Accurate Vibrational-Rotational Parameters and Infrared Intensities of 1-Bromo-1-fluoroethene: A Joint Experimental Analysis and Ab Initio Study.

The medium-resolution gas-phase infrared (IR) spectra of 1-bromo-1-fluoroethene (BrFC═CH2, 1,1-C2H2BrF) were investigated in the range 300-6500 cm-1, and the vibrational analysis led to the assignment of all fundamentals as well as many overtone and combination bands up to three quanta, thus giving an accurate description of its vibrational structure. Integrated band intensity data were determined with high precision from the measurements of their corresponding absorption cross sections. The vibrational analysis was supported by high-level ab initio investigations. CCSD(T) computations accounting for extrapolation to the complete basis set and core correlation effects were employed to accurately determine the molecular structure and harmonic force field. The latter was then coupled to B2PLYP and MP2 computations in order to account for mechanical and electrical anharmonicities. Second-order perturbative vibrational theory was then applied to the thus obtained hybrid force fields to support the experimental assignment of the IR spectra.

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.

The energetic of (CH2F2)2 investigated by TDL IR spectroscopy and DFT computations: From collision induced relaxation of ro-vibrational transitions to non-covalent interactions.

Difluoromethane (CH2F2) is an atmospheric pollutant presenting strong absorptions within the 8-12 µm atmospheric window, hence it can contribute to global warming. Its dimer, (CH2F2)2, is bound through weak hydrogen bonds (wHBs). Theoretically, wHBs are of paramount importance in biological systems, though their modeling at density functional theory (DFT) level requires dispersion correlations to be accounted for. In this work, the binding energy (3.1 ± 0.5 kcal mol(-1)) of (CH2F2)2 is experimentally derived from the foreign broadening coefficients of the monomer compound, collisionally perturbed by a range of damping gases. Measurements are carried out on CH2F2 ro-vibrational transitions by means of tunable diode laser spectroscopy. Six stationary points on the potential energy surface (PES) of the dimer are investigated at DFT level by using some of the last generation density functionals (DFs). The Minnesota M06 suite of functionals as well as range separated DFs and DFs augmented by the non-local (NL) van der Waals (vdW) dispersion corrections are considered. DFT results are compared to reference values at the estimated complete basis set (CBS) limit of CCSD(T) theory (coupled cluster with singles and doubles augmented by a perturbational estimate of connected triples) and to the experimental binding energy. The M06-2X, M06-HF, VV10, BLYP-NL, and B3LYP-NL DFs reproduce CCSD(T)/CBS binding energies with a mean absolute deviation <0.4 kcal mol(-1) and about the same deviation from the experimental value. The present results are of twofold relevance: (i) they show that binding energy of homodimers can be conveniently obtained from the monomer's foreign broadening coefficients and that the correct simulation of hydrogen bonds involved in (CH2F2)2 needs non-covalent interactions to be included into DFT; (ii) O2- and N2-pressure broadening parameters represent fundamental data for exploiting the efficacy of remote sensing measurements employed to retrieve temperature and concentration profiles of our atmosphere.

Insights into the interaction between CH2F2 and titanium dioxide: DRIFT spectroscopy and DFT analysis of the adsorption energetics.

Difluoromethane (CH2F2, HFC-32) has been proposed as a valid replacement for both CFCs and HCFCs (in particular HCFC-22), and nowadays it is widely used in refrigerant mixtures. Due to its commercial use, in the last years, the atmospheric concentration of HFC-32 has increased significantly. However, this molecule presents strong absorptions within the 8-12µm atmospheric window, and hence it is a greenhouse gas which contributes to global warming. Heterogeneous photocatalysis over TiO2 surface is an interesting technology for removing atmospheric pollutants since it leads to the decomposition of organic compounds into simpler molecules. In the present work, the adsorbate-substrate interaction between CH2F2 and TiO2 is investigated by coupling experimental measurements using DRIFT spectroscopy to first-principle simulations at DFT/B3LYP level. The experimental results confirm that CH2F2 interacts with the TiO2 surface (∼80% rutile, 20% anatase) through both F and H atoms and show that the DRIFT technique is well suited to study the adsorption of halogenated methanes over semiconductor surfaces. DFT calculations are carried out by considering different periodicities and surface coverages, according to a structure involving an acid-base interaction between the F and Ti(4+) atoms as well as an H-bond between the CH2 group and an O(2-) ion. Lateral effects and energetics are analyzed in the limit of low coverage according to a procedure taking into account the binding, interaction, and distortion energies. The simulation at the different surface coverages and periodicities suggests similar decomposition pathways for the different investigated ensemble configurations.

