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).

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.

How does the composition of a PAH influence its microsolvation? A rotational spectroscopy study of the phenanthrene–water and phenanthridine–water clusters

The influence of a nitrogen atom in the backbone of a PAH was revealed by the hydrated clusters of phenanthrene and phenanthridine in a rotational spectroscopy study. Background image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) – ESA/Hubble Collaboration.

The challenge of non-covalent interactions: theory meets experiment for reconciling accuracy and interpretation

In the past decade, many gas-phase spectroscopic investigations have focused on the understanding of the nature of weak interactions in model systems. Despite the fact that non-covalent interactions play a key role in several biological and technological processes, their characterization and interpretation are still far from being satisfactory. In this connection, integrated experimental and computational investigations can play an invaluable role. Indeed, a number of different issues relevant to unraveling the properties of bulk or solvated systems can be addressed from experimental investigations on molecular complexes. Focusing on the interaction of biological model systems with solvent molecules (e.g., water), since the hydration of the biomolecules controls their structure and mechanism of action, the study of the molecular properties of hydrated systems containing a limited number of water molecules (microsolvation) is the basis for understanding the solvation process and how structure and reactivity vary from gas phase to solution. Although hydrogen bonding is probably the most widespread interaction in nature, other emerging classes, such as halogen, chalcogen and pnicogen interactions, have attracted much attention because of the role they play in different fields. Their understanding requires, first of all, the characterization of the directionality, strength, and nature of such interactions as well as a comprehensive analysis of their competition with other non-covalent bonds. In this review, it is shown how state-of-the-art quantum-chemical computations combined with rotational spectroscopy allow for fully characterizing intermolecular interactions taking place in molecular complexes from both structural and energetic points of view. The transition from bi-molecular complex to microsolvation and then to condensed phase is shortly addressed.