Unraveling the origin of band shapes in infrared spectra of N2O-12CO2 and 12CO2-13CO2 ice particles

Band structures in the region of strong infrared absorption bands for different N2O-12CO2 and 12CO2-13CO2 composite particles are investigated by combining quantum mechanical exciton calculations with systematic experimental investigations. The ice particles are generated by collisional cooling and characterized with rapid-scan infrared spectroscopy. The size of the particles lies between approximately 10 and 100 nm. The calculated spectra show excellent agreement with the experimental data. This work leads to a detailed understanding on a molecular level of shape effects in pure and statistically mixed particles as well as of the characteristic features observed for core-shell particles.     

Infrared spectroscopy and modeling of co-crystalline CO2·C2H2 aerosol particles. II. The structure and shape of co-crystalline CO2·C2H2 aerosol particles

Infrared absorption spectra of co-crystalline CO(2)·C(2)H(2) aerosol particles were modeled using a combination of two methods. Density functional theory was used to model several bulk CO(2)·C(2)H(2) co-crystal structures and to calculate their lattice energies and frequency-dependent dielectric tensors. This was necessary as there currently exists no crystallographic or refractive index data on co-crystalline CO(2)·C(2)H(2)due to its metastability. The discrete dipole approximation was then used to calculate infrared absorption spectra of different model particles using the dielectric tensors calculated using density functional theory. Results from these simulations were compared to the experimental spectrum of co-crystalline CO(2)·C(2)H(2) aerosol particles. The aerosol particles after the decomposition of the co-crystalline phase were studied in Part I.     

Spectra and integrated band intensities of the low order OH stretching overtones in peroxyformic acid: an atmospheric molecule with prototypical intramolecular hydrogen bonding

The gas phase spectra of several vibrational bands of peroxyformic acid (PFA), an atmospheric molecule exhibiting intramolecular hydrogen bonding, are presented. In the fundamental region, Fourier transform infrared (FT-IR) spectroscopy is used to probe the C-O, O-H and C-H stretching vibrations, while in the region of the first and second OH-stretching overtones (2ν(OH) and 3ν(OH)) photoacoustic spectroscopy is used. Integrated absorption cross sections for the PFA vibrational bands are determined by comparing their respective peak areas with that for the OH-stretching bands of n-propanol for which the absorption cross section is known. The measured integrated intensities of the OH stretching bands are then compared with a local mode model using a one-dimensional dipole moment function in conjunction with the OH stretching potential computed at both the MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels. The data allow us to investigate changes in the OH stretch band position and intensity as a function of overtone order arising from the influence of hydrogen bonding. Furthermore, calculations at the MP2/aug-cc-pVDZ level show that there are three stable conformers of PFA with relative energies of 0, 13.54, and 13.76 kJ/mol, respectively. In the room temperature spectra, however, we see evidence for transitions from only the lowest energy conformer. The geometrical parameters and vibrational frequencies of the most stable conformer are presented.     

Infrared absorption of C6H5SO2 detected with time-resolved Fourier-transform spectroscopy

C6H5SO2 radicals were produced upon irradiation of three flowing mixtures: C6H5SO2Cl in N2, C6H5Cl and SO2 in CO2, and C6H5Br and SO2 in CO2, with a KrF excimer laser at 248 nm. A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to record the time-resolved infrared (IR) absorption spectra of reaction intermediates. Two transient bands with origins at 1087.7 and 1278.2 cm-1 are assigned to the SO2-symmetric and SO2-antisymmetric stretching modes, respectively, of C6H5SO2. Calculations with density-functional theory (B3LYP/aug-cc-pVTZ and B3P86/aug-cc-pVTZ) predict the geometry and vibrational wave numbers of C6H5SO2 and C6H5OSO. The vibrational wave numbers and IR intensities of C6H5SO2 agree satisfactorily with the observed new features. Rotational contours of IR spectra of C6H5SO2 simulated based on predicted molecular parameters agree satisfactorily with experimental results for both bands. The SO2-symmetric stretching band is dominated by a- and c-type rotational structures and the SO2-antisymmetric stretching band is dominated by a b-type rotational structure. When C6H5SO2Cl was used as a precursor of C6H5SO2, C6H5SO2Cl was slowly reproduced at the expense of C6H5SO2, indicating that the reaction Cl+C6H5SO2 takes place. When C6H5Br/SO2/CO2 was used as a precursor of C6H5SO2, features at 1186 and 1396 cm-1 ascribable to C6H5SO2Br were observed at a later period due to secondary reaction of C6H5SO2 with Br. Corresponding kinetics based on temporal profiles of observed IR absorption are discussed.     

