Insights into the non-covalent interactions of hydrogen sulfide with fenchol and fenchone from a gas-phase rotational study.

The gas-phase non-covalent interactions in the endo-fenchol-H2S and fenchone-H2S complexes have been unveiled using rotational spectroscopy in a supersonic jet expansion and quantum chemical calculations. In endo-fenchol, the hydrogen bond HSH⋯OH together with dispersive interactions stabilizes the system. In fenchone, the weak interaction HSH⋯OC allows an internal dynamic of H2S.

Gas-phase hydration of nopinone: the interplay between theoretical methods and experiments unveils the conformational landscape.

The structure of microsolvated nopinone formed in the supersonic jet expansion is investigated in the gas phase. The rotational spectra of nopinone(H2O)n (n = 1, 2, 3) were analysed by means of Fourier transform microwave spectroscopy. In the present study, three monohydrates, two dihydrates and two trihydrates were observed and characterized. The observed structures are the lowest energy conformers predicted by quantum chemical calculations. In all the observed hydrates of nopinone, water was found to be linked to the ketone group (C[double bond, length as m-dash]O) with a strong hydrogen bond (ONOPHW) and finishing with a dispersive one (OWHNOP). The structure of nopinone was found to alter the structure of water dimer and water trimer, which make nopinone be surrounded with a chain of water molecules. A remarkable decrease in the H-bonding length was observed when the number of attached water molecules is increased. Different DFT and ab initio calculations at the equilibrium structure allowed the identification of the observed conformers. Evaluation of the B3LYP-D3 and ωB97X-D results revealed deficiencies in reproducing the structure of one observed monohydrated structure while MP2 and M06-2X reproduce all the three observed structures. A comparison with similar bicyclic ketones highlights how a small change in the bicyclic ring leads to different effects in the microsolvation of biogenic VOCs. This study presents the first step of molecular aggregation to understand the atmospheric formation of aerosols at the molecular scale.

Disentangling the complex network of non-covalent interactions in fenchone hydrates via rotational spectroscopy and quantum chemistry.

The hydrates of the monoterpenoid fenchone (C10H16O)·(H2O)n (n = 1, 2, 3) were investigated by both computational chemistry and microwave spectroscopy. Two monohydrates, three dihydrates and for the first time three trihydrates were identified through the observation of the parent and 18O isotopologues in the rotational spectrum from 2 to 20 GHz. For each hydrate, the sets of rotational constants enabled the determination of the substitution coordinates of the oxygen water atoms as well as an effective structure accounting for the arrangement of the water molecules around fenchone. The hydrates consist of water chains anchored to fenchone by a -CO⋯H-O hydrogen bond and further stabilized by numerous -H-O⋯H-C- secondary hydrogen bonds with the alkyl hydrogen atoms of fenchone.

Conformational changes in hydroxyl functional groups upon hydration: the case study of endo fenchol.

The hydration of endo-fenchol has been studied in the gas phase using a combination of Fourier transform microwave spectroscopy coupled to a supersonic jet expansion and theoretical calculations in the 2 to 20 GHz range. An endo-fencholwater complex was observed. Multi-isotopic substitutions of deuterated species have also been studied in order to confirm the identity of the observed monohydrated endo-fenchol due to the flexibility of the OH group. Herein, the structure of the observed conformer was unveiled. Water induced an alteration in the arrangement of the hydroxyl group. The observed species is stabilized by a hydrogen bond between one water molecule and the highest energy conformer of endo-fenchol, which was not observed in our previous study of the fenchol monomer. This study highlights the flexibility of alcohol molecules and the effect of the strong (O-HO) and weak (C-HO) hydrogen bonds on the stabilization of the cluster with water.

Microsolvation of myrtenal studied by microwave spectroscopy highlights the role of quasi-hydrogen bonds in the stabilization of its hydrates.

