Probing the molecular structure of radical pairs has remained challenging because of their short lifetimes. Here we use the persistent nitroxyl radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) as model system, reporting the rotational characterization of its radical pairs in a supersonic molecular jet. The experiment employs isotopic-sensitive high-resolution microwave spectroscopy, revealing two distinct neutral pairing patterns avoiding electron transfer. In each radical pair the nitroxyl molecules establish non-covalent van der Waals interactions with N···O distances of 3.38 and 4.93 Å, respectively, as confirmed by 18O-labeling experiments. The absence of electron spin hyperfine effects is consistent with quantum mechanical density functional predictions favoring an unexpected open-shell singlet. These results starkly contrast with predictions for the dimethyl nitroxyl radical dimer, which undergoes a spontaneous hydrogen transfer to closed-shell species and confirms the kinetically hindered or frustrated character of the TEMPO dimer. The experiment thus provides a direct molecular-level picture of neutral radical pairs in isolation and open avenues for exploring the quantum structure and reactivity of correlated radical species.

The combined use of laboratory rotational spectroscopy and radio astronomical observations remains the most effective approach for identifying molecules in the interstellar medium (ISM). Following the recent detections of several polycyclic aromatic hydrocarbons (PAHs) and their cyano derivatives in the dense Taurus Molecular Cloud (TMC-1), it is reasonable to extend such searches to other PAHs within the same source. In this work, we report a rotational spectroscopy study of commercially available fluoranthene (FA) and its synthesized cyano derivative, 3-cyanofluoranthene (3-CNFA), using chirped-pulse Fourier-transform microwave spectroscopy. The analysis of the rotational spectra, supported by quantum chemical calculations, yielded molecular parameters for the parent species of both FA and 3-CNFA molecules. The experimental data of 3-CNFA were later used for its astronomical search in TMC-1 with the QUIJOTE line survey but proved unsuccessful. Despite the nondetection of 3-CNFA in this source, the upper limit to its abundance was established. The experimental data will support future astronomical searches in the ISM.

The detection of nitrile-containing species in the interstellar medium has highlighted the role of cyano derivatives as proxies for the corresponding non-polar hydrocarbons. Among them, cinnamonitrile can be regarded as a styrene analogue with enhanced polarity, and, thus, as a promising astrochemical candidate. Here, we report the first characterization of its rotational spectrum, combining quantum-chemical calculations and the so-called "Lego-brick" approach with broadband measurements in the 2-18 GHz range. Both E-and Z-cinnamonitrile have been identified in the spectrum, and accurate rotational, centrifugal distortion, and nitrogen quadrupole coupling constants have been obtained. Moreover, 15N-and 13C-containing isotopologues of the most stable E form have been observed in natural abundance and enabled the determination of a (partial) semi-experimental equilibrium structure. Special care was devoted to the treatment of the lowest-frequency torsional mode, whose anharmonicity required a tailored strategy in order to achieve a structure consistent with theoretical predictions. Our results provide both a reliable spectroscopic foundation for astronomical searches of cinnamonitrile and new insights into the structural flexibility of styrene derivatives which will be further explored in future works.

We report the discovery of the 3- and 4-cyano isomers of acenaphthylene (C12H8) using the QUIJOTE line survey of the Taurus molecular cloud 1 (TMC-1). The frequency range of the line survey in the Q band (31.2–50.3 GHz) was complemented with data from a new receiver installed at the Yebes 40m radio telescope that covers the K and Ka bands (18–32 GHz). The identification of 3-cyanoacenaphthylene was based on the detection of 135 individual lines that correspond to 194 rotational transitions. For 4-cyanoacenaphthylene, the lines are slightly weaker, but we identified 37 individual lines that correspond to 66 rotational transitions. As a first step for the identification, we compared the derived rotational constants with their expected theoretical values and obtained a match better than 0.2%. Finally, the new isomers were synthesized in our chemical laboratory and observed in our microwave spectroscopic facilities. The derived laboratory constants match those derived from TMC-1 data perfectly, which confirms our identifications. Lines in the K − Ka band from the previously detected isomers 1- and 5-cyanoacenaphthylene are also reported. The derived column density for 3-cyanoacenaphthylene is (7.0 ± 0.7) × 1011 cm−2, and for the 4-cyano isomer, we derive a column density of (5.0 ± 0.6) × 1011 cm−2. The four possible isomers of cyanoacenaphthylene are finally found in TMC-1. Despite their high relative energies, they exhibit similar abundances (within a factor of ∼2).

