The Unexplored World of Cycloalkene–Water Complexes: Primary and Assisting Interactions Unraveled by Experimental and Computational Spectroscopy

How Nonpolar CO2 Aggregates on Cycloalkenes: A Case Study with Cyclopentene-(CO2)1-3 Clusters

This study explores the molecular clusters of cyclopentene (CPE) with one to three CO2 molecules (CPE-(CO2)1-3) through their jet-cooled rotational spectra using Fourier transform microwave spectroscopy with supplementary quantum chemical calculations. The assembly of CPE-(CO2)1-3 clusters is predominantly driven by tetrel bonding networks, notably C···π(C═C) and C···O interactions, with additional stabilization from weak C─H(CH2)···C═O hydrogen bonds. Critically, the dispersive forces play a pivotal role in stabilizing CO2 aggregation on CPE, eclipsing the effects of electrostatic and orbital interactions. This highlights the complex balance of forces that govern the formation and stabilization of these molecular clusters. Our findings offer precise insights into noncovalent interactions that could enhance atmospheric chemistry models and sustain climate science through informed environmental chemistry strategies.

Rotational Spectroscopy of the 2,2,3,3,3-Pentafluoropropanol⋅⋅⋅Water Complex: Conformations and Large Amplitude Motions

The 2,2,3,3,3-pentafluoropropanol (PFP) monomer can exist in five conformations defined by the CCCO and CCOH dihedral angles: four mirror-imaged pairs (G+g+/G-g-, G+g-/G-g+, G+t/G-t, Tg+/Tg-) and an achiral Tt form. We examined the conformational landscape of the PFP⋅⋅⋅water complex using chirped pulsed Fourier transform microwave spectroscopy and theoretical calculations. Rotational spectra of two PFP⋅⋅⋅water conformers, PFPG+g+⋅⋅⋅W_H and PFPTg+⋅⋅⋅W_H , and seven deuterated isotopologues of each, were assigned. Tunneling splittings were observed for both conformers and are attributed to the exchange of the bonded and non-bonded hydrogen atoms of water. On the other hand, the tunneling splitting associated with the OH flipping motion in PFPTg+/Tg- appears to be quenched upon hydrogen bonding with water. The large amplitude motions associated with the water subunits were examined in detail to explain the very different magnitudes of the experimental and theoretical permanent electric dipole moment components. The study highlights the challenge in correctly identifying the conformers observed when large amplitude motions are involved. Quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses, as well as electrostatic potential (ESP) calculations were carried out to explore the nature of the non-covalent interactions and to appreciate the effects of fluorination.

Conformations and structures of 1,4-pentadien-3-ol and its water complex characterized by rotational spectroscopy

The 1,4-pentadien-3-ol and its monohydrate have been characterized by microwave spectroscopy in combination with theoretical computations. Experiments and ab initio calculations revealed that the 1,4-pentadien-3-ol monomer prefers a configuration with one vinyl being syn to the hydroxyl oxygen and the hydroxyl hydrogen toward the skew arranged vinyl, which therefore makes possible simultaneous CH···O and OH···π interactions. The observed monohydrate corresponds to the global minimum predicted theoretically, which is stabilized through a primary OH···O_w hydrogen bond together with a much weaker O_wH···π hydrogen bond. The NCI analyses, NBO calculation and SAPT method were applied to further elucidate the characteristics of hydrogen bonds in the 1,4-pentadien-3-ol···water complex.

Interaction Types in C6H5(CH2)nOH-CO2 (n = 0-4) Determined by the Length of the Side Alkyl Chain

C₆H₅(CH₂)_nOH-CO₂ complexes have been investigated using rotational spectroscopy (n = 0-2) complemented by quantum chemical calculations (n = 0-4), which implies that the side alkyl chain length can determine the types of intermolecular interactions. Unlike the in-plane C···O tetrel bond in phenol-CO₂, the π*_CO2···π_aromatic interaction has been shown to link CO₂ to phenylmethanol and 2-phenylethanol, which is, to the best of our knowledge, the first time it has been demonstrated by rotational spectroscopy. Further elongations of the side alkyl chain gradually increase the energies of intramolecular hydrogen bonds in 3-phenylpropanol and 4-phenylbutanol so that CO₂ cannot break it. CO₂ will be pushed farther from the monomers and link with the -OH group through a dominating C···O tetrel bond. Our observations would allow, with the choice of the proper length of the side alkyl chain, new strategies for engineering C···π_aromatic-centered noncovalent bonding schemes for the capture, utilization, and storage of CO₂.