Hydrates of myrtenal (C10H14O) · (H2O)n (n = 1, 2, 3) were experimentally investigated in a molecular jet using a cavity-based Fourier transform microwave spectrometer in the 2.6 GHz-15 GHz frequency range. The assignment of the spectra was made possible, thanks to computationally optimized structures at the B3LYP-D3BJ/def2-TZVP and MP2/6-311++G(d,p) levels using the Gaussian 16 software. The spectra of two mono- and two dihydrates and those of the lowest energy conformer among the two expected trihydrates could be assigned. A similar study replacing normal water by 18O labeled water allowed the identification of the spectra of all possible isotopomers, leading to the calculation of the substitution coordinates of water oxygen atoms and that of the effective structure of the water molecule arrangements around myrtenal, except for the trihydrate. The structure of the latter species was nevertheless confirmed by the analysis of the spectrum of the isotopomer with three H2 18O molecules. The computational rotational constants and structural parameters were found quite close to the experimental ones at the density functional theory B3LYP-GD3BJ/def2-TZVP and ab initio MP2/6-311++G(d,p) levels. Symmetry adapted perturbation theory calculations reveal that the aldehyde hydrogen atom strongly interacts with water oxygen atoms in the case of di- and trihydrates.

Gas-Phase Hydration of Perillaldehyde Investigated by Microwave Spectroscopy Assisted by Computational Chemistry.

The microsolvated complexes of two equatorial conformers of perillaldehyde were experimentally investigated in a supersonic molecular jet coupled to a cavity-based Fourier transform microwave spectrometer, in the 2.3-8 GHz frequency range. The structures of hydrates C10H14O·(H2O)n (n = 1,2,3) were first optimized at the MP2/6-311++G(d,p) and B3LYP-D3BJ/def2-TZVP levels of theory. The spectral signatures of four monohydrates and of two dihydrates could then be obtained. Additional rotational constants from the analysis of the spectra of their 18O isotopologues allowed the calculation of the substitution coordinates of the water oxygen atoms of each hydrate. They were found to be in good agreement with those of the optimized structures. SAPT2 calculations and noncovalent interaction analysis highlight the role of dispersion and quasi-hydrogen bonds in the stabilization of the structures.

Microhydration of verbenone: how the chain of water molecules adapts its structure to the host molecule.

The microsolvation of verbenone (C10H14O)·(H2O)n (n = 1, 2, 3) was experimentally investigated in a supersonic expansion using a cavity-based Fourier transform microwave spectrometer, in the 2.8-14 GHz frequency range. Thanks to computationally optimized structures at the B3LYP-D3BJ/def2-TZVP and MP2/6-311++G(d,p) levels using the Gaussian 16 software, the spectra of two mono- and two dihydrates, and that of the lowest energy conformer among the four expected trihydrates, could be assigned. A similar study replacing normal water with 18O labeled water allowed the identification of the spectra of all possible isotopomers, leading to the calculation of the substitution coordinates of water oxygen atoms, and of the effective structure of the water molecule arrangements around verbenone. The computed rotational constants and structural parameters were found to be quite close to the experimental ones both at the DFT and ab initio levels. A comparison between the structures of the hydrates of camphor previously studied by Pérez et al. [J. Phys. Chem. Lett., 2016, 7, 154-160] and of those of verbenone shows that the chain of water molecules adapt their structure according to the geometry of the host molecule. The general trend is that bond angles in the water chain are much wider in verbenone than in camphor.

The gas phase structure of α-pinene, a main biogenic volatile organic compound.

The gas phase structure of the bicyclic atmospheric aerosol precursor α-pinene was investigated employing a combination of quantum chemical calculation and Fourier transform microwave spectroscopy coupled to a supersonic jet expansion. The very weak rotational spectra of the parent species and all singly substituted 13C in natural abundance have been identified, from 2 to 20 GHz, and fitted to Watson's Hamiltonian model. The rotational constants were used together with geometrical parameters from density functional theory and ab initio calculations to determine the rs, r0, and rm(1) structures of the skeleton, without any structural assumption in the fit concerning the heavy atoms. The double C=C bond was found to belong to a quasiplanar skeleton structure containing 6 carbon atoms. Comparison with solid phase structure is reported. The significant differences of α-pinene in gas phase and other gas phase bicyclic monoterpene structures (ß-pinene, nopinone, myrtenal, and bicyclo[3.1.1]heptane) are discussed.

Conformational Landscape and Torsion-Rotation-Vibration Effects in the Two Conformers of Methyl Vinyl Ketone, a Major Oxidation Product of Isoprene.

