Infrared bands of CS2 dimer and trimer at 4.5 μm

We report observation of new infrared bands of (CS₂)₂ and (CS₂)₃ in the region of the CS₂ ν₁ + ν₃ combination band (at 4.5 µm) using a quantum cascade laser. The complexes are formed in a pulsed supersonic slit-jet expansion of a gas mixture of carbon disulfide in helium. We have previously shown that the most stable isomer of (CS₂)₂ is a cross-shaped structure with D_2d symmetry and that for (CS₂)₃ is a barrel-shaped structure with D₃ symmetry. The dimer has one doubly degenerate infrared-active band in the ν₁ + ν₃ region of the CS₂ monomer. This band is observed to have a rather small vibrational shift of -0.844 cm^-1. We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel bands and one perpendicular band. Much larger vibrational shifts of -8.953 cm^-1 for the perpendicular band and -8.845 cm^-1 and +16.681 cm^-1 for the parallel bands are observed. Vibrational shifts and possible vibrational assignments, in the case of the parallel bands of the trimer, are discussed using group theoretical arguments.

Micro-solvation of CO in water: infrared spectra and structural calculations for (D2O)2–CO and (D2O)3–CO

The weakly-bound molecular clusters (D₂O)₂–CO and (D₂O)₃–CO are observed in the C–O stretch fundamental region (≈2150 cm⁻¹), and their rotationally-resolved infrared spectra yield precise rotational parameters.

Discerning Inter- and Intramolecular Vibrations of Sulfur Polyaromatic Compounds

Thiophenes are an important class of molecules in fields as diverse as petrochemistry, molecular electronics, and optoelectronics. Thiophenic submolecular motifs are thought to play a role in molecular association and nanoaggregation phenomena in both pure materials and natural and synthetic mixtures. Vibrational (infrared and Raman) spectroscopy provides the means to characterize these species. In this work far-infrared photoacoustic and low-frequency Raman spectra of a series of polycyclic aromatic hydrocarbons containing sulfur have been measured and interpreted using DFT calculations based on a perturbational-variational method coupled with potential truncation. The approach and outcomes illustrate how inter- and intramolecular vibrations for thiophenic systems in single and multicomponent mixtures can be discriminated. This work offers the perspective to search the inter- and intramolecular signatures of the main submolecular motifs and heteroelements postulated as being present in the asphaltenes.

New Infrared Bands of the C-Bonded Isomer of OC-N2O and Determination of Two Intermolecular Frequencies

Three combination bands involving intermolecular modes of the most stable isomer of the OC-N₂O van der Waals complex have been observed, two in the carbon monoxide CO stretch region (∼2150 cm^-1) and one in the ν₃ asymmetric stretch region of N₂O (∼2223 cm^-1). Vibrational assignment is achieved by comparison with data recently published ( Barclay , A. ; et al. Chem. Phys. Lett. 2016 , 62 , 651 ), concerning OC-CO₂. Two of these bands involve the same intermolecular mode, one with the CO stretch and the other with the ν₃ asymmetric stretch of N₂O. The values of intermolecular frequency deduced from these two bands agree very well, showing no significant dependence on intramolecular vibrational excitation.

Detection of a higher energy isomer of the CO–N2O van der Waals complex and determination of two of its intermolecular frequencies

Infrared spectra in the carbon monoxide CO stretch region (∼2150 cm⁻¹) and in the ν₃ asymmetric stretch region of N₂O (∼2223 cm⁻¹) are assigned to the previously unobserved O-bonded form of the CO–N₂O dimer (“isomer 2”).

