Fermi Dyad Research Articles

Collision-Induced infrared absorption and Raman scattering of H2 in supercritical CO2

• The solvation of H 2 in supercritical CO 2 is investigated. • Co-localized infrared/Raman spectroscopic experiments are reported. • A collision induced infrared spectrum of H 2 in supercritical CO 2 is clearly put in evidence. • The effect of the local density fluctuations on the IR and Raman spectra are discussed. In this study, the solvation of H 2 in supercritical CO 2 was investigated by co-localized infrared absorption/Raman scattering spectroscopies. The Fermi dyad and combinations of vibrational modes of CO 2 were detected by Raman and infrared spectroscopy respectively, with increasing intensities according to CO 2 concentrations varying from 1 to 15 mol.L -1 . H 2 rotational bands and vibrons with almost constant intensities were detected by Raman spectroscopy according to a constant H 2 pressure of 3 MPa. In contrast, a collision-induced infrared band of H 2 was reported for the first time in supercritical CO 2 and its intensity was found to be directly proportional to the intensity of the CO 2 bands. In addition, the intensities of the H 2 and CO 2 contributions were found to be sensitive to the local density fluctuations existing near the critical temperature of CO 2 (Tc = 31 °C, Pc = 7.4 MPa).

The perfection of Raman spectroscopic gas densimeters

AbstractRaman spectroscopy can be used to determine density of gases, because the energy of fundamental vibrational modes is affected by intermolecular distances. The key problem is the estimation of exact peak positions of Raman bands, because the analyses require a precision that is mostly less than the pixel resolution of modern Raman spectrometers. A new method to determine peak positions of Raman bands and atomic emission lines in a discontinuous spectrum without numerical manipulations is tested in this study: modified scanning multichannel technique. Relocation of the gratings with a Sinus Arm Drive can be performed over a distance that is only a fraction of the pixel size that allows peak position estimations with precisions smaller than the pixel resolution and to determine the uncertainty in this estimation. This uncertainty was not determined in previous studies about gas densimeters, resulting in a large variety of inconsistent data. The new method is tested with fluid inclusions in quartz. A CO2 density of 0.1477 ± 0.0006 g·cm−3 and 0.8880 ± 0.0007 g·cm−3 determined with microthermometry correspond to a Fermi dyad of 103.12 ± 0.27 cm−1 and 104.71 ± 0.26 cm−1. A CH4 density of 0.3461 ± 0.0002 g·cm−3 and 0.4011 ± 0.0001 g·cm−3 correspond to peak positions of 2910.66 ± 0.12 cm−1 and 2910.57 ± 0.12 cm−1. The error in these numbers must be regarded as the best estimated uncertainties of peak positions, which are probably slightly adjusted to higher values due to mechanical irregularities of the Sinus Arm Drive in modern Raman systems.

How to VPT2: Accurate and Intuitive Simulations of CH Stretching Infrared Spectra Using VPT2+K with Large Effective Hamiltonian Resonance Treatments.

This article primarily discusses the utility of vibrational perturbation theory for the prediction of X-H stretching vibrations with particular focus on the specific variant, second-order vibrational perturbation theory with resonances (VPT2+K). It is written as a tutorial, reprinting most important formulas and providing numerous simple examples. It discusses the philosophy and practical considerations behind vibrational simulations with VPT2+K, including but not limited to computational method selection, cost-saving approximations, approaches to evaluating intensity, resonance identification, and effective Hamiltonian structure. Particular attention is given to resonance treatments, beginning with simple Fermi dyads and gradually progressing to arbitrarily large polyads that describe both Fermi and Darling-Dennison resonances. VPT2+K combined with large effective Hamiltonians is shown to be a reliable framework for modeling the complicated CH stretching spectra of alkenes. An error is also corrected in the published analytic formula for the VPT2 transition moment between the vibrational ground state and triply excited states.