junChS and junChS-F12 Models: Parameter-free Efficient yet Accurate Composite Schemes for Energies and Structures of Noncovalent Complexes

A recently developed model chemistry (denoted as junChS [Alessandrini, S.; et al. J. Chem. Theory Comput. 2020, 16, 988-1006]) has been extended to the employment of explicitly correlated (F12) methods. This led us to propose a family of effective, reliable, and parameter-free schemes for the computation of accurate interaction energies of molecular complexes ruled by noncovalent interactions. A thorough benchmark based on a wide range of interactions showed that the so-called junChS-F12 model, which employs cost-effective revDSD-PBEP86-D3(BJ) reference geometries, has an improved performance with respect to its conventional counterpart and outperforms well-known model chemistries. Without employing any empirical parameter and at an affordable computational cost, junChS-F12 reaches subchemical accuracy. Accurate characterizations of molecular complexes are usually limited to energetics. To take a step forward, the conventional and F12 composite schemes developed for interaction energies have been extended to structural determinations. A benchmark study demonstrated that the most effective option is to add MP2-F12 core-valence correlation corrections to fc-CCSD(T)-F12/jun-cc-pVTZ geometries without the need of recovering the basis set superposition error and the extrapolation to the complete basis set.

Integration of theory, simulation, artificial intelligence and virtual reality: a four-pillar approach for reconciling accuracy and interpretability in computational spectroscopy

The established pillars of computational spectroscopy are theory and computer based simulations. Recently, artificial intelligence and virtual reality are becoming the third and fourth pillars of an integrated strategy for the investigation of complex phenomena. The main goal of the present contribution is the description of some new perspectives for computational spectroscopy, in the framework of a strategy in which computational methodologies at the state of the art, high-performance computing, artificial intelligence and virtual reality tools are integrated with the aim of improving research throughput and achieving goals otherwise not possible. Some of the key tools (e.g., continuous molecular perception model and virtual multifrequency spectrometer) and theoretical developments (e.g., non-periodic boundaries, joint variational-perturbative models) are shortly sketched and their application illustrated by means of representative case studies taken from recent work by the authors. Some of the results presented are already well beyond the state of the art in the field of computational spectroscopy, thereby also providing a proof of concept for other research fields.

Hydrogen bonding networks and cooperativity effects in the aqueous solvation of trimethylene oxide and sulfide rings by microwave spectroscopy and computational chemistry

The intermolecular interactions responsible for the microsolvation of the highly flexible trimethylene oxide (TMO) and trimethylene sulfide (TMS) rings with one and two water (w) molecules were investigated using rotational spectroscopy (8-22 GHz) and quantum chemical calculations. The observed patterns of transitions are consistent with the most stable geometries of the TMO-w, TMO-(w)₂, and TMS-w complexes at the B2PLYP-D3(BJ)/aug-cc-pVTZ level and were confirmed using spectra of the ¹⁸O isotopologue. Due to its effectively planar backbone, TMO offers one unique binding site for solvation, while water can bind to the puckered TMS ring in either an axial or equatorial site of the heteroatom. In all clusters, the first water molecule binds in the σ_v symmetry plane of the ring monomer and serves as a hydrogen bond donor to the heteroatom. The second water molecule is predicted to form a cooperative hydrogen bonding network between the three moieties. Secondary C-H⋯O interactions are a key stabilizing influence in trimers and also drive the preferred binding site in the TMS clusters with the axial binding site preferred in TMS-w and the equatorial form calculated to be more stable in the dihydrate. Using an energy partition scheme from the symmetry-adapted perturbation theory for the O, S, and Se containing mono- and dihydrates, the intermolecular interactions are revealed to be mainly electrostatic, but the dispersive character of the contacts is enhanced with the increasing size of the ring's heteroatom due to the key role of longer-range secondary interactions.

