Microwave spectra have been observed for the dihydrate and trihydrate of triflic acid (CF₃SO₃H-(H₂O)₂ and CF₃SO₃H-(H₂O)₃). For the dihydrate, spectra for CF₃³⁴SO₃H-(H₂O)₂ and CF₃SO₃H-(D₂O)₂ were also identified. The fitted rotational constants for both the triflic acid dihydrate and trihydrate are in consistently good agreement with those derived from their respective global minimum structures calculated with various DFT methods. The two water molecules in the dihydrate bind to triflic acid in a cyclic arrangement to form an 8-membered ring, resembling an insertion of a second water molecule into the 6-membered ring of the triflic acid monohydrate. For the triflic acid trihydrate, however, rather than form a 10-membered ring via insertion of a third water molecule into the dihydrate geometry, the global minimum structure represents a hydronium triflate ion pair solvated by two water molecules ((CF₃SO₃⁻H₃O⁺)-(H₂O)₂). This work represents direct experimental evidence in support of the predicted transition from hydrogen bonding to ion pair formation when triflic acid is solvated with only three water molecules.

Gas phase trimethylammonium triflate, (CH₃)₃NH⁺ - ⁻OSO₂CF₃, has been observed by microwave spectroscopy. The ion pair was produced by on-the-fly mixing of trimethylamine and the superacid triflic acid in a supersonic jet. An initial fit with unresolved ¹⁴N hyperfine structure was obtained within several minutes from the chirped-pulse spectrum using a new fitting program for dense spectral, DAPPERS, and the nitrogen hyperfine structure was then measured by cavity FTMW spectroscopy. Rotational constants are in good agreement with those calculated at the MP2/6-311++G(df,pd) level, which indicate a covalent N-H bond distance. Moreover, the measured ¹⁴N nuclear quadrupole coupling constants, together with those calculated at a series of N-H distances, provide definitive evidence of the transfer of a proton in this cold, 1:1 complex.

Esterification is a reaction of great interest and importance in organic and biological chemistry, where typically an alcohol and a carboxylic acid react and form an ester as the product. Previously, by exploring gaseous mixtures of formic acid (FA) with various alcohols using rotational spectroscopy, it has been found that primary and secondary alcohols tend to readily form an ester by reacting with FA, whereas the tertiary alcohol-FA adduct remains unreacted.L. Evangelisti, L. Spada, W. Li, F. Vazart, V. Barone, and W. Caminati, Angew. Chem. Int. Ed. Engl. 56, 3872-3875 (2017).erein, we present the microwave spectroscopic study of FA with two primary alcohols, 3-methyl-3-oxetanemethanol (MOM) and cyclobutanemethanol (CBM), in the gas phase. The spectra in the frequency span of 18-26 GHz were collected on our newly built segmented chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer.M. Fatima, C. Pérez, B. E. Arenas, M. Schnell, and A. L. Steber, Phys. Chem. Chem. Phys. 22, 17042-17051 (2020).oth of the esterification products have been unambiguously assigned with multiple structural conformations. Interestingly, their pre-reaction complexes, MOM-FA and CBM-FA, are observed in the spectra as well. Different from CBM-FA, whose ester product is more pronounced than the adduct in the spectrum, the reaction of MOM with FA seems to be more slowly. This could be attributed to the presence of relatively stronger intermolecular interactions in MOM-FA, compared to CBM-FA. Quantum-chemical calculations were performed to have a deeper insight into their reactivity.

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L. Evangelisti, L. Spada, W. Li, F. Vazart, V. Barone, and W. Caminati, Angew. Chem. Int. Ed. Engl. 56, 3872-3875 (2017).H M. Fatima, C. Pérez, B. E. Arenas, M. Schnell, and A. L. Steber, Phys. Chem. Chem. Phys. 22, 17042-17051 (2020).B