Abstract The structural preferences of molecular assemblies are governed by a delicate balance between strong directional forces and diffuse dispersion contacts. Mixed trimers of pyrrole (Py) and benzene (Bz) provide an ideal benchmark to probe this interplay: the robust N-H⋯π interaction anchoring the Py-Bz dimer competes with the drive toward compact, dispersion-stabilized arrangements in larger clusters. Here, we report the first high-resolution structural characterization of the Py-(Bz)2 and (Py)2-Bz trimers and the (Py)2-(Bz)2 tetramer using chirped-pulse Fourier transform microwave spectroscopy combined with dispersion-corrected DFT calculations and intensity-based cross-correlation analysis. The results show that while N-H⋯π and C-H⋯π interactions serve as primary anchors, the overall geometries are dictated by space-filling principles that maximize dispersion contacts. These findings establish pyrrole-benzene hetero clusters as a rigorous benchmark for theoretical methods and provide fundamental insight into the forces guiding aromatic aggregation and self-assembly in complex molecular systems.

We used jet-cooled broadband rotational spectroscopy and quantum mechanical calculations to study the potential energy surface, molecular structure and intra-and intermolecular interactions of the biarylic thienyl pyridines of 2-(2-thienyl)pyridine and 2-(2-pyridyl)benzothiophene and their monohydrates. Two isomers of the bare molecules were identified in the gas phase, characterized by planar structures and zusammen (Z) or entgegen (E) orientations around the ring junction. A single Z-isomer was observed for both monohydrates, primary stabilized by a hydroxyl-to-nitrogen (O-H center dot center dot center dot N) hydrogen bond and secondary C-H center dot center dot center dot O interactions. The computational study included D3 dispersion-corrected hybrid (B3LYP) and double hybrid (B2PLYP) density functional methods, with additional calculations at the RI-MP2 and DLPNO-CCSD(T) levels. NBO calculations examined the donor-acceptor hyperconjugative effects involving the nitrogen and sulfur atoms, suggesting that their participation in the larger stability of the Z form is not decisive and may involve other intramolecular interactions. In particular, examination of the electronic density shifts (EDS) further suggests that non-covalent N center dot center dot center dot S chalcogen interactions partially contribute to the preference for the Z conformation.

Sulfur-containing heterocycles play a prominent role in biomedical research because of their wide range of pharmacological activities. In this work we report on oltipraz, a synthetic 1,2-dithiole-3-thione derivative recognized for its chemo and hepatoprotective effects. The investigation has addressed the stereochemistry, molecular structure and intramolecular interactions of the title compound using a combination of chirped-pulsed Fourier transform microwave spectroscopy and quantum chemical calculations. The parent and three 34S monosubstituted isotopologues were assigned in the rotational spectrum, confirming the presence of a single synclinal conformation in the gas phase. Density functional theory calculations (B3LYP-D3(BJ), ωB97X-D and B2PLYP-D3(BJ)) supplemented the experiment. The origin of the conformational preferences was further investigated using the Natural Bond Orbital (NBO) and Non-Covalent Interaction (NCI) analyses. Both approaches suggest a limited contribution of the N···S intramolecular interaction in stabilizing the observed molecular conformation.

We examined the competition between pi-stacking and hydrogen bonding in the phenolthiophenol heterodimer, generated in a jet-cooled expansion. The experiment used chirped-pulse broadband microwave spectroscopy, operating over the full 2-8 GHz cm-wave frequency region. Two different isomers were observed for the cluster, both stabilised by a combination of pi-stacking interactions and the alcohol-to-thiol (O-HS) hydrogen bond. The two isomers only differ in the parallel (cis) or antiparallel (trans) relative orientation of the thiol and alcohol groups. The trans isomer was confirmed as global minimum by observation of conformational relaxation in the jet with different carrier gases. The experimental results were complemented with electronic structure calculations using density-functional theory. Dispersion forces represent the primary contributor to the interaction energy, as established by energy decomposition using symmetry-adapted perturbation theory. The results provide insight into the sulfur-centered hydrogen bonds and the different intermolecular forces in the related homodimers of phenol and thiophenol, suggesting an anticorrelation between the hydrogen bond strength and the presence of pi-stacking.

