Intermolecular dissociation energies of hydrogen-bonded 1-naphthol complexes

We have measured the intermolecular dissociation energiesD0of supersonically cooled 1-naphthol(1NpOH) complexes with solvents S = furan, thiophene, 2,5-dimethylfuran, and tetrahydrofuran. Thenaphthol OH forms non-classical H-bonds with the aromaticπ-electrons of furan, thiophene, and2,5-dimethylfuran and a classical H-bond with the tetrahydrofuran O atom. Using the stimulated-emission pumping resonant two-photon ionization method, the ground-stateD0(S0) values werebracketed as 21.8±0.3 kJ/mol for furan, 26.6±0.6 kJ/mol for thiophene, 36.5±2.3 kJ/mol for2,5-dimethylfuran, and 37.6±1.3 kJ/mol for tetrahydrofuran. The dispersion-corrected density func-tional theory methods B97-D3, B3LYP-D3 (using the def2-TZVPP basis set), andωB97X-D [usingthe 6-311++G(d,p) basis set] predict that the H-bonded (edge) isomers are more stable than the faceisomers bound by dispersion; experimentally, we only observe edge isomers. We compare the cal-culated and experimentalD0values and extend the comparison to the previously measured 1NpOHcomplexes with cyclopropane, benzene, water, alcohols, and cyclic ethers. The dissociation energiesof the nonclassically H-bonded complexes increase roughly linearly with the average polarizabilityof the solvent, ̄α(S). By contrast, theD0values of the classically H-bonded complexes are larger,increase more rapidly at low ̄α(S), but saturate for large ̄α(S). The calculatedD0(S0) values forthe cyclopropane, benzene, furan, and tetrahydrofuran complexes agree with experiment to within1 kJ/mol and those of thiophene and 2,5-dimethylfuran are∼3 kJ/mol smaller than experiment. TheB3LYP-D3 calculatedD0values exhibit the lowest mean absolute deviation (MAD) relative toexperiment (MAD = 1.7 kJ/mol), and the B97-D3 andωB97X-D MADs are 2.2 and 2.6 kJ/mol,respectively

The furan microsolvation blind challenge for quantum chemical methods: First steps

© 2018 Author(s). Herein we present the results of a blind challenge to quantum chemical methods in the calculation of dimerization preferences in the low temperature gas phase. The target of study was the first step of the microsolvation of furan, 2-methylfuran and 2,5-dimethylfuran with methanol. The dimers were investigated through IR spectroscopy of a supersonic jet expansion. From the measured bands, it was possible to identify a persistent hydrogen bonding OH-O motif in the predominant species. From the presence of another band, which can be attributed to an OH-π interaction, we were able to assert that the energy gap between the two types of dimers should be less than or close to 1 kJ/mol across the series. These values served as a first evaluation ruler for the 12 entries featured in the challenge. A tentative stricter evaluation of the challenge results is also carried out, combining theoretical and experimental results in order to define a smaller error bar. The process was carried out in a double-blind fashion, with both theory and experimental groups unaware of the results on the other side, with the exception of the 2,5-dimethylfuran system which was featured in an earlier publication

The first microsolvation step for furans : new experiments and benchmarking strategies

The site-specific first microsolvation step of furan and some of its derivatives with methanol is explored to benchmark the ability of quantum-chemical methods to describe the structure, energetics, and vibrational spectrum at low temperature. Infrared and microwave spectra in supersonic jet expansions are used to quantify the docking preference and some relevant quantum states of the model complexes. Microwave spectroscopy strictly rules out in-plane docking of methanol as opposed to the top coordination of the aromatic ring. Contrasting comparison strategies, which emphasize either the experimental or the theoretical input, are explored. Within the harmonic approximation, only a few composite computational approaches are able to achieve a satisfactory performance. Deuteration experiments suggest that the harmonic treatment itself is largely justified for the zero-point energy, likely and by design due to the systematic cancellation of important anharmonic contributions between the docking variants. Therefore, discrepancies between experiment and theory for the isomer abundance are tentatively assigned to electronic structure deficiencies, but uncertainties remain on the nuclear dynamics side. Attempts to include anharmonic contributions indicate that for systems of this size, a uniform treatment of anharmonicity with systematically improved performance is not yet in sight

The reduced cohesion of homoconfigurational 1,2-diols

International audienceBy a combination of linear FTIR and Raman jet spectroscopy, racemic trans-1,2-cyclohexanediol is shown to form an energetically unrivalled S4-symmetric heterochiral dimer in close analogy to 1,2-ethanediol. Analogous experiments with enantiopure trans-1,2-cyclohexanediol reveal the spectral signature of at least three unsymmetric homochiral dimers. A comparison to signal-enhanced spectra of 1,2-ethanediol and to calculations uncovers at least three transiently homochiral dimer contributions as well. In few of these dimer structures, the intramolecular OHO contact present in monomeric 1,2-diols survives, despite the kinetic control in supersonic jet expansions. This provides further insights into the dimerisation mechanism of conformationally semi-flexible molecules in supersonic jets. Racemisation upon dimerisation is shown to be largely quenched under jet cooling conditions, whereas it should be strongly energy-driven at higher temperatures. The pronounced energetic preference for heterochiral aggregation of vicinal diols is also discussed in the context of chirality-induced spin selectivity

Rotational Signatures of Dispersive Stacking in the Formation of Aromatic Dimers

The aggregation of aromatic species is dictated by inter‐ and intramolecular forces. Not only is characterizing these forces in aromatic growth important for understanding grain formation in the interstellar medium, but it is also imperative to comprehend biological functions. We report a combined rotational spectroscopic and quantum‐chemical study on three homo‐dimers, comprising of diphenyl ether, dibenzofuran, and fluorene, to analyze the influence of structural flexibility and the presence of heteroatoms on dimer formation. The structural information obtained shows clear similarities between the dimers, despite their qualitatively different molecular interactions. All dimers are dominated by dispersion interactions, but the dibenzofuran dimer is also influenced by repulsion between the free electron pairs of the oxygen atoms and the π‐clouds. This study lays the groundwork for understanding the first steps of molecular aggregation in systems with aromatic residues

Aromatic embedding wins over classical hydrogen bonding – a multi-spectroscopic approach for the diphenyl ether–methanol complex

Dispersion interactions are omnipresent in intermolecular interactions, but their respective contributions are difficult to predict. Aromatic ethers offer competing docking sites for alcohols: the ether oxygen as a well known hydrogen bond acceptor, but also the aromatic π system. The interaction with two aromatic moieties in diphenyl ether can tip the balance towards π binding. We use a multi-spectroscopic approach to study the molecular recognition, the structure and internal dynamics of the diphenyl ether–methanol complex, employing infrared, infrared-ultraviolet and microwave spectroscopy. We find that the conformer with the hydroxy group of the alcohol binding to one aromatic π cloud and being coordinated by an aromatic C–H bond of the other phenyl group is preferred. Depending on the expansion conditions in the supersonic jet, we observe a second conformer, which exhibits a hydrogen bond to the ether oxygen and is higher in energy