The rotational spectra of the acrolein-methanol and acrolein-ethanol complexes in gas phase were investigated with pulsed supersonic-jet Fourier-transform microwave spectroscopy and quantum mechanical calculations. Two complexes involving the s-trans acrolein and methanol were observed, identified by methanol oxygen interacting with methylenic or aldehydic hydrogen. Two complexes involving the s-trans acrolein and the trans and gauche conformations of ethanol were observed. The OD-substituted species of two conformations of s-trans-acrolein···methanol and the OD- and ¹³C₁-substituted species of the s-trans-acrolein···trans-ethanol were measured. Ab initio calculations run at the MP2/aug-cc-pVTZ level allowed to explore the conformational space of the acrolein-alcohols complexes. The complexes involving s-cis acrolein are less stable than those involving s-trans acrolein, according to the energy difference in the monomer. The three-fold barriers to internal rotation of the methyl group, V₃, in s-trans-acrolein···methanol-1 (2.629(5) kJ mol⁻¹) and s-trans-acrolein···methanol-2 (2.722(5) kJ mol⁻¹) were determined, which are smaller than the value in methanol monomer (4.462 kJ mol⁻¹). Two hydrogen bonds, a conventional O-H···O and a weak C-H···O hydrogen bond, forming a planar and ring-like structure were determined in s-trans-acrolein···methanol-1, s-trans-acrolein···methanol-2 and s-trans-acrolein···trans-ethanol. Both two molecules in each complex are acting as proton donor and acceptor. Another one weak C-H···O hydrogen bond was identified in conformation s-trans -acrolein···gauche-ethanol, in which the acrolein oxygen interacts with the methyl and hydroxyl hydrogen of ethanol at the same time.

Glycerol (CH2OHCHOHCH2OH) is a colorless, odorless, and viscous liquid with a sweet taste and is widely used in food and drug industry. This polyol compound with three hydroxyl groups is capable of taking on many different intra- and intermolecular hydrogen bonding topologies, leading to highly complex conformational spaces for its monomer and dimer. Rotational spectra of five of its monomeric conformers had been reported before.[1],[2] In the current study, we apply CREST, a conformer-rotamer ensemble sampling tool developed by the Grimme group,[3] together with DFT calculations to systematically explore the conformational spaces of its monomer and dimer. Rotational spectra of glycerol in the 1-12 GHz range were recorded using a chirped-pulse Fourier transform microwave spectrometer and analyzed. Rotational transitions of several conformers of the binary glycerol complex were assigned. The structure-energy relationships of the conformers of the glycerol dimer and the corresponding monomer will be discussed. [1] V.V. Ilyushin, R.A. Motiyenko, F.J. Lovas, and D.F. Plusquellic, J. Mol. Spectrosc. 2018, 251,129. [2] G. Maccaferri, W. Caminati and P.G. Favero, J. Chem. Spc., Faraday Trans., 1997, 93(23), 4115. [3] S. Grimme, J. Chem. Theory Comput. 2019, 15, 2847.

Present in a variety of chemical systems, π-π stacking and hydrogen bonding are intermolecular forces critical to the formation and stabilization of various chemical structures. Within DNA these two forces work in unison to stabilize the infamous double helix structure;¹ however, these forces do not always act cooperatively and often compete with each other. This competition is clearly exhibited within the phenol dimer structure, where a predominance of hydrogen bonding over π-π stacking is observed.² However, it was unclear how the competition between of hydrogen bonding and π-π stacking would change as the size of the aromatic subunit increases. We use 1-naphthol and 2-naphthol, which can be considered analogues of phenol, as a tool to further understand the complex interplay between π-π stacking and hydrogen bonding Using chirped-pulse Fourier transform microwave spectroscopy, we detected pure rotational transitions of two conformers for both 1-naphthol and 2-naphthol. We additionally observed and subsequently assigned the dimers of 1-naphthol and 2-naphthol. The experimental identifications of the observed dimers were supplemented with a conformer search enabled by dispersion corrected DFT³ and the CREST sampling program.⁴ Through this study we find, somewhat surprisingly, that the structures of 1-naphthol and 2-naphthol are dictated mainly by π-π stacking, unlike the case of the phenol dimer. Furthermore, the dominance of the π -π stacking interactions not only provides information about 1-naphthol and 2-naphthol like structures, but also insights into the complicated and intricate dynamics of these intermolecular forces and the fine balance between them. 1. Ts'o, P. O., Elsevier: 2012; Vol. 2.; 2. Seifert, N. A., Steber, A. L., Neill, J. L., Pérez, C., Zaleski, D. P., Pate, B. H., Lesarri, A., Phys. Chem. Chem. Phys. 2013, 15 (27), 11468-11477.; 3. Becke, A. D., J Chem. Phys. 1992, 96 (3), 2155-2160.; 4. Pracht, P.; Bohle, F.; Grimme, S., Phys. Chem. Chem. Phys. 2020.