Molecular dynamics simulations for CO2 spectra. III. Permanent and collision-induced tensors contributions to light absorption and scattering

Classical molecular dynamics simulations have been performed for gaseous CO(2) starting from an accurate anisotropic intermolecular potential. Through calculations of the evolutions of the positions and orientations of a large number of molecules, the time evolutions of the permanent and collision-induced electric dipole vector and polarizability tensor are obtained. These are computed from knowledge of static molecular parameters taking only the leading induction terms into account. The Laplace transforms of the auto-correlation functions of these tensors then directly yield the light absorption and scattering spectra. These predictions are, to our knowledge, the first in which the contributions of permanent and collision-induced tensors are simultaneously taken into account for gaseous CO(2), without any adjusted parameter. Comparisons between computations and measurements are made for absorption in the region of the ν(3) infrared band and for depolarized Rayleigh scattering in the roto-translational band. They demonstrate the quality of the model over spectral ranges from the band center to the far wings where the spectrum varies by several orders of magnitude. The contributions of the permanent and interaction-induced (dipole and polarizability) tensors are analyzed for the first time, through the purely permanent (allowed), purely induced, and cross permanent∕induced components of the spectra. It is shown that, while the purely induced contribution is negligible when compared to the collision-broadened allowed component, the cross term due to interferences between permanent and induced tensors significantly participates to the wings of the bands. This successfully clarifies the long lasting, confusing situation for the mechanisms governing the wings of the CO(2) spectra considered in this work.     

Spectroscopic characterization of the isolated SF6- and C4F8- anions: observation of very long harmonic progressions in symmetric deformation modes upon photodetachment

Spectroscopic studies of the SF6- and c-C4F8- anions are reported to provide experimental benchmarks for theoretical predictions of their structures and electron binding energies. The photoelectron spectrum of SF6- is dominated by a long progression in the S-F stretching mode, with an envelope consistent with theoretical predictions that the anion preserves the Oh symmetry of the neutral, but has a longer S-F bond length. This main progression occurs with an unexpectedly strong contribution from a second mode, however, whose characteristic energy does not correspond to any of the neutral SF6 fundamental vibrations in its ground electronic state. The resulting doublet pattern is evident when the bare ion is prepared with low internal energy content (i.e., using N2 carrier gas in a free jet or liquid nitrogen-cooling in a flowing afterglow) but is much better resolved in the spectrum of the SF6-.Ar complex. The infrared predissociation spectrum of SF6-.Ar consists of a strong band at 683(5) cm(-1), which we assign to the nu3 (t1u) fundamental, the same mode that yields the strong 948 cm(-1) infrared transition in neutral SF6. One qualitatively interesting aspect of the SF6- behavior is the simple structure of its photoelectron spectrum, which displays uncluttered, harmonic bands in an energy region where the neutral molecule contains about 2 eV of vibrational excitation. We explore this effect further in the c-C4F8- anion, which also presents a system that is calculated to undergo large, symmetrical distortion upon electron attachment to the neutral. The photoelectron spectrum of this species is dominated by a long, single vibrational progression, this time involving the symmetric ring-breathing mode. Like the SF6- case, the c-C4F8- spectrum is remarkably isolated and harmonic in spite of the significant internal excitation of a relatively complex molecular framework. Both these perfluorinated anions thus share the property that the symmetrical deformation of the structural backbone upon photodetachment launches very harmonic motion in photoelectron bands that occur far above their respective adiabatic electron affinities.     

Vibrational delocalization in ammonia aerosol particles

The present contribution investigates shape effects and surface effects in the infrared spectra of pure (NH(3)) and mixed (NH(3)-CO(2) and NH(3)-NH(2)D-NHD(2)-ND(3)) ammonia particles with sizes between about 1 and 50 nm. The particles investigated have been generated in a collisional cooling cell as aerosols at temperatures between 20 and 80 K. The contribution reveals that only the combination with a microscopic model leads to a comprehensive understanding of the various features observed in the experimental infrared spectra. As one of the major results, the corresponding exciton model explains why pronounced shape effects observed for pure particles only play a minor role in the case of mixed particles.     

