Nitrogen-Broadening Parameters for Atmospheric Spectra Modelling of the ν3 Band of SF6.

The infrared absorption of the ν3 band region of SF6, at temperatures spanning the 130 to 297 K range, has been reexamined using improved instrumentation with one goal: to estimate the broadening of parameters by nitrogen gas. These parameters are compared to previous literature predictions and an extended set of IR cross-sections is proposed and compared to other existing datasets.

A Theoretical Study of HF-CH3Cl and (HF)2-CH3Cl Complexes.

The equilibrium geometries, relative stabilities, and vibrational properties (frequencies and intensities) of the HF-CH3Cl (1:1) and (HF)2-CH3Cl (1:2) complexes were reinvestigated at the MP2/Aug-cc-pVTZ level. The results are discussed in light of the results obtained in solid argon matrices by L. Andrews and co-workers and related to the bonding analysis. The stability and cooperative effects in the hydrogen bonding of the (HF)2-CH3Cl complex in its cyclic form are outlined.

Improved mid infrared detector for high spectral or spatial resolution and synchrotron radiation use.

When using bright, small effective size sources, such as synchrotron radiation light beam, for broadband spectroscopy at spectral or spatial high resolution for mid-IR FTIR measurements, a marked detectivity improvement can be achieved by setting up a device matching the detector optical étendue to that of the source. Further improvement can be achieved by reducing the background unmodulated flux and other intrinsic noise sources using a lower temperature cryogen, such as liquid helium. By the combined use of cooled apertures, cold reimaging optics, filters and adapted detector polarization, and preamplification electronics, the sensitivity of a HgCdTe photoconductive IR detector can be improved by a significant factor with respect to standard commercial devices (more than one order of magnitude on average over 6-20 µm region) and the usable spectral range extended to longer wavelengths. The performances of such an optimized detector developed on the AILES Beamline at SOLEIL are presented here.

On the electronic structure and photochemistry of coordinatively unsaturated complexes: the case of nickel bis-dinitrogen, Ni(N2 )2.

The electronic ground and excited states of the coordinatively unsaturated complex Ni(η(1) -N2 )2 , isolated in an Ar matrix, are analyzed in detail by vibrational and electronic absorption and emission spectroscopies allied with quantum chemical calculations. The bond force constants are determined from a normal coordinate analysis and compared with those of the isoelectronic carbonyl complex. The consequences for the bond properties are discussed, and the trend in the force constants is compared with the standard formation enthalpies. The linear complex Ni(η(1) -N2 )2 with two terminal dinitrogen ligands can be photoisomerized to two isomeric, metastable forms Ni(η(1) -N2 )(η(2) -N2 ) and Ni(η(2) -N2 )2 , with one and two side-on coordinated dinitrogen ligands, respectively.

A rugged, high precision capacitance diaphragm low pressure gauge for cryogenic use.

In order to carry out precise laboratory measurements of infrared absorption intensities, line profiles of molecules and organic volatile compounds for atmospheric chemistry in planetary and upper earth atmospheric layers, precise gas pressure measurement between 10(-3) and a few mbars in the 77-300 K temperature range is necessary. A prototype, rugged, precision capacitive pressure gauge for cryogenic use has been designed, built at SOLEIL and tested down to 77 K. The design includes corrosion-resistant materials and has been tailored to operate on a differential measurement scheme based on a simple, precision capacitance-to-digital converter chip, instead of high precision floating capacitive bridges, as are used in other designs. The designs conception and performance specifications are presented here, illustrated by a precision of better than 1% in the 0.2-40 mbar range, with a resolution of 2 × 10(-3) mbar. The gauge is tunable and can be adjusted for higher precision and a better resolution, at the expense of the maximum high-pressure range.

A new, low temperature long-pass cell for mid-infrared to terahertz spectroscopy and synchrotron radiation use.

