Millimeter-Wave Spectroscopy of Methylfuran Isomers: Local vs. Global Treatments of the Internal Rotation.

Methylfurans are methylated aromatic heterocyclic volatile organic compounds and primary or secondary pollutants in the atmosphere due to their capability to form secondary organic aerosols in presence of atmospheric oxidants. There is therefore a significant interest to monitor these molecules in the gas phase. High resolution spectroscopic studies of methylated furan compounds are generally limited to pure rotational spectroscopy in the vibrational ground state. This lack of results might be explained by the difficulties arisen from the internal rotation of the methyl group inducing non-trivial patterns in the rotational spectra. In this study, we discuss the benefits to assign the mm-wave rotational-torsional spectra of methylfuran with the global approach of the BELGI-Cs code compared to local approaches such as XIAM and ERHAM. The global approach reproduces the observed rotational lines of 2-methylfuran and 3-methylfuran in the mm-wave region at the experimental accuracy for the ground vt=0 and the first torsional vt=1 states with a unique set of molecular parameters. In addition, the V3 and V6 parameters describing the internal rotation potential barrier may be determined with a high degree of accuracy with the global approach. Finally, a discussion with other heterocyclic compounds enables the study of the influence of the electronic environment on the hindered rotation of the methyl group.

Laboratory study of iron isotope fractionation during dissolution of mineral dust and industrial ash in simulated cloud water.

Atmospheric deposition is a key mode of iron (Fe) input to ocean regions where low concentrations of this micronutrient limit marine primary production. Various natural particles (e.g., mineral dust, volcanic ash) and anthropogenic particles (e.g., from industrial processes, biomass burning) can deliver Fe to the ocean, and assessment of their relative importance in supplying Fe to seawater requires knowledge of both their deposition flux and their Fe solubility (a proxy for Fe bioavailability). Iron isotope (54Fe, 56Fe, 57Fe, 58Fe) analysis is a potential tool for tracing natural and anthropogenic Fe inputs to the ocean. However, it remains uncertain how the distinct Fe isotopic signatures (δ56Fe) of these particles may be modified by physicochemical processes (e.g., acidification, photochemistry, condensation-evaporation cycles) that are known to enhance Fe solubility during atmospheric transport. In this experimental study, we measure changes over time in both Fe solubility and δ56Fe of a Tunisian soil dust and an Fe-Mn alloy factory industrial ash exposed under irradiation to a pH 2 solution containing oxalic acid, the most widespread organic complexing agent in cloud- and rainwater. The Fe released per unit surface area of the ash (∼1460 µg Fe m-2) is ∼40 times higher than that released by the dust after 60 min in solution. Isotopic fractionation is also observed, to a greater extent in the dust than the ash, in parallel with dissolution of the solid particles and driven by preferential release of 54Fe into solution. After the initial release of 54Fe, the re-adsorption of A-type Fe-oxalate ternary complexes on the most stable surface sites of the solid particles seems to impair the release of the heavier Fe isotopes, maintaining a relative enrichment in the light Fe isotope in solution over time. These findings provide new insights on Fe mobilisation and isotopic fractionation in mineral dust and industrial ash during atmospheric processing, with potential implications for ultimately improving the tracing of natural versus anthropogenic contributions of soluble Fe to the ocean.

Intramolecular H-Bond Dynamics of Catechol Investigated by THz High-Resolution Spectroscopy of Its Low-Frequency Modes.

Catechol is an oxygenated aromatic volatile organic compound and a biogenic precursor of secondary organic aerosols. Monitoring this compound in the gas phase is desirable due to its appreciable reactivity with tropospheric ozone. From a molecular point of view, this molecule is attractive since the two adjacent hydroxy groups can interchangeably act as donor and acceptor in an intramolecular hydrogen bonding due to the tunnelling between two symmetrically equivalent structures. Using synchrotron radiation, we recorded a rotationally-resolved Fourier Transform far-infrared (IR) spectrum of the torsional modes of the free and bonded -OH groups forming the intramolecular hydrogen bond. Additionally, the room temperature, pure rotational spectrum was measured in the 70-220 GHz frequency range using a millimeter-wave spectrometer. The assignment of these molecular transitions was assisted by anharmonic high-level quantum-chemical calculations. In particular, pure rotational lines belonging to the ground and the four lowest energy, vibrationally excited states were assigned. Splitting due to the tunnelling was resolved for the free -OH torsional state. A global fit combining the far-IR and millimeter-wave data provided the spectroscopic parameters of the low-energy far-IR modes, in particular those characterizing the intramolecular hydrogen bond dynamics.

Full Conformational Landscape of 3-Methoxyphenol Revealed by Room Temperature mm-wave Rotational Spectroscopy Supported by Quantum Chemical Calculations.

Room temperature millimeter-wave rotational spectroscopy supported by high level of theory calculations have been employed to fully characterise the conformational landscape of 3-Methoxyphenol, a semi-volatile polar oxygenated aromatic compound precursor of secondary organic aerosols in the atmosphere arising from biomass combustion. While previous rotationally-resolved spectroscopic studies in the microwave and in the UV domains failed to observe the complete conformational landscape, the 70-330 GHz rotational spectrum measured in this study reveals the ground state rotational signatures of the four stable conformations theoretically predicted. Moreover, rotational transitions in the lowest energy vibrationally excited states were assigned for two conformers. While the inertial defect of methoxyphenol does not significantly change between conformers and isomers, the excitation of the methoxy out-of-plane bending is the main contribution to the non-planarity of the molecule.

