Understanding benzyl alcohol aggregation by chiral modification: the pairing step

Electrocatalytic Aromatic Alcohols Splitting to Aldehydes and H2 Gas

Selective electrocatalytic transformation of alcohols to aldehydes offers an efficient and environmentally friendly platform for the simultaneous production of fine chemicals and pure hydrogen gas. However, traditional alcohol oxidation reactions (AORs) in aqueous electrolyte unavoidably face competitive reactions (e.g., water oxidation and overoxidations reactions) for the presence of active oxygen species from water oxidation, causing an unwanted decrease in final efficiency and selectivity. Here, we developed an integrated all-solid proton generator-transfer electrolyzer to trigger the pure alcohol splitting reaction (ASR). In this splitting process, only O-H and C-H bonds can be cleaved at the proton generator (Pt nanoparticles), thereby completely avoiding all competitive reactions involving oxygen active species to give a > 99% selectivity to aldehydes. The as-generated protons are transported to the cathode by a three-dimensional (3D) conducting network (assemblies of ionomers and carbon spheres) for efficient hydrogen production. Unlike the poor selectivity (<22%) and durability (<3 h) of a conventional AOR electrolyzer, this ASR electrolyzer could be continuously operated at a low cell voltage of 1.2 V for at least 10 days to give a high Faradaic efficiency of 80-93% for aldehyde production.

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.

Homochiral vs. heterochiral preference in chiral self-recognition of cyclic diols

The structure and clustering propensity of a chiral derivative of cis-1,2-cyclohexanediol, namely, 1-phenyl-cis-1,2-cyclohexanediol (cis-PCD), has been studied under supersonic expansion conditions by combining laser spectroscopy with quantum chemistry calculations. The presence of the phenyl substituent induces conformational locking relative to cis-1,2-cyclohexanediol (cis-CD), and only one conformer of the bare molecule is observed by both Raman and IR-UV double resonance spectroscopy. The homochiral preference inferred for the dimer formation at low enough temperature is in line with the formation of a conglomerate in the solid state. The change in clustering propensity in cis-PCD relative to trans-1,2-cyclohexanediol (trans-CD), which shows heterochiral preference, is explained by the presence of the phenyl substituent rather than the effect of cis-trans isomerism. Indeed the transiently chiral cis-CD also forms preferentially heterodimers, whose structure is very close to that of the corresponding trans-CD dimer.

Advances and Prospects in Understanding London Dispersion Interactions in Molecular Chemistry

London dispersion (LD) interactions are the main contribution of the attractive part of the van der Waals potential. Even though LD effects are the driving force for molecular aggregation and recognition, the role of these omnipresent interactions in structure and reactivity had been largely underappreciated over decades. However, in the recent years considerable efforts have been made to thoroughly study LD interactions and their potential as a chemical design element for structures and catalysis. This was made possible through a fruitful interplay of theory and experiment. This review highlights recent results and advances in utilizing LD interactions as a structural motif to understand and utilize intra- and intermolecularly LD-stabilized systems. Additionally, we focus on the quantification of LD interactions and their fundamental role in chemical reactions.

Accurate Geometry and Non-Covalent Interactions in 1-Phenylethanol and Its Monohydrate: A Rotational Study

The pure rotational spectra of 1-phenylethanol and its monohydrate were measured by using a pulsed jet Fourier transform microwave spectrometer. One conformer of the 1-phenylethanol monomer with the trans form was observed in the pulsed jet. The experimental values of rotational constants of ten isotopologues, including eight mono-substituted ¹³ C and one D isotopologues, allow an accurate structure determination of the skeleton of 1-phenylethanol. For its monohydrate, only one isomer has been observed, of which 1-phenylethanol adopts the trans form and binds with water through an O-H⋅⋅⋅O_w and an O_w -H⋅⋅⋅π hydrogen bond. Each rotational transition displays a doublet with a relative intensity ratio of 1 : 3, due to a hindered internal rotation of water around its C₂ axis. This study provides the information on accurate geometry of 1-phenylethanol (PE) and large amplitude motion of water in the PE monohydrate.