N2-, O2- and He-collision-induced broadening of sulfur dioxide ro-vibrational lines in the 9.2 µm atmospheric window.

Sulfur dioxide (SO2) is a molecule of considerable interest for both atmospheric chemistry and astrophysics. In the Earth's atmosphere, it enters in the sulfur cycle and it is ubiquitous present in polluted atmospheres, where it is responsible for acid rains. It is also of astrophysical and planetological importance, being present on Venus and in interstellar clouds. In this work the collisional broadening of a number of ν1 ro-vibrational lines of SO2 perturbed by N2, O2 and He are investigated at room temperature in the 9 µm atmospheric region by means of high resolution tunable diode laser (TDL) infrared spectroscopy. From N2- and O2-broadening coefficients, the broadening parameters of sulfur dioxide in air, useful for atmospheric applications, are derived as well. From the present measurements some conclusions on the quantum number dependence of the N2-, O2- and He-broadening coefficients are drawn. While the J dependence is weak for all the perturbers investigated, different trends with Ka are reported. N2-broadening coefficients show a slight decrease with increasing values of Ka, whereas O2 and He broadening cross sections first increase up to Ka(″)≈6 and then they keep a nearly constant value. A comparison and a brief discussion on the efficiency of self-, N2-, O2- and He-collisional dynamics are given. The data obtained represent a significant analysis on foreign broadening of SO2 useful for atmospheric remote sensing and astrophysical applications.

An integrated experimental and quantum-chemical investigation on the vibrational spectra of chlorofluoromethane.

The vibrational analysis of the gas-phase infrared spectra of chlorofluoromethane (CH2ClF, HCFC-31) was carried out in the range 200-6200 cm(-1). The assignment of the absorption features in terms of fundamental, overtone, combination, and hot bands was performed on the medium-resolution (up to 0.2 cm(-1)) Fourier transform infrared spectra. From the absorption cross section spectra accurate values of the integrated band intensities were derived and the global warming potential of this compound was estimated, thus obtaining values of 323, 83, and 42 on a 20-, 100-, and 500-year horizon, respectively. The set of spectroscopic parameters here presented provides the basic data to model the atmospheric behavior of this greenhouse gas. In addition, the obtained vibrational properties were used to benchmark the predictions of state-of-the-art quantum-chemical computational strategies. Extrapolated complete basis set limit values for the equilibrium geometry and harmonic force field were obtained at the coupled-cluster singles and doubles level of theory augmented by a perturbative treatment of triple excitations, CCSD(T), in conjunction with a hierarchical series of correlation-consistent basis sets (cc-pVnZ, with n = T, Q, and 5), taking also into account the core-valence correlation effects and the corrections due to diffuse (aug) functions. To obtain the cubic and quartic semi-diagonal force constants, calculations employing second-order Møller-Plesset perturbation (MP2) theory, the double-hybrid density functional B2PLYP as well as CCSD(T) were performed. For all anharmonic force fields the performances of two different perturbative approaches in computing the vibrational energy levels (i.e., the generalized second order vibrational treatment, GVPT2, and the recently proposed hybrid degeneracy corrected model, HDCPT2) were evaluated and the obtained results allowed us to validate the spectroscopic predictions yielded by the HDCPT2 approach. The predictions of the deperturbed second-order perturbation approach, DVPT2, applied to the computation of infrared intensities beyond the double-harmonic approximation were compared to the accurate experimental values here determined. Anharmonic DFT and MP2 corrections to CCSD(T) intensities led to a very good agreement with the absorption cross section measurements over the whole spectral range here analysed.

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.

Microwave, high-resolution infrared, and quantum chemical investigations of CHBrF2: ground and v4 = 1 states.