r0pt Figure Following previous studies of gas-phase dimerization of sulfur-bearing aromatic thiols, like thiophenolR.T. Saragi, M. Juanes, C. Pérez, P. Pinacho, D.P. Tikhonov, W. Caminati, M. Schnell, A. Lesarri, J. Phys. Chem. Lett. 2021, 12, 5, 1367–1373. benzyl mercaptanR.T. Saragi, et al., in publication 2021.nd 2-phenylethyl mercaptanR.T. Saragi, et al., in publication 2021. we conducted a structural investigation of 2-naphthalenethiol using chirped-pulse Fourier transform microwave spectroscopy in a jet-cooled expansion. Two conformers of the monomer have been observed in the frequency region 2-8 GHz. All monosubstituted (³⁴S and ¹³C) isotopologues could also be observed. Finally, a single isomer of the homodimer of 2-naphthalenethiol was identified. The homodimer is stabilized by π-stacking interactions, with no hydrogen bond interaction between the two thiol groups. Supporting ab initio and DFT calculations will be presented.

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R.T. Saragi, M. Juanes, C. Pérez, P. Pinacho, D.P. Tikhonov, W. Caminati, M. Schnell, A. Lesarri, J. Phys. Chem. Lett. 2021, 12, 5, 1367–1373., R.T. Saragi, et al., in publication 2021.a R.T. Saragi, et al., in publication 2021.,

r0pt Figure The carbonyl oxygen of the asymmetrically substituted ketone acetophenone exhibits two hydrogen bond docking sites, where in combination with a phenol molecule two competing isomers can be formed. The two conformers can be probed experimentally by analyzing the differences in the OH-stretching frequency via FTIR spectroscopy in a supersonic jet and determining their abundance ratio at low temperatures. C. Zimmermann et al., Phys. Chem. Chem. Phys., 2020, 22, 2870. his allows for the experimental benchmarking of theoretically predicted relative conformational energies on a kJ mol⁻¹ or even finer scale. H. C. Gottschalk et al., J. Chem. Phys., 2018, 148, 014301.A. Poblotzki et al., J. Phys. Chem. Lett., 2017, 8, 5656. By halogenating the second or fourth position in the phenyl-ring of the acetophenone molecule with fluorine, chlorine or bromine, six closely related systems are formed, allowing to probe the influence of substituent effects on the favored docking site in addition to the London dispersion attraction and steric hindrances. C. Zimmermann et al., Phys. Chem. Chem. Phys., 2020, 22, 2870. T H. C. Gottschalk et al., J. Chem. Phys., 2018, 148, 014301.

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Noncovalent interactions topologies of C₆H₅(CH₂)_nOH-CO₂(n=0-2) complexes have been investigated by rotational spectroscopy complementary with quantum chemistry. Experimental evidences pointed out that with the extension of the alkyl chain, the primary binding sites are switched: phenol and CO₂ are linked through a dominant C···O tetrel bond and a secondary C-H···O weak hydrogen bond, while phenylmethanol and 2-phenylethanol link with CO₂ through a C···π(aromatic) interactions. In addition, the Symmetry-Adapted Perturbation theory analysis show that the dispersion term is the dominant force in such C···π(aromatic) complex, while the electrostatic term is dominating in phenol-CO₂.

- 0pt Figure Pnicogen bonding emerged as an important interaction as ubiquitous hydrogen bonding, the study of interactions of simple molecular model systems involving pnicogen bonding can be a platform to understand the complex mechanisms controlled by these non-covalent interactions. In the present work matrix isolation spectroscopy in combination with quantum chemical computations were used to elucidate the structures of P...N pnicogen bonded dimers of phosphoryl chloride(POCl₃) prototype with nitrogen-bases such as ammonia (NH₃), aniline (C₆H₇N), and pyridine (C₅H₅N), where phosphorus is predominantly present in pentavalent state. The basicities of the interacting partner (NH₃,C₆H₇N,C₅H₅N) completely influences the geometrical preference of all these dimers. The POCl₃-NH₃ dimer is anticipated to have a hydrogen bonded geometry, however with hydrogen bonding, a P...N pnicogen bonding plays a definite and a non-trivial role in their overall stabilization. An interesting paradigm transformation was noticed in POCl₃-C₆H₇N and POCl₃-C₅H₅N heterodimers, where P...N pnicogen bonding was observed to completely dominate the hydrogen bonding. Furthermore, the characteristic interactions were investigated through electrostatic potential mapping, energy decomposition and non covalent interaction analyses.