Disentangling the complex network of non-covalent interactions in fenchone hydrates via rotational spectroscopy and quantum chemistry

Water Arrangements upon Interaction with a Rigid Solute: Multiconfigurational Fenchone-(H2O)4-7 Hydrates

Insight into the arrangements of water molecules around solutes is important to understand how solvation proceeds and to build reliable models to describe water-solute interactions. We report the stepwise solvation of fenchone, a biogenic ketone, with 4-7 water molecules. Multiple hydrates were observed using broadband rotational spectroscopy, and the configurations of four fenchone-(H2O)4, three fenchone-(H2O)5, two fenchone-(H2O)6, and one fenchone-(H2O)7 complexes were characterized from the analysis of their rotational spectra in combination with quantum-chemical calculations. Interactions with fenchone deeply perturb water configurations compared with the pure water tetramer and pentamer. In two fenchone-(H2O)4 complexes, the water tetramer adopts completely new arrangements, and in fenchone-(H2O)5, the water pentamer is no longer close to being planar. The water hexamer interacts with fenchone as the least abundant book isomer, while the water heptamer adopts a distorted prism structure, which forms a water cube when including the fenchone oxygen in the hydrogen bonding network. Differences in hydrogen bonding networks compared with those of pure water clusters show the influence of fenchone's topology. Specifically, all observed hydrates except one show two water molecules binding to fenchone through each oxygen lone pair. The observation of several water arrangements for fenchone-(H2O)4-7 complexes highlights water adaptability and provides insight into the solvation process.

Benchmarking the anisotropy of nitroxyl radical solvation with IR spectroscopy

Two simple nitroxyl radicals, di-tert-butyl nitroxyl (DTBN) and 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) are solvated by one or two water, methanol, tert-butyl alcohol or phenol molecules. The resulting low temperature IR spectra of the vacuum-isolated microsolvates in the OH stretching range are assigned based on harmonic DFT predictions for closed shell solvent dimers and trimers and their offset from experiment, to minimise theory-guided assignment bias. Systematic conformational preferences for the first and second solvent molecule are observed, depending on the conformational rigidity of the radical. These assignments are collected into an experimental benchmark data set and used to assess the spectral predicting power of different DFT approaches. The goal is to find inexpensive computational methods which provide reliable spectral predictions for this poorly explored class of microsolvates.

Assignment of the absolute configuration of molecules that are chiral by virtue of deuterium substitution using chiral tag molecular rotational resonance spectroscopy

Chiral tag molecular rotational resonance (MRR) spectroscopy is used to assign the absolute configuration of molecules that are chiral by virtue of deuterium substitution. Interest in the improved performance of deuterated active pharmaceutical ingredients has led to the development of precision deuteration reactions. These reactions often generate enantioisotopomer reaction products that pose challenges for chiral analysis. Chiral tag rotational spectroscopy uses noncovalent derivatization of the enantioisotopomer to create the diastereomers of the 1:1 molecular complexes of the analyte and a small, chiral molecule. Assignment of the absolute configuration requires high-confidence determinations of the structures of these weakly bound complexes. A general search method, CREST, is used to identify candidate geometries. Subsequent geometry optimization using dispersion corrected density functional theory gives equilibrium geometries with sufficient accuracy to identify the isomers of the chiral tag complexes produced in the pulsed jet expansion used to introduce the sample into the MRR spectrometer. Rotational constant scaling based on the fact that the diastereomers have the same equilibrium geometry gives accurate predictions allowing identification of the homochiral and heterochiral tag complexes and, therefore, assignment of absolute configuration. The method is successfully applied to three oxygenated substrates from enantioselective Cu-catalyzed alkene transfer hydrodeuteration reaction chemistry.

Wetting vs Droplet Aggregation: A Broadband Rotational Spectroscopic Study of 3-Methylcatechol⋅⋅⋅Water Clusters

Two competing solvation pathways of 3-methylcatechol (MC), an atmospherically relevant aromatic molecule, with up to five water molecules were explored in detail by using a combination of broadband rotational spectroscopy and computational chemistry. Theoretically, two different pathways of solvation emerge: the commonly observed droplet pathway which involves preferential binding among the water molecules while the solute serves as an anchor point for the formation of a water cluster, and an unexpected wetting pathway which involves interactions between the water molecules and the aromatic face of MC, i.e., a wetting of the π-surface. Conclusive identification of the MC hydrate structures, and therefore the wetting pathway, was facilitated by rotational spectra of the parent MC hydrates and several H2 18 O and 13 C isotopologues which exhibit splittings associated with methyl internal rotation and/or water tunneling motions. Theoretical modelling and analyses offer insights into the tunneling and conversion barriers associated with the observed hydrate conformers and the nature of the non-covalent interactions involved in choosing the unusual wetting pathway.