Looking for the Elusive Imine Tautomer of Creatinine: Different States of Aggregation Studied by Quantum Chemistry and Molecular Spectroscopy

New spectroscopic experiments and state-of-the-art quantum-chemical computations of creatinine in different aggregation states unequivocally unveiled a significant tuning of tautomeric equilibrium by the environment: from the exclusive presence of the amine tautomer in the solid state and aqueous solution to a mixture of amine and imine tautomers in the gas phase. Quantum-chemical calculations predict the amine species as the most stable tautomer by about 30 kJ mol^-1 in condensed phases. On the contrary, moving to the isolated forms, both Z and E imine isomers become more stable by about 7 kJ mol^-1 . Since the imine isomers and one amine tautomer are separated by significant energy barriers, all of them should be present in the gas phase. This prediction has indeed been confirmed by high-resolution rotational spectroscopy, which provides the first experimental characterization of the elusive imine tautomer. The interpretation of the complicated hyperfine structure of the rotational spectrum, originated by three ¹⁴ N nuclei, makes it possible to use the spectral signatures as a sort of fingerprint for each individual tautomer in the complex sample.

Unveiling Bifunctional Hydrogen Bonding with the Help of Quantum Chemistry: The Imidazole-Water Adduct as Test Case

The ubiquitous role of water and its amphiprotic nature call for a deeper insight into the physical-chemical properties of hydrogen-bonded complexes formed with building blocks of biomolecules. In this work, the semiexperimental (SE) approach combined with the template model (TM) protocol allowed the accurate determination of the equilibrium structure of two isomeric forms of the imidazole-water complex. In this procedure, the integration of experiment (thanks to a recent rotational spectroscopy investigation) and theory is exploited, also providing the means of assessing the reliability and accuracy of different quantum-chemical approaches. Overall, this study demonstrated the robustness of the combined SE-TM approach, which can provide accurate results using affordable quantum-chemical methods. Finally, the structural and energetic characteristics of these complexes have been examined in detail and compared with those of analogous heterocycle-water adducts, also exploiting energy decomposition analyses.

Hydrogen versus tetrel bonds in complexes of 3-oxetanone with water and formaldehyde

The ability and preference of 3-oxetanone to form hydrogen or tetrel bonds have been investigated in its complexes with water and formaldehyde by using Fourier transform microwave spectroscopy complemented with quantum chemical calculations. Different types of interactions and internal dynamics have been observed in the targeted complexes. With water, the ether oxygen of 3-oxetanone is the favoured interaction site forming a classical O-HO hydrogen bond. Quite differently, the carbonyl group of 3-oxetanone plays the dual role as a tetrel donor and a proton acceptor in the 3-oxetanone-formaldehyde complex, featuring the CO tetrel bond and C-HO weak hydrogen bond interactions. Splittings originated from the internal rotation of formaldehyde around its C₂ axis were also observed. The V₂ barrier was estimated to be 375(10) cm^-1 based on Meyer's one-dimensional flexible model. The changes in geometries and electronic densities upon complexation would shed light on the impact of archetype solvent and organic substrate molecules on the reactivity of 3-oxetanone.

New Insights into Secondary Organic Aerosol Formation: Water Binding to Limonene

Limonene is an abundant monoterpene in the atmosphere and one of the main precursors of secondary organic aerosol. Understanding its interactions with atmospheric molecules is crucial to explain aerosol formation and the various products obtained from competing reaction pathways. Here, using broadband rotational spectroscopy in combination with computational calculations, we show that limonene effectively interacts with water, forming a variety of complexes. Seven different isomers of limonene-H₂O, where water and limonene are connected by O-H···π and C-H···O interactions, have been unambiguously identified. Water has been found to preferentially bind to the endocyclic double bond of limonene. Our findings demonstrate a striking ability of water to attach to limonene and enrich our knowledge on the possible interactions of limonene in the atmosphere.

Rotational Spectroscopy Meets Quantum Chemistry for Analyzing Substituent Effects on Non-Covalent Interactions: The Case of the Trifluoroacetophenone-Water Complex

The most stable isomer of the 1:1 complex formed by 2,2,2-trifluoroacetophenone and water has been characterized by combining rotational spectroscopy in supersonic expansion and state-of-the-art quantum-chemical computations. In the observed isomer, water plays the double role of proton donor and acceptor, thus forming a seven-membered ring with 2,2,2-trifluoroacetophenone. Accurate intermolecular parameters featuring one classical O-H···O hydrogen bond and one weak C-H···O hydrogen bond have been determined by means of a semi-experimental approach for equilibrium structure. Furthermore, insights on the nature of the established non-covalent interactions have been unveiled by means of different bond analyses. The comparison with the analogous complex formed by acetophenone with water points out the remarkable role played by fluorine atoms in tuning non-covalent interactions.