The observation of gas-phase water clusters has been instrumental in understanding water aggregation and cooperativity, paving the way for solvation models in the bulk. However, the characterization of hydrogen sulfide self-aggregation is still largely unexplored. Here, we investigate two mixed pentamers of hydrogen sulfide and water to examine the influence of the weaker, dispersion-based and less directional interactions caused by hydrogen sulfide. Unprecedented structural resolution was obtained by combination of jet-cooled broadband rotational spectroscopy and quantum-chemical calculations. Specifically, we compare the 4:1 and 1:4 hydrogen sulfide - water pentamers, offering comparison with the prototype homoclusters. Important structural differences are revealed in the hydrogen sulfide clusters, which reorganize to compensate for the weaker sulfur-centered hydrogen bonds. The noncovalent interactions in the pentamers were rationalized using density functional theory and reduced electronic density calculations. Moreover, a comprehensive many-body decomposition energy analysis revealed significant variations in molecule two- and three-body contributions to the total interaction energy based on the relative proportions of H2O and H2S. These findings offer new insights into the distinct cooperative forces in water and hydrogen sulfide clusters. The results will improve our understanding and modeling of sulfur-centered hydrogen bonds, which may be useful across various research fields, including protein folding, molecular aggregation, materials science, and computational benchmarking.

Characterizing the interactions between water and volatile anesthetics at a molecular level is crucial for understanding their mechanisms of action. We employed broadband molecular rotational spectroscopy (CP-FTMW) and extensive isotopic substitution experiments to generate and characterize the stepwise addition of up to four water molecules to the volatile anesthetic sevoflurane, a flexible molecule with multiple binding sites. The substantial amount of isotopic data enabled the conclusive derivation of accurate structural information. The observed structures contain the most stable conformer of the previously identified monomer, with water clusters favorably interacting with the molecule to form an open chain with up to three water molecules. Notably, two isomers were detected for the tetrahydrate, which exhibit a cyclic structure with either a clockwise or anticlockwise orientation, resembling that of the pure water tetramer. The four-water marks a transition where water-water interactions dominate over direct sevoflurane-water interactions driving the assembly of the water network.

Understanding the interactions of chiral molecules with water is crucial, given the central role that water plays in chemical and biological processes. We report the investigation of the amino alcohol prolinol, a widely used chiral catalyst and auxiliary in asymmetric synthesis, and its interactions with one to three water molecules by applying broadband rotational spectroscopy. Bare prolinol adopts two low-energy conformations stabilized by an intramolecular O-H···N hydrogen bond. Upon complexation with a single water molecule, four prolinol-H2O isomers are identified, showing addition and insertion structures, where the original prolinol conformations are conserved. Notably, complexation with two and three water molecules induces prolinol to adopt its highest energy conformations, which lie more than 9.5 kJ mol-1 above the global minimum and feature an intramolecular N-H···O hydrogen bond. In prolinol-(H2O)2,3, water acts as a conformational switch for prolinol, binding to both the amino and hydroxyl groups. Combined NMR studies and molecular dynamics simulations reveal that, in bulk water, prolinol exists as a highly flexible conformational ensemble, with no evidence of a stable intramolecular hydrogen bond, and mainly samples the same conformational space as that displayed in prolinol-(H2O)2,3. Our results illustrate how stepwise hydration proceeds and reveal the profound changes that water can induce in flexible chiral molecules. These findings provide a solid foundation for future experiments and modeling of solvation-induced processes.

We present the discovery of the unsubstituted polycyclic aromatic hydrocarbon (PAH) phenalene (C13H10) in TMC-1 as part of the QUIJOTE line survey. In spite of the low dipole moment of this three-ring PAH, we managed to identify a total of 267 rotational transitions with quantum numbers J and K-a up to 34 and 14, respectively, corresponding to 71 independent frequencies. The identification of this new PAH from our survey was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. Our subsequent chemical synthesis of this PAH and investigation of its laboratory microwave spectrum unequivocally support our identification. We report the column density of phenalene in TMC-1 as (2.8 +/- 1.6) x 10(13) cm(-2).

