Susana Blanco

Predicting site selectivity during the earliest stages of solvation in multifunctional organic molecules remains a complex challenge in chemical physics. In this work, we use high-resolution rotational spectroscopy and quantum-chemical calculations to decode the hydration landscape of N,N-diethylacetyloxyamine (DEAcA), a molecule featuring competing sp3 nitrogen and carbonyl oxygen acceptors. While the isolated monomer preferentially adopts a syn configuration driven by the electric dipole moment minimization and the synergistic alleviation of oxygen–oxygen electrostatic repulsion, microsolvation ((H2O)n, n = 1–3) reveals an exclusive preference for the highly basic nitrogen site. We demonstrate that the 14N nuclear quadrupole coupling (NQC) constants provide direct experimental evidence of the cooperative polarization that strengthens the nascent water chain. Crucially, at the tetrahydrated level (n = 4), we identify a sharp transition in the solvation regime: the global minimum shifts to a cyclic water tetramer anchored at the carbonyl oxygen. This structural pivot marks the boundary where localized stereoelectronic control yields to the collective stabilization of the solvent network. These findings provide a rigorous benchmark for modeling the transition from molecular recognition to bulk-like solvation in complex organic systems.

Aza-crown ethers are common building blocks of functional materials with ionophoric properties. The tuning of ion binding selectivity demands a careful control of the three-dimensional conformation of the macrocycle moiety and its solvation environment. In this study, broadband high-resolution microwave rotational spectroscopy is employed to assess the most stable configurations of the macrocycle 1-aza-12-crown-4 and of some of its hydrated clusters with up to three water molecules, generated in a supersonic jet. Nine rotamers are identified, which allows us to lay out a chart of hydration pathways that are consistent with the sequential incorporation of water molecules to the most stable conformations of the bare macrocycle during the early stages of the jet expansion. The preference of water binding on the amine moiety over the ether groups is observed in all the detected hydrates, highlighting the broad impact of N-substitution on the chemical behavior of macrocycles. In contrast to analogous non-substituted crown ethers, the aza-crown ether retains its conformation upon hydration, aided by a preorganized intramolecular N–H⋯O hydrogen bond that facilitates microsolvation without structural rearrangement.

The complex between pyrazole and carbon dioxide has been generated in a supersonic jet and characterized using Fourier transform microwave spectroscopy and state of the art CCSD(T) theoretical calculations. The complex presents a planar configuration showing a simultaneous N⋯C=O n → π* tetrel bond and a NH⋯O hydrogen bond. The TS internal rotation barrier that interconverts the oxygen atoms of CO2 has been calculated to be 10 kJ mol−1 at the CCSD(T) level. The electronic characteristics of the minimum and TS have been analyzed. Machine learning methods have been applied to predict the potential energy surface of the pyrazole⋯carbon dioxide complex.

Sotolon is a chiral furanone derivative featuring three distinct oxygen atoms at carbonyl, hydroxyl, and cyclic ether groups that can serve as hydrogen-bond acceptor sites, making it an ideal model system for probing water’s preferential interactions with competing functional groups. In this study, the rotational spectrum of sotolon and its microsolvated complexes, representing the early stages of hydration, was investigated using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. The conformational landscape of sotolon is dominated by a single conformer stabilized by an intramolecular O–H···O=C hydrogen bond. During hydration, water molecules disrupt this interaction by forming closed hydrogen-bonded cycles, resulting in mono- and dihydrated complexes. High-level theoretical calculations underscore the central role of electrostatic interactions in stabilizing these hydrated structures. Furthermore, A/E splittings observed in the rotational spectrum, arising from the internal rotation of one of sotolon’s methyl groups, provide insight into how hydration modulates the methyl internal rotation barrier.

The structure of the ferrocene‐water ((Cp) ₂ Fe:H ₂ O) complex has been characterized by combining state of the art rotational spectroscopy and computational chemistry. Two structures have been retrieved experimentally where water interacts with two distinguished binding sites, the exo Cp π cloud and the planetary‐like planar orbit around iron. The spectra of both conformers exhibit significant averaging effects due to the nearly free rotational dynamics of water. This is in good agreement with the exploration of the potential energy surface of (Cp) ₂ Fe:H ₂ O. For water interacting with the exo Cp π cloud, we found three axial conformers giving rise to twenty minima with small internal rotation barriers. Theoretical calculations reveal two minima for the observed equatorial complex. Water interacts with ferrocene quite strongly, exhibiting binding energies above −11 kJ/mol in all binding sites and contributing to small deformations in the structure of bare ferrocene.

Microsolvation of the carbamate moiety delivers precise information on complexation effects on the N–C=O backbone and is of relevance to the peptide bond functionality. In this context, the mono-, di-, and trihydrated complexes of methyl carbamate have been studied in molecular expansion by high-resolution microwave spectroscopy, using chirped-pulse and Fabry–Perot resonator Fourier transform microwave instruments covering the frequency range from 2 to 18 GHz. From the rotational constants of the parent and the 18Ow substituted monoisotopologues, accurate values have been derived for the geometries of the hydrogen bond interactions. The nuclear quadrupole coupling constant χcc of the nitrogen nucleus provides a direct measure of complexation changes and decreases with the degree of hydration, whereas the hindered internal rotation barrier increases slightly with microsolvation. Both tendencies could have a common origin in the π-cooperative inductive effects as the microsolvation series progresses. All transitions are split by the internal rotation of the methyl top and the nuclear quadrupole coupling, and in the largest cluster, they are additionally split by an inversion motion.

The structure of isolated 1‐chloronaphthalene has been investigated in a supersonic expansion by high‐resolution chirped‐pulse Fourier transform microwave (CP‐FTMW) spectroscopy in the 2–8 GHz frequency range. Accurate values of the rotational, centrifugal distortion, and nuclear quadrupole coupling constants for the only availabe conformer have been determined. The intensity of the spectrum allowed us to observe all the heavy atoms isotopologues in natural abundance, determining their rotational constants. From the extensive experimental dataset we derived accurate structures for 1‐chloronaphthalene using different methodologies and compared with related compounds.

We present a study of salicylic acid and its hydrates, with up to four water molecules, done by employing chirped-pulse Fourier transform microwave spectroscopy. We employed the spectral data set of the parent, ¹³C, and ²H isotopologues to determine the molecular structure and characterize the intra- and intermolecular interactions of salicylic acid and its monohydrate. Complementary theoretical calculations were done to support the analysis of the experimental results. For the monomer, we analyzed structural properties, such as the angular-group-induced bond alternation (AGIBA) effect. In the microsolvates, we analyzed their main structural features dominated by the interaction of water with the carboxylic acid group. This work contributes to seeding information on how water molecules accumulate around this group. Moreover, we discussed the role of cooperative effects further stabilizing the observed inter- and intramolecular hydrogen bond interactions.