Water Arrangements upon Interaction with a Rigid Solute: Multiconfigurational Fenchone-(H2O)4-7 Hydrates.

Insight into the arrangements of water molecules around solutes is important to understand how solvation proceeds and to build reliable models to describe water-solute interactions. We report the stepwise solvation of fenchone, a biogenic ketone, with 4-7 water molecules. Multiple hydrates were observed using broadband rotational spectroscopy, and the configurations of four fenchone-(H2O)4, three fenchone-(H2O)5, two fenchone-(H2O)6, and one fenchone-(H2O)7 complexes were characterized from the analysis of their rotational spectra in combination with quantum-chemical calculations. Interactions with fenchone deeply perturb water configurations compared with the pure water tetramer and pentamer. In two fenchone-(H2O)4 complexes, the water tetramer adopts completely new arrangements, and in fenchone-(H2O)5, the water pentamer is no longer close to being planar. The water hexamer interacts with fenchone as the least abundant book isomer, while the water heptamer adopts a distorted prism structure, which forms a water cube when including the fenchone oxygen in the hydrogen bonding network. Differences in hydrogen bonding networks compared with those of pure water clusters show the influence of fenchone's topology. Specifically, all observed hydrates except one show two water molecules binding to fenchone through each oxygen lone pair. The observation of several water arrangements for fenchone-(H2O)4-7 complexes highlights water adaptability and provides insight into the solvation process.

Microwave spectra of dinitrotoluene isomers: a new step towards the detection of explosive vapors.

The spectroscopic characterization of explosive taggants used for TNT detection is a research topic of growing interest. We present a gas-phase rotational spectroscopic study of weakly volatile dinitrotoluene (DNT) isomers. The pure rotational spectra of 2,4-DNT and 2,6-DNT were recorded in the microwave range (2-20 GHz) using a Fabry-Perot Fourier-transform microwave (FP-FTMW) spectrometer coupled to a pulsed supersonic jet. Rotational transitions are split by hyperfine quadrupole coupling at the two 14N nuclei leading to up to 9 hyperfine components. The spectral analysis was supported by quantum chemical calculations carried out at the B98/cc-pVTZ and MP2/cc-pVTZ levels of theory. Based on 2D potential energy surfaces at the B98/cc-pVTZ level of theory, the methyl group internal rotation barriers were calculated to be V3 = 515 cm-1 and 698 cm-1 for 2,4- and 2,6-DNT, respectively. Although no splitting due to internal rotation was observed for 2,6-DNT, several splittings were observed for 2,4-DNT. The microwave spectra of both species were fitted using a semi-rigid Hamiltonian accounting for the quadrupole coupling hyperfine structure. Based on the internal axis method (IAM), an additional analysis was performed to retrieve an accurate value of the rotationless A-E tunneling splitting which could be extracted from the rotational dependence of the tunneling splitting. This yielded in the case of 2,4-DNT to an experimental value of 525 cm-1 for the barrier height V3 which agrees well with the DFT value. The coupled internal rotations of -CH3 and -NO2 are investigated in terms of 2-D surfaces, as already done in the case of 2-nitrotoluene [A. Roucou et al., Chem. Phys. Chem., 2020, 21, 2523-2538].

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.

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.

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.

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.

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.

Rotational spectrum of NSF3 in the ground and v5 = 1 vibrational states: observation of Q-branch perturbation-allowed transitions with delta(k - l) = 0, +/-3, +/-6 and anomalies in the rovibrational structure of the v5 = 1 state.

The rotational spectrum of NSF3 in the ground and v5 = 1 vibrational states has been investigated in the centimeter- and millimeter-wave ranges. R-branch (J + 1 <-- J) transitions for J = 0, 1 and Q-branch rotational transitions for the v5 = 1 vibrational state have been measured by waveguide Fourier transform microwave spectroscopy in the range 8-26.5 GHz. The Q-branch transitions include 28 direct l-type doubling transitions (kl = +1, A1) <--> (kl = +1, A2) with J < or = 62, and 108 direct l-type resonance transitions following the selection rule delta k = delta l = +/-2 with J < or = 60 and G = |k - l| < or = 3. A process called "regional resonance" was observed in which a cluster of levels interacted strongly over a large range in J. This process led to the observation of 55 perturbation-allowed transitions following the selection rules delta(k - l) = +/-3, +/-6. In particular, (kl = +1, A+) <--> (kl = -2, A-), (kl = +4, A+) <--> (kl = +1, A-), (kl = +2) <--> (kl = -1), (kl = +3) <--> (kl = 0), (kl = +2) <--> (kl = -3), and (kl = +3) <--> (kl = -3). The various aspects of the regional resonances are discussed in detail. An accidental near-degeneracy of the kl = 0 and kl = -4 levels at J = 26/27 led to the observation of perturbation-allowed transitions following the selection rule delta(k-l) = +/-6 with (kl = +2) <--> (kl = -4). A corresponding near-degeneracy between kl = -1 and kl = -3 levels at J = 30/31 led to the detection of similar transitions, but with (kl = +3) <--> (kl = -3). In the range 230-480 GHz, R-branch rotational transitions have been measured by absorption spectroscopy up to J = 49 in the ground-state and up to J = 50 in the v5 = 1 vibrational state. The transition frequencies have been analyzed using various reduced forms of the effective Hamiltonians. The data for the v5 = 1 vibrational state have been fitted successfully using two models up to seventh order with delta k = +/-3 interaction parameters constrained (dt constrained to zero, and epsilon to zero or to the ground-state value). On the other hand, reductions with the (delta k = +/-1, deltal = -/+2) interaction parameter q12 fixed to zero failed to reproduce the experimental data since the parameters defining the reduction transformation do not arise in the correct order of magnitude. The ground-state data have been analyzed including parameters up to fourth order constraining either parameters of the delta k = +/-3 interactions to zero (reduction A), or of the delta k = +/-6 interactions to zero (reduction B). The unitary equivalence of the different parameter sets obtained is demonstrated for both vibrational states.

Millimeter wave spectrum of bromomethyl radical, CH2Br.

The rotational spectra of the two isotopic species of the bromomethyl radical, CH2 79Br and CH2 81Br, have been observed in their ground electronic state 2B1 in the 180-470 GHz frequency region, corresponding to a-type transitions from N=8-7 to N=21-20. The radical was produced by hydrogen abstraction of methylbromide (CH3Br) either by chlorine or by fluorine atoms in a free space cell. Hyperfine structure due to the bromine nucleus has been resolved in the observed spectra, and the rotational constants as well as the fine and hyperfine interaction constants were accurately determined for both isotopomers. The inertial defect was determined to be 0.028 96(20) and 0.028 95(20) amu A(2), for CH2 79Br and CH2 81Br, respectively, suggesting a planar structure. By fixing the [angle]HCH bond angle at 124.5 degrees , an effective molecular structure can be derived as r0(CBr)=1.848 A and r0(CH)=1.084 A. A comparison of the molecular structure of various halogen-substituted methyl radicals with respect to the planarity of these radicals is discussed.