Rotational Spectroscopy of 2-Furoic Acid and Its Dimer: Conformational Distribution and Double Proton Tunneling

Structural and tunneling properties of the 2-furoic acid (FA) monomer and dimer were investigated using rotational spectroscopy and DFT calculations. CREST, a conformational ensemble space exploration tool, was used to identify all possible low-energy conformations of the FA monomer and dimer, followed by the DFT geometry optimization and harmonic frequency calculations. Broadband rotational spectra in the 2–6 and 8–12 GHz regions were recorded in a supersonic jet expansion. The monomeric FA was found to exist dominantly as three different conformers: I , II , and III in a jet, with I and II taking on the cis-COOH configuration while III having the trans-COOH configuration. For the FA dimer, only the I – II conformer was observed experimentally, whereas the symmetric I – I and II – II conformers were not observed because of their zero dipole moments. The analysis of the splittings in the rotational transitions of I – II allowed one to extract the tunneling splitting to be 1056.0(12) MHz. The barrier height was determined to be ∼442 cm⁻¹ using the scaled potential energy scans at several different levels of theory.     

The Hydrogen Bond and Beyond: Perspectives for Rotational Investigations of Non-Covalent Interactions

In the last decade, experiment and theory have expanded our vision of non-covalent interactions (NCIs), shifting the focus from the conventional hydrogen bond to new bridging interactions involving a variety of weak donor/acceptor partners. Whereas most experimental data originate from condensed phases, the introduction of broadband (chirped-pulse) microwave fast-passage techniques has revolutionized the field of rotational spectroscopy, offering unexplored avenues for high-resolution studies in the gas phase. We present an outlook of hot topics for rotational investigations on isolated intermolecular clusters generated in supersonic jet expansions. Rotational spectra offer very detailed structural data, easily discriminating the isomeric or isotopic composition and effectively cancelling any solvent, crystal, or matrix bias. The direct comparison with quantum mechanical predictions provides insight into the origin of the inter- and intramolecular interactions with much greater precision than any other spectroscopic technique, simultaneously serving as test-bed for fine-tuning of theoretical methods. We present recent examples of rotational investigations around three topics: oligomer formation, chiral recognition, and identification of halogen, chalcogen, pnicogen, or tetrel bonds. The selected examples illustrate the benefits of rotational spectroscopy for the structural and energetic assessment of inter-/intramolecular interactions, which may help to move from fundamental research to applications in supramolecular chemistry and crystal engineering.     

Geometry of an Isolated Dimer of Imidazole Characterised by Rotational Spectroscopy and Ab Initio Calculations

An isolated, gas‐phase dimer of imidazole is generated through laser vaporisation of a solid rod containing a 1:1 mixture of imidazole and copper in the presence of an argon buffer gas undergoing supersonic expansion. The complex is characterised through broadband rotational spectroscopy and is shown to have a twisted, hydrogen‐bonded geometry. Calculations at the CCSD(T)(F12*)/cc‐pVDZ‐F12 level of theory confirm this to be the lowest‐energy conformer of the imidazole dimer. The distance between the respective centres of mass of the imidazole monomer subunits is determined to be 5.2751(1) Å, and the twist angle γ describing rotation of one monomer with respect to the other about a line connecting the centres of mass of the monomers is determined to be 87.9(4)°. Four out of six intermolecular parameters in the model geometry are precisely determined from the experimental rotational constants and are consistent with results calculated ab initio.     

Hydrogen Bonding in the Dimer and Monohydrate of 2-Adamantanol: A Test Case for Dispersion-Corrected Density Functional Methods

Weakly-bound intermolecular clusters constitute reductionist physical models for non-covalent interactions. Here we report the observation of the monomer, the dimer and the monohydrate of 2-adamantanol, a secondary alcohol with a bulky ten-carbon aliphatic skeleton. The molecular species were generated in a supersonic jet expansion and characterized using broadband chirped-pulse microwave spectroscopy in the 2–8 GHz frequency region. Two different gauche-gauche O-H···O hydrogen-bonded isomers were observed for the dimer of 2-adamantanol, while a single isomer was observed for the monomer and the monohydrate. The experimental rotational parameters were compared with molecular orbital calculations using density functional theory (B3LYP-D3(BJ), B2PLYP-D3(BJ), CAM-B3LYP-D3(BJ), ωB97XD), additionally providing energetic and electron density characterization. The shallow potential energy surface makes the dimer an interesting case study to benchmark dispersion-corrected computational methods and conformational search procedures.     