New combination bands of N2O-CO2, N2O-OCS, and N2O-N2 complexes in the N2O ν(1) region

Spectra of the weakly bound complexes N2O-CO2, N2O-OCS, and N2O-N2 were studied in the region of the ν1 fundamental of N2O (∼2224 cm(-1)) using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion with an effective rotational temperature of about 2.5 K. One new combination band was observed for each complex: a band involving an intermolecular in-plane bending mode for N2O-N2, a band involving the disrotation (in-plane geared bend) for of N2O-CO2, and a band involving the out-of-plane torsional vibration for isomer b of N2O-OCS. Small perturbations were noted for the N2O-OCS band. Because of the absence of theoretical prediction, the nature of the intermolecular bending mode for N2O-N2 has not been identified. The resulting intermolecular frequencies are 34.175(1), 17.107(1), and 22.334(1) cm(-1) for N2O-CO2, N2O-OCS, and N2O-N2, respectively. In addition, the previously known fundamental band of N2O-N2 at 2225.99 cm(-1) was analyzed in improved detail. This band exhibits very weak a-type transitions which were not detected in the first infrared observation of this complex, indicating that N2O-N2 is not exactly T-shaped. That is, the N2O molecular axis is not exactly perpendicular to the a-inertial axis, in agreement with a previous structural determination of this complex by rotational spectroscopy.

Dual beam photoacoustic infrared spectroscopy of solids using an external cavity quantum cascade laser

Quantum cascade laser-based instrumentation for dual beam photoacoustic (PA) spectroscopy is described in this article. Experimental equipment includes a 4.55 μm (2141-2265 cm(-1)) continuous wave external cavity quantum cascade laser (EC-QCL), two gas-microphone PA cells, and two lock-in amplifiers. Correction for the time and wavenumber dependence of the laser output is effected through real-time division of the PA signals derived from the sample and reference channels. Source-compensated mid-infrared absorption spectra of carbon black powder and aromatic hydrocarbon solids were obtained to confirm the reliability of the method. Absorption maxima in the EC-QCL PA spectra of hydrocarbons are better defined than those in Fourier transform infrared spectra acquired under similar conditions, enabling the detection of several previously unknown bands.

Infrared spectra of ethylene clusters: (C2D4)2 and (C2D4)3

Spectra of ethylene dimers and trimers are studied in the ν(11) fundamental band region of C(2)D(4) (≈2200 cm(-1)) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The dimer spectrum is that of a prolate symmetric top perpendicular band, with a distinctive appearance because the A rotational constant is almost exactly equal to six times the B constant. The analysis supports the previously determined cross-shaped dimer structure with D(2d) symmetry. An ethylene trimer has not previously been observed with rotational resolution. The spectrum is that of an oblate symmetric top parallel band. It leads to a proposed trimer structure which is barrel shaped and has C(3h) or C(3) symmetry, with the ethylene monomer C-C axes approximately aligned along the trimer symmetry axis.

Soret effect and photochemical reaction in liquids with laser-induced local heating

We report a theoretical model and experimental results for laser-induced local heating in liquids, and propose a method to detect and quantify the contributions of photochemical and Soret effects in several different situations. The time-dependent thermal and mass diffusion equations in the presence and absence of laser excitation are solved. The two effects can produce similar transients for the laser-on refractive index gradient, but very different laser-off behavior. The Soret effect, also called thermal diffusion, and photochemical reaction contributions in photochemically reacting aqueous Cr(VI)-diphenylcarbazide, Eosin Y, and Eosin Y-doped micellar solutions, are decoupled in this work. The extensive use of lasers in various optical techniques suggests that the results may have significance extending from physical-chemical to biological applications.

Photoacoustic infrared spectroscopy at the Canadian Light Source: commissioning experiments

The commissioning of synchrotron radiation (SR) photoacoustic (PA) infrared spectroscopy at the Canadian Light Source is described in this article. Aperture tests demonstrated an exponential relationship between the wavenumber where SR and thermal-source PA intensities are equal and beam diameter. Total PA intensity increased linearly with aperture size up to 1.5 mm (SR) or 3 mm (thermal source). At larger apertures, this intensity approached a limiting value. The SR beam diameter in the spectrometer was estimated to be about 0.8 mm in these tests. The low-frequency noise and dc offset that characterize SR PA spectra are suppressed through the calculation of average interferograms prior to Fourier transformation.

