Hydrogen Bonding in Alcohol Clusters: A Comparative Study by Infrared Cavity Ringdown Laser Absorption Spectroscopy

Dispersion Control over Molecule Cohesion: Exploiting and Dissecting the Tipping Power of Aromatic Rings

ConspectusWe have learned over the past years how London dispersion forces can be effectively used to influence or even qualitatively tip the structure of aggregates and the conformation of single molecules. This happens despite the fact that single dispersion contacts are much weaker than competing polar forces. It is a classical case of strength by numbers, with the importance of London dispersion forces scaling with the system size. Knowledge about the tipping points, however difficult to attain, is necessary for a rational design of intermolecular forces. One requires a careful assessment of the competing interactions, either by sensitive spectroscopic techniques for the study of the isolated molecules and aggregates or by theoretical approaches. Of particular interest are the systems close to the tipping point, when dispersion interactions barely outweigh or approach the strength of the other interactions. Such subtle cases are important milestones for a scale-up to realistic multi-interaction situations encountered in the fields of life and materials science. In searching for examples that provide ideal competing interactions in complexes and small clusters, aromatic systems can offer a diverse set of molecules with a variation of dispersion and electrostatic forces that control the dominant and peripheral interactions. Our combined spectroscopic and theoretical investigations provide valuable insights into the balance of intermolecular forces because they typically allow us to switch the aromatic substituent on and off. High-resolution rotational spectroscopy serves as a benchmark for molecular structures, as correct calculations should be based on correct geometries. When discussing the competition with other noncovalent interactions, obvious competitors are directional hydrogen bonds. As a second counterweight to aryl interactions, we will discuss aurophilic/metallophilic interactions, which also have a strong stabilization with a small number of atoms involved. Vibrational spectroscopy is most sensitive to interactions of light atoms, and the competition of OH hydrogen bonds with dispersion forces in a molecular aggregate can be judged well by the OH stretching frequency. Experiments in the gas phase are ideal for gauging the accuracy of quantum chemical predictions free of solvent forces. A tight collaboration utilizing these three methods allows experiment vs experiment vs theory benchmarking of the overall influence of dispersion in molecular structures and energetics.

An investigation into the adsorption mechanism of n-butanol by ZIF-8: a combined experimental and ab initio molecular dynamics approach

The zeolitic imidazolate framework, ZIF-8, has been shown by experimental methods to have a maximum saturation adsorption capacity of 0.36 g g^-1 for n-butanol from aqueous solution, equivalent to a loading of 14 butanol molecules per unit cell or 7 molecules per sodalite β-cage. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) shows the presence of hydrogen bonding between adsorbed butanol molecules within the cage; the presence of three different O-H stretching modes indicates the formation of butanol clusters of varying size. Ab initio molecular dynamics simulations show the formation of intermolecular hydrogen bonding between the butanol molecules, with an average hydrogen-bond coordination number of 0.9 after 15 ps simulation time. The simulations also uniquely demonstrate the presence of weaker interactions between the alcohol O-H group and the π-orbital of the imidazole ring on the internal surface of the cage during early stages of adsorption. The calculated adsorption energy per butanol molecule is -33.7 kJ mol^-1, confirming that the butanol is only weakly bound, driven primarily by the hydrogen bonding. Solid-state MAS NMR spectra suggest that the adsorbed butanol molecules possess a reasonable degree of mobility in their adsorbed state, rather than being rigidly held in specific sites. 2D ¹³C-¹H heteronuclear correlation (HETCOR) experiments show interactions between the butanol aliphatic chain and the ZIF-8 framework experimentally, suggesting that O-H interactions with the π-orbital are only short lived. The insight gained from these results will allow the design of more efficient ways of recovering and isolating n-butanol, an important biofuel, from low-concentration solutions.

On the relationship between the Debye process in dielectric response and a dissociation-association phenomenon in phenyl alcohols

In this paper, we have examined a series of phenyl-substituted primary monohydroxy alcohols (phenyl alcohols, PhAs), from ethanol to hexanol by means of dielectric and Fourier transform infrared (FTIR) spectroscopies supported by the mechanical investigations. The combination of both dielectric and mechanical data allows calculation of the energy barrier, E_a, for dissociation by the Rubinstein approach developed to describe the dynamical properties of self-assembling macromolecules. It was observed that the determined activation energy remains constant, |E_a,RM| ∼ 12.9-14.2 kJ mol^-1, regardless of the molecular weight of the examined material. Surprisingly, the obtained values agree very well with E_a of the dissociation process determined from the FTIR data analysed within the van't Hoff relationship, where E_a,vH ∼ 9.13-13.64 kJ mol^-1. Thus, the observed agreement between E_a determined by both applied approaches clearly implies that in the case of the examined series of PhAs, the dielectric Debye-like process is governed by the association-dissociation phenomenon as proposed by the transient chain model.

