Molecular structure and non-covalent interaction of 2-thiophenecarboxaldehyde and its monohydrated complex

Fluorination effects on non-covalent binding forces: A rotational study on the 2-(trifluoromethyl)acrylic acid-water complex

This study explores the effects of the -CF3 group on non-covalent interactions through a comprehensive rotational investigation of the 2-(trifluoromethyl)acrylic acid-water complex. Employing Fourier transform microwave spectroscopy complemented by quantum chemical calculations, two isomers, i.e., s-cis and s-trans structures, have been observed in the pulsed jet. Based on relative intensity measurements, the s-cis to the s-trans population ratio was experimentally estimated to be ∼ 1:1.2. Subsequently, a comparison of the non-covalent interactions was carried out between the three similar complexes, acrylic acid-water, methacrylic acid-water, and 2-(trifluoromethyl)acrylic acid-water, offering quantitative insights into fluorination affecting the strength of the formed hydrogen bonds important, e.g., in molecular recognition.

Exploring the conformational landscape, hydrogen bonding, and internal dynamics in the diallyl ether and diallyl sulfide monohydrates

The conformational spaces of the diallyl ether (DAE) and diallyl sulfide (DAS) monohydrates were explored using rotational spectroscopy from 6 to 19 GHz. Calculations at the B3LYP-D3(BJ)/aug-cc-pVTZ level suggested significant differences in their conformational behavior, with DAE-w exhibiting 22 unique conformers and DAS-w featuring three stable structures within 6 kJ mol-1. However, only transitions from the lowest energy conformer of each were experimentally observed. Spectral analysis confirmed that binding with water does not alter the conformational preference for the lowest energy structure of the monomers, but it does influence the relative stabilities of all other conformers, particularly in the case of DAE. Non-covalent interaction and quantum theory of atoms in molecules analyses showed that the observed conformer for each complex is stabilized by two intermolecular hydrogen bonds (HBs), where water primarily interacts with the central oxygen or sulfur atom of the diallyl compounds, along with secondary interactions involving the allyl groups. The nature of these interactions was further elucidated using symmetry-adapted perturbation theory, which suggests that the primary HB interaction with S in DAS is weaker and more dispersive in nature compared to the primary HB in DAE. This supports the experimental observation of a tunneling splitting exclusively in the rotational spectrum of DAS-w, as the weaker contact allows water to undergo internal motions within the complex, as shown based on calculated transition state structures for possible tunneling pathways.

Noncovalent interactions of aromatic heterocycles: rotational spectroscopy and theoretical calculations of the thiazole-CF4 and thiazole-SF6 complexes

The complexes of thiazole with CF4 and SF6 have been investigated by Fourier transform microwave spectroscopy and quantum chemical calculations. One rotational spectrum was observed for the thiazole-CF4 complex. Experiments and theoretical computations confirmed that the observed structure of thiazole-CF4 is primarily formed due to N⋯CCF4 interaction with the C atom of CF4 located in the plane of the thiazole ring. The rotational transitions of thiazole-CF4 exhibit A/E torsional splitting induced by the internal rotation of the -CF3 top. The potential barrier of the -CF3 internal rotation is 0.2411(1) kJ mol-1, consistent with the calculated value (∼0.3 kJ mol-1). For the thiazole-SF6 complex, one conformer with SF6 located above the thiazole ring is detected. The observed structure of thiazole-SF6 is mainly stabilized by van der Waals interactions. The energy decomposition analysis reveals that the electrostatics and dispersion are the dominant attractive contributions to the formation of thiazole-CF4 and thiazole-SF6 dimers, whereas the weight of the dispersion term becomes more significant in the thiazole-SF6 complex compared to that of the thiazole-CF4 complex.

Three non-bonding interaction topologies of the thiazole-formaldehyde complex observed by rotational spectroscopy

When an aldehyde molecule interacts with a nitrogen atom inserted in an aromatic ring, they form a number of non-bonding topologies. We measured the rotational spectra of three different isomers of the thiazole-formaldehyde adduct. In all of them, formaldehyde interacts specifically with thiazole through an n → π* interaction (along the Bürgi-Dunitz trajectory) and a C-H⋯O (acting as a proton acceptor) weak hydrogen bond, or through C-H⋯N (acting as a proton donor) and C-H⋯O (acting as a proton acceptor) weak hydrogen bonds. The spectra of isotopic substituted species were also measured to draw the molecular structures. Two n → π* stabilized isomers show a vertical structure in which the two molecular planes are perpendicular to each other, and the hydrogen bonded isomers feature a co-planar architecture. The competition between these non-bonding interactions was unveiled from experiments and theoretical calculations.

Equilibrium Structure in the Presence of Methyl Internal Rotation: Microwave Spectroscopy and Quantum Chemistry Study of the Two Conformers of 2-Acetylfuran

For 2-acetylfuran, quantum chemistry predicted and proton magnetic resonance study reported two conformers, anti and syn, differing in the position of the carbonyl group with respect to the O1-C2 bond of the furan ring. The microwave spectrum of the title molecule was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer, confirming the presence of both conformers. Spectroscopic parameters such as the rotational and centrifugal distortion constants could be determined with high precision. The spectra of all ¹³C- and ¹⁸O-isotopologues of the energetically more favorable anti-conformer could be assigned, allowing the experimental determination of bond lengths and bond angles from the heavy atom substitution r_s and the semi-experimental equilibrium r_e^SE structures. Splittings arising from the internal rotation of the acetyl methyl group could be resolved for both conformers as well as for all assigned isotopologues, from which the barrier to methyl internal rotation was determined. The torsional barrier is largely invariant at around 319 cm^-1 in the parent species of anti-2-acetylfuran and its isotopologues, showing that though isotopic substitution greatly influences the rotational properties of the molecule and causes a different microwave spectrum, its effect on the methyl torsion is negligible. On the other hand, conformational effects play a decisive role, as the torsional barrier of 239.780(13) cm^-1 found for syn-2-acetylfuran differs significantly from the value for anti-2-acetylfuran. The results are compared and discussed with other methyl-substituted furan derivatives and acetyl group containing ketones for a better understanding of different effects influencing molecular geometry parameters and methyl internal rotations.

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.

Unveiling the structural and energetic properties of thiazole-water complex by microwave spectroscopy and theoretical calculations

The structure and non-covalent bonding features of the complex of thiazole and water were studied by using supersonic jet Fourier transform microwave spectroscopy and theoretical calculations. One isomer was observed which corresponds to the global minimum of the complex predicted theoretically. The rotational spectra of 9 additional isotopologues, including 5 mono-substituted heavy atoms of thiazole (³⁴S, ¹³C and ¹⁵N), and 4 water isotopic species (H₂¹⁸O, DOH, HOD and D₂O), were also measured and analyzed. The experimental spectroscopic parameters were used to determine the structural parameters of the observed isomer. Theoretical analyses based on quantum theory of atoms in molecules and natural bond orbital revealed that the two moieties are linked by a N···H-O hydrogen bond. The total interaction energy of the complex is calculated to be -25.1 kJmol^-1 with electrostatics being the major term according to energy decomposition analysis.

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.