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Cu2ZnSn(S,Se)4 (CZT(S,Se)) thin films exhibit the characteristics necessary to be effective absorbers in solar cells. In this report, the room temperature experimental Raman scattering spectra, recorded at different excitation wavelengths, are systematically analyzed theoretically using the results of DFT harmonic frequencies calculations at the Γ-point for various modifications of kesterite (KS), stannite (ST), and pre-mixed Cu-Au (PMCA) crystal structures. The specific anharmonism‐induced features in the spectra of CZT(S,Se) crystals are identified, and the spectral lineshapes at varied strengths of anharmonic interaction are simulated. A robust agreement between the experimental Raman spectrum and the theoretical results is demonstrated, ensuring the reliability of estimating parameters related to anharmonic effects. Therefore, our findings show that incorporating anharmonism as an additional contribution to the phonon spectra, particularly in non-resonant cases, allows for a more accurate description of the vibrational properties of CZT(S,Se). This could play a crucial role in distinguishing between different phases of CZT(S,Se) materials and open new possibilities for the fabrication of solar cells with enhanced characteristics. [ABSTRACT FROM AUTHOR]

The Raman spectra in the C-H stretching region are of great importance for the study of the structure and dynamics of organic compounds. However, the Fermi resonance between the first overtone mode of C-H bending vibration and C-H stretching vibration typically results in the disturbance of Raman bands in the C-H stretching region. In this context, a specific deuterated molecule with only one C–H bond was proposed, and it was found that the frequency of the first overtone mode of the C-H bending vibration was significantly different from the frequency of the C-H stretching vibration. Due to the significant discrepancy, Fermi resonance in the C-H stretching region was eliminated from the experimental and theoretical Raman spectra of deuterated leucine, deuterated benzoin, deuterated methanol, and deuterated ethanol. Hence then, the Raman spectra of these specific deuterated compounds in the C-H stretching region can be used to study the structure or the dynamics of the organic compounds. [ABSTRACT FROM AUTHOR]

Studying the vibrational energy flow within molecules can help us better understand their reactivity and facilitate the design of novel functional groups for potential drug candidates. This study examined the conformation of benzene sulfonamide in solvents and vacuum. Potential energy surfaces were computed for the two possible conformations, showing that the conformers' stability varied with the medium. Additionally, the vibrational energy redistribution of halogen‐substituted benzene sulfonamide conformers was studied in solvents and vacuums. The lifetimes of the halogen stretching mode C−X (where X=F, Cl, or Br) in the substituted benzene sulfonamides were calculated and compared. The solvent and vacuum media lifetimes followed the order: C−Cl>C−Br>C−F. However, the degree of delocalization of vibrational energy was found to vary between them. [ABSTRACT FROM AUTHOR]

The IR transmittance spectrum of an isotopic mixture of chlorodifluoromethane (CHF2Cl, Ereon-22) with a 33% fraction of 13C and a natural ratio of chlorine isotopes was measured in the frequency range 1400–740 cm–1 with a resolution of 0.001 cm–1 at a temperature of 20°C. An ab initio calculation of the structure and sextic potential energy surface and surfaces of the components of the dipole moment has been carried out by the electronic quantum-mechanical method of Moller–Plesset, MP2/cc-pVTZ. Then the potential was optimized by replacing the harmonic frequencies with the frequencies calculated by the electronic method of coupled clusters, CCSD(T)/aug-cc-pVQZ. The fundamental and combination frequencies were calculated using the operator perturbation theory of Van Vleck (CVPTn) of the second and fourth order (n = 2, 4). Resonance effects were modeled using an additional variational calculation in the basis up to fourfold VCI excitation (4). The average prediction error for the fundamental frequencies of the 12C isotopologues was ~1.5 cm–1. The achieved accuracy made it possible to reliably predict the isotopic frequency shifts of the 13C isotopologues. It is shown that the strong Fermi resonance ν4/2ν6 dominates in the 12C isotopologues and is practically absent in 13C. The literature assumption [Spectrochim. Acta A, 44:553] about the splitting of ν1(CH) due to the resonance ν1/ν2 + ν7 + ν9 is confirmed. The coefficients of the polyadic quantum number are determined. The analysis made it possible to carry out a preliminary identification of the centers of the vibrational-rotational bands of isotopologues 13CHF235Cl and 13CHF237Cl in the spectrum of the mixture in preparation for individual analyzes of the vibrational-rotational structures of individual vibrational transitions. [ABSTRACT FROM AUTHOR]

The radiative forcing (RF) of climate change due to increases in carbon dioxide (CO2) concentration is primarily in the wavenumber region 500–850 cm−1 (wavelengths of approximately 12 to 20 μm). It originates from absorption and emission of infrared radiation due to vibrational–rotational transitions of the CO2 molecule. While this RF has been the subject of intense and detailed study, to date, the contribution of different vibrational transitions to this forcing has not been explored. This article presents an analysis of radiative transfer calculations that quantify the role of different vibrational transitions and illustrates that while the fundamental bending mode contributes nearly 90% of the total infrared intensity, it contributes less than half of the RF at present‐day CO2 concentrations; this is because the absorption at the centre of this fundamental band is so intense that the effect of additional CO2 is strongly muted. By successively adding in additional CO2 bands to the calculations, it is demonstrated that a key spectroscopic phenomenon, known as Fermi Resonance (an interaction between excited states of the bending and the symmetric stretching modes of CO2) leads to a significant spreading of the infrared intensity to both higher and lower wavenumbers, where the fundamental bending mode is less important. The Fermi Resonance transitions contribute only about 4% of the total infrared intensity in this spectral region but cause more than half of the present‐day RF. The less‐abundant isotopologues of CO2 have little impact on the spectrally integrated RF, but this small contribution results from a compensation between more significant positive and negative contributions to the spectral RF. This work does not alter the results of detailed RF calculations available in the literature; rather, it helps explain the physical basis of that forcing. [ABSTRACT FROM AUTHOR]