A new cell has been designed for accurate spectroscopic measurements in the 80-400 K temperature range with variable path lengths from 3 to more than 141 m. The spectral coverage at these temperatures ranges from the visible to less than 10 cm(-1), thanks to the use of diamond windows. The design of the cryostat and vacuum setups allows vibration-free operation. The equipment provides temperature homogeneity and pressure control to better than 2% over the 100-400 K and the 0.1-1000 mbar ranges. Remote-controlled opto-mechanical systems enable in situ adjustments as well as changes of the optical path length within half an hour, in order to optimize measurement time in an open user facility. It allows then to meet the specific requirements of high resolution measurements on the Far-Infrared AILES beamline at SOLEIL as well at the LISA facility, in Créteil, in the mid-IR. This new instrument opens up the way for many experiments in the field of high-resolution gas-phase IR spectroscopy, in particular, in quantitative spectroscopy for atmospheric applications: measurements of absorption line parameters (absolute intensities, cross sections, and pressure-induced widths) using Fourier transform spectroscopy. The design and performance of the equipment are briefly presented and illustrated on spectroscopic examples.

High-resolution spectroscopy and structure of osmium tetroxide. A benchmark study on 192OsO4.

Osmium tetroxide (OsO(4)) is a heavy tetrahedral molecule that constitutes a benchmark for quantum chemistry calculations. Its favorable spin statistics (due to the zero nuclear spin of oxygen atoms) is such that only A(1) and A(2) (T(d) symmetry) rovibrational levels are allowed, leading to a dense but quite easily resolvable spectrum. We reinvestigate here the ν(1)/ν(3) stretching fundamental (940-980 cm(-1)) dyad region and perform new assignments and effective Hamiltonian parameter fits for the main isotopologue ((192)OsO(4)). We also investigate the ν(2)/ν(4) bending fundamental dyad (300-360 cm(-1)) for the first time and perform a preliminary analysis. New experimental data have been obtained at 0.001 cm(-1) resolution using an isotopically pure (192)OsO(4) sample and the Synchrotron SOLEIL light source. Assignments and analyses were performed using SPVIEW and XTDS software, respectively. We provide precise effective Hamiltonian parameters, including the band centers for all of the fundamental levels and rotational constants for the ground state and for all four fundamental levels. We discuss isotopic shifts, estimate the equilibrium rotational constant B(e), and derive a precise value for the equilibrium bond length r(e)(Os-O) = 1.70919(16) Å. We also performed experiments to measure for the first time the IR integrated intensities for the ν(2)/ν(4) bending fundamental dyad. These new data are compared to current ab initio predictions.

Vibrational spectrum and structure of CoO6: a model compound for molecular oxygen reversible binding on cobalt oxides and salts; a combined IR matrix isolation and theoretical study.

The formation and structure of a novel species, a disuperoxo-cobalt dioxide complex (CoO(6)), has been investigated using matrix isolation in solid neon and argon, coupled to infrared spectroscopy and by quantum chemical methods. It is found that CoO(6) can be formed by successive complexation of cobalt dioxide by molecular oxygen without activation energy by diffusion of ground state O(2) molecules at 9K in the dark. The IR data on one combination and seven fundamentals, isotopic effects, and quantum chemical calculations are both consistent with an asymmetrical structure with two slightly nonequivalent oxygen ligands complexing a cobalt dioxide subunit. Evidence for other, metastable states is also presented, but the data are not complete. The electronic structure and formation pathway of this unique, formally +VI oxidation state, complex has been investigated using several functionals of current DFT within the broken-symmetry unrestricted formalism. It has been shown that the M06L pure local functional well reproduce the experimental observations. The ground electronic state is predicted to be an open shell (2)A'' doublet with the quartet states above by more than 9 kcal/mol and the sextet lying even higher in energy. The ground state has a strong and complex multireference character that hinders the use of more precise multireference approaches and requires caution in the methodology to be used. The geometrical, energetic, and vibrational properties have been computed.

Formation and structure of Co2O4: a combined IR matrix isolation and theoretical study.

