Kinetics of electron transfer reactions by humic substances: Implications for their biogeochemical roles and determination of their electron donating capacity.

Humic substances (HS) possess redox active groups covering a wide range of potentials and are used by facultative anaerobic microorganisms as electron acceptors. To serve as suitable electron shuttles for anaerobic respiration, HS should be able to re-oxidize relatively quickly to prevent polarization of the surrounding medium. Mediated electrochemical oxidation and decolorization assays, based on the reduction of the radical ion of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS•-) allow to determine the electron donating capacity (EDC) of HS, but uncertainties remain about the reaction time that should be allowed to obtain environmentally meaningful EDC values. In this work, we performed a kinetic analysis of the time trend of the reduction of ABTS•- by HS by Vis and Electron Paramagnetic Resonance (EPR) spectroscopies and by cyclic voltammetry. We found evidences of two concomitant separate mechanisms of electron exchange: a fast and a slow transfer processes which may have different environmental roles. These results can set a base to identify the appropriate conditions for the spectrophotometric determination of the fast and slow components of the EDC of HS.

Molecular synthons for accurate structural determinations: the equilibrium geometry of 1-chloro-1-fluoroethene.

The equilibrium structure for 1-chloro-1-fluoroethene is reported. The structure has been obtained by a least-squares fit procedure using the available experimental ground-state rotational constants of eight isotopologues. Vibrational effects have been removed from the rotational constants using the vibration-rotation interaction constants derived from computed quadratic and cubic force fields obtained with the required quantum chemical calculations carried out by using both coupled cluster and density functional theory. The semi-experimental geometry obtained in this way has been also compared with the corresponding theoretical predictions obtained at the CCSD(T) level after extrapolation to the complete basis set limit and inclusion of core-valence corrections. These results allow completion of the molecular geometries of the isomers of chlorofluoroethene in addition to the cis and trans forms of 1-chloro-2-fluoroethene already published.

Study of the Vibrational Spectra and Absorption Cross Sections of 1-Chloro-1-fluoroethene by a Joint Experimental and Ab Initio Approach.

The gas-phase infrared spectra of 1-chloro-1-fluoroethene (geminal chloro-fluoroethene, ClFC═CH2, 1,1-C2H2ClF) were recorded at medium resolution in the range of 400-6400 cm-1, and the vibrational analysis led to revised assignments for the ν11 (A″ symmetry), ν2 (A' symmetry), and ν1 (A' symmetry) bands. Besides the fundamentals, all the most important spectral features were interpreted in terms of overtone and combination bands, thus obtaining an accurate description of the vibrational structure of ClFC═CH2. Accurate measurements of absorption cross-sectional spectra were carried out, and integrated band intensity data were determined. High-level ab initio calculations of harmonic and anharmonic force fields thoroughly supported and guided the analysis and the disentangling of the several strongly coupled polyads involving many vibrational levels. Diagonalization of the effective Hamiltonian with the off-diagonal elements involving several Fermi and Darling-Dennison resonance coefficients computed by the theoretical cubic and quartic force constants provided the predicted energy levels in good agreement with the vibrational assignments. The calculated infrared intensities, obtained by taking into account anharmonic corrections, were compared to the accurate experimental absorption cross-sectional data determined here.

Anharmonic force field and vibrational dynamics of CH2F2 up to 5000 cm(-1) studied by Fourier transform infrared spectroscopy and state-of-the-art ab initio calculations.