In situ high‐pressure Raman spectroscopy experiments have been performed to investigate the structural changes of parabanic acid (PA) up to ~12.0 GPa. The analysis of Raman spectroscopy reveals that PA undergoes two phase transitions. The collapse of spacing between molecular layers and the distortion of hydrogen‐bonding networks almost in ac‐plane are the main causes of the first phase transition above 2.1 GPa. The changes of Fermi resonance parameters also provide evidence for the first phase transitions. The second phase transition around 4.0 GPa can attribute to the further reduction of the molecular interlayer spacing and the resulting distortion of hydrogen‐bonding network. First‐principle calculations and Hirshfeld surfaces further confirm the analysis of the experimental results. The analysis of high‐pressure phase transition contributes a better explanation for the high‐pressure phase transition process of layered supramolecular materials with hydrogen‐bond self‐assembly. This study provides the possibility of two new polymorphs for PA under high pressure and is helpful for broadening its potential application of this material, especially in the field of supramolecular chemistry. [ABSTRACT FROM AUTHOR]

[Display omitted] • High-pressure Raman spectra of xylitol from atmospheric pressure to 9.08 GPa. • An anharmonic coupling between peaks of fundamental modes of xylitol at 2945 cm−1 and 2955 cm−1 at 0.99 GPa. • The influence of pressure on hydrogen bonding in xylitol molecules. Xylitol, as a typical polyol, has a broad range of application prospects. However, the molecular states of xylitol under different environments are rarely reported until now. In this work, the state changes of xylitol molecules under high pressure were analyzed by Raman spectra. A Fermi resonance phenomenon in the fundamental mode of xylitol at 2945 (±0.06) cm−1 and 2955 (±0.41) cm−1 was observed at 0.99 GPa. The Fermi doublets possess the same symmetry and close energy levels, which had not been changed by pressures. However, the high pressure shortened the atomic distances and applied the extra disturbance, providing the necessary conditions for energy transfer. Besides, the Fermi doublets decoupling happened at 4 GPa due to the breaking of hydrogen bonding. This work provides an important reference for studying molecular states and weak interactions of polyols under high pressures. [ABSTRACT FROM AUTHOR]

The facet-specific interaction between molecules and crystalline catalysts, such as titanium dioxides (TiO2), has attracted much attention due to possible facet-dependent reactivity. Using surface-sensitive sum-frequency vibrational spectroscopy, we have studied how methanol interacts with different common facets of crystalline TiO2, including rutile(110), (001), (100), and anatase(101), under ambient temperature and pressure. We found that methanol adsorbs predominantly in the molecular form on all of the four surfaces, while spontaneous dissociation into methoxy occurs preferentially when these surfaces become defective. Extraction of Fermi resonance coupling between stretch and bending modes of the methyl group in analyzing adsorbed methanol spectra allows determination of the methanol adsorption isotherm. The isotherms obtained for the four surfaces are nearly the same, yielding two adsorbed Gibbs free energies associated with two different adsorption configurations singled out by ab initio calculations. They are (i) ∼-20 kJ/mol for methanol with its oxygen attached to a low-coordinated surface titanium, and (ii) ∼-5 kJ/mol for methanol hydrogen-bonded to a surface oxygen and a neighboring methanol molecule. Despite similar adsorption energetics, the Fermi resonance coupling strength for adsorbed methanol appears to depend sensitively on the surface facet and coverage.

In this work, the structural dynamics of the chloromethanes CCl4, CHCl3 and CH2Cl2 were evaluated through a computational spectroscopy approach by comparing experimental inelastic neutron scattering (INS) spectra with the corresponding simulated spectra obtained from periodic DFT calculations. The overall excellent agreement between experimental and calculated spectra allows a confident assignment of the vibrational features, including not only the molecular fundamental modes but also lattice and combination modes. In particular, an impressive overtone sequence for CHCl3 is fully described by the simulated INS spectrum. In the CCl4 spectrum, the splitting of the ν3 mode at ca. 765–790 cm−1 is discussed on the basis of the Fermi resonance vs. crystal splitting controversy. [ABSTRACT FROM AUTHOR]