Correlating vibrational properties with temperature and pressure dependent CO2 adsorption in zeolitic imidazolate frameworks

Zeolitic imidazolate frameworks (ZIFs) feature a rigid porous structure where the interplay of pore merits and wall functionality, determined by the different imidazolate functional groups, results in superior CO2 capture ability. In this work, the vibrational properties of ZIF-68 and ZIF-69, two characteristic complex gmelinite (GME) type ZIFs comprising of benzimidazolate (bIm) and chloro-benzimidazolate (cbIm) linkers, respectively, were investigated by micro-Raman spectroscopy as a function of CO2 pressure and temperature, in combination with macroscopic adsorption experiments and extended molecular simulations, in order to explore the underlying host-guest interactions and particularly the variation of the framework lattice dynamics and flexibility to CO2 loading. The CO2 isosteric heat of adsorption (Qst) was quantitatively determined by the temperature dependence of the CO2 Fermi dyad intensity at constant pressure. ZIF-69 was consistently found to present higher Qst than ZIF-68 due to the cbIm polar functionality, in close agreement with macroscopic CO2 adsorption experiments and Monte Carlo analysis. More importantly, high CO2 uptake was found to cause significant blue shifts and enhancement of the frequency shift temperature gradients of several low-frequency Raman modes, which according to detailed polarization analysis of ZIF microcrystals, arise from free breathing vibrations of the functionalized ligands in the large ZIF pores. Low-frequency micro-Raman spectroscopy may accordingly constitute a sensitive spectroscopic tool for unveiling lattice dynamics upon CO2 sorption in ZIFs.

Infrared Spectroscopy of Gas-Phase MnxOy(CO2)z+ Complexes

The interaction of manganese oxide clusters MnxOy+ (x = 2-5, y ≥ x) with CO2 is studied via infrared multiple-photon dissociation spectroscopy (IR-MPD) in the spectral region of 630-1860 cm-1. Along with vibrational modes of the manganese oxide cluster core, two bands are observed around 1200-1450 cm-1 and they are assigned to the characteristic Fermi resonance of CO2 arising from anharmonic coupling between the symmetric stretch vibration and the overtone of the bending mode. The spectral position of the lower frequency band depends on the cluster size and the number of adsorbed CO2 molecules, whereas the higher frequency band is largely unaffected. Despite these effects, the observation of the Fermi dyad indicates only a small perturbation of the CO2 molecule. This finding is confirmed by the theoretical investigation of Mn2O2(CO2)+ revealing only small orbital mixing between the dimanganese oxide cluster and CO2, indicative of mainly electrostatic interaction.

Understanding the anharmonic vibrational structure of the carbon dioxide dimer.

Understanding the vibrational structure of the CO2 system is important to confirm the potential energy surface and interactions in such van der Waals complexes. In this work, we use our previously developed mbCO2 potential function to explore the vibrational structure of the CO2 monomer and dimer. The potential function has been trained to reproduce the potential energies at the CCSD(T)-F12b/aug-cc-pVTZ level of electronic structure theory. The harmonic approximation, as well as anharmonic corrections using vibrational structure theories such as vibrational self-consistent field, vibrational second-order Møller-Plesset perturbation, and vibrational configuration interaction (VCI), is applied to address the vibrational motions. We compare the vibrational results using the mbCO2 potential function with traditional electronic structure theory results and to experimental frequencies. The anharmonic results for the monomer most closely match the experimental data to within 3 cm-1, including the Fermi dyad frequencies. The intermolecular and intramolecular dimer frequencies were treated separately and show good agreement with the most recent theoretical and experimental results from the literature. The VCI treatment of the dimer vibrational motions accounts for vibrational mixing and delocalization, such that we observe the dimer Fermi resonance phenomena, both in the intramolecular and intermolecular regions.

