-alcohols decreases on increasing of the chain length, but for long chain alcohols this effect is less evident. More apparent solubility reduction appears in going from the primary to secondary alcohols. The effective shielding of the OH group in the linear alcohols is achieved when on both sides of the OH group are ethyl or longer substituents, while the shielding by methyl groups is less efficient. Water is much better soluble in the cyclic alcohols as compared with the linear ones due to better accessibility of the OH group. The soft confinement of water in aliphatic alcohols allows for flexible structural arrangements and interactions. Even at low water content, we did not observe free molecules of water. At these conditions, the molecules of water are singly or doubly bonded to the OH groups from the alcohol. Increasing solubility of water reduces the number of the free OH groups and leads to formation of water clusters. Obtained results allow concluding that in alcohols with sizable aliphatic part the molecules of water are confined in the vicinity of the OH groups.

Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion

We have quantum chemically investigated the origin of the atypical blueshift of the H-C bond stretching frequency in the hydrogen-bonded complex X- •••H3 C-Y (X, Y=F, Cl, Br, I), as compared to the corresponding redshift occurring in Cl- •••H3 N and Cl- •••H3 C-H, using relativistic density functional theory (DFT) at ZORA-BLYP-D3(BJ)/QZ4P. Previously, this blueshift was attributed, among others, to the contraction of the H-C bonds as the H3 C moiety becomes less pyramidal. Herein, we provide quantitative evidence that, instead, the blueshift arises from a direct and strong X- •••C interaction of the HOMO of A- with the backside lobe on carbon of the low-lying C-Y antibonding σ* LUMO of the H3 C-Y fragment. This X- •••C bond, in essence a tetrel bond, pushes the H atoms towards a shorter H-C distance and makes the H3 C moiety more planar. The blueshift may, therefore, serve as a diagnostic for tetrel bonding.

C-H Groups as Donors in Hydrogen Bonds: A Historical Overview and Occurrence in Proteins and Nucleic Acids

Hydrogen bonds constitute a unique type of non-covalent interaction, with a critical role in biology. Until fairly recently, the canonical view held that these bonds occur between electronegative atoms, typically O and N, and that they are mostly electrostatic in nature. However, it is now understood that polarized C-H groups may also act as hydrogen bond donors in many systems, including biological macromolecules. First recognized from physical chemistry studies, C-H…X bonds were visualized with X-ray crystallography sixty years ago, although their true significance has only been recognized in the last few decades. This review traces the origins of the field and describes the occurrence and significance of the most important C-H…O bonds in proteins and nucleic acids.

Vibrational Solvatochromism Study of the C-H···O Improper Hydrogen Bond

The improper C-H···O hydrogen bond is an important weak interaction, with broad implications for protein and nucleic acid structure, molecular recognition, enzyme catalysis, and drug interaction. Despite its wide identification in crystal structures, the general existence of C-H···O hydrogen bonds remains elusive especially for natural C-H groups in bulk aqueous solutions at room temperature. Vibrational spectroscopy is a promising methodology to tackle this challenge, as formation of C-H···O hydrogen bonds usually causes shifts of the C-H stretch frequency. Yet, prior observations are inconclusive, as they are all based on a simple blue-shift in aqueous solution and cannot distinguish if it is an effect caused by solvent reorganization or a specific hydrogen-bonding interaction. In this work, we used vibrational solvatochromism as a calibration of the solvent reorganization effect and identified a specific H-bonding interaction. We performed vibrational solvatochromism study of C-H(D) of multiple alcohol molecules including the CH mode of CD₃CH(OH)CD₃ and the CD₃ modes of CD₃OH, CD₃CH₂OH, and CD₃CH(OH)CD₃ in a series of solvents. We found an abnormal blue-shift of the Raman frequency of the C-H and C-D bonds at both the C_α and C_β positions of alcohols in water, which lies in an opposite direction to the expected trend due to vibrational solvatochromism. This experimental evidence supports that the improper C-H···O hydrogen bonds might generally exist between nonpolarized C-H and water in liquid solutions at room temperature.

Boosting Zinc Electrode Reversibility in Aqueous Electrolytes by Using Low-Cost Antisolvents

Antisolvent addition has been widely studied in crystallization in the pharmaceutical industries by breaking the solvation balance of the original solution. Here we report a similar antisolvent strategy to boost Zn reversibility via regulation of the electrolyte on a molecular level. By adding for example methanol into ZnSO₄ electrolyte, the free water and coordinated water in Zn²⁺ solvation sheath gradually interact with the antisolvent, which minimizes water activity and weakens Zn²⁺ solvation. Concomitantly, dendrite-free Zn deposition occurs via change in the deposition orientation, as evidenced by in situ optical microscopy. Zn reversibility is significantly boosted in antisolvent electrolyte of 50 % methanol by volume (Anti-M-50 %) even under harsh environments of -20 °C and 60 °C. Additionally, the suppressed side reactions and dendrite-free Zn plating/stripping in Anti-M-50 % electrolyte significantly enhance performance of Zn/polyaniline coin and pouch cells. We demonstrate this low-cost strategy can be readily generalized to other solvents, indicating its practical universality. Results will be of immediate interest and benefit to a range of researchers in electrochemistry and energy storage.