Methyl vinyl ketone is the second major oxidation product of isoprene, and as such an important volatile organic compound present in the troposphere. In the present study, quantum chemical calculations coupled to high-resolution millimeter-wave spectroscopy have been performed to characterize the ground and first excited vibrational states of the two stable conformers. Equilibrium structures, internal rotation barriers, and relative energies have been calculated at the MP2 and M062X levels of theory. Experimental molecular parameters have been obtained that model the rotational and torsional structures, including splitting patterns due to the internal rotation of the methyl group. For the most stable antiperiplanar (s-trans) conformer, the set of parameters obtained for the ground state should be useful to further model IR spectra up to room temperature. By combining theoretical and experimental data, we obtained a relative energy value of 164 ± 30 cm-1 in the gas phase between the more stable antiperiplanar and the less stable synperiplanar conformers. Moreover, we compared our system with related molecules for the variation in the barriers of methyl rotors in different molecular environments. In addition, the inverse sequence of A and E tunneling substates for the rotational lines of the first excited skeletal torsional state and Coriolis-type coupling with methyl torsion have been observed. For the less stable synperiplanar (cis) conformer, molecular parameters for the ground and first excited torsional states as well as of the first excited skeletal torsional state are presented.

Characterising molecules for fundamental physics: an accurate spectroscopic model of methyltrioxorhenium derived from new infrared and millimetre-wave measurements.

Precise spectroscopic analysis of polyatomic molecules enables many striking advances in physical chemistry and fundamental physics. We use several new high-resolution spectroscopic devices to improve our understanding of the rotational and rovibrational structure of methyltrioxorhenium (MTO), the achiral parent of a family of large oxorhenium compounds that are ideal candidate species for a planned measurement of parity violation in chiral molecules. Using millimetre-wave and infrared spectroscopy in a pulsed supersonic jet, a cryogenic buffer gas cell, and room temperature absorption cells, we probe the ground state and the Re[double bond, length as m-dash]O antisymmetric and symmetric stretching excited states of both CH3187ReO3 and CH3185ReO3 isotopologues in the gas phase with unprecedented precision. By extending the rotational spectra to the 150-300 GHz range, we characterize the ground state rotational and hyperfine structure up to J = 43 and K = 41, resulting in refinements to the rotational, quartic and hyperfine parameters, and the determination of sextic parameters and a centrifugal distortion correction to the quadrupolar hyperfine constant. We obtain rovibrational data for temperatures between 6 and 300 K in the 970-1015 cm-1 range, at resolutions down to 8 MHz and accuracies of 30 MHz. We use these data to determine more precise excited-state rotational, Coriolis and quartic parameters, as well as the ground-state centrifugal distortion parameter DK of the 187Re isotopologue. We also account for hyperfine structure in the rovibrational transitions and hence determine the upper state rhenium atom quadrupole coupling constant eQq'.

Conformational Flexibility of Limonene Oxide Studied By Microwave Spectroscopy.

Monoterpenoids are biogenic volatile organic compounds that play a major role in atmospheric chemistry by participating in the formation of aerosols. In this work, the monoterpenoid (R)-(+)-limonene oxide (C10 H16 O) was characterized in the gas phase by Fourier-transform microwave spectroscopy in a supersonic jet. Five conformers of limonene oxide, four equatorial and one axial considering the configuration of the isopropenyl group, were unambiguously identified from analysis of the rotational spectrum. The observed conformers include cis and trans forms, which are stabilized by a subtle balance of hydrogen bonds, dispersive interactions, and steric effects. Estimated conformational relative abundances surprisingly reveal that the abundance of the axial conformer is similar to that of some of the equatorial conformers. In addition, the potential energy surface was extensively explored by using density functional theory and ab initio methods.

Competition between inter- and intra-molecular hydrogen bonding: An infrared spectroscopic study of jet-cooled amino-ethanol and its dimer.