Laser Spectroscopy of Helium Solvated Clusters of Methanol and Methanol-Water in the Symmetric Methyl Stretching Band

Mid-infrared spectra of methanol and methanol-water clusters have been investigated in the symmetric CD₃ stretching band of CD₃OH and CD₃OD. We find that the position of this band provides a useful signature of the general type of hydrogen-bonded cluster it is associated with. Our results are consistent with those previously reported in the OH stretching region (Sulaiman, M. I.; Yang, S.; Ellis, A. M. J. Phys. Chem. A 2017, 121, 771-776) in that methanol clusters from the trimer to the pentamer are cyclic and that mixed clusters with one water molecule (and at least two methanol molecules) are also cyclic. We additionally provide evidence that the methanol trimer adopts a chair-like structure (as opposed to bowl-like), that mixed clusters with a larger number of water molecules are also cyclic, and that branched methanol clusters contribute to the depletion signal in larger methanol clusters. We performed double-hybrid DFT calculations which support these interpretations.

Infrared spectroscopy and theoretical structure analyses of protonated fluoroalcohol clusters: the impact of fluorination on the hydrogen bond networks

To explore the impact of fluorination on the hydrogen bond networks of protonated alkylalcohols, infrared spectroscopy and theoretical computations of protonated 2,2,2-trifluoroethanol clusters, H⁺(TFE)_n, (n = 4-7), were performed. It has been demonstrated that the development of the hydrogen bond networks from a linear type to cyclic types occurs in this size region for the protonated alkylalcohol clusters. In contrast, infrared spectroscopy of H⁺(TFE)_n in the OH/CH stretch region clearly indicated that the linear type structures are held in the whole size range, irrespective of temperature of the clusters. The extensive stable isomer structure search of H⁺(TFE)_n based on our latest sampling approach supported the strong preference of the linear type hydrogen bond networks. Detailed analyses of the free OH stretching vibrational bands evidenced the intra- and intermolecular OH⋯FC interactions in the clusters. In addition, infrared spectra of protonated clusters of 2,2-difluoroethanol, 2,2-difluoropropanol, and 3,3,3-trifluoropropanol were measured for n = 4 and 5, and their spectra also indicated the effective inhibition of the cyclic hydrogen bond network formation by the fluorination.

Activity coefficients of binary methanol alcohol mixtures from cluster weighting

The hydrogen bond network of different small alcohols is investigated via cluster analysis. Methanol/alcohol mixtures are studied with increasing chain length and branching of the molecule. Those changes can play an important role in different fields, including solvent and metal extraction. The extended tight binding method GFN2-xTB allows the evaluation and geometry optimization of thousands of clusters built via a genetic algorithm. Interaction energies and geometries are evaluated and discussed for the neat systems. Thermodynamic properties, such as vaporization enthalpies and activity coefficients, are calculated with the binary quantum cluster equilibrium (bQCE) approach using our in-house code peacemaker 2.8. Combined distribution functions of the distances against the angles of the hydrogen bonds are evaluated for neat and mixed clusters and weighted by the equilibrium populations achieved from bQCE calculations.

Exploration of the potential energy surfaces of small ethanol clusters

The potential energy surfaces of small ethanol clusters, from dimer to pentamer, have been thoroughly explored using two different levels of theory. There is a clear relative energy gap between cyclic, linear and branched cyclic structures.