The Role of State-of-the-Art Quantum-Chemical Calculations in Astrochemistry: Formation Route and Spectroscopy of Ethanimine as a Paradigmatic Case

The gas-phase formation and spectroscopic characteristics of ethanimine have been re-investigated as a paradigmatic case illustrating the accuracy of state-of-the-art quantum-chemical (QC) methodologies in the field of astrochemistry. According to our computations, the reaction between the amidogen, NH, and ethyl, C2H5, radicals is very fast, close to the gas-kinetics limit. Although the main reaction channel under conditions typical of the interstellar medium leads to methanimine and the methyl radical, the predicted amount of the two E,Z stereoisomers of ethanimine is around 10%. State-of-the-art QC and kinetic models lead to a [E-CH3CHNH]/[Z-CH3CHNH] ratio of ca. 1.4, slightly higher than the previous computations, but still far from the value determined from astronomical observations (ca. 3). An accurate computational characterization of the molecular structure, energetics, and spectroscopic properties of the E and Z isomers of ethanimine combined with millimeter-wave measurements up to 300 GHz, allows for predicting the rotational spectrum of both isomers up to 500 GHz, thus opening the way toward new astronomical observations.

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.

Toward Fully Unsupervised Anharmonic Computations Complementing Experiment for Robust and Reliable Assignment and Interpretation of IR and VCD Spectra from Mid-IR to NIR: The Case of 2,3-Butanediol and trans-1,2-Cyclohexanediol

The infrared (IR) and vibrational circular dichroism (VCD) spectra of 2,3-butanediol and trans-1,2-cyclohexanediol from 900 to 7500 cm^–1 (including mid-IR, fundamental CH and OH stretchings, and near-infrared regions) have been investigated by a combined experimental and computational strategy. The computational approach is rooted in density functional theory (DFT) computations of harmonic and leading anharmonic mechanical, electrical, and magnetic contributions, followed by a generalized second-order perturbative (GVPT2) evaluation of frequencies and intensities for all the above regions without introducing any ad hoc scaling factor. After proper characterization of large-amplitude motions, all resonances plaguing frequencies and intensities are taken into proper account. Comparison of experimental and simulated spectra allows unbiased assignment and interpretation of the most interesting features. The reliability of the GVPT2 approach for OH stretching fundamentals and overtones is confirmed by the remarkable agreement with a local mode model purposely tailored for the latter two regions. Together with the specific interest of the studied molecules, our results confirm that an unbiased assignment and interpretation of vibrational spectra for flexible medium-size molecules can be achieved by means of a nearly unsupervised reliable, robust, and user-friendly DFT/GVPT2 model.

Theory meets experiment for elucidating the structure and stability of non-covalent complexes: water–amine interaction as a proof of concept

A joint experimental-theoretical spectroscopic investigation has focused on a better understanding of the nature of weak, non-covalent interactions in amine-water model systems.

Unbiased Determination of Absolute Configurations by vis-à-vis Comparison of Experimental and Simulated Spectra: The Challenging Case of Diplopyrone

A new experimental-computational strategy for the determination of the absolute configuration (AC) of complex chiral molecules is proposed by combining diverse experimental spectroscopies with quantum-mechanical simulations well beyond the current computational practice. Key features are the conformer search and relative stability evaluation performed by a new stochastic two-level tool followed by a vis-à-vis comparison of experimental and computed spectra without any ad hoc adjustment. The entire computational procedure is embedded in the user-friendly VMS software, and its reliability is granted by the inclusion of mechanic/electric/magnetic anharmonicity as well as ro-vibrational and vibronic couplings by means of generalized perturbation theory in conjunction with double-hybrid functionals combined with empirical dispersion contributions and suitable basis sets. To test and validate the new approach, the puzzling case of diplopyrone, a fungal phytotoxic metabolite, has been chosen: the close match between new experimental and simulated infrared absorption and vibrational circular dichroism spectra has led to the unbiased evaluation of its AC.