N-methyl inversion in pseudo-pelletierine

We have previously conducted rotational studies of several tropanes, \footnote {E.~J.~Cocinero, A.~Lesarri, P.~\'Ecija, J.-U.~Grabow, J.~A.~Fern\'andez, F.~Casta\~{n}o, \textit{Phys. Chem. Chem. Phys.} \textbf{2010}, \textit{49}, 4503}$^{,}$\footnote { P.~\'Ecija, E.~J.~Cocinero, A.~Lesarri, F. ~J. ~Basterretxea, J.~A.~Fern\'andez, F.~Casta\~{n}o, \textit{Chem. Phys. Chem.} \textbf{2013}, \textit{14}, 1830}$^{,}$\footnote { P.~\'Ecija, M. ~Vallejo-Lopez, I. ~Uriarte, F. ~J. ~Basterretxea, A. Lesarri, J.~A.~Fern\'andez, E.~J.~Cocinero, \textit{submitted} \textbf{2016}} since this bicyclic structural motif forms the core of different alkaloids of pharmaceutical interest. Now we report on the conformational properties and molecular structure of pseudo-pelletierine (9-methyl-9-azabicyclo[3.3.1]nonan-3-one), probed in a jet expansion with Fourier-transform microwave spectroscopy. Pseudo-pelletierine is an azabicycle with two fused six-membered rings, where the \textit{N}-methyl group can produce inverting axial o equatorial conformations. The two conformations were detected in the rotational spectrum, recorded in the region 6-18 GHz. Unlike tropinone and \textit{N}-methylpiperidone, where the most stable conformer is equatorial, the axial species was found dominant for pseudo-pelletierine. All monosubstituted isotopic species ($^{13}$C, $^{15}$N and $^{18}$O) were identified for the axial conformer, leading to an accurate determination of the effective and substitution structures. An estimation of conformational populations was derived from relative intensities. The experimental results will be compared with \textit{ab initio} (MP2) and DFT (M06-2X, B3LYP) calculations

The first stages of nanomicelle formation captured in the sevoflurane trimer

Producción CientíficaSelf-aggregation of sevoflurane, an inhalable, fluorinated anesthetic, provides a challenge for current state-of-the-art high-resolution techniques due to its large mass and the variety of possible hydrogen bonds between monomers. Here we present the observation of sevoflurane trimer by chirped-pulse Fourier transform microwave spectroscopy, identified through the interplay of experimental and computational methods. The trimer (>600 Da), one of the largest molecular aggregates observed through rotational spectroscopy, does not resemble the binding (C–H···O) motif of the already characterized sevoflurane dimer, instead adapting a new binding configuration created predominantly from 17 CH···F hydrogen bonds that resembles a nanomicellar arrangement. The observation of such a heavy aggregate highlights the potential of rotational spectroscopy to study larger biochemical systems in the limit of spectroscopic congestion but also showcases the challenges ahead as the mass of the system increases.NSF Major Research Instrumentation program (grant CHE0960074)Ministerio de Ciencia, Innovación y Universidades - Fondo Europeo de Desarrollo Regional (grant PGC2018-098561-B-C22

Furanosic forms of sugars: conformational equilibrium of methyl beta-D-ribofuranoside

The investigation of an isolated ribofuranose unit in the gas phase reveals the intrinsic conformational landscape of the biologically active sugar form.We report the rotational spectra of two conformers of methyl b-D-ribofuranoside in a supersonic jet expansion. Both conformers adopt a near twisted (3T2) ring conformation with the methoxy and hydroxymethyl substituents involved in various intramolecular hydrogen bonds.MINECO-FEDER CTQ2015-68148-C2-

Six-fold-symmetry internal rotation in toluenes: the low barrier challenge of 2,6-and 3,5-difluorotoluene

Pure six-fold symmetry (V6) internal rotation poses significant challenges to experimental and theoretical determination, as the very low torsional barriers result in huge tunneling splittings difficult to identify and to model. Here we resolved the methyl group internal rotation dynamics of 2,6- and 3,5-difluorotoluene using a newly developed computer code especially adapted to V6 problems. The jet-cooled rotational spectra of the title molecules in the 5–25 GHz region revealed internal rotation tunneling doublings of up to 3.6 GHz, which translated in methyl group potential barriers of V6 = 0.14872(24) and 0.0856(10) kJ mol−1, respectively, in the vibrational ground-state. Additional information on Stark effects and carbon isotopic species in natural abundance provided structural data and the electric dipole moments for both molecules. Ab initio calculations at the MP2 level do not reproduce the tiny torsional barriers, calling for experiments on other systems and additional theoretical models.DFGMINECO/CTQ2012-39132-C02-0

Rotational spectrum and intramolecular hydrogen bonding in 1,2-butanedithiol

Producción CientíficaThe jet-cooled rotational spectrum of 1,2-butanedithiol was observed in the frequency region 2–8 GHz. Two conformers were detected for the molecule, corresponding to anti- and gauche-carbon molecular skeletons, both featuring a gauche arrangement of the two thiol groups. All 13C and 34S monosubstituted isotopologues were additionally assigned in natural abundance for the most stable anti isomer, while only the two 34S species were detected for the weaker gauche conformation. The structural analysis included ground-state effective structures, isotopic substitution coordinates, B3LYP-D3(BJ) density functional molecular orbital calculations and non-covalent interactions mapping with NCIPlot. The structural data confirm that the two thiol groups synchronize their orientation either parallel or antiparallel to support intramolecular S–H⋯S weak hydrogen bonding, reminiscent of the intramolecular hydrogen bond networks observed in adjacent alcohol groups. DFT calculations on 1,2-butanediol and 1,2-ethanedithiol offered structural comparisons with the title compound.MICIU-FEDER (grant PGC2018- 098561-B-C22) and JCyL (grant VA056G18