High resolution study of the ν2 and ν5 rovibrational fundamental bands of thionyl chloride: Interplay of an evolutionary algorithm and a line-by-line analysis.

The ν2 and ν5 fundamental bands of thionyl chloride (SOCl2) were measured in the 420 cm-1-550 cm-1 region using the FT-far-IR spectrometer exploiting synchrotron radiation on the AILES beamline at SOLEIL. A straightforward line-by-line analysis is complicated by the high congestion of the spectrum due to both the high density of SOCl2 rovibrational bands and the presence of the ν2 fundamental band of sulfur dioxide produced by hydrolysis of SOCl2 with residual water. To overcome this difficulty, our assignment procedure for the main isotopologues 32S16O35Cl2 and 32S16O35Cl37Cl alternates between a direct fit of the spectrum, via a global optimization technique, and a traditional line-by-line analysis. The global optimization, based on an evolutionary algorithm, produces rotational constants and band centers that serve as useful starting values for the subsequent spectroscopic analysis. This work helped to identify the pure rotational submillimeter spectrum of 32S16O35Cl2 in the v2=1 and v5=1 vibrational states of Martin-Drumel et al. [J. Chem. Phys. 144, 084305 (2016)]. As a by-product, the rotational transitions of the v4=1 far-IR inactive state were identified in the submillimeter spectrum. A global fit gathering all the microwave, submillimeter, and far-IR data of thionyl chloride has been performed, showing that no major perturbation of rovibrational energy levels occurs for the main isotopologue of the molecule.

Ab initio rovibronic states of the thioketenyl radical in its X2Π electronic ground state.

We present an ab initio study of the thioketenyl (HCCS) radical in its degenerate X(2)Π electronic ground state. All rotational and vibrational degrees of freedom are taken into account including the electronic orbital and spin angular momenta. The structure of the rovibronic levels and the nature of the corresponding wave functions show resonances even at very low energies due to strong couplings between the bending, rotation, and spin terms in the Hamiltonian. Assignments from the dominant contributions of the eigenvectors are discussed in parallel with previously published data. The rotational structures for the first excited vibronic states are computed as well as transition intensities for the fundamental one.

Ab initio potential-energy surface and rovibrational states of the HCN-HCl complex.

A four-dimensional intermolecular potential-energy surface has been calculated for the HCN-HCl complex, with the use of the coupled cluster method with single and double excitations and noniterative inclusion of triples. Data for more than 13,000 geometries were represented by an angular expansion in terms of coupled spherical harmonics; the dependence of the expansion coefficients on the intermolecular distance R was described by the reproducing kernel Hilbert space method. The global minimum with De=1565 cm(-1) and Re=7.47a0 has a linear HCN-HCl hydrogen-bonded structure with HCl as the donor. A secondary hydrogen-bonded equilibrium structure with De=564 cm(-1) and Re=8.21a0 has a T-shaped geometry with HCN as the donor and the acceptor HCl molecule nearly perpendicular to the intermolecular axis. This potential surface was used in a variational approach to compute a series of bound states of the isotopomers HCN-H35Cl, DCN-H35Cl, and HCN-H37Cl for total angular momentum J=0,1,2 and spectroscopic parities e, f. The results could be analyzed in terms of the approximate quantum numbers of a linear polyatomic molecule with two coupled bend modes, plus a quantum number for the intermolecular stretch vibration. They are in good agreement with the recent high resolution spectrum of Larsen et al. [Phys. Chem. Chem. Phys. 7, 1953 (2005)] in the region of 330 cm(-1) corresponding to the HCl libration. The (partly anomalous) effects of isotopic substitutions on the properties of the complex were explained with the aid of the calculations.

Ab initio calculation of the NH(3sigma-)-NH(3sigma-) interaction potentials in the quintet, triplet, and singlet states.

We present the ab initio potential-energy surfaces of the NH-NH complex that correlate with two NH molecules in their 3sigma- electronic ground state. Three distinct potential-energy surfaces, split by exchange interactions, correspond to the coupling of the S(A) = 1 and S(B) = 1 electronic spins of the monomers to dimer states with S = 0, 1, and 2. Exploratory calculations on the quintet (S = 2), triplet (S = 1), and singlet (S = 0) states and their exchange splittings were performed with the valence bond self-consistent-field method that explicitly accounts for the nonorthogonality of the orbitals on different monomers. The potential surface of the quintet state, which can be described by a single Slater determinant reference function, was calculated at the coupled cluster level with single and double excitations and noniterative treatment of the triples. The triplet and singlet states require multiconfiguration reference wave functions and the exchange splittings between the three potential surfaces were calculated with the complete active space self-consistent-field method supplemented with perturbative configuration interaction calculations of second and third orders. Full potential-energy surfaces were computed as a function of the four intermolecular Jacobi coordinates, with an aug-cc-pVTZ basis on the N and H atoms and bond functions at the midpoint of the intermolecular vector R. An analytical representation of these potentials was given by expanding their dependence on the molecular orientations in coupled spherical harmonics, and representing the dependence of the expansion coefficients on the intermolecular distance R by the reproducing kernel Hilbert space method. The quintet surface has a van der Waals minimum of depth D(e) = 675 cm(-1) at R(e) = 6.6a0 for a linear geometry with the two NH electric dipoles aligned. The singlet and triplet surfaces show similar, slightly deeper, van der Waals wells, but when R is decreased the weakly bound NH dimer with S = 0 and S = 1 converts into the chemically bound N2H2 diimide (also called diazene) molecule with only a small energy barrier to overcome.