• In situ Raman spectra of xylitol at the temperatures range of 83–363 K. • Phase transition of xylitol at 253 K. • The FR appeared between two fundamental vibration modes of xylitol at 308 K. Xylitol is one of the widely used sugar substitutes in food and drug industries. Exploring behavior of xylitol molecules under low- and high-temperature conditions is conducive to predicting the changes in applications. Herein, the low- and high- temperature Raman spectra of xylitol were investigated in the 83 to 363 K temperature interval. The conformation transition of xylitol was firstly observed at 253 K. Besides, the Fermi resonance between two fundamental vibration modes (the symmetric vibration of the CH 2 and the antisymmetric vibration of the CH 2) was identified for the first time at 308 K. Two-dimensional correlation Raman spectroscopy was used to reveal the Fermi doublets dynamics in the temperature field, and analyze the specific sequence of spectra intensity changes. [Display omitted] [ABSTRACT FROM AUTHOR]

The mode selectivity of the H+CH3D→H2+CH2D reaction was studied using a recently developed ten-dimensional time-dependent wave packet method. The reaction dynamics are studied for the reactant CH3D initially from the ground state, the CH3 symmetry and asymmetry stretching excitation, the CD stretching excitation and the fundamental and the first overtone of the CH3 bending mode. The calculated reaction probabilities show that exciting either of the CH3 stretching modes enhances the reactivity in the collision energy range below 1.0 eV, while the CD stretching excitation does not obviously prompt the reaction. Fundamental CH3 bending excitation has nearly no effect on promoting reactivity. However, a significant enhancement is observed for the first overtone excitation of the CH3 bending mode, resulting from the Fermi resonance between the fundamental state of the CH3 symmetry stretching mode and the first overtone state of the CH3 bending mode. [ABSTRACT FROM AUTHOR]

Vibrational spectra of the methyl groups in CH3SH, CH3SCH3, CH3OH, and CH3OCH3 monomers were measured to analyze their complex features as a result of bend/umbrella‐stretch Fermi resonance. Multiple bands were recorded between 2800–3000 cm−1 associated with the vibrations of the methyl groups in CH3OH, and CH3OCH3. On the other hand, the corresponding spectra of CH3SH and CH3SCH3 are relatively simpler with main peaks between 2900 to 3000 cm−1. Theoretical ab initio anharmonic algorithm using six normal modes on a methyl group with potential energy surface at CCSD(T)‐F12/aug‐cc‐pVTZ quality is able to account for all the experimentally observed features across these four species. In CH3SCH3 and CH3SH, all three CH stretch fundamentals are above 2900 cm−1, but the fundamental of umbrella mode is lower than 1350 cm−1, thus only the overtone states of two bending modes are able to borrow sufficient intensities. In CH3OCH3 and CH3OH, umbrella modes have frequencies similar to the bending modes and the CH stretch fundamentals are red‐shifted to 2850–2950 cm−1 leading to more complex vibrational features due to better energy‐matching condition. [ABSTRACT FROM AUTHOR]

Unintended local temperature enhancement by excitation laser might change Raman spectral features and potentially lead to misinterpretation of the data. To evaluate robustness of Raman CO2 densimeters in the presence of laser heating, we investigate the relation between temperature (T, °C), density (ρ, g/cm3), and Fermi diad split (Δ, cm−1) using a high‐pressure optical cell at 23°C to 200°C and 7.2–248.7 MPa. Results indicate that Δ decreases concomitantly with increasing temperature for a constant density in all density regions investigated. This result suggests that the density estimated based on Δ might be underestimated if the fluid is heated locally by the laser. Combining results of earlier studies with those of the present study indicates that the temperature dependence of Δ (|(∂Δ/∂T)ρ|) has a maximum value around 0.6–0.7 g/cm3. Consequently, at very high densities such as 1.1–1.2 g/cm3, |(∂Δ/∂T)ρ| is small. Thus, Δ at such densities is less affected by laser heating. However, at densities below approximately 0.7 g/cm3, although |(∂Δ/∂T)ρ| becomes smaller at lower densities, the relative density decrease becomes larger even for a small density decrease because the density itself becomes smaller. Therefore, at such densities, a density decrease of more than 10% was observed for some fluid inclusions, even at typical laser powers for inclusion analysis. Finally, to accurately estimate the density even in the presence of laser heating, we show that it is effective to estimate the intercept Δ from the correlation between Δ and laser power and substitute it into Δ–ρ relations. [ABSTRACT FROM AUTHOR]

The sequences of Fermi resonances νs ≈ 2νb in the IR spectrum of a fluoroform solution (CHF3) in liquefied krypton have been studied. Here, νs is the CH-stretching vibration and νb is the bending vibration. It has been shown that it is necessary to use an extended set of spectroscopic parameters for the correct description of resonant multiplets at a high degree of vibrational excitation. Among other factors, it is necessary to consider the dependence of the anharmonic interaction constant αsbb on the vibrational quantum numbers. The conclusions are generalized for the arbitrary case of the CH-chromophore CHX3. [ABSTRACT FROM AUTHOR]

The sequences of Fermi resonances νs ≈ 2νb in the IR spectrum of a fluoroform solution (CHF3) in liquefied krypton have been studied. Here, νs is the CH-stretching vibration and νb is the bending vibration. It has been shown that it is necessary to use an extended set of spectroscopic parameters for the correct description of resonant multiplets at a high degree of vibrational excitation. Among other factors, it is necessary to consider the dependence of the anharmonic interaction constant αsbb on the vibrational quantum numbers. The conclusions are generalized for the arbitrary case of the CH-chromophore CHX3. [ABSTRACT FROM AUTHOR]