Difluoromethane (CH(2)F(2), HFC-32) is a molecule used in refrigerant mixtures as a replacement of the more environmentally hazardous, ozone depleting, chlorofluorocarbons. On the other hand, presenting strong vibration-rotation bands in the 9 µm atmospheric window, it is a greenhouse gas which contributes to global warming. In the present work, the vibrational and ro-vibrational properties of CH(2)F(2), providing basic data for its atmospheric modeling, are studied in detail by coupling medium resolution Fourier transform infrared spectroscopy to high-level electronic structure ab initio calculations. Experimentally a full quantum assignment and accurate integrated absorption cross sections are obtained up to 5000 cm(-1). Ab initio calculations are carried out by using CCSD(T) theory and large basis sets of either the correlation consistent or atomic natural orbital hierarchies. By using vibrational perturbation theory to second order a complete set of vibrational and ro-vibrational parameters is derived from the ab initio quartic anharmonic force fields, which well compares with the spectroscopic constants retrieved experimentally. An excellent agreement between theory and experiment is achieved for vibrational energy levels and integrated absorption cross sections: transition frequencies up to four quanta of vibrational excitation are reproduced with a root mean square deviation (RMSD) of 7 cm(-1) while intensities are predicted within few km mol(-1) from the experiment. Basis set performances and core correlation effects are discussed throughout the paper. Particular attention is focused in the understanding of the anharmonic couplings which rule the vibrational dynamics of the |ν(1)>, |2ν(8)>, |2ν(2)> three levels interacting system. The reliability of the potential energy and dipole moment surfaces in reproducing the vibrational eigenvalues and intensities as well as in modeling the vibrational and ro-vibrational mixings over the whole 400-5000 cm(-1) region is also demonstrated by spectacular spectral simulations carried out by using the ro-vibrational Hamiltonian constants, and the relevant coupling terms, obtained from the perturbation treatment of the ab initio anharmonic force field. The present results suggest CH(2)F(2) as a prototype molecule to test ab initio calculations and theoretical models.

Toward a complete understanding of the vinyl fluoride spectrum in the atmospheric region.

A deep and comprehensive investigation of the vinyl fluoride (CH(2)CHF) spectrum in the atmospheric window around 8.7 µm is presented. At first, the ro-vibrational patterns are modelled to an effective Hamiltonian, which also takes into account the coupling of the C-F stretching vibration, ν(7), with the neighbouring vibrational combination ν(9)+ν(12). The obtained Hamiltonian gives very accurate simulations and predictions of the ro-vibrational quantum energies. Then, in the main part of the work, an experimental and theoretical study of vinyl fluoride self-broadening collisions is carried out for the first time. The broadening coefficients obtained experimentally are compared with those calculated by a semiclassical theory, demonstrating a significant contribution of collisional coupling effects between lines connecting pairs of degenerate (or nearly degenerate) rotational levels. Finally, the experimentally retrieved integrated absorption coefficients are used to calculate the absorption cross-section of the ν(7) normal mode, from which dipole transition moments are derived. The obtained results provide a deep insight into the spectral behaviour of vinyl fluoride, in a spectral region of primary relevance for atmospheric and environmental determinations. Indeed, the data presented constitute an accurate model for the remote sensing of vinyl fluoride--a molecule of proved industrial importance which can lead to hazardous effects in the atmosphere and affects human's health.

Spectroscopic study of CHBrF(2) up to 9500 cm(-1): Vibrational analysis, integrated band intensities, and ab initio calculations.

The gas-phase infrared spectra of bromodifluoromethane, CHBrF(2), have been examined at medium resolution in the range of 200-9500 cm(-1). The assignment of the absorptions in terms of fundamental, overtone, combination, and hot bands, assisted by quantum chemical calculations is consistent all over the region investigated. Accurate values of integrated band intensities have also been determined for the first time in the range of 500-6000 cm(-1). Structural and molecular spectroscopic properties have been calculated at high level of theory. The coupled cluster CCSD(T) method in conjunction with a hierarchical series of correlation consistent basis sets has been employed and extrapolation to complete basis set has been considered for the equilibrium geometry. Vibrational analysis based on the second order perturbation theory has been carried out with the ab initio anharmonic force constants calculated using the second order Moller-Plesset perturbation as well as coupled cluster [CCSD(T)] theory. A good agreement between the computed and the experimental data also including the integrated infrared band intensities has been obtained.

Infrared spectrum and anharmonic force field of CH2DBr.