Invited article: Linearization and signal recovery in photoacoustic infrared spectroscopy

Photoacoustic (PA) infrared spectroscopy enables the characterization of a wide variety of materials, affording the spectroscopist several advantages over more traditional infrared methods. While PA spectra are readily acquired using commercial instrumentation, the quality of the data can be improved substantially through the use of specialized numerical and experimental procedures. Two of these methods are the subject of this review. Specifically, this article describes (a) linearization of PA infrared spectra, a calculation that incorporates phase and amplitude information to extend the range of linearity for strongly absorbing samples, and (b) lock-in and digital signal-recovery procedures in step-scan phase-modulation PA infrared spectroscopy. Linearization yields significant improvement in band definition, especially in the low-wavenumber region. This numerical method succeeds in situations where the PA phase of the sample is less than that of the reference (carbon black). When this criterion is not met initially, the sample or reference interferograms can be manipulated prior to the calculation. The steps involved in linearization are illustrated in detail and approximations are discussed. Lock-in demodulation of the step-scan phase-modulation signal is compared to digital (software) demodulation in this study; the lock-in technique is found to be superior in several cases. The imaginary interferograms in these experiments sometimes lack a strong central feature, a situation that necessitates the application of less commonly used methods for phase correction and spectrum calculation. These methods, which are available in commercial software, include two-quadrant and stored-phase corrections. The PA phase spectrum resembles amplitude and absorption spectra when real and imaginary PA spectra are correctly calculated.

Fourier transform Raman spectroscopy of Syncrude Sweet Blend distillation fractions: the 200-1800 cm-1 region

The fingerprint (200-1800 cm(-1)) region in FT-Raman spectra of Syncrude Sweet Blend (SSB) and its three constituent distillation fractions (naphtha, light gas oil and heavy gas oil) was analyzed in detail in this study. Approximately 50 bands were observed and assigned to functional groups in saturated (alkanes) and unsaturated (aromatics) species. Characteristic bands for mono-, bi-, and tricyclic aromatics were identified and used to quantify these groups in SSB and the distillation fractions. Total aromatics content was determined using the carbon-carbon stretching bands in the 1600-cm(-1) region, and shown to agree with earlier results obtained from the C-H stretching region. The bands due to mono- and bicyclic aromatics permitted calculation of the relative abundances of these species with an accuracy equivalent to that obtained using NMR spectroscopy, a traditional method for measuring this quantity.

Fourier transform Raman spectroscopy of Syncrude sweet blend distillation fractions derived from Athabasca bitumen

The C-H stretching region in FT-Raman spectra of Syncrude sweet blend (SSB) and three distillation fractions (naphtha, light gas oil and heavy gas oil) was analyzed in detail in this investigation. The frequencies and intensities of the 11 aliphatic and three aromatic C-H bands used to fit the spectrum of SSB were equal to the averages (weighted sums) of the corresponding quantities in the spectra of the fractions. The additivity of the spectra, thought to be a consequence of the large number of discrete compounds contained in each fraction, makes it possible to estimate the composition of other SSB samples using the spectra of the fractions reported in this work. In the aromatic C-H region, total intensities can be used to calculate the distribution of aromatics among the distillation fractions; these data also permit calculation of the fractional aromaticity (per cent aromatic carbon) for SSB and each fraction, with accuracies comparable to those obtained using NMR spectroscopy.

A Fifth OH-Stretching Band in IR Spectra of Kaolinites

Transmission infrared spectra of different kaolinites were studied by curve fitting. These spectra generally exhibit four hydroxyl stretching bands. In this article we show that a fifth OH band (already identified in Raman and photoacoustic IR spectra of kaolinites) is also observed in transmission IR spectra of hydrothermal and authigenic kaolinites, which have a high degree of crystallinity. This additional band is weak or undetectable for kaolinites with a low degree of crystallinity. Copyright 1999 Academic Press.