We investigated the 300 K high-pressure behavior of ilmenite using Raman spectroscopy to 54 GPa. Upon compression, we observe a Fermi resonance between the lowest frequency Ag symmetry peaks (ν4 and ν5) between ~ 10 and ~ 30 GPa: bands that involve major components of Ti–O and Fe–O-related displacements, respectively. The peaks’ relative intensities switch at ~ 18 GPa and they also reach their minimum separation at ~ 20 GPa, indicating that their maximum resonance occurs between 18 and 20 GPa. The negative shift of the Ti–O-associated ν4 vibration under compression is fully consistent with a shift in valence of Ti from 4 + to 3 + under compression. Anomalously small mode shifts of other, more localized vibrations are also consistent with a charge transfer from Fe to Ti under compression. At higher pressures, we have not found definitive evidence for a transition to the perovskite-structure at 300 K, which has been well characterized at high pressures and temperatures. At 40 GPa, we observe an apparent reversible disordering that persists up to our highest pressure. The 300 K mode shifts of the Raman active modes in FeTiO3 under pressure are notably different from those of other ABO3 compounds (where A = Mg, Mn and B = Ti, Si); in other ilmenite-structured compounds, the peaks shift at a faster rate and there has not been any observation of Fermi resonance. Thus, iron’s complex electronic structure, and its charge transfer with titanium, appears to play a primary role in the behavior of phonons in FeTiO3 ilmenite. [ABSTRACT FROM AUTHOR]

• The effect of H-Bond on the FR in the Py-Water was performed via DFT method. • At the pyridine mole fraction of 0.7, water preferentially affinity to the pyridine molecule, and the Fermi doublet changes obviously. • At the pyridine mole fraction of 0.5, the structure of the H-Bond network is most favorable for the bonding of Py molecules. The Fermi resonance (FR) occurs between the ring breathing vibrational (ν 1) and the triangular stretching (ν 12) mode of pyridine in aqueous pyridine solution was studied using Raman spectroscopy combined with density functional theory (DFT) calculations. The effect of H-Bond on the FR in the mixture was performed by MD analysis. Combined with the FR parameters, it was found that due to the H-Bond effect the FR coupling changed significantly when the pyridine mole fractions were near 0.7 and 0.5, respectively. The water H-Bond network structure is most favorable for pyridine molecule bonding at a pyridine mole concentration around 0.5, and water preferentially affinity to the pyridine molecule at x = 0.7. In addition, we analyzed the O H vibrational region at different mole fractions and theoretically calculated Py associated O H in specific molecular bonds. [Display omitted] [ABSTRACT FROM AUTHOR]

• Investigating the impact of methanol(CH 3 OH) hydrogen bonding network on the Fermi resonance of acetonitrile(CH 3 CN). • Analyze the changes of hydrogen bonding network in CH 3 CN-CH 3 OH binary solution by Raman spectroscopy. • Using density functional theory (DFT) and reduced density gradient (RDG) to analyze the hydrogen bonding network in acetonitrile-methanol solutions of different concentrations. Fermi resonance (FR) is a molecular vibrational coupling and energy transfer phenomenon existing within or between molecules. Raman spectra of acetonitrile-methanol (CH 3 CN-CH 3 OH) solutions were measured with different volume fractions, and the FR effect between nitrile stretching mode (ν 2) and the combination modes (ν 3 + ν 4) in acetonitrile was obtained. The Fermi doublets of ν 2 ∼ ν 3 + ν 4 exhibit a blue-shift as the concentration of CH 3 CN decreases. The calculated FR parameters indicate that the hydrogen bonding (HB) network between CH 3 CN and CH 3 OH molecules represents a linear modulation effect on the FR. Meanwhile, we also investigated the O–H stretching region of CH 3 OH and utilized Density Functional Theory (DFT) and reduced density gradient (RDG) analysis to reveal the changes in the HB network of CH 3 OH. The result strongly implies a robust correlation between the O–H band and the FR. The results are of significance for understanding influence of HB on FR effect in binary solutions and for further studying the modulatory role of organic solvents on the FR effect of CH 3 CN. [ABSTRACT FROM AUTHOR]

[Display omitted] • Four frequencies have been observed in the VV and VH component Raman spectra of VC. • Established the model for the conjunction of intramolecular and intermolecular vibrational coupling. • Proposed the Schrödinger equation and obtained four energy solutions for the antiparallel alignment mode. • Intramolecular and intermolecular interaction constants have been determined individually at different concentration. • Dimer structure and DFT calculations can reproduce the NCE and concentration effects. The positions of splitting bands resulting from simultaneous intramolecular (such as Fermi resonance) and intermolecular interactions were calculated by a variational method. Hamiltonian calculation model employed an antiparallel arrangement. Four nondegenerate energy solutions were obtained, which correspond exactly to the four peaks observed in polarized Raman spectroscopy and high resolution infrared spectroscopy: two in the VV component and two in the VH component. The intramolecular coupling constant W, unperturbed intensity ratios R, and unperturbed band difference X 0 for vinylene carbonate (VC) were calculated from the derived equations and the experimental data at infinite dilution concentration. Additionally, the intermolecular interaction constants V of VC in different polar solvents (VC/CH 3 CN, VC/CHCl 3 and VC/CH 3 OH binary mixtures) at varying concentrations were calculated. The curvature of V vs concentration was found to fit the Logan model very well. [ABSTRACT FROM AUTHOR]

