Water-Water and Water-Solute Interactions in Microsolvated Organic Complexes

Structural and Electronic Evolution of Ethanolamine upon Microhydration: Insights from Hyperfine Resolved Rotational Spectroscopy

Ethanolamine hydrates containing from one to seven water molecules were identified via rotational spectroscopy with the aid of accurate quantum chemical methods considering anharmonic vibrational corrections. Ethanolamine undergoes significant conformational changes upon hydration to form energetically favorable hydrogen bond networks. The final structures strongly resemble the pure (H2O)3-9 complexes reported before when replacing two water molecules by ethanolamine. The 14N nuclear quadrupole coupling constants of all the ethanolamine hydrates have been determined and show a remarkable correlation with the strength of hydrogen bonds involving the amino group. After addition of the seventh water molecule, both hydrogen atoms of the amino group actively contribute to hydrogen bond formation, reinforcing the network and introducing approximately 21-27 % ionicity towards the formation of protonated amine. These findings highlight the critical role of microhydration in altering the electronic environment of ethanolamine, enhancing our understanding of amine hydration dynamics.

Electric nuclear quadrupole coupling reveals dissociation of HCl with a few water molecules

Investigating the dissociation of acids in the presence of a limited number of water molecules is crucial for understanding various elementary chemical processes. In our study, focusing on HCl(H2O)n clusters (where HCl is hydrogen chloride and H2O is water) formed in a cold and isolated jet expansion, we used the nuclear quadrupole coupling tensor obtained through rotational spectroscopy to decipher the nature of the hydrogen-chlorine (H-Cl) chemical bond in a microaqueous environment. For n = 1 to 4, the H-Cl bond is covalent. At n = 5 and 7, the contact ion pair of H3O+Cl- is spontaneously formed within the hydrogen bond networks of book and cube acid-water clusters, respectively.

Understanding the n → π* non-covalent interaction using different experimental and theoretical approaches

Herein, a perspective on the recent understanding of weak n → π* interaction obtained using different experimental and theoretical approaches is presented. This interaction is purely an orbital interaction that involves the delocalization of the lone pair electrons (n) on nitrogen, oxygen, and sulfur to the π* orbitals of CO, CN, and aromatic rings. The n → π* interaction has been found to profoundly influence the stabilization of peptides, proteins, drugs, and various small molecules. Although the functional properties of this non-covalent interaction are still quite underestimated, there are recent demonstrations of applying this interaction to the regulation of synthetic chemistry, catalysis, and molecular recognition. However, the identification and quantification of the n → π* interaction remain a demanding task as this interaction is quite weak and based on the electron delocalization between the two orbitals, while hyperconjugation interactions between neighboring atoms and the group involved in the n → π* interaction are simultaneously present. This review provides a comprehensive picture of understanding the n → π* interaction using different experimental approaches such as the X-ray diffraction technique, and electronic, NMR, microwave, and IR spectroscopy, in addition to quantum chemistry calculations. A detailed understanding of the n → π* interaction can help in modulating the strength of this interaction, which will be further helpful in designing efficient drugs, synthetic peptides, peptidomimetics, etc.

Gas-phase hydration of nopinone: the interplay between theoretical methods and experiments unveils the conformational landscape

The structure of microsolvated nopinone formed in the supersonic jet expansion is investigated in the gas phase. The rotational spectra of nopinone(H₂O)_n (n = 1, 2, 3) were analysed by means of Fourier transform microwave spectroscopy. In the present study, three monohydrates, two dihydrates and two trihydrates were observed and characterized. The observed structures are the lowest energy conformers predicted by quantum chemical calculations. In all the observed hydrates of nopinone, water was found to be linked to the ketone group (C[double bond, length as m-dash]O) with a strong hydrogen bond (O_NOPH_W) and finishing with a dispersive one (O_WH_NOP). The structure of nopinone was found to alter the structure of water dimer and water trimer, which make nopinone be surrounded with a chain of water molecules. A remarkable decrease in the H-bonding length was observed when the number of attached water molecules is increased. Different DFT and ab initio calculations at the equilibrium structure allowed the identification of the observed conformers. Evaluation of the B3LYP-D3 and ωB97X-D results revealed deficiencies in reproducing the structure of one observed monohydrated structure while MP2 and M06-2X reproduce all the three observed structures. A comparison with similar bicyclic ketones highlights how a small change in the bicyclic ring leads to different effects in the microsolvation of biogenic VOCs. This study presents the first step of molecular aggregation to understand the atmospheric formation of aerosols at the molecular scale.