Competition between hydrogen bonds and van der Waals forces in intermolecular structure formation of protonated branched-chain alcohol clusters

To investigate the influence of bulky alkyl groups on hydrogen-bonded (H-bonded) network structures of alcohols, infrared (IR) spectra of protonated clusters of 2-propanol (2-PrOH) and tert-butyl alcohol (t-BuOH) were observed in the OH and CH stretch regions. In addition, by varying the tag species, the temperature dependence profile of the isomer population of H+(t-BuOH)n was revealed. An extensive search for stable isomers was performed using dispersion-corrected density functional theory methods, and temperature-dependent IR spectral simulations were done on the basis of the harmonic superposition approximation. The computational results qualitatively agreed with the observed size and temperature dependence of the H-bonded network structures of these protonated bulky alcohol clusters. However, the difficulty in the quantitative evaluation of dispersion was also demonstrated. It was shown that H+(2-PrOH)n (n = 4-7) have essentially the same network structures as the protonated normal alcohol clusters studied so far. On the other hand, H+(t-BuOH)n (n = 4-8) showed a clear preference for the smaller-membered ring structures, that is very different from the preference of the protonated normal alcohol clusters. The origin of the different structure preferences was discussed in terms of the steric effect and dispersion.

Hydrogen bonding between hydrides of the upper-right part of the periodic table

One of the most important electrostatic interactions between molecules is most definitely the hydrogen bond. Understanding the basis of this interaction may offer us the insight needed to understand its effect on the macroscopic scale. Hydrogen bonding is for example the reason for anomalous properties in compounds like water and naturally life as we know it. The strength of the bond depends on numerous factors, among them the electronegativity of participating atoms. In this work we calculated the strength of hydrogen bonds between hydrides of the upper-right part of the periodic table (C, N, O, F, P, S, Cl, As, Se, Br) using quantum-chemical methods. The aim was to determine what influences the strength of strong and weak hydrogen bonds in simple hydrides. Various relationships were checked. A relation between the strength of the bond and the electronegativity of the participating atoms was found. We also observed a correlation between the strength of hydrogen bonds and the inter-atomic distances, along with the dependence on the charge transfer on the atom of the donor. We also report characteristic geometries of different dimers.

Hydrogen bond network structures of protonated short-chain alcohol clusters

Protonated alcohol clusters enable extraction of the physical essence of the nature of hydrogen bond networks.

Hydrogen bonding and dominant conformations of hydrated sugar analogue complexes using tetrahydrofurfuryl alcohol as the model sugar molecule

Water molecules, which serve as both hydrogen bond donors and acceptors, have been found to influence the conformational landscape of gas-phase phenyl-β-d-glucopyranoside.

Cooperativity in Alcohol-Nitrogen Complexes: Understanding Cryomatrices through Slit Jet Expansions

FTIR spectroscopy of supersonic expansions is used to characterize alcohol dimers with one, two, and several nitrogen molecules attached to them. The nitrogen coating causes progressive spectral downshifts of the OH stretching fundamentals which are related to and explain matrix isolation shifts. Comparison of methanol, tert-butyl alcohol and ethanol as well as deuteration of methanol assist in the assignment. Alcohol monomers and trimers are significantly more resistant to nitrogen coating due to a lack of cooperativity and dangling bonds, respectively. In the case of ethanol, the role of conformational isomerism and combination bands is further elucidated. The experimental findings help rationalize the anomalously small OH stretching dimerization shift of methanol in the gas phase, in comparison to that of tert-butyl alcohol.

Infrared absorption of methanol-water clusters (CH3OH)n(H2O), n = 1-4, recorded with the VUV-ionization/IR-depletion technique

We recorded infrared (IR) spectra in the CH- and OH-stretching regions of size-selected clusters of methanol (M) with one water molecule (W), represented as M_nW, n = 1-4, in a pulsed supersonic jet using the photoionization/IR-depletion technique. Vacuum ultraviolet emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer to detect clusters M_nW as protonated forms M_n-1WH⁺. The variations in intensities of M_n-1WH⁺ were monitored as the wavelength of the IR laser light was tuned across the range 2700-3800 cm^-1. IR spectra of size-selected clusters were obtained on processing of the observed action spectra of the related cluster-ions according to a mechanism that takes into account the production and loss of each cluster due to IR photodissociation. Spectra of methanol-water clusters in the OH region show significant variations as the number of methanol molecules increases, whereas those in the CH region are similar for all clusters. Scaled harmonic vibrational wavenumbers and relative IR intensities predicted with the M06-2X/aug-cc-pVTZ method for the methanol-water clusters are consistent with our experimental results. For dimers, absorption bands of a structure WM with H₂O as a hydrogen-bond donor were observed at 3570, 3682, and 3722 cm^-1, whereas weak bands of MW with methanol as a hydrogen-bond donor were observed at 3611 and 3753 cm^-1. For M₂W, the free OH band of H₂O was observed at 3721 cm^-1, whereas a broad feature was deconvoluted to three bands near 3425, 3472, and 3536 cm^-1, corresponding to the three hydrogen-bonded OH-stretching modes in a cyclic structure. For M₃W, the free OH shifted to 3715 cm^-1, and the hydrogen-bonded OH-stretching bands became much broader, with a weak feature near 3179 cm^-1 corresponding to the symmetric OH-stretching mode of a cyclic structure. For M₄W, the observed spectrum agrees unsatisfactorily with predictions for the most stable cyclic structure, indicating significant contributions from branched isomers, which is distinctly different from M₅ of which the cyclic form dominates.