The recent interstellar detection of individual polycyclic aromatic hydrocarbons (PAHs) in the Taurus Molecular Cloud (TMC-1) brings with it interest in related species that could be present in this astronomical environment. The interstellar PAHs detected in TMC-1 consist of a few pure PAHs while the majority that have been detected are their cyano-derivative counterparts due to their larger dipole moment components. Bowl-shaped PAHs, such as sumanene (C21H12), represent another important target for radio astronomy as they are very polar species, in spite of their high symmetry, increasing their chances of detection. Here, we present the laboratory rotational spectroscopic study of the PAH sumanene, characterized in the gas-phase using a chirped-pulse Fourier-transform microwave spectrometer operating between 2 and 8 GHz. Accurate spectroscopic parameters are derived from the spectral analysis and compared to those obtained for corannulene. These parameters have been employed to achieve reliable frequency predictions for their astronomical search in TMC-1. We do not detect either sumanene or corannulene in our QUIJOTE line survey of TMC-1 but upper limits to their abundance in this source are derived.

The interactions between water and aromatic rings are pervasive across various scientific and technological disciplines, including biochemistry, materials science, and environmental chemistry. In this study, we combine broadband rotational spectroscopy and quantum-chemical calculations to reveal detailed structural and binding motifs in the aggregation of benzene, as the prototypical aromatic molecule, in the presence of a few water molecules. The benzene dimer and trimer structures with up to two water molecules are conclusively identified through isotopic substitution. We observe that the π-stacking interactions are substituted by more favorable CH···π contacts, allowing the insertion of water molecules acting as bridges between aromatic rings. This induces a shortening of the O···O distances for the complexes with two water molecules compared to that of the isolated water dimer. A many-body decomposition analysis of the interaction energy reveals the interactions of water with the aromatic partners through three-body contributions. While in the prototypical hydrogen-bonded pure water clusters this contribution amounts to 20-25% of the total interaction energy, we observe a significant contribution on the order of 10% in the interactions with the benzene rings. These results experimentally rationalize the binding strength of π-systems with water.

Durch Wasser hervorgerufene Strukturveränderungen spielen eine zentrale Rolle in der Chemie und Biologie, sind aber auf molekularer Ebene nach wie vor schwer vorherzusagen, zu messen und zu kontrollieren. Hier untersuchen wir in der Gasphase die größenabhängige Wasseraggregation am flexiblen Molekül 4‐Hydroxy‐2‐butanon und modellieren die Konformationsanpassungsfähigkeit flexibler Substrate an Wirtswassergerüste, sowie die Präferenzen für sequentielles Tröpfchenwachstum. Das Experiment wurde mittels breitbandiger Rotationsspektroskopie durchgeführt und mit quantenchemischen Berechnungen nachvollzogen. Experimentell wurden vom Di‐ bis zum Pentahydrat (4‐Hydroxy‐2‐butanon‐(H2O) n=2–5 ) zwei verschiedene Isomere beobachtet, einschließlich der 18 O‐Isotopologen für die Di‐ und Trihydrate. Um Wassermoleküle effektiv unterzubringen, verändert sich interessanterweise das schwere Atomgerüst von 4‐Hydroxy‐2‐butanon in jedem beobachteten Isomer und entspricht nicht der stabilen Konformation des freien Monomers. Alle Solvate gehen von der Alkoholgruppe (Protonendonor) aus, behalten aber die Carbonylgruppe als sekundären Bindungspunkt bei. Die Wassergerüste ähneln stark denen in reinen Wasserclustern und balancieren zwischen der Fähigkeit von 4‐Hydroxy‐2‐butanon, die Orientierung und Position der Wassermoleküle zu steuern, und der Fähigkeit von Wasser, die Konformation des Monomers zu modulieren. Die vorliegende Arbeit liefert somit eine genaue molekulare Beschreibung, wie sich torsionsflexible Moleküle im Verlauf der fortschreitenden Solvatation dynamisch an Wasser anpassen.

The interplay between laboratory rotational spectroscopy and radio astronomical observations provides the most effective procedure for identifying molecules in the interstellar medium (ISM). Following the recent interstellar detections of several Polycyclic Aromatic Hydrocarbons (PAHs) and cyano derivatives in the dense molecular cloud TMC-1, it is reasonable to consider searching for other cyano-PAHs in this astronomical source. We present a rotational spectroscopy investigation of the two cyano derivatives of the PAH biphenylene, a plausible reaction product of interstellar benzyne. The rotational spectrum provided molecular parameters for the parent species and 14 monosubstituted isotopologues for each isomer. An accurate equilibrium structure was determined for both isomers using Watson's mass-dependence method (r(m)((2))), offering information on its uncommon ring union. Astronomical searches for the cyanobiphenylene isomers have been undertaken in TMC-1, using the QUIJOTE line survey. No lines of any isomer were found in this astronomical source, but the experimental data will serve to enable future searches for these species in the ISM.