The Fourier transform IR vibrational spectra of amino-ethanol (AE) and its dimer have been recorded at room temperature and under jet-cooled conditions over the far and mid infrared ranges (50-4000 cm-1) using the White-type cell and the supersonic jet of the Jet-AILES apparatus at the synchrotron facility SOLEIL. Assignment of the monomer experimental frequencies has been derived from anharmonic frequencies calculated at a hybrid CCSD(T)-F12/MP2 level. Various thermodynamical effects in the supersonic expansion conditions including molar dilution of AE and nature of carrier gas have been used to promote or not the formation of dimers. Four vibrational modes of the observed dimer have been unambiguously assigned using mode-specific scaling factors deduced from the ratio between experimental and computed frequencies for the monomer. The most stable g'Gg' monomer undergoes strong deformation upon dimerization, leading to a homochiral head to head dimer involving two strong hydrogen bonds.

Standard free energy of the equilibrium between the trans-monomer and the cyclic-dimer of acetic acid in the gas phase from infrared spectroscopy.

Survey jet-cooled spectra of acetic acid have been recorded in the infrared region (200-4000 cm(-1)) over a wide range of expansion conditions. From the variations of the relative intensities of the signals, vibrational transitions have been assigned unambiguously to the trans-monomer and cyclic-dimer. The IR-active fundamental frequencies have been determined at the instrumental accuracy of 0.5 cm(-1). This analysis of the jet-cooled spectra supported by electronic structure calculations permitted us to characterize the trans-monomer/cyclic-dimer equilibrium. From static cell spectra at 298 K, variations of the molar fractions ratio as a function of the total pressure were used to estimate the equilibrium constant and the Gibbs free energy of dimerization at 298 K. The very good agreement with the literature data shows that the present method is able to produce, from a single study, a free energy value as reliable as the one obtained from a large collection of data. In addition, the semi-empirical free energy value was used to estimate the accuracy of electronic structure calculations and in turn the accuracy of the derived useful information such as the dissociation energy of the complex (i.e. the strength of the hydrogen bonds) or the relative energies within the conformational landscape.

High resolution spectroscopy of methyltrioxorhenium: towards the observation of parity violation in chiral molecules.

Originating from the weak interaction, parity violation in chiral molecules has been considered as a possible origin of biohomochirality. We have proposed the observation of molecular parity violation using the two-photon Ramsey fringes technique on a supersonic beam. As a first step in this direction, a detailed spectroscopic study of methyltrioxorhenium (MTO) has been undertaken. It is an ideal test molecule as the achiral parent molecule of chiral candidates for a parity violation experiment. For the (187)Re MTO isotopologue, a combined analysis of Fourier transform microwave and infrared spectra as well as ultra-high resolution CO(2) laser absorption spectra enabled the assignment of 28 rotational lines and 71 rovibrational lines, some of them with a resolved hyperfine structure. A set of spectroscopic parameters in the ground and first excited state, including hyperfine structure constants, was obtained for the ν(as) antisymmetric Re=O stretching mode of this molecule. This result validates the experimental approach to be followed once a chiral derivative of MTO is synthesized, and shows the benefit of the combination of several spectroscopic techniques in different spectral regions, with different set-ups and resolutions. The first high resolution spectra of jet-cooled MTO, obtained on a set-up being developed for the observation of molecular parity violation, are shown, which constitutes a major step towards the targeted objective.

Progress toward the first observation of parity violation in chiral molecules by high-resolution laser spectroscopy.

Parity violation (PV) effects in chiral molecules have so far never been experimentally observed. To take up this challenge, a consortium of physicists, chemists, theoreticians, and spectroscopists has been established and aims at measuring PV energy differences between two enantiomers by using high-resolution laser spectroscopy. In this article, we present our common strategy to reach this goal, the progress accomplished in the diverse areas, and point out directions for future PV observations. The work of André Collet on bromochlorofluoromethane (1) enantiomers, their synthesis, and their chiral recognition by cryptophanes made feasible the first generation of experiments presented in this article.

Microwave spectrum, structure, and quantum chemical studies of a compound of potential astrochemical and astrobiological interest: Z-3-amino-2-propenenitrile.