In this work, the effect of electric field induced by localized plasmon excitation in nanostructured metal film on the vibrational spectrum of adsorbed organic molecule is studied theoretically. By using the Green function formalism, we show that electric field can significantly influence both the frequencies of the molecular vibrations and the anharmonicity constant. Consequently, if Fermi resonance can take place in the adsorbed molecule, it is spectral manifestation (in Raman spectra) will also be modified by the electric field of plasmon. We illustrate the effect with a CO2 molecule adsorbed on nanostructured gold surface, a situation relevant to the application of Surface‐Enhanced Raman Scattering (SERS) or other plasmon‐mediated phenomena. The obtained in these work analytical expressions are shown to describe well spectral features in earlier experimental SERS studies on CO2. [ABSTRACT FROM AUTHOR]

Infrared spectra for a series of asymmetric proton‐bound dimers with protonated trimethylamine (TMA‐H+) as the proton donor were recorded and analyzed. The frequency of the N–H+ stretching mode is expected to red shift as the proton affinity of proton acceptors increases. The observed band, however, shows a peculiar splitting of approximately 300 cm−1 with the intensity shifting pattern resembling a two‐level system. Theoretical investigation reveals that the observed band splitting and its extraordinarily large gap of around 300 cm−1 is a result of strong coupling between the fundamental of the proton stretching mode and overtone states of the two proton bending modes, that is commonly known as Fermi resonance (FR). We also provide a general theoretical model to link the strong FR coupling to the quasi‐two‐level system. Since the model does not depend on the molecular specification of TMA‐H+, the strong coupling we observed is an intrinsic property associated with proton motions. [ABSTRACT FROM AUTHOR]

Dependence of residual pressures of fluid inclusions on their size and host mineral species provides valuable information related to the depth provenance and P–T–t path of the rocks. Although Raman‐based barometry is an effective method for ascertaining the internal pressure of H2O–CO2 fluid inclusions, few studies have elucidated Raman spectral features of CO2 in a system of high‐pressure H2O–CO2. New experiments using a high‐pressure optical cell in this binary system with compositions of 100, 75 ± 2, and 60 ± 2 mol% CO2 were conducted for this study to verify the availability of Raman CO2 barometers for use in assessing the temperature and pressure conditions of approximately 22°C and 17.3–141.4 MPa. Our results demonstrate that the existence of H2O does not affect the relation between Fermi diad splits (Δ, cm−1) and total pressure of pure CO2. These results suggest that the Δ–total pressure relation obtained from pure CO2 is also applicable to H2O–CO2 systems, even at high pressure. However, unlike Δ, because the peak positions of the Fermi diad in the system of H2O–CO2 shift to a higher wavenumber than those of pure CO2 at given pressure higher than 30 MPa, the peak positions are not very suitable for the pressure scale in an H2O–CO2 system. Additionally, we confirmed the availability of bandwidths of CO2 as an indicator of compositions that can identify the presence of very small amounts of H2O (at least 0.3 mol% H2O), even at room temperature. [ABSTRACT FROM AUTHOR]

Dependence of residual pressures of fluid inclusions on their size and host mineral species provides valuable information related to the depth provenance and P–T–t path of the rocks. Although Raman‐based barometry is an effective method for ascertaining the internal pressure of H2O–CO2 fluid inclusions, few studies have elucidated Raman spectral features of CO2 in a system of high‐pressure H2O–CO2. New experiments using a high‐pressure optical cell in this binary system with compositions of 100, 75 ± 2, and 60 ± 2 mol% CO2 were conducted for this study to verify the availability of Raman CO2 barometers for use in assessing the temperature and pressure conditions of approximately 22°C and 17.3–141.4 MPa. Our results demonstrate that the existence of H2O does not affect the relation between Fermi diad splits (Δ, cm−1) and total pressure of pure CO2. These results suggest that the Δ–total pressure relation obtained from pure CO2 is also applicable to H2O–CO2 systems, even at high pressure. However, unlike Δ, because the peak positions of the Fermi diad in the system of H2O–CO2 shift to a higher wavenumber than those of pure CO2 at given pressure higher than 30 MPa, the peak positions are not very suitable for the pressure scale in an H2O–CO2 system. Additionally, we confirmed the availability of bandwidths of CO2 as an indicator of compositions that can identify the presence of very small amounts of H2O (at least 0.3 mol% H2O), even at room temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • Weak hydrogen bonding is present in binary solutions. • Simulations of the HB modes with DFT are in good agreement with experiment. • HB and high pressure tune the FR of the CS 2 molecule differently. In the spectroscopic study of polyatomic molecules, Fermi resonance (FR) is a vibrational coupling and energy transfer phenomenon that widely exists intra- and intermolecular. In particular, the FR coupling between the fundamental mode ν 1 and the doubling mode 2ν 2 of the CS 2 molecule has attracted extensive research. In this work, we investigate the effect of local field on tuning the FR of CS 2. By analyzing the Raman spectra of CS 2 mixed with methanol and ethanol with different mole fractions, the results indicated that weak HBs interactions in binary solutions can be reflected by the linear frequency shift of the C–H bond vibrations (in methanol and ethanol) with different molar concentrations. Furthermore, the geometrical structure was optimized using DFT simulation, and the vibration analysis and interaction energy were carried out. The simulated Raman spectra are in good agreement with the experiments. In addition, high-pressure Raman spectra of CS 2 were obtained by diamond anvil cell technique (up to 9.19 GPa) and a pressure-induced phase transition was observed at 1.71 GPa. The results demonstrated that the pressure-induced polymerization phase transition of CS 2 molecules causes the close packing and more orderly arrangement of molecules, resulting in the enhancement of FR coupling. HB and high pressure tune the FR of the CS 2 molecule differently. [ABSTRACT FROM AUTHOR]