The formation and structure of dicobalt tetroxide (Co2O4) has been investigated using matrix isolation in solid neon and argon coupled to infrared spectroscopy and quantum chemical methods. It is found that Co2O4 can be formed by dimerization of cobalt dioxide without activation energy by diffusion of ground state CoO2 molecules at 9 K in the dark. The IR data on eight fundamentals, isotopic effects and quantum chemical calculations are both consistent with a centro-symmetrical structure with two pairs of equivalent oxygen atoms, engaged in a stronger terminal Co-O bond and in a weaker bridging Co-O-Co position. Evidence for other, metastable states is also presented, but the data are not conclusive. The electronic structure and formation pathway has been investigated using the Tao-Perdew-Staroverov-Scuseria/triple-zeta valence polarived basis set (TPSS/TZVP) and broken symmetry unrestricted density functional theory (BS-UDFT) approach and the ground electronic state is predicted to be an open shell ¹Ag singlet with the quintet, triplet, septet, and nonet states above by 3.3, 4.9, 9.3, and 27.7 kcal/mol, respectively, but certainly has a complex multireference character that hinders the use of more precise multireference approaches. Different formation pathways have been considered, and the 2(O═Co═O) → Co2O4 dimerization reaction is found to be the only barrierless channel and to be strongly exothermic. Comparisons with another transition metal (TM) oxide system (V2O4) suggests that the difference in predicted ground state geometries in TM2O4 systems might be due in HOMO-LUMO shapes of the isolated dioxide subunits and optimal overlap configurations.

Evaluation of CO coordination energies from spectroscopic data: on the use of vibrational isotopic effects.

For molecules containing a linearly coordinated carbonyl group, relationships linking 13C and 18O isotopic effects on the CO stretching vibration to the force constant of the M-C coordination bond are proposed. These relationships are rationalized by simple considerations involving the mechanical coupling of the CO and M-C oscillators, tested on model triatomic molecules, and generalized to larger systems. Previous theoretical considerations and several examples presented here show that the long-accepted relation between the shift in the stretching frequency of the coordinated CO with respect to that of isolated CO and the coordination strength has no general predictive power. In contrast, the force constant of the coordination bond can be correlated with the coordination binding energy, and a method for empirically estimating this important parameter from spectroscopic observations of the strongly absorbing CO stretching vibrations of molecular systems or adsorbates is proposed.

The Ni2+O2 reaction: the IR spectrum and structure of Ni2O2. A combined IR matrix isolation and theoretical study.

The formation of Ni2O2 can be observed from the condensation of effusive beams of Ni and O2 in neon or argon matrices. Observation of 58Ni(2)16O2, 58Ni60Ni16O2, 60Ni2(16)O2, Ni(2)18O2 and Ni(2)16O18O isotopic data for five fundamental transitions enable a discussion of structural parameters for matrix-isolated Ni2O2 in its cyclic ground state. Analysis of the nickel isotopic effects on the 58,60Ni2(16)O18O fundamentals suggest an elongated rhombic structure with a Ni-O bond force constant (240+/-10 N m-1) and NiONi bond angles around 79 degrees. The latter points to a Ni-Ni internuclear distance shorter than the O-O one. Low-lying singlet, triplet and quintet states have been studied using density functional theory with an unrestricted wave function and broken symmetry formalism. The high spin states and closed shell singlet states have been also investigated at the CCSD(T) level. The Ni2O2 ground state is calculated to be an antiferromagnetic singlet state with all the hybrid functionals. The first order properties (energies, geometry) calculated with a hybrid functional are very similar when different exchange-correlation functionals with different exact exchange fractions are used and the calculated ground state geometry (NiONi bond angle near 80 degrees, NiO bond distance around 179.5 pm) is in good agreement with the experimental estimate. Nevertheless, a correct reproduction of the experimental vibrational properties is found only when a hybrid functional containing an exact exchange fraction in the 0.4-0.5 range is used. The orbital and topological bonding analyses of Ni2O2 reveal that the relatively short Ni-Ni internuclear distance within the molecule should not be interpreted as a remaining metal-metal bonding interaction, but clearly indicate that the bonding driving force is due to the formation of four strong and highly polarized Ni-O bonds. Even in such an early stage of metal oxidation, the Ni-Ni interaction has virtually disappeared.