Molecular Clustering in Formaldehyde-Methanol-Water Mixtures Revealed by High-Intensity, High-q Small-Angle Neutron Scattering

Methanol-Water (mw) mixtures, with or without a solute, display a nonideal thermodynamic behavior, typically attributed to the structure of the microphase. However, experimental observation of the microphase structures at the molecular length scale has been a challenge. We report the presence of molecular clusters in mw and formaldehyde-methanol-water (fmw) mixtures using small-angle neutron scattering (SANS) experiments and molecular dynamics (MD) simulations. Hydrophobic clusters of methanol in mw and formaldehyde-methanol in fmw mixtures were observed at low methanol compositions (x_m ≤ 0.3). A three-dimensional hydrogen-bonded network of water with the solute is observed at x_m = 0.5. Linear chains of methanol surrounding the formaldehyde and water molecules were observed at high methanol compositions (x_m ≥ 0.7). The calculated size of the molecular clusters (r ≈ 0.5 nm, spherical) from the SANS data and their volume fraction closely matched the MD simulation results.

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.

Internal electric fields in methanol [MeOH]2-6 clusters

Water and methanol are well known solvents showing cooperative hydrogen bonding, however the differences in the hydrogen bonding pattern in water and methanol are due to the presence of the methyl group in methanol. The presence of the methyl group leads to formation of C-HO hydrogen bonds apart from the usual O-HO hydrogen bonds. The electric fields evaluated along the hydrogen bonded donor OH and CH groups reveal that the C-HO hydrogen bonds can significantly influence the structure and energetics (by about 20%) of methanol clusters. A linear Stark effect was observed on the hydrogen bonded OH groups in methanol clusters with a Stark tuning rate of 3.1 cm^-1 (MV cm^-1)^-1 as an average behaviour. Furthermore, the Stark tuning of the OH oscillators in methanol depends on their hydrogen bonding environment wherein molecules with the DAA motif show higher rates than the rest. The present work suggests that the OH group of methanol has higher sensitivity as a vibrational probe relative to the OH group of water.

Probing Blue-Shifting Hydrogen Bonds with Adiabatic Energy Decomposition Analysis

The physical origin of blue-shifting hydrogen bonds remains a subject of debate, although many plausible explanations have been proposed. Using a molecular property decomposition analysis based on absolutely localized molecular orbitals, we investigated several representative F₃CH···Y (Y = H₂O, NH₃, Cl^-) complexes. We reveal that features of a blue-shifting H-bond already appear on the frozen surface where both polarization and charge transfer (CT) are "turned off", and that the final frequency shift observed depends on the strength of CT. Further decomposition of forces at the frozen level shows that Pauli repulsion is the only component that shortens the C-H bond in the short-range, while both permanent electrostatics and dispersion lengthen the bond. The effects of these forces from the medium to long-range are also discussed. Our analysis provides a complete picture for blue-shifting H-bonds and suggests two necessary conditions for their features to be observed at equilibrium structures: (i) stronger Pauli repulsion than the combination of electrostatic and dispersion forces; (ii) relatively weak CT that is insufficient to compensate for the blue-shifting effect of the frozen interaction.

Site-specific X-ray induced dynamics in liquid methanol

Complex chemical and biochemical systems are susceptible to damage from ionising radiation. However, questions remain over the extent to which such damage is influenced by the nature of the surrounding chemical environment, which can consist of both hydrophobic and hydrophilic domains. To gain fundamental insight into the first crucial mechanistic steps of radiation damage in such systems, we need to understand the initial radiation response, i.e. dynamics occurring on the same timescale as electronic relaxation, which occur in these different environments. Amphiphilic molecules contain both hydrophobic and hydrophilic domains, but the propensity for charge delocalisation and proton dynamics to occur in these different domains has been largely unexplored so far. Here, we present carbon and oxygen 1s Auger spectra for liquid methanol, one of the simplest amphiphilic molecules, as well as its fully deuterated equivalent d4-methanol, in order to explore X-ray induced charge delocalisation and proton dynamics occurring on the few femtosecond timescale. Unexpectedly, we find a similar propensity for proton dynamics to occur at both the carbon and oxygen site within the lifetime of the core hole. Our results could serve as a model for decay processes that are likely to occur in other more complex amphiphilic systems.

Probe of Alcohol Structures in the Gas and Liquid States Using C⁻H Stretching Raman Spectroscopy

Vibrational spectroscopy is a powerful tool for probing molecular structures and dynamics since it offers a unique fingerprint that allows molecular identification. One of important aspects of applying vibrational spectroscopy is to develop the probes that can characterize the related properties of molecules such as the conformation and intermolecular interaction. Many examples of vibrational probes have appeared in the literature, including the azide group (⁻N₃), amide group (⁻CONH₂), nitrile groups (⁻CN), hydroxyl group (⁻OH), ⁻CH group and so on. Among these probes, the ⁻CH group is an excellent one since it is ubiquitous in organic and biological molecules and the C⁻H stretching vibrational spectrum is extraordinarily sensitive to the local molecular environment. However, one challenge encountered in the application of C⁻H probes arises from the difficulty in the accurate assignment due to spectral congestion in the C⁻H stretching region. In this paper, recent advances in the complete assignment of C⁻H stretching spectra of aliphatic alcohols and the utility of C⁻H vibration as a probe of the conformation and weak intermolecular interaction are outlined. These results fully demonstrated the potential of the ⁻CH chemical group as a molecular probe.