[Display omitted] The vibrational dynamics of diborane have been extensively studied both theoretically and experimentally ever since the bridge structure of diborane was established in the 1950s. Numerous infrared and several Raman spectroscopic studies have followed in the ensuing years at ever increasing levels of spectral resolution. In parallel, ab initio computations of the underlying potential energy surface have progressed as well as the methods to calculate the anharmonic vibration dynamics beyond the double harmonic approximation. Nevertheless, even 70 years after the bridge structure of diborane was established, there are still significant discrepancies between experiment and theory for the fundamental vibrational frequencies of diborane. In this work we use parahydrogen (pH 2) matrix isolation infrared spectroscopy to characterize six fundamental vibrations of B 2 H 6 and B 2 D 6 and compare them with results from configuration-selective vibrational configuration interaction theory. The calculated frequencies and intensities are in very good agreement with the pH 2 matrix isolation spectra, even several combination bands are well reproduced. We believe that the reason discrepancies have existed for so long is related to the large amount of anharmonicity that is associated with the bridge BH stretching modes. However, the calculated frequencies and intensities reported here for the vibrational modes of all three boron isotopologues of B 2 H 6 and B 2 D 6 are within ± 2.00 cm−1 and ± 1.44 cm−1, respectively, of the experimental frequencies and therefore a refined vibrational assignment of diborane has been achieved. [ABSTRACT FROM AUTHOR]

Abstract Raman spectra of acetonitrile-water mixtures are measured in different acetonitrile concentration fractions. The nitrile stretch (v 2) and the combination (v 3 + v 4) modes are coupled by Fermi resonance. We extract the Fermi resonance parameters and find that the frequency separation shows non-linear dependence as a function of acetonitrile concentration fractions. v 2 ~ v 3 + v 4 Fermi doublets intensity ratio exhibits nonlinearity, which is accelerated with the decreasing of the Fermi coupling strength. The OH stretching region is also investigated in order to reveal the variation of the water structure. Our data show that there are strong relationships between the intensity of ~3225 cm−1 band and the v 2 ~v 3 + v 4 Fermi resonance. Because the nitrile stretch and Fermi resonance are very environmental sensitive, the nonlinear modulation on Fermi resonance and the non-regular intensity ratio are attributed to the change of water hydrogen-bonding network. The results provide useful structural and spectral insights for the design of new experiments and the molecular structure modeling. Graphical abstract Unlabelled Image Highlights • Acetonitrile CN stretching (v 2) and the combination (v 3 + v 4) Fermi resonance shows nonlinear dependence on water structure. • The H-bonded and free species in acetonitrile-water mixtures exhibit different Fermi coupling strength. • Second derivative analysis and Lorentzian peak fits are introduced to find the species variation. [ABSTRACT FROM AUTHOR]

Abstract The conformational analysis of some 2‑(methoxy)‑2‑[(4‑substituted)‑phenylsulfanyl]‑(4′‑substituted) acetophenones was performed through infrared (IR) spectroscopic analysis of the carbonyl stretching band (ν CO), supported by B3LYP/6-31+G(d,p) calculations and X-ray diffraction. Five (1 – 5) of the seven studied compounds (1 – 7) presented Fermi resonance (FR) on the ν CO fundamental transition band. Deuteration of these compounds (1a – 5a) precluded the occurrence of FR, revealing a ν CO doublet for all compounds in all solvents used. The computational results indicated the existence of three conformers (c 1 , c 2 and c 3) for the whole series whose relative abundances varied with solvent permittivity. The higher ν CO frequency c 1 conformer was assigned to the higher frequency component of the carbonyl doublet, while both c 2 and c 3 were assigned to the lower frequency one. Anharmonic vibrational frequencies and Potential Energy Distribution (PED) calculations of compound 3 indicated that the combination band (cb) between the methyne δ CH and one skeletal mode couples with the ν CO mode giving rise to the FR on the c 2 conformer in vacuum and on the c 1 one in non-polar solvents. The experimental data indicated a progressive increase in c 1 conformer stability with the increase of the solvent dielectric constant, which is in good agreement with the polarizable continuum model (PCM) calculations. The higher ν CO frequency and the stronger solvation of the c 1 conformer is a consequence of the repulsive field effect (RFE) originated by the alignment and closeness of the Cδ+ Oδ− and Cδ+ Oδ− dipoles. Finally, the balance between orbital and electrostatic interactions dictates the conformational preferences. X-ray single crystal analysis for compound 6 revealed the c 1 geometry in the solid state and its stabilization by C H...O hydrogen bonds. Graphical Abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The diffuse reflectance infrared spectra of substitutional solid solutions of K1-X RbXClO4 (0 ≤ x ≤ 1) were studied at room temperature. The main wave pattern data of the IR spectra has linear dependence on solid solution composition. Fermi resonance is discussed as well.