Cooperative hydrogen bonding in thiazole⋯(H2O)2 revealed by microwave spectroscopy

Two isomers of a complex formed between thiazole and two water molecules, thi⋯(H₂O)₂, have been identified through Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The complex was generated by the co-expansion of a gas sample containing trace amounts of thiazole and water in an inert buffer gas. For each isomer, rotational constants, A₀, B₀, and C₀; centrifugal distortion constants, D_J, D_JK, d₁, and d₂; and nuclear quadrupole coupling constants, χ_aa(N) and [χ_bb(N) - χ_cc(N)], have been determined through fitting of a rotational Hamiltonian to the frequencies of observed transitions. The molecular geometry, energy, and components of the dipole moment of each isomer have been calculated using Density Functional Theory (DFT). The experimental results for four isotopologues of isomer I allow for accurate determinations of atomic coordinates of oxygen atoms by r₀ and r_s methods. Isomer II has been assigned as the carrier of an observed spectrum on the basis of very good agreement between DFT-calculated results and a set of spectroscopic parameters (including A₀, B₀, and C₀ rotational constants) determined by fitting to measured transition frequencies. Non-covalent interaction and natural bond orbital analyses reveal that two strong hydrogen bonding interactions are present within each of the identified isomers of thi⋯(H₂O)₂. The first of these binds H₂O to the nitrogen of thiazole (OH⋯N), and the second binds the two water molecules (OH⋯O). A third, weaker interaction binds the H₂O sub-unit to the hydrogen atom that is attached to C2 (for isomer I) or C4 (for isomer II) of the thiazole ring (CH⋯O).

Connections between the accuracy of rotational constants and equilibrium molecular structures

Rotational spectroscopy is the technique of choice for investigating molecular structures in the gas phase. Indeed, rotational constants are strongly connected to the geometry of the molecular system under consideration. Therefore, they are powerful tools for assessing the accuracy that quantum chemical approaches can reach in structural determinations. In this review article, it is shown how it is possible to measure the accuracy of a computed equilibrium geometry based on the comparison of rotational constants. But, it is also addressed what accuracy is required by computations for providing molecular structures and thus rotational constants that are useful to experiment. Quantum chemical methodologies for obtaining the "0.1% accuracy" for rotational constants are reviewed for systems ranging in size from small molecules to small polycyclic aromatic hydrocarbons. This accuracy for systems containing two dozen or so atoms opens the way towards future applications such as the accurate characterization of non-covalent interactions, which play a key role in several biological and technological processes.

Competition between In-Plane vs Above-Plane Configurations of Water with Aromatic Molecules: Non-Covalent Interactions in 1,4-Naphthoquinone-(H2O)1-3 Complexes

Non-covalent interactions between aromatic molecules and water are fundamental in many chemical and biological processes, and their accurate description is essential to understand molecular relative configurations. Here we present the rotational spectroscopy study of the water complexes of the polycyclic aromatic hydrocarbon 1,4-naphthoquinone (1,4-NQ). In 1,4-NQ-(H₂O)_1,2, water molecules bind through O-H···O and C-H···O hydrogen bonds and are located on the plane of 1,4-NQ. For 1,4-NQ-(H₂O)₃, in-plane and above-plane water configurations are observed exhibiting O-H···O, C-H···O, and lone pair···π-hole interactions. The observation of different water arrangements for 1,4-NQ-(H₂O)₃ allows benchmarking theoretical methods and shows that they have great difficulty in predicting energy orderings due to the strong competition of C-H···O binding with π and π-hole interactions. This study provides important insight into water interactions with aromatic systems and the challenges in their modeling.

Water binding to the atmospheric oxidation product methyl vinyl ketone

Methyl vinyl ketone is one of the major oxidation products of isoprene, and therefore, an important precursor of secondary organic aerosol. Understanding its interactions with water is relevant to gain insight into aerosol formation and improve the predictive power of atmospheric chemistry models. The molecular complex formed between methyl vinyl ketone and water has been generated in a supersonic jet and characterized using high-resolution microwave spectroscopy in combination with quantum chemistry calculations. In this study, we show that methyl vinyl ketone interacts with water forming four 1:1 isomers connected by O - H···O and C - H···O hydrogen bond interactions. Water has been found to preferentially bind to the antiperiplanar conformation of methyl vinyl ketone. Evidence of a large amplitude motion arising from the methyl internal rotation has been found in the rotational spectra of the dimer. The threefold methyl internal rotation barrier heights have been further determined and discussed for all the species.

Interactions of limonene with the water dimer

The interactions of limonene with the water dimer have been characterised through the identification of seven different isomers.

Setting up the HyDRA blind challenge for the microhydration of organic molecules

The procedure leading to the first HyDRA blind challenge for the prediction of water donor stretching vibrations in monohydrates of organic molecules is described. A training set of 10 monohydrates...

Geminal Diol Formation from the Interaction of a Ketone with Water in the Gas Phase: Structure and Reactivity of Cyclooctanone-(H2O)1,2 Clusters

The hydration of ketones is known to occur in condensed phases, but it is not considered to be favorable in the gas phase due to restricted water content. We report the first evidence of geminal diol formation upon ketone hydration in the gas phase, obtained through the investigation of the interactions of cyclooctanone with water using broadband rotational spectroscopy. Oxygen-atom exchange between water and cyclooctanone was observed for two isomers of cyclooctanone-H₂O and two isomers of cyclooctanone-(H₂O)₂. All complexes were unambiguously identified from the analysis of the rotational spectrum of the parent species and all their ¹³C and ¹⁸O isotopologues, and their heavy-atom substitution and effective structures were determined as well as their binding interactions. The production of gem-diols from gas-phase hydration of ketones has implications for atmospheric chemistry and opens a new channel for secondary aerosol formation.