Raman Spectra of Nitrogen, Carbon Dioxide, and Hydrogen in a Methane Environment

Changes in the Raman spectra of N2, H2, and CO2 are studied in the range of 200–3800 cm–1 depending on the concentration of surrounding CH4 molecules at a fixed medium pressure of 25 atm and temperature of 300 K. It has been found that changes in the spectral characteristics of purely rotational H2 lines in a CH4 medium are negligible, while the Q-branches of the v1/2v2 Fermi dyad in СO2 become narrower and wavenumbers of its high-frequency component and v1 band of N2 decrease. In addition, under these conditions, the ratio of intensities of the CO2 Fermi dyad Q-branch varies in proportion to the concentration of surrounding molecules of CH4. The obtained data will be used in diagnosing the composition of natural gas using Raman spectroscopy.

Band shift and bandwidth broadening in Raman spectra of CO2 induced by absorption into an imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate, up to 15 MPa

Raman spectral changes of CO2 induced by the absorption were evaluated for the Fermi dyad continuously up to 15MPa of equilibrium CO2 pressure. Little pressure dependence was observed for both the band positions and the bandwidth of the absorbed CO2, compared to those for neat supercritical CO2. Furthermore, the band positions lay at much lower values than those for neat CO2. The bandwidth broadening was observed for the absorbed CO2 even in the lower pressure region. The absorption state of CO2 is discussed from the viewpoints of structural fluctuations and interaction forces induced by the surrounding ionic liquid.

Fermi resonance in CO2: Mode assignment and quantum nuclear effects from first principles molecular dynamics.

Vibrational spectroscopy is a fundamental tool to investigate local atomic arrangements and the effect of the environment, provided that the spectral features can be correctly assigned. This can be challenging in experiments and simulations when double peaks are present because they can have different origins. Fermi dyads are a common class of such doublets, stemming from the resonance of the fundamental excitation of a mode with the overtone of another. We present a new, efficient approach to unambiguously characterize Fermi resonances in density functional theory (DFT) based simulations of condensed phase systems. With it, the spectral features can be assigned and the two resonating modes identified. We also show how data from DFT simulations employing classical nuclear dynamics can be post-processed and combined with a perturbative quantum treatment at a finite temperature to include analytically thermal quantum nuclear effects. The inclusion of these effects is crucial to correct some of the qualitative failures of the Newtonian dynamics simulations at a low temperature such as, in particular, the behavior of the frequency splitting of the Fermi dyad. We show, by comparing with experimental data for the paradigmatic case of supercritical CO2, that these thermal quantum effects can be substantial even at ambient conditions and that our scheme provides an accurate and computationally convenient approach to account for them.

Determination of pressure in aqueo-carbonic fluid inclusions at high temperatures from measured Raman frequency shifts of CO2

Due to the presence of additional volatiles and/or electrolytes in CO 2 -H 2 O fluids, the total pressure of many natural aqueo-carbonic fluid inclusions at high temperatures as determined using microthermometry is usually made with considerable uncertainty. In this paper, we present the results of our high P - T in situ Raman scattering study of high-density aqueo-carbonic fluids, with and without a small amount of CH 4 and NaCl, whose objective is to derive a new method for pressure determination in aqueo-carbonic fluid inclusions at high temperatures. The measurement of the Fermi dyad bands at temperatures up to 400 °C and pressures up to 1200 MPa is described. The manner in which the frequency shifts and intensity of Raman bands are governed by pressure, temperature, presence of CH 4 in carbonic and NaCl in aqueous fluids is discussed. From the monotonic dependence of the frequency shifts of the lower Fermi dyad band ν − and the Fermi resonant splitting D ( D = ν + − ν − ) with pressure and temperature, the pressure (in MPa) in aqueo-carbonic fluid inclusions at elevated temperatures can be determined directly by using the following two polynomial equations: P ( MPa ) = − 16 + 1 . 232 × T − 53 . 72 × ( Δ ν − ) − 1 . 83 × 10 − 3 × T 2 + 24 . 46 × ( Δ ν − ) 2 − 0 . 292 × T × ( Δ ν − ) , P ( MPa ) = − 26 + 1 . 501 × T + 193 . 24 × ( Δ D ) − 1 . 61 × 10 − 3 × T 2 + 5 . 436 × ( Δ D ) 2 + 0 . 158 × T × ( Δ D ) , where T is in °C, Δ ν − and Δ D represent frequency shifts (in cm −1 ) of the lower band and the resonant splitting relative to the reference values measured at 23 °C and 6 MPa, respectively. Based on the attainable accuracy of the fitted peak positions and the results from fitting of Raman frequency shifts’ dependence with pressure and temperature, the uncertainty in pressure determination is about 50 MPa for pressures determined from ν − and 40 MPa from that determined from D .