Infrared analysis of CO ice particles in the aerosol phase

Fourier transform infrared extinction spectra of a variety of CO ice aerosols, generated at low temperatures in a liquid helium cooled collisional-cooling cell, have been analyzed. Different operation modes of the cooling system were used for the generation of spherical and nonspherical CO nanoparticles at temperatures between 5 and 35 K and with diameters between 10 and 1000 nm. In contrast to the predominantly amorphous CO films described in the literature the presented CO particles are (poly)crystalline. A Mie inversion iterative scheme is presented and used to infer the optical constants of CO ice for the cases compact particles have been produced. The spectra of nonspherical CO aerosol particles are interpreted by modeling the extinction using the discrete dipole approximation procedure combined with the retrieved optical constants. A global positive matrix factorization scheme allows us to infer the dominant shapes in the observed particle distribution and can be used as a guide for further experiments. Near 25 K a pronounced shape evolution of smaller particles from spherical toward longish structures was observed at low buffer-gas pressure over 400 s.     

High resolution infrared spectroscopy of carbon dioxide clusters up to (CO2)13

Thirteen specific infrared bands in the 2350 cm(-1) region are assigned to carbon dioxide clusters, (CO(2))(N), with N = 6, 7, 9, 10, 11, 12 and 13. The spectra are observed in direct absorption using a tuneable infrared laser to probe a pulsed supersonic jet expansion of a dilute mixture of CO(2) in He carrier gas. Assignments are aided by cluster structure calculations made using two reliable CO(2) intermolecular potential functions. For (CO(2))(6), two highly symmetric isomers are observed, one with S(6) symmetry (probably the more stable form), and the other with S(4) symmetry. (CO(2))(13) is also symmetric (S(6)), but the remaining clusters are asymmetric tops with no symmetry elements. The observed rotational constants tend to be slightly (≈2%) smaller than those from the predicted structures. The bands have increasing vibrational blueshifts with increasing cluster size, similar to those predicted by the resonant dipole-dipole interaction model but significantly larger in magnitude.     

Infrared absorption of gaseous benzoyl radical C6H5CO recorded with a step-scan Fourier-transform spectrometer

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the gaseous transient species benzoyl radical, C(6)H(5)CO. C(6)H(5)CO was produced either from photolysis of acetophenone, C(6)H(5)C(O)CH(3), at 248 nm or in reactions of phenyl radical (C(6)H(5)) with CO; C(6)H(5) was produced on photolysis of C(6)H(5)Br at 248 nm. One intense band at 1838 ± 1 cm(-1), one weak band at 1131 ± 3 cm(-1), and two extremely weak bands at 1438 ± 5 and 1590 ± 10 cm(-1) are assigned to the C═O stretching (ν(6)), the C-C stretching mixed with C-H deformation (ν(15)), the out-of-phase C(1)C(2)C(3)/C(5)C(6)C(1) symmetric stretching (ν(10)), and the in-phase C(1)C(2)C(3)/C(4)C(5)C(6) antisymmetric stretching (ν(7)) modes of C(6)H(5)CO, respectively. These observed vibrational wavenumbers and relative IR intensities agree with those reported for C(6)H(5)CO isolated in solid Ar and with values predicted for C(6)H(5)CO with the B3LYP/aug-cc-pVDZ method. The rotational contours of the two bands near 1838 and 1131 cm(-1) simulated according to rotational parameters predicted with the B3LYP/aug-cc-pVDZ method fit satisfactorily with the experimental results. Additional products BrCO, C(6)H(5)C(O)Br, and C(6)H(5)C(O)C(6)H(5) were identified in the C(6)H(5)Br/CO/N(2) experiments; the kinetics involving C(6)H(5)CO and C(6)H(5)C(O)Br are discussed.     

Matrix isolation infrared spectroscopic and density functional theory studies on the reactions of yttrium and lanthanum hydrides with carbon monoxide

Laser-ablated yttrium and lanthanum hydrides have been co-deposited at 4 K with carbon monoxide in excess argon. Products, such as HYCO, (HY)2CO, HLaCO, HLa(CO)2, and H2LaCO, have been formed in the present experiments and characterized using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, the change of reagent concentration and laser energy, and the comparison with theoretical predictions. Density functional theory calculations have been performed on these molecules. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these molecules from the matrix infrared spectra. Plausible reaction mechanisms have been proposed to account for the formation of these molecules.     