双色双共振-多光子电离光谱研究小分子的碰撞诱导转动能量转移

用双色双共振多光子电离光谱方法测量了NO分子A~(2∑+)(v=0)态的转动能量转移, 得到了由R-F能量转移导致的转动可分辨的弛豫光谱, 计算了转动态-态转移速率常数。用以转移能量为基础的指数和幂指数能隙模型, 对碰撞弛豫态分布进行计算机模拟, 并从计算值与实验值的比较讨论了能隙模型存在的不足。用同法对I_2分子B∏(O_u~+)态的测量, 得到由转动能量转移导致的谱线展宽及交叠并作了分析。     

Accurate Geometry and Non-Covalent Interactions in 1-Phenylethanol and its Monohydrate: A Rotational Study

The pure rotational spectra of 1-phenylethanol and its monohydrate were measured by using a pulsed jet Fourier transform microwave spectrometer. One conformer of the 1-phenylethanol monomer with the trans form was observed in the pulsed jet. The experimental values of rotational constants of ten isotopologues, including eight mono-substituted ¹³C and one D isotopologues, allow an accurate structure determination of the skeleton of 1-phenylethanol. For its monohydrate, only one isomer has been observed, of which 1-phenylethanol adopts the trans form and binds with water through an O−H⋅⋅⋅O_w and an O_w−H⋅⋅⋅π hydrogen bond. Each rotational transition displays a doublet with a relative intensity ratio of 1 : 3, due to a hindered internal rotation of water around its C₂ axis. This study provides the information on accurate geometry of 1-phenylethanol (PE) and large amplitude motion of water in the PE monohydrate.     

The Effect of Dispersion on the Structure of Diphenyl Ether Aggregates

Dispersion interactions can play an important role in understanding unusual binding behaviors. This is illustrated by a systematic study of the structural preferences of diphenyl ether (DPE)–alcohol aggregates, for which OH???O‐bound or OH???π‐bound isomers can be formed. The investigation was performed through a multi‐spectroscopic approach including IR/UV and microwave methods, combined with a detailed theoretical analysis. The resulting solvent‐size‐dependent trend for the structural preference turns out to be counter‐intuitive: the hydrogen‐bonded OH???O structures become more stable for larger alcohols, which are expected to be stronger dispersion energy donors and thus should prefer an OH???π arrangement. Dispersion interactions in combination with the twisting of the ether upon solvent aggregation are key for understanding this preference.     

Interactions between Freons: A Rotational Study of CH2F2–CH2Cl2

The rotational spectra of two isotopologues of a 1:1 difluoromethane–dichloromethane complex have been investigated by pulsed‐jet Fourier‐transform microwave spectroscopy. The assigned (most stable) isomer has C_s symmetry and it displays a network of two C? H???Cl? C and one C? H???F? C weak hydrogen bonds, thus suggesting that the former interactions are stronger. The hyperfine structures owing to ³⁵Cl (or ³⁷Cl) quadrupolar effects have been fully resolved, thus leading to an accurate determination of the three diagonal (χ_gg; g=a, b, c) and the three mixed quadrupole coupling constants (χ_gg′; g, g′=a, b, c; g≠g′). Information on the structural parameters of the hydrogen bonds has been obtained. The dissociation energy of the complex has been estimated to be 7.6 kJ mol^?1.     

Infrared Spectroscopy Evidence of Weak Interactions in Frustrated Lewis Pairs Formed by Tris(pentafluorophenyl)borane

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Since their discovery, many works have been investigating how these Lewis pairs (intermolecular pairs) are held together in an encounter complex. This prompted several studies based on theoretical investigations, but experimental ones are limited yet. In this communication we show evidence of weak intermolecular interactions between Lewis acids and Lewis bases, distinguishing the Lewis adduct from FLPs, by probing fluorine-carbon vibrational modes using infrared spectroscopy. The main evidence is based on the band shifts occurring in FLPs due to weak hydrogen bonds between the hydrogen atoms of the Lewis base and the fluorine atoms of Lewis acid.     