Intramolecular dehydration of biomass-derived sugar alcohols in high-temperature water

The intramolecular dehydration of biomass-derived sugar alcohols d-sorbitol, d-mannitol, galactitol, xylitol, ribitol, l-arabitol, erythritol, l-threitol, and dl-threitol was investigated in high-temperature water at 523–573 K without the addition of any acid catalysts.

Spectroscopic Evidence for Clusters of Like-Charged Ions in Ionic Liquids Stabilized by Cooperative Hydrogen Bonding

Direct spectroscopic evidence for hydrogen-bonded clusters of like-charged ions is reported for ionic liquids. The measured infrared O-H vibrational bands of the hydroxyethyl groups in the cations can be assigned to the dispersion-corrected DFT calculated frequencies of linear and cyclic clusters. Compensating the like-charge Coulomb repulsion, these cationic clusters can range up to cyclic tetramers resembling molecular clusters of water and alcohols. These ionic clusters are mainly present at low temperature and show strong cooperative effects in hydrogen bonding. DFT-D3 calculations of the pure multiply charged clusters suggest that the attractive hydrogen bonds can compete with repulsive Coulomb forces.

Novel Microwave Thermodynamic Model for Alcohol with Clustering Structure in Nonpolar Solution

A solution containing ethanol as polar material and either benzene or n-dodecane as nonpolar solvent was heated by microwave irradiation employing a single-mode resonance microwave device. Although the microwave heating efficiency was expected from the just value of the relative dielectric constant (εr') or relative dielectric loss (εr″) for liquid system, it was revealed that the clustering structure of alcohol molecules expected from the excess parameter such as the excess relative dielectric loss is the important factor in the decision for efficiency of the microwave heating for the solution. This assumption and novel theory were strongly supported from the thermodynamic data such as vapor pressure and the partial enthalpy.

Positively Charged Phosphorus as a Hydrogen Bond Acceptor

Phosphorus (P) is an element that is essential to the life of all organisms, and the atmospheric detection of phosphine suggests the existence of a volatile biogeochemical P cycle. Here, we investigate the ability of P to participate in the formation of OH···P hydrogen bonds. Three bimolecular alcohol-trimethylphosphine complexes have been detected. Initially, the complexes were detected using matrix isolation spectroscopy, which favors complex formation. Subsequently, the fundamental OH-stretching vibration was observed in room-temperature gas-phase spectra. On the basis of our measured OH-stretching frequency red shifts and quantum chemical calculations, we find that P is an acceptor atom similar in strength to O and S and that all three P, O, and S atoms are weaker acceptors than N. The quantum chemical calculations show that both H and P in the OH···P hydrogen bond have partial positive charges, as expected from their electronegativities. However, the electrostatic potentials show a negative potential area on the electron density surface around P that facilitates formation of hydrogen bonds.

FTIR Spectra of n-Octanol in Liquid and Solid States

The investigation of the temperature dependence of FTIR spectrum of n-octanol in the temperature range from −150°C to 50°C is presented. The observed changes in the registered spectra during gradual heating of the sample were analysed. The structure transformation at the phase transition from solid to liquid phase is detected.