Z-3-Amino-2-propenenitrile, H2NCH=CHCN, a compound of astrochemical and astrobiological interest, has been studied by Stark and Fourier transform microwave spectroscopy along with eight of its isotopologues; the synthesis of five of these are reported. The spectra of the ground vibrational state and of three vibrationally excited states belonging to the two lowest normal modes were assigned for the parent species, whereas the ground states were assigned for the isotopologues. The frequency of the lowest in-plane bending fundamental vibration was determined to be 152(20) cm(-1) and the frequency of the lowest out-of-plane fundamental mode was found to be 176(20) cm(-1) by relative intensity measurements. A delicate problem is whether this compound is planar or slightly nonplanar. It was found that the rotational constants of the nine species cannot be used to conclude definitely whether the molecule is planar or not. The experimental dipole moment is mu(a) = 16.45(12), mu(b) = 2.86(6), mu(c) = 0 (assumed), and mu(tot.) = 16.70(12) x 10(-30) C m [5.01(4) D]. The quadrupole coupling constants of the two nitrogen nuclei are chi(aa) = -1.4917(21) and chi(cc) = 1.5644(24) MHz for the nitrogen atom of the cyano group and chi(aa) = 1.7262(18) and chi(cc) = -4.0591(17) MHz for the nitrogen atom of the amino group. Extensive quantum-chemical calculations have been performed, and the results obtained from these calculations have been compared with the experimental values. The equilibrium structures of vinylamine, vinyl cyanide, and Z-3-amino-2-propenenitrile have been calculated. These calculations have established that the equilibrium structure of the title compound is definitely nonplanar. However, the MP2/VQZ energy difference between the planar and nonplanar forms is small, only -423 J/mol. Z-Amino-2-propenenitrile and E-3-amino-2-propenenitrile are formed simply by mixing ammonia and cyanoacetylene at room temperature. A plausible reaction path has been modeled. G3 calculations indicate that the enthalpy (298.15 K, 1 atm) of the transition state is about 130 kJ/mol higher than the sum of the enthalpies of the reactants ammonia and cyanoacetylene. This energy difference is comparatively high, which indicates that both E- and Z-3-aminopropenenitrile are not likely to be formed in the gas phase in cold interstellar clouds via a collision between ammonia and cyanoacetylene. An alternative reaction between protonated cyanoacetylene (H-C[triple bond]C-C[triple bond]NH+) and ammonia is predicted to have a much lower activation energy than the reaction between the neutral molecules. Although protonated E- and Z-3-aminopropenenitrile in principle may be formed this way, it is more likely that a collision between NH3 and H-C[triple bond]C-C[triple bond]NH+ leads to NH4+ and H-C[triple bond]C-C[triple bond]N.

Conformational flexibility in hydrated sugars: the glycolaldehyde-water complex.

Conformational flexibility in the smallest hydrated sugar, the glycolaldehyde-water complex, has been investigated in the gas phase by means of a combination of a microwave Fourier transform spectroscopy experiment in a supersonic molecular beam and ab initio quantum chemistry calculations. The water molecule inserts into glycolaldehyde using H-bonding selectivity: the two lowest energy conformations are stabilized by two intermolecular hydrogen bonds, and the next two by one intra- plus one intermolecular hydrogen bond. A dynamical flexibility associated with the two lowest energy conformations has been experimentally observed and accurately modeled with a two-dimensional potential energy surface involving the hydroxyl group and the free OH water group coordinates. The conclusions drawn from the role played in the conformational flexibility by the hydroxyl and carbonyl groups are extended to other carbohydrates and biomolecules.

Chlorofluoroiodomethane as a potential candidate for parity violation measurements.

CHFClI is among the more favorable molecules for parity violation (PV) measurements in molecules. Despite the fact that calculated PV effects are two orders of magnitude smaller than in some organometallic compounds, CHFClI displays interesting features which could make possible a new experimental PV test on this molecule. Indeed, ultrahigh resolution spectroscopy using an ultrastable CO(2) laser is favored by several intrinsic properties of this molecule. For example, the high vapor pressure of CHFClI allows investigation by supersonic beam spectroscopy. Indeed, the spectroscopic constants have been accurately determined by microwave and millimetre wave spectroscopy. This is important for the subsequent selection of an appropriate absorption band of CHFClI that could be brought to coïncide with the absorption of CO(2). Partially resolved (+)- and (-)-CHFClI enantiomers with respectively 63.3 and 20.5% ee's have been recently prepared and analyzed by molecular recognition using chiral hosts called cryptophanes. Finally, the S-(+)/R-(-) absolute configuration was ascertained by vibrational circular dichroïsm (VCD) in the gas phase.