In Silico Design of Cylindrophanes: The Role of Functional Groups in a Fluoride Selective Host

Molecular recognition is the key driver in the formation of supramolecular complexes, enabling the selective encapsulation of specific guests. Here, we explore the delicate balance between different energetic terms in the formation of an efficient host for fluoride anions based on a cylindrophane structure, which can be achieved by the incorporation of ligand sites into a cyanuric acid based cyclophane framework, resulting a close proximity between the ammonium hydrogens and the anion. This study describes the character and contribution of different energetic and repulsive terms that favor the efficient inclusion of fluoride. Our findings are useful for further rational design and synthesis of efficient and highly selective fluoride hosts, which have been generally less well described than complexing agents for other halides.     

Chloromethane–Water Adduct: Rotational Spectrum,Weak Hydrogen Bonds,and Internal Dynamics

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 (R_H???Cl=2.638(2) Å) and C? H???O (R_H???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 uŲ rather than ?3.22 uŲ) allowed the estimation of the barrier to the internal rotation of the CH₃ group, V₃≈8 cm^?1.     

Seven Conformations of the Macrocycle Cyclododecanone Unveiled by Microwave Spectroscopy

The physicochemical properties and reactivity of macrocycles are critically shaped by their conformations. In this work, we have identified seven conformations of the macrocyclic ketone cyclododecanone using chirped-pulse Fourier transform microwave spectroscopy in combination with ab initio and density functional theory calculations. Cyclododecanone is strongly biased towards adopting a square configuration of the heavy atom framework featuring three C–C bonds per side. The substitution and effective structures of this conformation have been determined through the observation of its ¹³C isotopologues. The minimisation of transannular interactions and, to a lesser extent, HCCH eclipsed configurations drive conformational preferences. Our results contribute to a better understanding of the intrinsic forces mediating structural choices in macrocycles.     

The Shapes of Sulfonamides: A Rotational Spectroscopy Study

Benzenesulfonamides are a class of molecules of extreme interest in the biochemical field because many of them are active against a variety of diseases. In this work, the pharmacophoric group benzensulfonamide, its derivatives para-toluensulfonamide and ortho-toluensulfonamide, and the bioactive molecule sulfanilamide, were investigated using rotational spectroscopy to determine their conformations and the influence of different substituents on their structures. For all species, the hyperfine structure due to the ¹⁴N atom was analyzed, and this provided crucial information for the unambiguous identification of the observed conformation of all molecules. In addition, for ortho-toluensulfonamide, the vibration–rotation hyperfine structure related to the methyl torsion was analyzed, and the methyl group rotation barrier was determined. For benzensulfonamide, partial r_S and r₀ structures were established from the experimental rotational constants of the parent and two deuterated isotopic species. In all compounds except ortho-toluensulfonamide, the amino group of the sulfonamide group lies perpendicular to the benzene plane with the aminic hydrogens eclipsing the oxygen atoms. In ortho-toluensulfonamide, where weak attractive interactions occur between the nitrogen lone pair and the methyl hydrogen atoms, the amino group lies in a gauche orientation, retaining the eclipsed configuration with respect to the SO₂ frame. A comparison of the geometrical arrangements found in the PDB database allowed us to understand that the bioactive conformations are different from those found in isolated conditions. The conformations within the receptor are reached with an energy cost, which is balanced by the interactions established in the receptor.     

From Molecular to Cluster Properties: Rotational Spectroscopy of 2-Aminopyridine and of Its Biomimetic Cluster with Water

We report the observation and analysis of the rotational spectrum of a 1:1 cluster between 2-aminopyridine and water (AMW) carried out with supersonic expansion Fourier transform microwave spectroscopy at 4.7–16.5 GHz. Measurements of the 2-aminopyridine monomer (AMP) were also extended up to 333 GHz for the room-temperature rotational spectrum and to resolved hyperfine splitting resulting from the presence of two ¹⁴N quadrupolar nuclei. Supersonic expansion measurements for both AMP and AMW were also carried out for two synthesized isotopic species with single deuteration on the phenyl ring. Nuclear quadrupole hyperfine structure has also been resolved for AMW and the derived splitting constants were used as an aid in structural analysis. The structure of the AMW cluster was determined from the three sets of available rotational constants and the hydrogen bonding configuration is compared with those for clusters with water of similarly sized single-ring molecules. Experimental results aided by quantum chemistry computations allow the conclusion that the water molecule is unusually strongly bound by two hydrogen bonds, OH^...N and O^...HN, to the NCNH atomic chain of AMP with the potential to replace hydrogen bonds to the identical structural segment in cytosine and adenine in CT and AT nucleic acid base pairs.     