Vibrational quasi-degenerate perturbation theory: applications to fermi resonance in CO2, H2CO, and C6H6

A quasi-degenerate perturbation method with vibrational self-consistent field (VSCF) reference wavefunction is developed. It simultaneously accounts for strong anharmonic mode-mode coupling among a few states (static correlation) by a configuration interaction theory and for weak coupling with a vast number of the other states (dynamic correlation) by a perturbation theory. A general formula is derived based on the van Vleck perturbation theory. An algorithm that selects a compact set of the most important VSCF configurations which contribute to the static correlation is proposed and a scheme to limit the number of configurations considered for dynamic correlation is also implemented. This method reproduces the vibrational frequencies of CO2 and H2CO that are subject to the strongest anharmonic mode-mode coupling within 10 cm(-1) of vibrational configuration interaction results in a computational expense reduced by a factor of one to two orders of magnitude. The method also reproduces the infrared absorption of C6H6 in the CH stretching (nu12) frequency region, in which combination tones nu13nu16 and nu2nu13nu18 appear on account of an intensity borrowing from nu12via the anharmonic coupling.     

Isotopic mixed crystal exciton spectra in the far infrared: Naphthalene

The far infrared vibrational exciton spectra of isotopic mixed crystals of naphthalene-h₈ and d₈ were studied. The two observed translational phonon modes were determined to fall into the amalgamated band limit while the lowest energy B_3u, A_u and B_1u vibrational exciton bands were found to be in the separated band limit. The lowest energy B_3u “butterfly” mode with its large (15 cm^?1) exciton splitting was found to agree well with CPA calculations of mixed crystal spectra. A peak at 185 cm^?1 was also assigned as a peak in the vibrational exciton density-of-states of the B_3u mode.     

Structure of CO2 adsorbed on the KCl(100) surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.     

SiH2Cl2: ab initio anharmonic force field, dipole moments, and infrared vibrational transitions

The vibrational spectra of SiH2Cl2 have been recorded in the 1000-13,000 cm(-1) region, utilizing the Fourier-transform spectroscopy and Fourier-transform intracavity laser absorption spectroscopy. Totally 61 band centers and intensities are derived from the infrared spectra. An ab initio quartic force field is obtained by applying the second-order Moller-Plesset perturbation theory and correlation-consistent polarized valence triplet-zeta basis sets [J. Chem. Phys. 90, 1007 (1989); 98, 1358 (1993)]. Most observed bands are assigned by the vibration analysis based on the second-order perturbation theory. Reduced-dimensional ab initio dipole moment functions (two dimensional and three dimensional) have also been calculated to investigate the absolute band intensities of the SiH2 chromophore. The calculated values agree reasonably with the observed ones.     

Size effects in the infrared spectra of NH3 ice nanoparticles studied by a combined molecular dynamics and vibrational exciton approach

Infrared extinction spectra of ammonia ice nanoparticles with radii between 2 and 10 nm show pronounced band shape variations depending on the conditions of particle formation by collisional cooling. We present experimental and theoretical evidence showing that the variations in the region of the nu2 (umbrella) fundamental are due to changes in the particle size. The effect is analyzed in terms of an explicit atomistic model of the particles' structure and vibrational dynamics. An explicit potential function combined with a novel extension of the vibrational exciton approach allows us to simulate extinction spectra for particles containing up to 16,000 atoms. It is shown that the particles formed under the conditions of our experiments consist of a crystalline core surrounded by an amorphous shell with an approximately constant thickness of 1-2 nm. For the nu2 fundamental, this shell gives rise to a broad band [full width at half maximum (FWHM) 72 cm(-1)] blueshifted by about 19 cm(-1) relative to a narrow peak (FWHM of 19 cm(-1)) which arises from the crystalline core.     

Vibrational spectra and gas phase structure of N-cyanoimidosulfurous difluoride, NCN=SF2

The vibrational spectra, IR (gas) and Raman (liquid) of N-cyanoimidosulfurous difluoride, NCN=SF2, were recorded, and the molecular structure was determined by gas electron diffraction. The spectra were assigned by comparing the vibrational frequencies with those in related molecules and with calculated (HF, MP2, B3LYP with 6-31G(d) basis sets) values, and a normal coordinate analysis was performed. The molecule possesses a syn conformation (Ctriple bondN syn with respect to the bisector of the SF2 angle). This has been rationalized by orbital interactions of the electron lone pairs of sulfur and nitrogen with the N-C and S-F bonds, respectively, which are antiperiplanar or anticlinal to these lone pairs (anomeric effects). Quantum chemical calculations with the B3LYP and MP2 methods reproduce the experimental structure reasonably well if large basis sets (6-311G(2d,f)) are used.     

