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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. [ABSTRACT FROM AUTHOR]

The rotational spectrum of 2-thiophenecarboxaldehyde was investigated by using supersonic jet Fourier transform microwave spectroscopy. The measurements were extended to the 34S, 33S, 13C, and 18O isotopologs for the cis conformer, as well as to the 34S and 13C isotopologs for the trans conformer, leading to an accurately structural determination of the two observed conformers. The unchanged experimental Pcc values upon isotopic substitution indicate effective planar geometries of the two conformers. The ring structures of thiophene are slightly different between the cis and trans conformers. Two isomers of the monohydrated complex of 2-thiophenecarboxaldehyde, formed between a cis or trans monomer with water stabilized by an O—H⋯O hydrogen bond (HB) and an additional (C=O)CH⋯O(H2O) or (Cring)CH⋯O(H2O) HB, respectively, were observed in jet expansion. The noncovalent interactions attributed to the stabilization of the monomer and the monohydrated complex are revealed by quantum chemical methods. The interaction energy for trans-w-1 is 4 kJ mol−1 larger than that of cis-w-1, attributed to the relative stronger CH⋯O HB. The relative abundance of the two conformers of the 2-thiophenecarboxaldehyde monomer and the two isomers of the complex was estimated in the jet. [ABSTRACT FROM AUTHOR]

The disulfide‐centered hydrogen bonds in the three different model systems of diethyl disulfide⋅⋅⋅H2O/H2CO/HCONH2 clusters were characterized by high‐resolution Fourier transform microwave spectroscopy and quantum chemical computations. The global minimum energy structures for each cluster are experimentally observed and are characterized by one of the three different S−S⋅⋅⋅H−C/N/O disulfide‐centered hydrogen bonds and two O⋅⋅⋅H−C hydrogen bonds. Non‐covalent interaction and natural bond orbital analyses further confirm the experimental observations. The symmetry‐adapted perturbation theory (SAPT) analysis reveals that electrostatic is dominant in diethyl disulfide⋅⋅⋅H2O/HCONH2 clusters being consistent with normal hydrogen bonds, whilst dispersion takes over in diethyl disulfide⋅⋅⋅H2CO cluster. Our study gives accurate structural parameters for the disulfide bond involved non‐covalent clusters providing important benchmarking data for the theoretical evaluation of more complex systems. [ABSTRACT FROM AUTHOR]

The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, OH⋅⋅⋅Cl ( RH⋅⋅⋅Cl=2.638(2) Å) and CH⋅⋅⋅O ( RH⋅⋅⋅O=2.501(2) Å), forming a five-membered ring. All transitions display the hyperfine structure due to the 35Cl (or 37Cl) nuclear quadrupole effects. Dynamical features in the spectrum are caused by two large-amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V2=320(10) cm−1. Finally, an unexpected small value of the inertial defect (−0.96 uÅ2 rather than −3.22 uÅ2) allowed the estimation of the barrier to the internal rotation of the CH3 group, V3≈8 cm−1. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two isomers of the 2-(trifluoromethyl)acrylic acid–water complex are observed by rotational spectroscopy complemented by quantum chemical calculations. • The structures, populations, and non-covalent interactions of the two isomers are determined. • The fluorination effect is proved to only enhance the energy of O-H···O w hydrogen bond by compared the energies of the corresponding hydrogen bonds formed in similar complexes. This study explores the effects of the −CF 3 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. [ABSTRACT FROM AUTHOR]

1,1,1,2-Tetrafluoroethane dimer was investigated by pulsed jet Fourier transform microwave spectroscopy. One conformer, stabilized through a network of four C-H···F-C interactions, was observed, although several almost isoenergetic configurations were suggested by ab initio calculations. The measurements, extended to four 13C species in natural abundance, allowed determination of the carbon skeleton structures and evaluation of the weak hydrogen bond parameters. Information on the dissociation energy is also provided. [ABSTRACT FROM AUTHOR]