A combined electron paramagnetic resonance and fourier transform infrared study of the co(c(6)h(6))(1,2) complexes isolated in neat benzene or in cryogenic matrixes.

The products obtained in the reaction of cobalt atoms in neat benzene or in a benzene/argon mixture at low temperature have been reinvestigated. At least three cobalt-containing species were detected by IR, namely, Co(C(6)H(6)), Co(C(6)H(6))(2), and Co(x)(C(6)H(6)), x>1. The IR bands were assigned to these complexes by monitoring their behavior as a function of (a) Co and C(6)H(6) concentration, (b) isotopic substitution, and (c) photoirradiation. We were able to analyze the sample in neat benzene by both electron paramagnetic resonance (EPR) and IR spectroscopy and to determine the magnetic parameters (g tensor and Co hyperfine interaction) for the Co(C(6)H(6))(2) sandwich compound. The large number of fundamental bands observed in the IR spectrum of Co(C(6)H(6))(2), the absorption pattern observed in the Co-ring stretching region of the IR spectrum of the mixed complex, Co(C(6)H(6))(C(6)D(6)) and the orthorhombic g-values extracted from the EPR spectrum are most consistent with nonequivalent benzene ligands in Co(C(6)H(6))(2), i.e., C(s) symmetry. A bonding scheme consistent with both the EPR and IR data for Co(C(6)H(6))(2) is discussed.

Reactivity of titanium dimer and molecular nitrogen in rare gas matrices. Vibrational and electronic spectra and structure of Ti2N2.

The reactivity of diatomic titanium with molecular nitrogen has been investigated in rare gas matrices. The formation of Ti2N2 from the condensation of effusive beams of Ti and N2 in neon and argon matrices is observed after sample deposition. Our results also show that the in situ formation results from the spontaneous reaction at 9 K of ground state Ti2 with N2. Several low-lying excited states of Ti2N2 are also observed between 0.78 and 1.1 eV above the ground state, leading to a complex sequence of interacting vibronic transitions, merging into a broad continuum above 1.25 eV. Observations of Ti2(14)N2, Ti2(15)N2 and Ti2(14)N(15)N isotopic data enable the determination of all fundamental vibrations in the ground electronic state. Semi-empirical harmonic potential calculations lead to estimates of 3.22 N cm(-1) for the Ti-N bond force constant and 90 +/- 5 degrees for the bond angles. Comparisons with TiN diatomic data suggest a near square-planar structure with 175 +/- 3 pm TiN bond distance. Quantum chemical calculations at various levels indicate a 1A(g) ground state with a Ti-N distance close to 180 pm and 89 degrees for the NTiN bond angle, and give fundamental frequencies in excellent agreement with the experimentally observed values. Further MRCI calculations on all low-lying states enable an interpretation of the complex electronic spectrum in the NIR region.

The Ni + O2 reaction: a combined IR matrix isolation and theoretical study of the formation and structure of NiO2.

The reaction of Ni atoms with molecular oxygen has been reinvestigated experimentally in neon matrices and theoretically at the DFT PW91PW91/6311G(3df) level. Experimental results show that i) the nature of the ground electronic state of the superoxide metastable product is the same in neon and argon matrices, ii) two different photochemical pathways exist for the conversion of the superoxide to the dioxide ground state (involving 1.6 or 4 eV photons) and iii) an important matrix effect exists in the Ni + O(2)--> Ni(O(2)) or ONiO branching ratios. Theoretical results confirm that the electronic ground state of the metastable superoxide corresponds to the singlet state, in agreement with former CCSD(T) calculations, but in contradiction with other recent works. Our results show that the ground electronic state of the dioxide is (1)Sigma(+)(g) with the lowest triplet and quintet states at slightly higher energy, consistent with the observation of weak vibronic transitions in the near infrared. The potential energy profiles are modelled for the ground state and nine electronic excited states and a pathway for the Ni(triplet) + O(2)(triplet) --> Ni(O(2)) or ONiO (singlet) reaction is proposed, as well as for the Ni(O(2)) --> ONiO photochemical reaction, accounting for the experimental observations.