London Dispersion-Assisted Low-Temperature Gas Phase Synthesis of Hydrogen Bond-Inserted Complexes

Supersonic expansions of organic molecules in helium carrier gas mixtures are used to synthesize model (pre)reactive complexes at low temperature. Whether or not barriers for hydrogen bond rearrangements can be overcome in this collisional process is not well understood. Using the example of alcohols inserting into intramolecular hydrogen bonds of α-hydroxy esters, we explore whether dispersion energy donors can assist the process in a systematic way. Bromo, iodo, and tert-butyl substitution of benzyl alcohol in the para-position is used to show that the insertion process into methyl glycolate is controllable, whereas it is largely avoided for the chiral methyl lactate homologue. Methyl lactate appears to steer the transient chirality of benzyl alcohol derivatives in a uniform direction relative to the lactate handedness for the OH∙∙∙O=C insertion product, as well as for the competing attachment to the hydroxy group of the ester. A simple rule based on the total binding energy in relation to the rearrangement barrier is tentatively proposed to estimate whether the insertion is feasible or not in such molecular complexes during expansion.

The evolution towards cyclic structures in the aggregation of aromatic alcohols: the dimer, trimer and tetramer of 2-phenylethanol

Gas-phase spectroscopic studies of alcohol clusters offer accurate information on the influence of non-covalent interactions on molecular recognition, and are of paramount importance to model supramolecular and biological chemical processes. Here, we examine the role of the aliphatic side chain in the self-aggregation of aromatic alcohols, using a multi-methodological gas-phase approach which combines microwave spectroscopy and mass-resolved electronic and vibrational laser spectroscopy. Spectroscopic and electronic structure computations were carried out for the dimer, trimer and tetramer of 2-phenylethanol, extending previous investigations on smaller aromatic alcohols. While the conformational flexibility of the ethyl group anticipates a variety of torsional isomers, the intra- and inter-molecular interactions restrict molecular conformations and favour particularly stable isomers. The conformational landscape of the clusters is very shallow and multiple competing isomers were rotationally and/or vibrationally detected, including three dimer species, two trimers and two tetramers. Cluster growth is associated with a tendency to form cyclic hydrogen bond structures.

Alternating 1-Phenyl-2,2,2-Trifluoroethanol Conformational Landscape With the Addition of One Water: Conformations and Large Amplitude Motions

The 1:1 adduct of 1-phenyl-2,2,2-trifluoroethanol (PhTFE), a chiral fluoroalcohol, with water was investigated using chirped pulse Fourier transform microwave spectroscopy and computational methods. While PhTFE itself was predicted to have three minima, I (gauche+), II (trans), and III (gauche-), only I and II were stable and only I was observed experimentally. A systematic search of the PhTFE···H₂O conformational landscape identified 110 stable minima, 14 of which are within a 15 kJ mol^-1 energy window. Rotational spectra of the two PhTFE···H₂O conformers along with several deuterium and ¹⁸O isotopologues were assigned, and the isotopic data were used to verify the corresponding structures. In the two observed monohydrate conformers, one contains PhTFE I where the water subunit is inserted into the existing intramolecular OH···F contact of I, and the binary adduct is stabilized by two intermolecular contacts: OH···O_W and H_W···F, whereas the other contains PhTFE II where the water subunit interacts with both the alcohol hydrogen and phenyl ring of II, demonstrating that interaction with water sufficiently stabilizes II for its observation in a jet expansion. Interestingly, the predicted electric dipole moment components at the identified minima deviate considerably from the experimental ones. Such deviations were analyzed in terms of dynamic effects associated with the large amplitude motions of the unbound H_W. In addition, tunnelling effects associated with the exchange of the bonded and nonbonded H_W were also discussed.