Unlocking the Water Trimer Loop: Isotopic Study of Benzophenone-(H2 O)1-3 Clusters with Rotational Spectroscopy

Examined here are the structures of complexes of benzophenone microsolvated with up to three water molecules by using broadband rotational spectroscopy and the cold conditions of a molecular jet. The analysis shows that the water molecules dock sideways on benzophenone for the water monomer and dimer moieties, and they move above one of the aromatic rings when the water cluster grows to the trimer. The rotational spectra shows that the water trimer moiety in the complex adopts an open‐loop arrangement. Ab initio calculations face a dilemma of identifying the global minimum between the open loop and the closed loop, which is only solved when zero‐point vibrational energy correction is applied. An OH⋅⋅⋅π bond and a Bürgi‐Dunitz interaction between benzophenone and the water trimer are present in the cluster. This work shows the subtle balance between water–water and water–solute interactions when the solute molecule offers several different anchor sites for water molecules.

London Dispersion and Hydrogen-Bonding Interactions in Bulky Molecules: The Case of Diadamantyl Ether Complexes

Diadamantyl ether (DAE, C20 H30 O) represents a good model to study the interplay between London dispersion and hydrogen-bond interactions. By using broadband rotational spectroscopy, an accurate experimental structure of the diadamantyl ether monomer is obtained and its aggregates with water and a variety of aliphatic alcohols of increasing size are analyzed. In the monomer, C-H⋅⋅⋅H-C London dispersion attractions between the two adamantyl subunits further stabilize its structure. Water and the alcohol partners bind to diadamantyl ether through hydrogen bonding and non-covalent Owater/alcohol ⋅⋅⋅H-CDAE and C-Halcohol ⋅⋅⋅H-CDAE interactions. Electrostatic contributions drive the stabilization of all the complexes, whereas London dispersion interactions become more pronounced with increasing size of the alcohol. Complexes with dominant dispersion contributions are significantly higher in energy and were not observed in the experiment. The results presented herein shed light on the first steps of microsolvation and aggregation of molecular complexes with London dispersion energy donor (DED) groups and the kind of interactions that control them.

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.

A Direct Link from the Gas to the Condensed Phase: A Rotational Spectroscopic Study of 2,2,2-Trifluoroethanol Trimers

Rotational spectra of the three most stable conformers (I, II, III) of the ternary 2,2,2-trifluoroethanol (TFE) cluster were measured using Fourier transform microwave spectrometers, and unambiguously assigned with the aid of ab initio calculations. The most stable conformer, I, contains one trans-TFE subunit which is unstable in its isolated gas phase form. The study offers new insights into how the trans conformation is stabilized in TFE clusters of increasing size, and eventually becomes a dominant conformation in the liquid phase. A detailed analysis shows that while O-H⋅⋅⋅O-H and O-H⋅⋅⋅F-C hydrogen bonds are the most significant attractive interactions which stabilize all three conformers, the main driving force for the stability of I over III, which has all gauche-TFE subunits, is the attractive interaction of C-F⋅⋅⋅F-C contact pairs. A new type of three-point F⋅⋅⋅F⋅⋅⋅F attractive contact interaction is also identified.

The Clusters-in-a-Liquid Approach for Solvation: New Insights from the Conformer Specific Gas Phase Spectroscopy and Vibrational Optical Activity Spectroscopy

Vibrational optical activity spectroscopies, namely vibrational circular dichroism (VCD) and Raman optical activity (ROA), have been emerged in the past decade as powerful spectroscopic tools for stereochemical information of a wide range of chiral compounds in solution directly. More recently, their applications in unveiling solvent effects, especially those associated with water solvent, have been explored. In this review article, we first select a few examples to demonstrate the unique sensitivity of VCD spectral signatures to both bulk solvent effects and explicit hydrogen-bonding interactions in solution. Second, we discuss the induced solvent chirality, or chiral transfer, VCD spectral features observed in the water bending band region in detail. From these chirality transfer spectral data, the related conformer specific gas phase spectroscopic studies of small chiral hydration clusters, and the associated matrix isolation VCD experiments of hydrogen-bonded complexes in cold rare gas matrices, a general picture of solvation in aqueous solution emerges. In such an aqueous solution, some small chiral hydration clusters, rather than the chiral solutes themselves, are the dominant species and are the ones that contribute mainly to the experimentally observed VCD features. We then review a series of VCD studies of amino acids and their derivatives in aqueous solution under different pHs to emphasize the importance of the inclusion of the bulk solvent effects. These experimental data and the associated theoretical analyses are the foundation for the proposed "clusters-in-a-liquid" approach to account for solvent effects effectively. We present several approaches to identify and build such representative chiral hydration clusters. Recent studies which applied molecular dynamics simulations and the subsequent snapshot averaging approach to generate the ROA, VCD, electronic CD, and optical rotatory dispersion spectra are also reviewed. Challenges associated with the molecular dynamics snapshot approach are discussed and the successes of the seemingly random "ad hoc explicit solvation" reported before are also explained. To further test and improve the "clusters-in-a-liquid" model in practice, future work in terms of conformer specific gas phase spectroscopy of sequential solvation of a chiral solute, matrix isolation VCD measurements of small chiral hydration clusters, and more sophisticated models for the bulk solvent effects would be highly valuable.