Ni(N2)4 revisited: an analysis of the Ni-N2 bonding properties of this benchmark system on the basis of UV/Vis, IR and Raman spectroscopy.

Matrix isolation of Ni atoms in an N2 matrix leads to the formation of Ni(N2)4. This compound, being isoelectronic with the well known Ni(CO)4, represents an important bench-mark system. It has been characterised experimentally by UV/Vis, IR and Raman spectra. The vibrational spectra give evidence for both a1 modes, three of the four t2 modes, and one of the two e modes of the Td symmetric molecule. The experimental data obtained for Ni(14N2)4 and Ni(15N2)4 were used to determine the valence force constants f(Ni-N) and f(N-N), which are compared with those derived for Ni(N2) and for the corresponding carbonyl complexes Ni(CO) and Ni(CO)4. In addition, several overtones and combination modes of Ni(N2)4 were observed for the first time, providing further valuable information about the bond properties. The data allow for the first time a direct estimate of the Ni-N2 bond energy in Ni(N2)4 (120 kJ mol(-1)), that compares with a value of 148 kJ mol(-1) determined by the same method for Ni(CO)4.

Absorption spectra of ground-state and low-lying electronic States of copper nitrosyl: a rare gas matrix isolation study.

The reaction of ground-state Cu atoms with NO during condensation in solid argon, neon, and binary argon/neon mixtures has been reinvestigated. In addition to the ground-state already characterized in rare gas matrixes by its nu1 mode in reactions of laser-ablated Cu with nitric oxide, another very low lying electronic state is observed for CuNO in solid argon. Photoconversion and equilibrium processes are observed between the two lowest lying electronic states following photoexcitations to second and third excited states in the visible and near-infrared. The electronic spectrum of the CuNO complex was also recorded to understand the photoconversion processes. In solid neon, only the ground state (probably 1A') and the second and third excited states are observed. This suggests that interaction with the argon cage stabilizes the triplet state to make 1A' and 3A' ' states almost isoenergetic in solid argon. On the basis of previous predictions founded on DFT calculations on the very low lying 1A' and 3A' ', a mechanism is proposed, involving the singlet-triplet state manifolds. For these two lower and one higher electronic states, 14N/15N, 16O/18O, and 63Cu/65Cu isotopic data on nu1, nu2, and nu3 have been measured. On the basis of harmonic force-field calculations and relative intensities in the vibronic progressions, some structural parameters are estimated. The molecule is bent in all electronic states, with Cu-N-O bond angles varying slightly around 130 +/- 10 degrees , but the Cu-N bond force constants are substantially different, denoting larger differences in bond lengths.

Reactivity of atomic cobalt with molecular oxygen: a combined IR matrix isolation and theoretical study of the formation and structure of CoO2.

The reactivity of atomic cobalt toward molecular oxygen in rare gas matrices has been reinvestigated. Experiments confirm that Co atoms in their a(4)F ground state are inert toward O(2) in solid argon and neon but reactive in the b(4)F first excited state, in agreement with the previous gas-phase study of Honma and co-workers. The formation of CoO(2) starting from effusive beams of Co and O(2) has been followed by IR absorption spectroscopy, both in neon and argon matrices. Our observations show that only the dioxo form, OCoO, is stabilized in the matrix and that IR absorptions previously assigned to the peroxo and superoxo forms are due to other, larger species. The present data strongly support the linear geometry in rare gas matrices proposed by Weltner and co-workers. We report on measurements on all IR-active fundamental modes for (16)OCo(16)O, (18)OCo(18)O, and (16)OCo(18)O with additional combination transitions supplying anharmonicity correction. This allows for a 5.93 +/- 0.02 mdyne/A CoO harmonic bond force constant in solid neon. Using the empirical relationship previously optimized for the CoO diatomics, an approximate value for the CoO internuclear bond distance is proposed (1.615 +/- 0.01A). In light of recent theoretical studies predicting (2)A(1) or (6)A(1) electronic ground states, the geometry and electronic structure of the OCoO molecule has also been reconsidered. Calculations carried out at the CCSD(T)/6-311G(3df) level indicate a linear structure with an r(e) = 1.62 A bond distance, consistent with the experimental estimate. For later studies of larger systems, where CCSD(T) calculations become too time-consuming, an effective DFT-based method is proposed which reproduces the basic electronic and geometrical properties of cobalt dioxide. Quantitative results are compared to the experimental data and high-level results regarding bond length and frequencies. This DFT method is used to propose a reaction pathway.