Sniffing out camphor: the fine balance between hydrogen bonding and London dispersion in the chirality recognition with α-fenchol

Binary complexes between the chiral monoterpenoids camphor and α-fenchol were explored with vibrational and rotational jet spectroscopy as well as density functional theory in order to explore how chirality can influence the binding preferences in gas-phase complexes. The global minimum structures of the two diastereomers were assigned. It is found that chirality recognition leads to different compromises in the fine balance between intermolecular interactions. While one isomer features a stronger hydrogen bond, the other one is more tightly arranged and stabilized by larger London dispersion interactions. These new spectroscopic results help understand the influence of chirality in molecular aggregation and unveil the kind of interactions involved between a chiral alcohol and a chiral ketone with large dispersion contributions.

Attaching Onto or Inserting Into an Intramolecular Hydrogen Bond: Exploring and Controlling a Chirality-Dependent Dilemma for Alcohols

Prereactive complexes in noncovalent organocatalysis are sensitive to the relative chirality of the binding partners and to hydrogen bond isomerism. Both effects are present when a transiently chiral alcohol docks on a chiral α-hydroxy ester, turning such 1:1 complexes into elementary, non-reactive model systems for chirality induction in the gas phase. With the help of linear infrared and Raman spectroscopy in supersonic jet expansions, conformational preferences are investigated for benzyl alcohol in combination with methyl lactate, also exploring p-chlorination of the alcohol and the achiral homolog methyl glycolate to identify potential London dispersion and chirality effects on the energy sequence. Three of the four combinations prefer barrierless complexation via the hydroxy group of the ester (association). In contrast, the lightest complex predominantly shows insertion into the intramolecular hydrogen bond, such as the analogous lactate and glycolate complexes of methanol. The experimental findings are rationalized with computations, and a uniform helicality induction in the alcohol by the lactate is predicted, independent of insertion into or association with the internal lactate hydrogen bond. p-chlorination of benzyl alcohol has a stabilizing effect on association because the insertion motif prevents a close contact between the chlorine and the hydroxy ester. After simple anharmonicity and substitution corrections, the B3LYP-D3 approach offers a fairly systematic description of the known spectroscopic data on alcohol complexes with α-hydroxy esters.

Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol

Rotational microwave jet spectroscopy studies of the monoterpenol α-fenchol have so far failed to identify its second most stable torsional conformer, despite computational predictions that it is only very slightly higher in energy than the global minimum. Vibrational FTIR and Raman jet spectroscopy investigations reveal unusually complex OH and OD stretching spectra compared to other alcohols. Via modeling of the torsional states, observed spectral splittings are explained by delocalization of the hydroxy hydrogen atom through quantum tunneling between the two non-equivalent but accidentally near-degenerate conformers separated by a low and narrow barrier. The energy differences between the torsional states are determined to be only 16(1) and 7(1) cm-1hc for the protiated and deuterated alcohol, respectively, which further shrink to 9(1) and 3(1) cm-1hc upon OH or OD stretch excitation. Comparisons are made with the more strongly asymmetric monoterpenols borneol and isopinocampheol as well as with the symmetric, rapidly tunneling propargyl alcohol. In addition, the third-in contrast localized-torsional conformer and the most stable dimer are assigned for α-fenchol, as well as the two most stable dimers for propargyl alcohol.

Rovibronic signatures of molecular aggregation in the gas phase: subtle homochirality trends in the dimer, trimer and tetramer of benzyl alcohol

Molecular aggregation is of paramount importance in many chemical processes, including those in living beings. Thus, characterization of the intermolecular interactions is an important step in its understanding. We describe here the aggregation of benzyl alcohol at the molecular level, a process governed by a delicate equilibrium between OH⋯O and OH⋯π hydrogen bonds and dispersive interactions. Using microwave, FTIR, Raman and mass-resolved double-resonance IR/UV spectroscopic techniques, we explored the cluster growth up to the tetramer and found a complex landscape, partly due to the appearance of multiple stereoisomers of very similar stability. Interestingly, a consistently homochiral synchronization of transiently chiral monomer conformers was observed during cluster growth to converge in the tetramer, where the fully homochiral species dominates the potential energy surface. The data on the aggregation of benzyl alcohol also constitute an excellent playground to fine-tune the parameters of the most advanced functionals.