Structural changes induced by water play a pivotal role in chemistry and biology but remain challenging to predict, measure, and control at molecular level. Here we explore size-governed gas-phase water aggregation in the flexible molecule 4-hydroxy-2-butanone, modeling the conformational adaptability of flexible substrates to host water scaffolds and the preference for sequential droplet growth. The experiment was conducted using broadband rotational spectroscopy, rationalized with quantum chemical calculations. Two different isomers were observed experimentally from the di- to the pentahydrates (4-hydroxy-2-butanone-(H2O)n=2-5), including the 18O isotopologues for the di- and trihydrates. Interestingly, to accommodate water molecules effectively, the heavy atom skeleton of 4-hydroxy-2-butanone reshapes in every observed isomer and does not correspond to the stable conformer of the free monomer. All solvates initiate from the alcohol group (proton donor) but retain the carbonyl group as secondary binding point. The water scaffolds closely resemble those found in the pure water clusters, balancing between the capability of 4-hydroxy-2-butanone for steering the orientation and position of the water molecules and the ability of water to modulate the monomer's conformation. The present work thus provides an accurate molecular description on how torsionally flexible molecules dynamically adapt to water along progressing solvation.

Chiral molecules with low enantiomer interconversion barriers racemize even at cryogenic temperatures due to quantum tunneling, forming a racemic mixture that is impossible to separate using conventional chemical methods. Here we both experimentally and theoretically demonstrate a method to create and probe a state-specific enantiomeric enrichment for such molecular systems. The coherent, non-linear, and resonant approach is based on a microwave six-wave mixing scheme and consists of five phase-controlled microwave pulses. The first three pulses induce a chiral wavepacket in a chosen rotational state, while the consecutive two pulses induce a polarization for a particular rotational transition (listen transition) with a magnitude proportional to the enantiomeric excess created. The experiments are performed with the transiently chiral molecule benzyl alcohol, where a chiral molecular response was successfully obtained. This signal demonstrates that enantiomeric excess can be induced in a quantum racemic mixture of a transiently chiral molecule using the developed microwave six-wave mixing scheme, which is an important step towards controlling non-rigid chiral molecular systems.

Most biomolecular activity takes place in aqueous environments, and it is strongly influenced by the surrounding water molecules. The hydrogen bond networks that these water molecules form are likewise influenced by their interactions with the solutes, and thus, it is crucial to understand this reciprocal process. Glycoaldehyde (Gly), often considered the smallest sugar, represents a good template to explore the steps of solvation and determine how the organic molecule shapes the structure and hydrogen bond network of the solvating water cluster. Here, we report a broadband rotational spectroscopy study on the stepwise hydration of Gly with up to six water molecules. We reveal the preferred hydrogen bond networks formed when water molecules start to form three-dimensional (3D) topologies around an organic molecule. We observe that water self-aggregation prevails even in these early stages of microsolvation. These hydrogen bond networks manifest themselves through the insertion of the small sugar monomer in the pure water cluster in a way in which the oxygen atom framework and hydrogen bond network resemble those of the smallest three-dimensional pure water clusters. Of particular interest is the identification, in both the pentahydrate and hexahydrate, of the previously observed prismatic pure water heptamer motif. Our results show that some specific hydrogen bond networks are preferred and survive the solvation of a small organic molecule, mimicking those of pure water clusters. A many-body decomposition analysis of the interaction energy is also performed to rationalize the strength of a particular hydrogen bond, and it successfully confirms the experimental findings.

π-Stacking is a common descriptor for face-to-face attractive forces between aromatic hydrocarbons. However, the physical origin of this interaction remains debatable. Here we examined π-stacking in a model homodimer formed by two thiol-substituted naphthalene rings. Two isomers of the 2-naphthalenethiol dimer were discovered using rotational spectroscopy, sharing a parallel-displaced crossed orientation and absence of thiol-thiol hydrogen bonds. One of the isomers presents C2 symmetry, structurally analogous to the global minimum of the naphthalene dimer. The experimental data were rationalized with molecular orbital calculations, revealing a shallow potential energy surface. Noncovalent interactions are dominated by dispersion forces according to SAPT energy decomposition. In addition, the reduced electronic density shows a diffuse and extended region of inter-ring interactions, compatible with the description of π-stacking as a competition between dispersion and Pauli repulsion forces.