Broadening of vibration Q-bands in the Raman spectrum of carbon dioxide due to critical density fluctuations

Detailed analysis of CARS-spectroscopy experimental data of the carbon dioxide Fermi dyad showed that critical density variance should not exceed 0.1 and the lifetime of critical fluctuations is more than 10 ns. To interpret the experimental data, an original calculation of the critical fluctuation contribution to spectral line, considering the lifetime of critical fluctuations, was performed.

Raman spectroscopic study of CO2 in hydrate cages

The Raman spectra of CO2 molecules in hydrate cages was studied for structure I CO2 hydrate and structure II tetrahydrofuran (THF)+CO2 binary hydrate. The results obtained for sII hydrate indicated that the Fermi dyad peaks of CO2 in the small cage of THF–CO2 hydrate are located at 1274cm−1 and 1380cm−1. Numerical fitting of the sI CO2 hydrate Raman spectrum shows CO2 Fermi dyad peaks in the small cage are located at 1275cm−1 and 1382cm−1. The unperturbed frequencies of the symmetric stretching v1 and overtone bending v2 modes were calculated for CO2 in various states. It was found that the v1 frequency does not follow the loose cage–tight cage model for CO2 in hydrate cages. Explanations are given as to why such a relationship is not expected in terms of the incomplete sampling of the void space of the non-spherical cages by the linear guest molecule and the contribution of H-bonding to the guest–host interactions.

Quantitative Raman spectroscopic investigation of geo-fluids high-pressure phase equilibria: Part I. Accurate calibration and determination of CO2 solubility in water from 273.15 to 573.15 K and from 10 to 120 MPa

We developed an accurate method to calibrate Raman system for measuring the concentrations of dissolved gases in aqueous geo-fluids at high pressure, and determined CO2 solubility in water at temperatures from 273.15 to 573.15K and pressures from 10 to 120MPa. The Raman peak area ratio (PAR) and peak intensity ratio (peak height ratio, HR) of the upper band (at ∼1390cm−1) of CO2 Fermi dyad to the OH stretching band of water in homogeneous CO2 solutions were obtained over wide temperature, pressure, and concentration ranges. Pressure and concentration (mCO2) show little effects on PAR/mCO2 and HR/mCO2, while PAR/mCO2 increases linearly with temperature from 273.15 to 573.15K, and HR/mCO2 decreases linearly with temperature at a different rate at temperatures below and above 404.45K. The solubility data obtained from 298 to 523.15K are consistent with previous experimental and thermodynamic studies. However, predictions from commonly-used thermodynamic models for temperatures below 298.15K and above 523.15K substantially deviate from the new measurements, suggesting that new or refined parameters are needed to improve these models.

Infrared laser spectroscopy of the helium-solvated allyl and allyl peroxy radicals

Infrared spectra in the C-H stretch region are reported for the allyl (CH2CHCH2) and allyl peroxy (CH2=CH-CH2OO·) radicals solvated in superfluid helium nanodroplets. Nine bands in the spectrum of the allyl radical have resolved rotational substructure. We have assigned three of these to the ν1 (a1), ν3 (a1), and ν13 (b2) C-H stretch bands and four others to the ν14/(ν15+2ν11) (b2) and ν2/(ν4+2ν11) (a1) Fermi dyads, and an unassigned resonant polyad is observed in the vicinity of the ν1 band. Experimental coupling constants associated with Fermi dyads are consistent with quartic force constants obtained from density functional theory computations. The peroxy radical was formed within the He droplet via the reaction between allyl and O2 following the sequential pick-up of the reactants. Five stable conformers are predicted for the allyl peroxy radical, and a computed two-dimensional potential surface for rotation about the CC-OO and CC-CO bonds reveals multiple isomerization barriers greater than ≈300 cm(-1). Nevertheless, the C-H stretch infrared spectrum is consistent with the presence of a single conformer following the allyl + O2 reaction within helium droplets.