Abstract Raman spectra of the solution of LiClO 4 in acetonitrile (CH 3 CN) at different concentrations have been measured. With increasing the concentration of Li+, it was noted that several vibrational modes of CH 3 CN had significant changes in Raman shifts and some new Raman peaks emerged due to the CH 3 CN⋯Li+ complex formation. In addition, Fermi resonance phenomenon between the ν 2 ' and (ν 3 + ν 4)' Raman bands of CH 3 CN⋯Li+ complex was observed. Based on the Bertran's equations, Fermi resonance parameters of free CH 3 CN and CH 3 CN⋯Li+ complex at different concentrations have been calculated, respectively. Compared the Fermi resonance coupling coefficients W of free CH 3 CN with CH 3 CN⋯Li+ complex at different concentrations, the free CH 3 CN had a little smaller value, which indicated that the ν 2 '/(ν 3 + ν 4)' Fermi resonance in CH 3 CN⋯Li+ complex was much stronger than the ν 2 / ν 3 + ν 4 Fermi resonance in CH 3 CN. From the detailed analysis of the effect of Li+ on the spectral features of CH 3 CN, the effect mechanism of Li+ coordination to CH 3 CN at the nitrogen of the CN group on the ν 2 / ν 3 + ν 4 Fermi resonance of CH 3 CN has been elucidated. Graphical Abstract Unlabelled Image Highlights • Raman spectra of CH 3 CN with dissolved of LiClO 4 at different concentrations were obtained. • The complex CH 3 CN ⋯ Li + formation on the Raman spectrum of CH 3 CN has been analyzed. • ν 2 '/(ν 3 + ν 4)' Fermi resonance in CH 3 CN ⋯ Li + complex was observed. • Fermi resonance parameters of CH 3 CN and CH 3 CN ⋯ Li + at different concentrations were calculated. • The effect mechanisms of the complex formation on the Fermi resonance were elucidated. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Mid-Infrared spectra of pyrimidine (PM) and pyrazine (PZ) were recorded in the gas phase using a multi-pass long path gas cell. The IR band structure of these compounds above and below 3000 cm−1 is very broad and contains many humps and shoulders. These humps and shoulders are due to various higher quantum excitation of low-frequency vibrational modes, which participate in Fermi resonance with the nearby C H stretch fundamentals and appears in this region. We constructed an Effective Spectroscopic Hamiltonian (ESH) in a mixed local mode (LM) normal mode (NM) basis to assign the various overtone and combination bands in the C H stretching region of these compounds. The C H stretching vibrations of both PZ and PM were treated as symmetrized anharmonic Morse oscillators in local coordinates and the in-plane deformations down to 1000 cm−1 were treated as normal coordinates. The ESHs were diagonalized and the resulting eigenvalues were subsequently fitted in a given parameter space with the experimentally observed bands. The eigenvalues of the converged Hamiltonian are the anharmonic frequencies and the transition intensities were obtained by summing the squared eigenvector components. The overtone and combination transitions near 3000 cm−1 of both PM and PZ were identified and assigned from the eigenvector coefficients of the ESH matrix. The wavefunctions of a pure C H stretch, overtone of the H C C in-plane bend and due to Type 1 Fermi coupling (resonance between a fundamental with an overtones of a low frequency mode, in this case resonance between the C H strech and the overtone of H C C in-plane bending modes) has been demonstrated pictorially. [ABSTRACT FROM AUTHOR]

Highlights • Unprecedented agreement of theoretical IR spectra for H+(H 2 O) 3 with experiment. • Complete assignment of the observed fundamental and combination bands. • Discovery of two "giant" Fermi resonances dominating these spectra. • Possible role for Fermi resonance in gating proton transport. Abstract The protonated water trimer is a prototype "proton wire" that can transport two protons nearly concertedly. This "proton transfer mode" (PTM) is an important contributor to the infrared spectrum of the isolated gas-phase cluster. We have simulated its infrared spectrum for both the hydrated and deuterated isotopologues, using vibrational 2nd order perturbation theory (VPT2) and ab initio molecular dynamics (AIMD) trajectories. VPT2 calculations explain quantitatively the experimental spectra at both high and low frequencies, provided that high-level quantum chemistry is utilized. In the D 2 -tagged hydrated cluster, the PTM undergoes giant Fermi resonances (FR's) with two combination bands. In the deuterated analogue, one observes a single FR of "normal" intensity, manifested as the doublet recently reported experimentally. We provide band assignment for both isotopologues, with and without the D 2 tag applied in experiment. We discuss possible manifestations of the giant resonance on proton transfer through water wires. [ABSTRACT FROM AUTHOR]