Infrared Study of the Intercalation of Kaolinite by Caesium Bromide and Caesium Iodide

CsBr- and CsI-kaolinite intercalation complexes were synthesized by gradually heating caesium halide disks of the DMSO-kaolinite intermediate up to 330 degreesC. Infrared spectroscopy revealed two types of complexes with the caesium salts: almost nonhydrous, obtained during thermal treatment of the DMSO complex, and hydrated, produced by regrinding the disk in air. Comparison of band positions for CsBr-kaolinite and CsI-kaolinite with those for the CsCl complex (observed in a previous study) shows that the strength of the hydrogen bond between the intercalated halide and the inner surface hydroxyl decreases on the order CsCl > CsBr > CsI. The nonreactivity of CsI in mechanochemical intercalation may arise from weak interaction between I- and inner surface hydroxyl groups, resulting from the fact that caesium is a very soft acid and iodide is a very soft base. Consequently, the very strong interaction between the two ions in the crystal is not disrupted during mechanochemical treatment. Copyright 1998 Academic Press and Minister of Natural Resources, Canada.

Diffuse reflectance infrared spectra of kaolinite and kaolinite/alkali halide mixtures. Curve-fitting of the OH stretching region

Diffuse reflectance infrared spectra of kaolinite and binary mixtures of this clay with alkali halides have been analysed using curve-fitting and deconvolution. For caesium chloride and caesium bromide, decomposition of the hydroxyl stretching region has made it possible to correlate the positions and intensities of bands due to structural hydroxyl groups of kaolinite, or to intercalated water, with the degree of intercalation. One of these bands, arising from uncoupled inner surface OH stretching in kaolinite, is not identifiable in spectra of the uncomplexed clay or mixtures where intercalation did not occur. A new band revealed by curve-fitting spectra of intercalated samples is attributed either to water at the interface between the clay and the alkali halide, or to an FeO—H impurity in the kaolinite. Key words: kaolinite, infrared, curve-fitting.

The Raman spectrum of kaolinite #9 at 21°C

The Raman spectrum of kaolinite #9, a layer silicate of composition Al2Si2O5(OH)4 from Mesa Alta, New Mexico, USA, is reported and compared to previously published Raman and infrared spectra, as well as calculated lattice vibration frequencies, of other kaolinite samples. In the OH stretching region, a Raman band is observed at 3684 cm−1, a frequency which is generally unknown in infrared spectra of kaolinite. The two most likely origins of this band are (a) uncoupled inner-surface hydroxyl stretching, and (b) transverse/longitudinal splitting involving the 3695 cm−1 band, which occurs in both Raman and infrared spectra of kaolinite. The existing data do not conclusively show which of these explanations is correct. In the lattice vibration region, most of the observed Raman bands of kaolinite #9 have been tentatively assigned by comparison with published frequency calculations and existing assignments of infrared spectra of various kaolinites. The descriptions of many of the vibrational modes are approximate, partly because extensive mixing of vibrations makes a simple description of them impossible.

A consistent derivation of the Wilson–Decius s vectors, including new out-of-plane wag formulae

A consistent derivation of the complete set of s vectors needed for vibrational analysis by the Wilson GF matrix method is presented. The vectors obtained for the out-of-plane wag are given by new formulae which are considerably simpler than those presently in use. The formulae for the linear bending coordinate are also derived.

Raman resonance of electron donor/acceptor complex

The resonance Raman excitation profiles of a number of charge transfer transitions in electron donor/acceptor complexes with tetracyanoethylene as acceptor in solution at room temperature are reported and compared with absorption and fluorescence spectra of these complexes. All complexes show distinct anomalies which cannot be accounted for by existing theories unless they are extended. In particular, the excitation profiles peak at the low energy side of the absorption profiles by amounts of the order of 1000-2000 cm-1 and also, in the cases where two charge transfer bands are present, resonance occurs independently for the two bands. The latter observation suggests that the two bands are due to distinct species of the complex with differing geometrical configurations. The former observations is interpreted, in connection with the known asymmetry of the absorption profile and the large Stokes gap between absorption and fluorescence peaks, as arising from the relatively stronger contributions of the pure electronic and vibronic levels in the Stokes gap to the Raman scattering cross-section of the complex, and a frequency dependent damping of the vibronic transitions contributing to resonance. This provides important physical insights into the nature of charge transfer transitions of electron donor/acceptor complexes in general. In our discussion we also refer to similar anomalies in the work of others on the resonance Raman effect of the iodine visible absorption band in solution.