Fermi resonance in solid CO2 under pressure

The symmetric-stretching fundamental (ν1) and the bending first overtone (2ν2) of CO2, which are accidentally degenerate with the same symmetry, undergo a Fermi resonance and give rise to two Raman bands with a frequency difference of 107 cm(-1) and an intensity ratio of 2.1. Both the frequency difference and intensity ratio can be varied by pressure applied to CO2 in condensed phases, which has been utilized as a spectroscopic geobarometer for minerals with CO2 inclusion. This study calculates the pressure dependence of the Fermi dyad frequency difference and intensity ratio by combining the embedded-fragment second-order Mo̸ller-Plesset perturbation calculations of harmonic frequencies of solid CO2 under pressure and the coupled-cluster singles and doubles with noniterative triples and vibrational configuration-interaction calculations of anharmonic frequencies of molecular CO2. It reproduces frequency difference quantitatively and intensity ratio qualitatively up to 10 GPa. The analysis of the results is shown to render strong support for one particular order of unperturbed frequencies, ν1 > 2ν2, in both the gas and solid phases, which has been a matter of controversy for decades.

Microfluidic approach for studying CO2 solubility in water and brine using confocal Raman spectroscopy

This Letter reports a fast and accurate method to quantify dissolved carbon dioxide in water and salted water (NaCl) using microfluidic systems probed by confocal Raman spectroscopy. The relationship of CO2 solubility and Raman band intensity ratios (the νu of the CO2 Fermi dyad over the water stretching band) can be fit from third order polynomial forms, depending on NaCl concentration. This approach allows for a much faster determination of CO2 solubility measurements in pressurized aqueous media. More generally, by easily managing lithospheric fluids in microdevices, this strategy opens avenues towards true ‘Geological Lab on chip’.

Following 18O uptake in scCO2–H2O mixtures with Raman spectroscopy

The uptake of 18O by scC16O2 in mixtures containing liquid H218O was followed with Raman spectroscopy using a specially designed high-pressure optical cell. Characteristic bands from the C16O18O and C18O2 molecules were identified in the supercritical phase and measured in the spectra as a function of time after introducing the liquid H218O into the scC16O2. Temporal dependence indicated the process was diffusion-limited in our cell for both C16O18O and C18O2. The ratio of concentrations of the 18O-labeled CO2 molecules, C18O2/C16O18O, was much higher than a random distribution of the isotopes for the system expected at equilibrium. The results are consistent with previous studies showing both rapid kinetics for oxygen exchange in aqueous solutions and the role of CO2 transport at liquid water interfaces. More importantly, they demonstrate the potential for using Raman spectroscopy with 18O isotopic labeling in scCO2 reaction studies with the recently determined frequency and intensity characteristics of the Fermi dyad peaks from 18O-containing CO2 molecules.

Carbon Dioxide in 1-Butyl-3-methylimidazolium Acetate. I. Unusual Solubility Investigated by Raman Spectroscopy and DFT Calculations