Frontiers in Rotational Spectroscopy: Shapes and Tunneling Dynamics of the Four Conformers of the Acrylic Acid—Difluoroacetic Acid Adduct

The rotational spectra of four conformers of the acrylic acid—difluoroacetic acid adduct (CH₂=CHCOOH–CHF₂COOH, AA‐DFA) are reported and information on their internal dynamics is supplied. This represents an unprecedented result for the conformational analysis, with microwave spectroscopy, of such a heavy molecular adduct.     

Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran–Formaldehyde Complex

The 1:1 benzofuran–formaldehyde complex has been chosen as model system for analyzing π→π* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint “rotational spectroscopy–quantum chemistry” strategy unveiled the dominant role of π→π* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low-energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone-pair→π interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi-experimental equilibrium structure for a molecular complex of such a dimension.     

Synthesis and structure of cis-[RuCl(bpzm)(κ1-P-dpim)(κ2-P,N-dpim)]Cl·(CHCl3)5. Stability of [Cl(HCCl3) n ]− aggregates

The reaction of [RuCl₂(cod)(bpzm)] [cod = 1,5-cyclooctadiene, bpzm = bis(pyrazol-1-yl)methane] with 2-diphenylphosphino-1-methylimidazole (dpim) and crystallisation from CHCl₃ yielded crystals of cis-[RuCl(κ²-N,N-bpzm)(κ¹-P-dpim)(κ²-P,N-dpim)][Cl(CHCl₃)₄]·CHCl₃, (1√(CHCl₃)₅), in which the Cl^?  counteranion was solvated by four CHCl₃ molecules and interacted with the most positive region of the cation. The structure of the anionic entity and the presence of non-covalent interactions were studied. Theoretical calculations allowed the evaluation of the stability of [Cl(CHCl₃) _n ]^?  aggregates. A pronounced stability was found for aggregates with n = 6 with an increasing charge transfer from the chloride ion to the CHCl₃ molecules from n = 1 to 6. A literature survey on the occurrence of anionic species [Cl(CXCl₃) _n (HB) _m ]^?  (X = H or D; HB = hydrogen bonds with the cation) in solid state structures was carried out and the findings correlated with the results of computational studies. A stabilisation effect of a Cl…Cl interaction was demonstrated by a natural bond orbitals (NBO) analysis.     

Interactions of Aromatic Heterocycles with Water: The Driving Force from Free‐Jet Rotational Spectroscopy and Model Electrostatic Calculations

The interaction of isolated aromatic nitrogen atoms with water is explored within free jets by using rotational spectroscopy. To the existing data on diazines, we add the case of the 1:1 complex of 1,3,5‐triazine and water (where water donates a proton to one of the nitrogen heterocyclic atoms to form a planar adduct). An electrostatic model based on distributed multipoles accurately reproduces the structures of the four azine–water complexes and allows us to understand the forces that stabilize these structures. The applied intermolecular potential allows us to estimate the changes in the thermodynamic functions of the complexes—compared to the separated constituents—and evaluate the temperature at which the complexes are stable under standard conditions.     

The σ‐Hole Interaction Between Sulfur Hexafluoride and Ammonia Characterised by Broadband Rotational Spectroscopy

A weakly‐bound complex of SF₆ and NH₃ is generated within an expanding gas jet and characterised by broadband rotational spectroscopy. The spectra of isotopologues ³²SF₆???¹⁴NH₃, ³²SF₆???¹⁴ND₃, ³²SF₆???¹⁵NH₃ and ³⁴SF₆???¹⁵NH₃ are observed and assigned to determine the spectroscopic parameters. These parameters are consistent with the complex having a C_3v symmetric rotor geometry, in which the nitrogen atom of NH₃ coordinates to SF₆ such that the C_3v axis of the NH₃ sub‐unit is aligned with a local C₃ axis on the SF₆ sub‐unit. The geometry of the complex is rationalized in terms of a σ‐hole interaction. The observed spectra and ab initio calculations also reveal evidence of internal dynamics involving internal rotation of one monomer sub‐unit with respect to the other about the symmetry axis of the complex.