Characterization of ground and low-lying excited states of CoO4: a combined matrix isolation and DFT study.

The nature of the reaction products between CoO(2) and molecular O(2), isolated in rare gas matrices, have been investigated using IR absorption spectroscopy. In this paper, we report on the vibrational spectrum of the CoO(4) molecule in its ground and first low-lying excited states. Isotopic substitutions using (16)O(2) and (18)O(2) precursors, as well as (16)O(2) + (18)O(2) and (16)O(2) +( 16)O(18)O + (18)O(2) mixtures in either excess argon or neon, enable demonstration of C(2)(v)() and C(s)() structures for the respective states. CoO(4) is formed following molecular diffusion by complexation of ground-state CoO(2) by an O(2) molecule. The molecule is first formed in the excited state and then spontaneously relaxes to the ground state after remaining in the dark. The kinetics of relaxation can be fitted to a first-order exponential decay with an excited-state lifetime estimated around 23 +/- 2 min in argon and 15 +/- 2 min in neon, indicative of a slow, spin-forbidden process. Population of the excited state is induced by photons around 4250 +/- 250 cm(-1). Experimental results are compared to density functional theory (DFT) calculations at the BPW91/6-311G(3df) level. Electronic and geometrical optimizations were carried out starting from the ground-state precursors (i.e., (3)Sigma(g)(-) for O(2) and (2)Sigma(g)(+) for CoO(2)). Calculations predict a (2)A(2) (C(2)(v)()) ground state and a (4)A' (C(s)()) first excited state 0.37 eV above, close to the 4250 +/- 250 cm(-1) experimental excitation energy. The transition pathway is found to involve two supplementary states with crossed potential energy surfaces (PESs): a (2)B(1) excited state, 0.48 eV above the ground state, reached first through an adiabatic transition with a photon around 4800 cm(-1), and a (4)B(1) transition state into which the system relaxes before finally attaining the (4)A' (C(s)()) excited state. Harmonic frequencies and absolute intensities are also calculated and compared with the experimental data, indicating however that the DFT underestimates the internuclear distances for both configurations. Force and interaction constants were obtained with a semiempirical harmonic force-field potential calculation. They were then used in an empirical rule of plot linking force constants and internuclear distances in order to obtain an estimate of the Co-O bond lengths for each state and are compared to the DFT predictions.

Reactivity of cobalt dimer and molecular oxygen in rare gas matrices. IR spectrum, photophysics and structure of Co2O2.

The reactivity of cobalt dimer towards molecular oxygen has been investigated in rare gas matrices. If the formation of Co2O2 from the condensation of effusive beams of Co and O2 in neon and argon matrices is observed after sample deposition, our results show that the in situ formation does not result from the reaction of ground state Co nor Co2 with molecular oxygen. One reaction channel has been evidenced through reaction of Co2 in excited states, close or above the dissociation limit. Two metastable states of Co2O2 with low-symmetry structures, stabilized by interaction with the matrix cage have also been evidenced between 1.4 and 2 eV above the ground state. Observation of Co2(16)O2, Co2(18)O2 and Co2(16)O(18)O isotopic data for five fundamental and three combination transitions enable determination of all fundamental vibrations for matrix-isolated Co2O2 in its cyclic ground state. Semi-empirical harmonic potential calculations lead to estimates of 2.435 N cm(-1) for the Co-O bond force constant, and 93 +/- 5 degrees OCoO bond angle. In comparison with the CoO diatomic molecule, this suggests a near square-planar structure with a 1.765 +/- 0.01 A CoO bond distance.