Molecular Recognition, Transient Chirality and Sulfur Hydrogen Bonding in the Benzyl Mercaptan Dimer

The homodimers of transiently chiral molecules offer physical insight into the process of molecular recognition, the preference for homo or heterochiral aggregation and the nature of the non-covalent interactions stabilizing the adducts. We report the observation of the benzyl mercaptan dimer in the isolation conditions of a supersonic jet expansion, using broadband (chirped-pulse) microwave spectroscopy. A single homochiral isomer was observed for the dimer, stabilized by a cooperative sequence of S-H···S and S-H···π hydrogen bonds. The structural data, stabilization energies and energy decomposition describe these non-covalent interactions as weak and dispersion-controlled. A comparison is also provided with the benzyl alcohol dimer.

Subtle hydrogen bonds: benchmarking with OH stretching fundamentals of vicinal diols in the gas phase

The theoretical description of spectral signatures for weakly bound hydrogen contacts between alcohol groups is challenging and remains poorly characterised. By combining Raman jet spectroscopy with appropriately scaled harmonic DFT predictions and relaxation path analyses for 16 vicinal diols (ethylene glycol (ethane-1,2-diol), propane-1,2-diol, 3,3,3-trifluoro-propane-1,2-diol, rac-butane-2,3-diol, 2-methyl-propane-1,2-diol, 2-methyl-butane-2,3-diol, pinacol (2,3-dimethyl-butane-2,3-diol), 3-butene-1,2-diol, 1-phenyl-ethane-1,2-diol, trans-cyclobutane-1,2-diol, trans-cyclopentane-1,2-diol, trans-cyclohexane-1,2-diol, trans-cycloheptane-1,2-diol, cis-cyclohexane-1,2-diol, 1-(1-hydroxy-1-methylethyl)-cyclopentanol and [1,1'-bicyclopentyl]-1,1'-diol), 69 conformational assignments become possible in a two-tier approach with a 5 diol training and an 11 diol test set. The latter reveals systematic deviations for ring strain and secondary π interactions, but otherwise a remarkably robust correction and correlation model based on hybrid DFT with a minimally augmented triple-zeta basis set is obtained, whereas GGA functionals perform significantly worse. Raw experimental data in the 3560-3700 cm^-1 wavenumber range as well as computed geometries of all conformations invite further vibrational and structural benchmarking at the onset of hydrogen bonding. Beyond this diol-probed threshold, the accurate prediction of hydrogen bond induced shifts of different magnitudes remains one of the challenges for DFT functionals.

Dynamic chiral self-recognition in aromatic dimers of styrene oxide revealed by rotational spectroscopy

Chiral molecular recognition is a pivotal phenomenon in biomolecular science, governed by subtle balances of intermolecular forces that are difficult to quantify. Non-covalent interactions involving aromatic moieties are particularly important in this realm, as recurring motifs in biomolecular aggregation. In this work, we use high-resolution broadband rotational spectroscopy to probe the dynamic conformational landscape enclosing the self-pairing topologies of styrene oxide, a chiral aromatic system. We reach a definite assignment of four homochiral and two heterochiral dimers using auxiliary quantum chemistry calculations as well as structure-solving methods based on experimental isotopic information. A complete picture of the dimer conformational space is obtained, and plausible routes for conformational relaxation are derived. Molecular structures are discussed in terms of conformational flexibility, the concerted effort of weak intermolecular interactions, and their role in the expression of the molecular fit.