Molecular Recognition, Transient Chirality and Sulfur Hydrogen Bonding in the Benzyl Mercaptan Dimer

The homodimers of transiently chiral molecules offer physical insight into the process of molecular recognition, the preference for homo or heterochiral aggregation and the nature of the non-covalent interactions stabilizing the adducts. We report the observation of the benzyl mercaptan dimer in the isolation conditions of a supersonic jet expansion, using broadband (chirped-pulse) microwave spectroscopy. A single homochiral isomer was observed for the dimer, stabilized by a cooperative sequence of S-H···S and S-H···π hydrogen bonds. The structural data, stabilization energies and energy decomposition describe these non-covalent interactions as weak and dispersion-controlled. A comparison is also provided with the benzyl alcohol dimer.This research was funded by the Spanish Ministerio de Ciencia e Innovación MICINN-FEDER, grants numbers PGC2018-098561-B-C21 and PGC2018-098561-B-C22. The APC were funded by PGC2018-098561-B-C22

Water binding to the atmospheric oxidation product methyl vinyl ketone

Producción CientíficaMethyl vinyl ketone is one of the major oxidation products of isoprene, and therefore, an important precursor of secondary organic aerosol. Understanding its interactions with water is relevant to gain insight into aerosol formation and improve the predictive power of atmospheric chemistry models. The molecular complex formed between methyl vinyl ketone and water has been generated in a supersonic jet and characterized using high-resolution microwave spectroscopy in combination with quantum chemistry calculations. In this study, we show that methyl vinyl ketone interacts with water forming four 1:1 isomers connected by O − H···O and C − H···O hydrogen bond interactions. Water has been found to preferentially bind to the antiperiplanar conformation of methyl vinyl ketone. Evidence of a large amplitude motion arising from the methyl internal rotation has been found in the rotational spectra of the dimer. The threefold methyl internal rotation barrier heights have been further determined and discussed for all the species.Ministerio de Ciencia, Innovación y Universidades (grants PID2020-117925GA-I00 and PGC2018-098561-B-C22

Sulfur hydrogen bonding and internal dynamics in the monohydrates of thenyl mercaptan and thenyl alcohol

Producción CientíficaThe monohydrates of thenyl alcohol and thenyl mercaptan have been probed in a supersonic jet expansion using chirped-pulsed and Fabry-Perot Fourier-transform microwave spectroscopy. The rotational spectra revealed a single isomer for each of the dimers. The thenyl alcohol hydrate is stabilized by an O-H···Ow hydrogen bond between the alcohol and water, with water acting as proton acceptor and additionally engaging in a Ow-H···pi interaction with the thenyl ring. Conversely, water behaves as proton donor in the thenyl mercaptan hydrate, linking to the thiol group though a Ow-H···S hydrogen bond and secondary Ow-H··· interactions to the ring. In both dimers water retains internal mobility, as tunneling doublings in the spectrum confirm an internal rotation motion of water inside the cluster. The experimental results have been complemented with density-functional-theory molecular orbital calculations, binding energy decomposition and a topological analysis of the electronic density, providing a comparative description of the effects of hydrogen bonding of water to the alcohol and thiol groups in the dimers, relevant to understand hydrogen bonding to sulfur centers.MICINN-FEDER (PGC2018-098561- B-C22) and JCyL (grant VA056G18

Sulfur–arene interactions: the S⋯π and S–H⋯π interactions in the dimers of benzofuran⋯sulfur dioxide and benzofuran⋯hydrogen sulfide

Producción CientíficaNon-covalent interactions between sulfur centers and aromatic rings play important roles in biological chemistry. We examined here the sulfur–arene interactions between the fused aromatic heterocycle benzofuran and two prototype sulfur divalent triatomics (sulfur dioxide and hydrogen sulfide). The weakly-bound adducts were generated in a supersonic jet expansion and characterized with broadband (chirped-pulsed) time-domain microwave spectroscopy. The rotational spectrum confirmed the detection of a single isomer for both heterodimers, consistent with the computational predictions for the global minima. The benzofuran⋯sulfur dioxide dimer exhibits a stacked structure with sulfur closer to benzofuran, while in benzofuran⋯hydrogen sulfide the two S–H bonds are oriented towards the bicycle. These binding topologies are similar to the corresponding benzene adducts, but offer increased interaction energies. The stabilizing interactions are described as S⋯π or S–H⋯π, respectively, using a combination of density-functional theory calculations (dispersion corrected B3LYP and B2PLYP), natural bond orbital theory, energy decomposition and electronic density analysis methods. The two heterodimers present a larger dispersion component, but nearly balanced by electrostatic contributions.National Natural Science Foundation of China (No. 22273009)Ministerio de Ciencia e Innovación (MCIN-AEI) y FEDER (grant PID2021-125015NB-I00)FEDER - Junta de Castilla y León (grants INFRARED IR2021-UVa13 e IR2020-1-UVa02