The gas-phase infrared spectrum of monodeuteromethyl bromide, CH2DBr, has been examined at medium resolution in the range 400-10000 cm(-1), leading to the identification of 70 vibrational transitions. The assignment of the absorptions in terms of fundamentals, overtones, combinations, and hot bands, assisted by quantum chemical calculations, is consistent all over the region investigated. The (79/81)Br isotopic splitting for the lowest fundamental nu6 and the value for the v8 = 1 level have been now precisely determined. Anharmonic resonances are very marginal for all fundamentals and the Coriolis interaction effects are clearly evident in the nu4/nu8 band system, in the nu2 and nu7 fundamentals. Spectroscopic parameters, obtained from the analysis of partially resolved rotational structure, have been derived in the symmetric tops limit approximation. High-quality ab initio calculations have been performed, and harmonic and anharmonic force fields have been predicted from coupled cluster CCSD(T) calculations employing the cc-pVTZ basis set. A good agreement between computed and experimental data, also including the C-H stretching overtones at 6000 and 9000 cm(-1), has been obtained.

Infrared spectra, integrated band intensities, and anharmonic force field of H2C=CHF.

The gas-phase infrared spectra of vinyl fluoride, H(2)C=CHF, have been examined at medium resolution in the range 400-8000 cm(-1). The assignment of the absorptions in terms of fundamental, overtone, and combination bands, assisted by quantum chemical calculations, is consistent all over the region investigated. Spectroscopic parameters, obtained from the analysis of partially resolved rotational structure of some bands, have been derived and compared with the corresponding calculated values. Accurate values of integrated band intensities have also been determined for the first time. High-level ab initio calculations with large basis sets have been performed. Correlated harmonic force fields have been obtained from coupled cluster CCSD(T) calculations with the cc-pVQZ basis set, while anharmonic force constants have been computed employing the less resource demanding cc-pVTZ basis set. A good agreement between the computed and the experimental data has been obtained including those for the integrated infrared band intensities.

High-resolution infrared study of vinyl fluoride in the 750-1050 cm(-1) region: rovibrational analysis and resonances involving the nu8, nu10, and nu11 fundamentals.

The FTIR spectra of CH2[double bond]CHF have been investigated in the nu(8), nu(10), and nu(11) region between 750 and 1050 cm(-1) at a resolution of about 0.002 cm(-1). The nu(8) vibration of symmetry species A' gives rise to an a/b-type hybrid band, while the nu(10) and nu(11) modes of A' ' symmetry produce c-type absorptions. Due to the proximity of their band origins, the three vibrations perturb each other by Coriolis and high-order anharmonic resonances. In particular, the interactions between the nu(8) and nu(10) modes are very strong and widespread with band origins separated by only 1.37 cm(-1). Besides the expected c-type characteristics, the nu(10) band shows a very intense pseudo a-type component caused by the strong first-order Coriolis resonances with the nu(8) state. Furthermore, the 2nu(9) "dark state" was found to be involved in the interacting band systems. The spectral analysis resulted in the identification of 3144, 3235, and 3577 transitions of the nu(8), nu(10), and nu(11) vibrations, respectively. Almost all the assigned data were simultaneously fitted using the Watson's A-reduction Hamiltonian in the Ir representation and the perturbation operators. The model employed includes nine types of resonances within the tetrad nu(8)/nu(10)/nu(11)/2nu(9) and a set of spectroscopic constants for the nu(8), nu(10), and nu(11) fundamentals as well as parameters for the "dark state" 2nu(9), and fourteen coupling terms have been determined.

Rotational spectra of 1-chloro-2-fluoroethylene. II. Equilibrium structures of the cis and trans isomer.

Equilibrium structures for the cis and trans isomer of 1-chloro-2-fluoroethylene are reported. The structures are obtained within a least-squares fit procedure using the available experimental ground-state rotational constants for various isotopic species of both forms. Vibrational effects were eliminated before the analysis using vibration-rotation interaction constants derived from computed quadratic and cubic force fields with the required quantum chemical calculations carried out using second-order Moller-Plesset perturbation as well as coupled-cluster (CC) theory. The semiexperimental or empirical equilibrium geometries obtained in this way agree well with the corresponding theoretical predictions obtained from CC calculations [at the CCSD(T) level] after extrapolation to the complete basis-set limit and inclusion of core-valence correlation corrections. The present results allow a detailed analysis of the geometrical differences between the two forms of 1-chloro-2-fluoroethylene. They are also compared to the structural data available for other halogenated ethylenes.

