Rotational Spectroscopy Probes Water Flipping by Full Fluorination of Benzene

Stepwise hydrations of anhydride tuned by hydrogen bonds: rotational study on maleic anhydride-(H2O)1-3

The rotational spectra of maleic anhydride-(H₂O)_1-3 have been investigated for the first time by using pulsed jet Fourier transform microwave spectroscopy with complementary computational analyses. The experimental evidence points out that water tends to self-aggregate with hydrogen bonds and form homodromic cycles. Differences in bond lengths and charge distribution between the two carbonyl sites have been observed upon stepwise hydrations, which might further introduce a selectivity on the nucleophilic attack sites of hydrolysis. This study provides an important insight into the incipient solvation process (microsolvation) of maleic anhydride in water by understanding the cooperation and rearrangement of intermolecular hydrogen bonds in its stepwise hydrates.

σ-Hole activation and structural changes upon perfluorination of aryl halides: direct evidence from gas phase rotational spectroscopy

Enhancement of the σ-hole on the halogen atom of aryl halides due to perfluorination of the ring is demonstrated by use of the Extended Townes-Dailey (ETD) model coupled to a Natural Atomic Orbital Bond analysis on two perfluorinated aryl halides (C₆F₅Cl and C₆F₅Br) and their hydrogenated counterparts. The ETD analysis, which quantifies the halogen p-orbitals populations, relies on the nuclear quadrupole coupling constants which in this work are accurately determined experimentally from the rotational spectra. The rotational spectra investigated by Fourier-transform microwave spectroscopy performed in supersonic expansion are reported for the parent species of C₆F₅Cl and C₆F₅Br and their ¹³C, ³⁷Cl or ⁸¹Br substituted isotopologues observed in natural abundance. The experimentally determined rotational constants combined with theoretical data at the MP2/aug-cc-pVTZ level provide precise structural information from which an elongation of the ring along its symmetry axis due to perfluorination is proved.

Macrolactonization Reactions Driven by a Pentafluorobenzoyl Group*

Macrolactones constitute a privileged class of natural and synthetic products with a broad range of applications in the fine chemicals and pharmaceutical industry. Despite all the progress made towards their synthesis, notably from seco-acids, a macrolactonization promoter system that is effective, selective, flexible, readily available, and, insofar as possible, compatible with manifold functional groups is still lacking. Herein, we describe a strategy that relies on the formation of a mixed anhydride incorporating a pentafluorophenyl group which, due to its high electronic activation enables a convenient access to macrolactones, macrodiolides and esters with a broad versatility. Kinetic studies and DFT computations were performed to rationalize the reactivity of the pentafluorophenyl group in macrolactonization reactions.

Do Docking Sites Persist Upon Fluorination? The Diadamantyl Ether-Aromatics Challenge for Rotational Spectroscopy and Theory

Fluorinated derivatives of biological molecules have proven to be highly efficient at modifying the biological activity of a given protein through changes in the stability and the kind of docking interactions. These interactions can be hindered or facilitated based on the hydrophilic/hydrophobic character of a particular protein region. Diadamantyl ether (C₂₀H₃₀O) possesses both kinds of docking sites, serving as a good template to model these important contacts with aromatic fluorinated counterparts. In this work, an experimental study on the structures of several complexes between diadamantyl ether and benzene as well as a series of fluorinated benzenes is reported to analyze the effect of H→F substitution on the interaction and structure of the resulting molecular clusters using rotational spectroscopy. All experimentally observed complexes are largely dominated by London dispersion interactions with the hydrogen‐terminated surface areas of diadamantyl ether. Already single substitution of one hydrogen atom with fluorine changes the preferred docking site of the complexes. However, the overall contributions of the different intermolecular interactions are similar for the different complexes, contrary to previous studies focusing on the difference in interactions using fluorinated and non‐fluorinated molecules.

Analysis of thyme essential oils using gas-phase broadband rotational spectroscopy

A semi-quantitative analysis of the components of two natural essential oils has been carried out using broadband rotational spectroscopy, which is inherently molecule specific. The samples under study were two thyme essential oils from Spain with different compositions: (a) with thymol as the most abundant species (thyme I) and (b) with linalool and 4-carvomenthenol being the most abundant ones (thyme II). Relative intensity measurements of selected rotational transitions were carried out to estimate the abundances of the different species present in these complex mixtures, taking into account the square of the respective dipole moment components. One strength of rotational spectroscopy is its structure sensitivity. Here, we also re-investigated the microwave spectrum of linalool and determined the accurate experimental gas-phase structures of thymol and linalool through the assignment of all 13C isotopologues of their lowest energy conformers. A characteristic splitting pattern of the rotational transitions due to internal rotation of two non-equivalent methyl groups of linalool was observed in the thyme II spectrum. Their internal rotation barriers were experimentally determined to 4.7703(96) kJ mol-1 and 9.2581(74) kJ mol-1, respectively.

Progressive Hydrophobicity of Fluorobenzenes

The potentials of mean force for the dimers of fluorobenzenes sample both π-stacked and T-shaped structures for partially fluorinated benzenes, namely, 1,4-difluorobenzene, 1,3,5-trifluorobenzene, and 1,2,4,5-tetrafluorobenzene, and sample only the T-shaped structures for benzene and hexafluorobenzene. While the free energy for the dimerization in water is very weakly dependent on the number of fluorine atoms, the formation of π-stacked structures is entropy-driven and the T-shaped structures appear due to an enthalpic minimum. Interestingly, the solvation behavior suggests that the accumulation of water around the contact and solvent-separated pairs decreases with the increase in the number of fluorine atoms, which signifies progressive hydrophobicity of fluorobenzenes.

Water Docking Bias in [4]Helicene

We report on the one- and two-water clusters of [4]helicene, the smallest polycyclic aromatic hydrocarbon with a helical sense, which were captured in the gas phase using high-resolution rotational spectroscopy. The structures of the complexes are unambiguously revealed using microwave spectra of isotopically enriched species. In the one-water cluster, the apparent splitting pattern is consistent with a tunneling motion that encompasses an exchange of strongly and weakly bonded water hydrogens. This motion is "locked" in the two-water cluster. The relevant intermolecular contacts, symmetry, and aromaticity effects are unveiled for the microsolvated chiral topologies. These observations entail the first glance at the structures and internal dynamics of the water binding motifs of a chiral polycyclic aromatic hydrocarbon.