A combined microwave, infrared, and computational investigation of CHBrF(2) is reported. For the vibrational ground state, measurements in the millimeter- and sub-millimeter-wave regions for CH(79)BrF(2) and CH(81)BrF(2) provided rotational and centrifugal-distortion constants up to the sextic terms as well as the hyperfine parameters (quadrupole-coupling and spin-rotation interaction constants) of the bromine nucleus. The determination of the latter was made possible by recording of spectra at sub-Doppler resolution, achieved by means of the Lamb-dip technique, and supporting the spectra analysis by high-level quantum chemical calculations at the coupled-cluster level. In this context, the importance of relativistic effects, which are of the order of 6.5% and included in the present work using second-order direct perturbation theory, needs to be emphasized for accurate predictions of the bromine quadrupole-coupling constants. The infrared measurements focused on the ν(4) fundamental band of CH(79)BrF(2). Fourier transform investigations using a synchrotron radiation source provided the necessary resolution for the observation and analysis of the rotational structure. The spectroscopic parameters of the v(4) = 1 state were found to be close to those of the vibrational ground state, indicating that the ν(4) band is essentially unaffected by perturbations.

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.

Spectroscopic measurements of SO(2) line parameters in the 9.2 mum atmospheric region and theoretical determination of self-broadening coefficients.

Sulfur dioxide is still the subject of numerous spectroscopic studies since it plays an active role in the chemistry of Earth's atmosphere and it is a molecule of proven astrophysical importance. In the present work we have determined the self-broadening and integrated absorption coefficients for several lines in the nu(1) band spectral region around 9.2 mum. Besides the parameters of the lines belonging to the nu(1) fundamental of (32)SO(2), also those for some rovibrational lines of the nu(1)+nu(2)-nu(2) hot band of the (32)SO(2) isotopologue and the nu(1) band of the (34)SO(2) isotopic species have been determined. The measurements have been carried out at 297 K using a tunable diode laser spectrometer. The self-broadening parameters have also been theoretically determined employing a semiclassical formalism based on the Anderson-Tsao-Curnutte approximation. The study has been completed with the determination of the vibrational cross sections of the three fundamental bands measured from the spectra recorded at a resolution of 0.2 cm(-1) using a Fourier transform infrared spectrometer.

Adsorption of difluoromethane on titanium dioxide: investigation of the FTIR spectra and quantum-mechanical studies of the adsorbate-substrate structures.

The interaction of difluoromethane (CH(2)F(2)) with the TiO(2) surface (P25 Degussa) at room temperature has been studied by Fourier-transform infrared spectroscopy for the first time. From the comparison between the adsorption characteristics and the gas-phase spectra it can be deduced that the molecule adsorbs through an acid-base interaction between one F atom and the surface Lewis acid site (Ti(4+)) and an H-bond between the CH(2) group and the surface Lewis basic site (OH(-) or O(2-)). In order to obtain more information about the orientation geometry and the variation of the molecular structural parameters, a quantum-mechanical investigation at DFT/B3LYP level has been also performed, considering the anatase (101) surface and focusing on the O(2-) as Lewis basic site. The resulting adsorbate-substrate structures involve the formation of an acid-base interaction between one F atom and the Ti(4+) ion and differ for the number of the involved H-bonds. According to the scaled vibrational frequencies, the simulated adsorption model which better agrees with the experimental data corresponds to that in which the CH(2) group interacts with the surface by only one H-bond.

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 FTIR, microwave, and ab initio investigations of CH2 79BrF: ground, v(5) = 1, and v(6) = 1, 2 state constants.

A spectroscopic study of CH279BrF in the infrared and microwave regions has been carried out. The rovibrational spectrum of the nu5 fundamental interacting with 2nu6 has been investigated by high-resolution FTIR spectroscopy. Owing to the weakness of the 2nu6 band, the v6 = 2 state constants have been derived from v6 = 1. For this reason, the rotational spectra of the ground and v6 = 1 states have been observed by means of microwave spectroscopy. Highly accurate ab initio computations have also been performed at the CCSD(T) level of theory in order to support the experimental investigation. As far as the nu5 band is concerned, the analysis of the rovibrational structure led to the identification of more than 3000 transitions, allowing the determination of a set of spectroscopic parameters up to sextic distortion terms and pointing out first-order c-type Coriolis interaction with the v6 = 2 state. With regard to the pure rotational spectra measurements, the assignment of several DeltaJ = 0, +1 transitions allowed the determination of the rotational, all the quartic, and most of the sextic centrifugal distortion constants, as well as the full bromine quadrupole coupling tensor for both the ground and v6 = 1 states.

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.