Rotational spectra of 1-chloro-2-fluoroethylene. I. Main isotopologues and deuterated species of the trans isomer.

Guided by theoretical predictions, the rotational spectra of the mono- and bideuterated species of trans-1-chloro-2-fluoroethylene, CH35Cl=CDF, CH37Cl=CDF, CD35Cl=CHF, CD37Cl=CHF, CD35Cl=CDF, and CD37Cl=CDF, have been recorded for the first time. Assignment of the Delta J = 0 and Delta K(-1) = +1 bands with K(-1) = 3,4,5,... (all isotopic species) as well as of several Delta J = +/-1 and Delta K(-1) = +1 transitions (all isotopic species except CH37Cl=CDF, CD37Cl=CHF, and CD37Cl=CDF) led to the accurate determination of the ground-state rotational constants, the quartic, and some sextic centrifugal distortion constants, as well as the nuclear quadrupole coupling constants for both 35Cl and 37Cl in good agreement with corresponding theoretical predictions based on high-level coupled-cluster calculations. Inconsistencies of the present spectroscopic parameters with respect to those reported earlier for the two main isotopologues, i.e., CH35Cl=CHF and CH37Cl=CHF, necessitated a reinvestigation of the rotational spectra for these two isotopic species. Supported by quantum chemical calculations, the previously recorded spectra are reassigned to a vibrationally excited state, while analysis of the Delta J = 0 and Delta K(-1) = +1 as well as some Delta J = +/-1 and Delta K(-1) = +1 transitions provided a revised set of spectroscopic parameters for the vibrational ground state of these two isotopic species.

The energetics and structural properties of diazomethyl (HCNN) and cyanomidyl (HNCN) radicals and their related cations and anions from ab initio calculations.

The molecular structures and energetics of diazomethyl (HCNN) and cyanomidyl (HNCN) radicals and their related cations (HCNN(+),HNCN(+)) and anions (HCNN(-),HNCN(-)) are reported at a high level of accuracy. The singles and doubles coupled-cluster method including a perturbational correction for connected triple excitations with systematic sequences of correlation consistent basis sets have been employed. Extrapolation to the complete basis set limit has been used with accurate treatments of core-valence correlation effects in order to accurately predict molecular properties, ionization potentials, electron affinities as well as C-H and N-H bond dissociation energies. For all the species studied, harmonic vibrational frequencies have also been evaluated in order to obtain zero-point corrections to ionization potentials, electron affinities, and dissociation energies.

Trans-1-chloro-2-fluoroethylene: microwave spectra and anharmonic force field.

For the first time the millimeter-wave spectra of the trans-35ClHC=CHF and trans-37ClHC=CHF isotopomers have been observed in natural abundance. Many DeltaJ=0, +/-1 DeltaK(-1)=+1 transitions for 35ClHC=CHF and DeltaJ=0 DeltaK(-1)=+1 transitions for 37ClHC=CHF have been detected and assigned. This allowed us to accurately determine the vibrational ground-state rotational constants, quartic and some sextic centrifugal distortion constants, and nuclear quadrupole coupling constants for both 35Cl and 37Cl. The experimental investigation has been supported by highly accurate theoretical predictions. As far as ab initio computations are concerned, the complete set of cubic and quartic force constants have been evaluated by numerical differentiation of the analytic second-order Møller-Plesset many-body perturbation theory/correlation consistent polarized valence triple zeta second derivatives. The anharmonic part of the force field completes the theoretical study on the equilibrium structure, dipole moment, chlorine quadrupolar tensor, and harmonic force field previously carried out by the same authors.