A theoretical anharmonic study of the infrared absorption spectra of FHF-, FDF-, OHF-, and ODF- anions

Anharmonic vibrational frequencies, equilibrium bond lengths, rotational constants, and vibrational absorption spectra have been calculated for the triatomic anions, FHF(-) and OHF(-), and the heavier isotopomers FDF(-) and ODF(-). The triatomic anions are assumed to maintain a collinear configuration throughout all calculations, so only the symmetric (nu(1)) and asymmetric (nu(3)) stretching modes are considered. The two-dimensional permanent dipole surfaces and potential energy surfaces are then constructed along bond coordinates, using high-level ab initio methods. Fundamental and combination bands are obtained from the vibrational eigenfunctions, resulting in anharmonic frequencies, which can be compared with the available theoretical and experimental data. The agreement is very good, especially for the pure symmetric modes, while the asymmetric ones show larger discrepancies, presumably due to the neglected coupling between stretching and bending modes. Strong inverse anharmonicity is found in the level spacing of the asymmetric modes, for both FHF(-) and OHF(-) anions. The calculated mixed modes (nnu(1)+mnu(3), n, m=0-3) also agree reasonably with the few available experimental data, supporting our model. Based on the vibrational eigenfunctions, isotope effects are also rationalized. Infrared absorption spectra are calculated from the dipole autocorrelation function for FHF(-) and FDF(-), and for OHF(-) and ODF(-). Peak locations and relative intensities are assigned in terms of the fundamental and mixed transitions.     

Fourier transform-infrared and Raman spectra, ab initio calculations and assignments for 6-methyl-4-bromomethylcoumarin

Fourier transform-infrared (4000-400 cm-1) and Raman (3500-50 cm-1) spectral measurements have been made for 6-methyl-4-bromomethylcoumarin. Equilibrium structures, harmonic vibrational frequencies, infrared intensities, and depolarization ratios have been computed at RHF/6-31G* and B3LYP/6-31G* levels of theory. Twisting CH2Br moiety in the geometry optimization leads to the most stable conformer lacking symmetry (C1). This is reflected in the richness of bands in the experimental spectra. A complete assignments of the bands, aided by the ab initio calculations, has been proposed for the 6-methyl-4-bromomethylcoumarin. Due to lack of symmetry, several normal vibrations have been found to be mixed ones.     

Infrared absorption of CH3OSO detected with time-resolved Fourier-transform spectroscopy

A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to detect temporally resolved infrared absorption spectra of CH(3)OSO produced upon irradiation of a flowing gaseous mixture of CH(3)OS(O)Cl in N(2) or CO(2) at 248 nm. Two intense transient features with origins near 1152 and 994 cm(-1) are assigned to syn-CH(3)OSO; the former is attributed to overlapping bands at 1154 ± 3 and 1151 ± 3 cm(-1), assigned to the S=O stretching mixed with CH(3) rocking (ν(8)) and the S=O stretching mixed with CH(3) wagging (ν(9)) modes, respectively, and the latter to the C-O stretching (ν(10)) mode at 994 ± 6 cm(-1). Two weak bands at 2991 ± 6 and 2956 ± 3 cm(-1) are assigned as the CH(3) antisymmetric stretching (ν(2)) and symmetric stretching (ν(3)) modes, respectively. Observed vibrational transition wavenumbers agree satisfactorily with those predicted with quantum-chemical calculations at level B3P86∕aug-cc-pVTZ. Based on rotational parameters predicted at that level, the simulated rotational contours of these bands agree satisfactorily with experimental results. The simulation indicates that the S=O stretching mode of anti-CH(3)OSO near 1164 cm(-1) likely makes a small contribution to the observed band near 1152 cm(-1). A simple kinetic model of self-reaction is employed to account for the decay of CH(3)OSO and yields a second-order rate coefficient k=(4 ± 2)×10(-10) cm(3)molecule(-1)s(-1).