The unusual solubility of carbon dioxide in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been studied by Raman spectroscopy and DFT calculations. It is shown that the solubility results from the existence of two distinct solvation regimes. In the first one (CO(2) mole fraction ≤ 0.35), the usual Fermi dyad is not observed, a fact never reported before for binary mixtures with organic liquids or ionic liquids (IL). Strong experimental evidence complemented by effective DFT modeling shows that this regime is dominated by a chemical reaction leading to the carboxylation of the imidazolium ring accompanied by acetic acid formation. The reactive scheme proposed involves two concerted mechanisms, which are a proton exchange process between the imidazolium cation and the acetate anion and the carboxylation process itself initiated from the formation of "transient" CO(2)-1-butyl-3-methylimidazole 2-ylidene carbene species. In that sense, CO(2) triggers the carboxylation reaction. Moreover, this dynamic picture circumvents consideration of a long-lived carbene formation in dense phase. The second regime is characterized by the detection of the CO(2) Fermi dyad showing that the carboxylation reaction has been strongly moderated. This finding has been interpreted as due to the interaction of the acetic acid molecules with the COO group of acetate anions involved in monodentate forms with the cation. The observation of the Fermi doublet allows us to infer that CO(2) essentially preserves its linear geometry and that the nature and strength of the interactions with its environment should be comparable to those existing in organic liquids and other IL as well. These results have been supported by DFT calculations showing that the CO(2) molecule interacts with energetically equivalent coexisting structures and that its geometry departs only slightly from the linearity. Finally, we find that the CO(2) solvation in Bmim Ac and 1-butyl-3-methylimidazolium trifluoroacetate (Bmim TFA) cannot be straightforwardly compared neither in the first regime due to the existence of a chemical reaction nor in the second regime because CO(2) interacts with a variety of environments not only consisting of ions pairs like in Bmim TFA but also with carboxylate and acetic acid molecule.

Raman spectrum of supercritical C18O2and re-evaluation of the Fermi resonance

We report the first Raman spectra of fully (18)O-labeled supercritical CO(2) (scCO(2)) and various isotopic mixtures. The experimental results, coupled with ab initio molecular dynamics calculations, demonstrate that the frequencies assigned to the Fermi dyad of the CO(2) molecule transpose upon isotopic labeling of both oxygen atoms. Although the transposition of the Fermi dyad of CO(2) gas due to isotopic substitution has been discussed before, this is the first confirmation of the effect in the Raman spectrum of the supercritical fluid and provides necessary groundwork for future Raman spectroscopy studies of reactions in this important medium. More importantly, the work yields a quantitative assessment of the mixing of states upon labeling that provides the needed clarification concerning the pedigree of the assignments for the dyad of CO(2) under supercritical conditions.

Probing local structure of sub and supercritical CO 2 by using two-dimensional Raman correlation spectroscopy

Using two-dimensional correlation spectroscopy and curve fitting procedure, we analyzed the hypothesis that each of the two Raman Fermi dyad modes of CO 2 may be resolved in two contributions. The contribution to the highest frequency side of each vibration dyad mode of CO 2 is associated with molecules experiencing a low local density and the contribution to the lowest frequency side is associated with molecules experiencing a high local density. The Raman spectra were recorded along the liquid–gas coexistence curve of CO 2 in the temperature range between 250 K and 303 K, and along the critical isochore between 306 K and 320 K. The two-dimensional synchronous and asynchronous contour maps obtained by processing the dynamic spectra revealed that these two contributions undergo band shift, band width changes and intensity variation. Particularly, it was shown that the temperature affects in an asymmetric way the high and low frequency sides of the Fermi dyad centered at 1385 cm − 1 . The Fermi dyads were also decomposed into two Lorentzian components. The effect of temperature on the position, the width and the intensity of each of these components was quantified. The results show a systematic red shift of the two Fermi dyads along the coexistence curve and almost no dependence along the critical isochore. More interestingly, in the case of the Fermi dyad centered at 1280 cm − 1 , the width of these two contributions increases along the coexistence curve and remains almost constant along the critical isochore. This behavior was traced back to the increase of the heterogeneous distributions of CO 2 molecules and indicates that there are more different environment of individual CO 2 molecules when approaching the critical temperature. Finally, our results demonstrate the complementarities between two-dimensional spectroscopy and curve fitting procedure in order to analyze the Raman Fermi dyads and to characterize the changes in the local structure of sub and supercritical CO 2.