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.

By 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.

Intermolecular dissociation energies of hydrogen-bonded 1-naphthol complexes.

We have measured the intermolecular dissociation energies D 0 of supersonically cooled 1-naphthol (1NpOH) complexes with solvents S = furan, thiophene, 2,5-dimethylfuran, and tetrahydrofuran. The naphthol OH forms non-classical H-bonds with the aromatic π-electrons of furan, thiophene, and 2,5-dimethylfuran and a classical H-bond with the tetrahydrofuran O atom. Using the stimulated-emission pumping resonant two-photon ionization method, the ground-state D 0(S 0) values were bracketed as 21.8 ± 0.3 kJ/mol for furan, 26.6 ± 0.6 kJ/mol for thiophene, 36.5 ± 2.3 kJ/mol for 2,5-dimethylfuran, and 37.6 ± 1.3 kJ/mol for tetrahydrofuran. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3 (using the def2-TZVPP basis set), and ωB97X-D [using the 6-311++G(d,p) basis set] predict that the H-bonded (edge) isomers are more stable than the face isomers bound by dispersion; experimentally, we only observe edge isomers. We compare the calculated and experimental D 0 values and extend the comparison to the previously measured 1NpOH complexes with cyclopropane, benzene, water, alcohols, and cyclic ethers. The dissociation energies of the nonclassically H-bonded complexes increase roughly linearly with the average polarizability of the solvent, α ¯ (S). By contrast, the D 0 values of the classically H-bonded complexes are larger, increase more rapidly at low α ¯ (S), but saturate for large α ¯ (S). The calculated D 0(S 0) values for the cyclopropane, benzene, furan, and tetrahydrofuran complexes agree with experiment to within 1 kJ/mol and those of thiophene and 2,5-dimethylfuran are ∼3 kJ/mol smaller than experiment. The B3LYP-D3 calculated D 0 values exhibit the lowest mean absolute deviation (MAD) relative to experiment (MAD = 1.7 kJ/mol), and the B97-D3 and ωB97X-D MADs are 2.2 and 2.6 kJ/mol, respectively.

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

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.

Tipping the Scales: Spectroscopic Tools for Intermolecular Energy Balances.

Intermolecular energy balances are supramolecular complexes with a nearly degenerate bistable docking structure and low barriers in between, which can be tuned by chemical substitution to prefer one or the other site. The docking preference can be probed by forming the complexes in a supersonic jet expansion and by measuring their spectroscopic signature. Linear spectroscopies are shown to be well suited for this purpose, in particular when they are assisted by more sensitive techniques and by approximate computed photon interaction cross sections. Molecular analogues of conventional beam balances, seesaw balances, and torsional balances are discussed, all based on noncovalent interactions. The discrimination of energy differences down to the sub-kJ/mol level is demonstrated. The correspondence to intramolecular torsional balances in NMR spectroscopy is outlined. Besides highlighting conformational preferences, the results of intermolecular balance experiments can serve as critical benchmarks for an accurate description of intermolecular forces and zero-point vibrational energies.

Multi-spectroscopic and theoretical analyses on the diphenyl ether-tert-butyl alcohol complex in the electronic ground and electronically excited state.

Aromatic ethers such as diphenyl ether (DPE) represent molecules with different docking sites for alcohols leading to competing OH-O and OH-π interactions. In a multi-spectroscopic approach in combination with quantum chemical calculations the complex of DPE with tert-butyl alcohol (t-BuOH) is investigated in the electronic ground state (S0) and the electronically excited state (S1). FTIR, microwave as well as mass- and isomer-selective IR/R2PI spectra are recorded, revealing co-existing OH-O and OH-π isomers in the S0 state. Surprisingly, they are predicted to be of almost equal stability in contrast to the previously investigated DPE-MeOH complex, where the OH-π structure is preferred by both theory and experiment. The tert-butyl group in t-BuOH allows for a simultaneous optimization of hydrogen-bonding and dispersion interactions, which provides a sensitive meeting point between theory and experiment. In the electronically excited state of DPE-t-BuOH, vibrational spectra could be recorded separately for both isomers using UV/IR/UV spectroscopy. In the S1 state the same structural binding motifs are obtained as in the S0 state with the OH-O bond being weakened for the OH-O arrangement and the OH-π interaction being strengthened in the case of the OH-π isomer compared to the S0 state.

Subtle solvation behaviour of a biofuel additive: the methanol complex with 2,5-dimethylfuran.

Methanol is shown to engage two nearly equivalent solvation sites in 2,5-dimethylfuran, the electron-rich π cloud and the electron-deficient oxygen site. The latter only wins by a slight margin, thanks to the methyl group undergoing secondary interactions with the ring. These secondary attractions reduce the hydrogen bond-induced OH frequency shift of the OH-O contact, whereas the π cloud allows for a combined action of both binding mechanisms in the OH-π arrangement. In total, the hydrophobic character of 2,5-dimethylfuran is well reflected in the weak pair interactions, as judged by the small solvation shifts. Methanol solvation of 2,3-benzofuran is revisited and shown to be more ambiguous than previously thought, involving competition between five- and six-ring π clouds and the oxygen site for the OH group. The six-ring π cloud is slightly preferred. FTIR spectroscopy in supersonic jets is in systematic agreement with dispersion-corrected harmonic B3LYP and also B2PLYP predictions for these competing furan docking sites. Deuteration of the OH group helps to identify the docking sites because of its attenuated zero-point energy weakening effect on localized hydrogen bonds. Extension to less methylated furans is proposed in the context of a future forecasting competition for the performance of quantum chemical methods for intermolecular interactions.

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.

To π or not to π--how does methanol dock onto anisole?

Anisole offers two similarly attractive hydrogen bond acceptor sites to an incoming hydrogen bond donor: its oxygen atom and its delocalized π electron system. Electronic structure calculations up to the CCSD(T)/AVTZ level suggest an isoenergetic situation for methanol after harmonic zero point energy correction, within less than 1 kJ mol(-1). Linear infrared absorption spectroscopy in the OH stretching fundamental range applied to a cold supersonic jet expansion of anisole and methanol in helium shows that the oxygen binding site is preferred, with about 20 times less π-bonded than O-bonded dimers despite the non-equilibrium collisional environment. Accidental band overlap is ruled out by OH overtone and OD stretching spectroscopy. Furthermore, the diagonal anharmonicity constant of the OH stretching mode is derived from experiment and reaches 80% of the monomer distortion found in the methanol dimer, as expected for a weaker hydrogen bond to the aromatically substituted oxygen. To reconcile these experimental findings with ab initio theory, accurate nuclear and electronic structure calculations involving AVQZ basis sets are required. Dispersion-corrected double-hybrid density functional theory provides a less expensive successful structural approach.