Formic acid and dimethyl ether are combined in a supersonic expansion to form a molecular adduct with the two subunits held together by a "classical" OH···O hydrogen bond and a bifurcated weak CH2···O hydrogen bond. The rotational spectra of the parent and of two 13C isotopologues in natural abundance show that the complex has Cs symmetry, with the heavy atom symmetry planes of HCOOH and (CH3)2O perpendicular to each other. [ABSTRACT FROM AUTHOR]

Abstract: The rotational supersonic-jet Fourier transform microwave (FT-MW) spectra of the aromatic 2,6-difluorophenol (DFP) have been recorded and analyzed in the frequency range of 8–26GHz. A new design of broadband FT-MW spectroscopy with in-phase/quadrature-phase-modulation passage-acquired-coherence technique (IMPACT) has been used to obtain the spectra rapidly at high-resolution, obsoleting subsequent measurements with a resonator spectrometer. The structural analysis shows that the H-atom of the hydroxyl-group is located in the benzene plane forming an intramolecular hydrogen bond with one of the F-atoms. Due to the tunneling motion of this H-atom between two the equivalent positions, a large part of the rotational transitions are split into two signals being separated by ∼17MHz. The rotational constants, centrifugal distortion constants and the Coriolis coupling constant have been determined. [Copyright &y& Elsevier]

Abstract: The Fourier transform rotational spectra of the normal and the 13C species of the 1:1 complex between difluoromethane and water have been measured. The rotational transitions are split into two component lines, due to the internal rotation of the water moiety. The barrier to this motion has been estimated to be 340(20)cm−1. An improved structure of the complex has been obtained. [Copyright &y& Elsevier]

Rotational spectra of 2-methoxypyridine and its monohydrate have been characterized by pulsed jet Fourier transform microwave spectroscopy and complementary ab initio calculations. Rotational spectra of the parent monomer and seven mono-substituted isotopologues (13C and 15N) were measured in natural abundance, which allow determining the accurate structure of the skeleton of 2-methyoxypyridine. The barrier to the methyl internal rotation was determined from the A / E torsional symmetry species of the rotational transitions. For the 2-methyoxypyridine⋯H 2 O complex, rotational spectra of the parent dimer and isotopologues formed with isotopically enriched water (H 2 18O, HOD, DOH and D 2 O) indicate that the observed isomer is stabilized by a strong O H⋯N and a secondary bifurcated (C H) 2 ⋯O weak hydrogen bonds. Bader's quantum theory of atoms in molecules and Johnson's non-covalent interaction analyses were applied and visualized to have a better understanding of the non-covalent interactions in 2-methyoxypyridine⋯H 2 O. Unlabelled Image • The cis conformer of 2-methoxypyridine has been observed in the pulsed jet. • The barrier to the methyl internal rotation was determined from the A / E tunneling splittings. • Observed isomer of the complex is stabilized by a strong O H∙∙∙N HB and a secondary bifurcated (C H) 2 ∙∙∙O weak HB [ABSTRACT FROM AUTHOR]

The rotational spectrum of the acetophenone–CO 2 complex was investigated by pulsed jet-expansion Fourier transform microwave spectroscopy combined with ab initio calculations. In the supersonically cooled jet, only one isomer has been observed which is characterized by a dominant (CO 2)C···O tetrel bond and a secondary (C-H) methyl ∙∙∙O weak hydrogen bond. Johnson's non-covalent interaction, Bader's quantum theory of atoms in molecules and Symmetry-Adapted Perturbation Theory analyses have been applied to understand better the nature of non-covalent interactions in the acetophenone-CO 2 complex. Unlabelled Image • The rotational spectrum of acetophenone···CO 2 was analyzed. • The complex is characterized by a dominant C···O tetrel bond and a secondary C-H∙∙∙O weak hydrogen bond. • Theoretical calculations are still facing severe challenges in accurately describing tetrel bonded complexes. [ABSTRACT FROM AUTHOR]