Isothermal evaporation of ethanol in a dynamic gas atmosphere

Optimization of evaporation and pyrolysis conditions for ethanol are important in carbon nanotube (CNT) synthesis. The activation enthalpy (ΔH(‡)), the activation entropy (ΔS(‡)), and the free energy barrier (ΔG(‡)) to evaporation have been determined by measuring the molar coefficient of evaporation, k(evap), at nine different temperatures (30-70 °C) and four gas flow rates (25-200 mL/min) using nitrogen and argon as carrier gases. At 70 °C in argon, the effect of the gas flow rate on k(evap) and ΔG(‡) is small. However, this is not true at temperatures as low as 30 °C, where the increase of the gas flow rate from 25 to 200 mL/min results in a nearly 6 times increase of k(evap) and decrease of ΔG(‡) by ~5 kJ/mol. Therefore, at 30 °C, the effect of the gas flow rate on the ethanol evaporation rate is attributed to interactions of ethanol with argon molecules. This is supported by simultaneous infrared spectroscopic analysis of the evolved vapors, which demonstrates the presence of different amounts of linear and cyclic hydrogen bonded ethanol aggregates. While the amount of these aggregates at 30 °C depends upon the gas flow rate, no such dependence was observed during evaporation at 70 °C. When the evaporation was carried out in nitrogen, ΔG(‡) was almost independent of the evaporation temperature (30-70 °C) and the gas flow rate (25-200 mL/min). Thus the evaporation of ethanol in a dynamic gas atmosphere at different temperatures may go via different mechanisms depending on the nature of the carrier gas.

Weak Hydrogen Bonds Make a Difference: Dimers of Jet-Cooled Halogenated Ethanols

Hydrogen-bonded clusters of fluorinated and chlorinated ethanols exhibit rich isomerism in terms of monomer conformation, secondary contacts between the OH and CH groups and the halogen atoms, hydrogen bond topology, chirality recognition and acceptor lone electron pair choice. By expanding the six alcohols involving one to three fluorine or chlorine atoms at the methyl group in a supersonic slit jet expansion and by probing their monomer, dimer and trimer IR spectra between 800 and 4000 cm−1, this isomerism is unravelled in substantial detail. Argon relaxation experiments and complementary cluster Raman spectroscopy provide further information on the individual dimer conformations and on trimer assignments. Energy sequences, helicity- and topology-dependent OH red-shifts, differences between fluorine and chlorine, the influence of dispersion-like interactions and halogen number trends are uncovered and compared to systematic quantum-chemical calculations up to MP2/6–311+G* level. The experimental data provide rigorous reference values for an accurate and balanced quantum-mechanical description of weak hydrogen bond interactions to halogens in the presence of a strong hydrogen bond between oxygen atoms.

Chirality recognition between neutral molecules in the gas phase

Noncovalent interactions are particularly intriguing when they involve chiral molecules, because the interactions change in a subtle way upon replacing one of the partners by its mirror image. The resulting phenomena involving chirality recognition are relevant in the biosphere, in organic synthesis, and in polymer design. They may be classified according to the permanent or transient chirality of the interacting partners, leading to chirality discrimination, chirality induction, and chirality synchronization processes. For small molecules, high-level quantum chemical calculations for such processes are feasible. To provide reliable connections between theory and experiment, such phenomena are best studied in vacuum isolation at low temperature, using rotational, vibrational, electronic, and photoionization spectroscopy. We review these techniques and the results which have become available in recent years, with special emphasis on dimers of permanently chiral molecules and on the influence of conformational flexibility. Analogies between the microscopic mechanisms and macroscopic phenomena and between intra- and intermolecular cases are drawn.

Hydrogen-bonded OH stretching modes of methanol clusters: a combined IR and Raman isotopomer study

A comprehensive study of the OH and OD stretching fundamentals in clusters of methanol and its isotopomers CH(3)OD, CD(3)OH, and CD(3)OD provides detailed insights into the hydrogen-bond mediated coupling as a function of cluster size. The combination of infrared and Raman supersonic jet spectroscopy enables the observation and assignment of all hydrogen-bonded OH stretching modes of isolated methanol trimer and methanol tetramer. A consistent explanation for the spectral complexity observed more than a decade ago in methanol trimer in terms of low-frequency methyl umbrella motions is provided. Previous explanations based on cluster isomerism or anharmonic resonances are ruled out by dedicated jet experiments. The first experimental lower bound for concerted quadruple proton transfer in S(4) symmetric methanol tetramer is derived and compared with theoretical predictions. The observed isotope effects offer insights into the anharmonicity of the localized OH bond. The performance of harmonic B3LYP and MP2 calculations in predicting hydrogen-bond-induced spectral shifts and couplings is investigated.