Hexafluoroacetone (HFA, O C(CF 3 ) 2 ) is widely utilized in organic synthesis, chemistry of heat resistant polymers and biochemistry. In this work, infrared (IR) spectra of gaseous HFA were recorded in the range 2000–340 cm −1 with a resolution of 1 cm −1 . For the first time, experimental absolute IR intensities of fourteen fundamental bands of HFA were measured and compared with intensities predicted by anharmonic ab initio calculations. Theoretical molecular structures, semi-diagonal quartic potential energy surfaces (PES), and cubic surfaces of dipole moment (DM) components of HFA were calculated using MP2/cc-pVTZ ab initio quantum-mechanical model. In addition, harmonic frequencies were also calculated at MP2/cc-pVQZ level of theory and used for refining the PES. The numerical-analytic implementation of the second-order operator canonical Van Vleck perturbation theory (CVPT2) was employed for predicting anharmonic IR spectra of HFA. The deviation from observations of predicted IR intensities for ten fundamentals (ν 1 , ν 2 , ν 8 , ν 13 –ν 21 ) was less than 25% on average. The significant difference between ab initio harmonic intensities calculated with MP2/cc-pVTZ and MP2/cc-pVQZ models for some normal modes indicates that an inaccuracy of the quantum-mechanical model is probably the major source of errors in theoretical intensities. An interpretation of the IR experimental spectra and associated literature data for HFA is accomplished; the ν 4 , ν 7 , ν 10 and ν 15 fundamentals were reassigned. Three predicted Fermi resonances were identified in experimental spectra, ν 3 /ν 5  + ν 8 , ν 17 /ν 10  + ν 19 and ν 20 /ν 9  + ν 23 . An empirical fit of harmonic frequencies within anharmonic model assisted in a reliable identification of a number of overtones and binary combination tones. The fitted harmonic frequencies were compared with MP2/cc-pVQZ and a simplified complete basis set (CBS) extrapolation. It was found that MP2/cc-pVQZ frequencies were more reliable. The efficiency and convenience of the theoretical model employed and associated techniques for interpretation of complex spectra of HFA are demonstrated. [ABSTRACT FROM AUTHOR]

The facet-specific interaction between molecules and crystalline catalysts, such as titanium dioxides (TiO2), has attracted much attention due to possible facet-dependent reactivity. Using surfacesensitive sum-frequency vibrational spectroscopy, we have studied how methanol interacts with different common facets of crystalline TiO2, including rutile(110), (001), (100), and anatase(101), under ambient temperature and pressure. We found that methanol adsorbs predominantly in the molecular form on all of the four surfaces, while spontaneous dissociation into methoxy occurs preferentially when these surfaces become defective. Extraction of Fermi resonance coupling between stretch and bending modes of the methyl group in analyzing adsorbed methanol spectra allows determination of the methanol adsorption isotherm. The isotherms obtained for the four surfaces are nearly the same, yielding two adsorbed Gibbs free energies associated with two different adsorption configurations singled out by ab initio calculations. They are (i) ~-20 kJ/mol for methanol with its oxygen attached to a low-coordinated surface titanium, and (ii) ~-5 kJ/mol for methanol hydrogen-bonded to a surface oxygen and a neighboring methanol molecule. Despite similar adsorption energetics, the Fermi resonance coupling strength for adsorbed methanol appears to depend sensitively on the surface facet and coverage. [ABSTRACT FROM AUTHOR]

Abstract: The vibrational spectra of a molecule and related molecular crystal are systematically analyzed theoretically with the account of the influence of anharmonic interactions between the vibrations. The specific anharmonism‐induced features in the spectra of a molecule and crystal are revealed, and spectral lineshapes at varied strength of anharmonic interaction are simulated. The proposed general theoretical approach is used for fitting the experimental vibrational spectra of CO2 substance in gas and molecular crystal phases. Pretty good coincidence of experimental data with theoretical results is demonstrated, and the constants of anharmonic interaction are obtained. The proposed theoretical approach can be used to interpret the bands in the Raman and infrared absorption spectra of various complex molecules and crystals. [ABSTRACT FROM AUTHOR]

[Display omitted] • There were two phase transitions of α-HQ found at 3.61 and 12.46 GPa. • Pressure induced 831 cm−1 and 854 cm−1 with the same symmetry at 3.61 GPa. • Pressure provided an effective way to induce fundamental FR phenomenon. Fermi resonance (FR), a prevalent phenomenon in molecules, has an important effect in spectrum analysis. As an effective way to change the molecular structure and tune symmetry, high-pressure techniques can often induce FR. Hydroquinone (HQ) is a hydrogen-bonded crystal that tends to form a solid inclusion compound with a suitable guest and has wide applications. In this work, a high-pressure technique was used to investigate α-HQ using high pressure to tune the symmetry to produce FR. Raman and infrared spectra of α-HQ were investigated at ambient pressure, and then Raman spectra under high pressure of α-HQ were investigated up to 19.64 GPa. Results indicated that there were two phase transitions found at about 3.61 and 12.46 GPa. Fundamental FR was not present in α-HQ molecules at ambient pressure. At 3.61 GPa, the first-order phase transition occurred due to the pressure-induced symmetry change, resulting in two Raman modes at 831 cm−1 and 854 cm−1 with the same symmetry, thereby providing evidence that the fundamental FR phenomenon occurred. Furthermore, the pressure-induced changes of the FR parameters were elucidated. Thus pressure provided an effective way to study FR between two asymmetric species. [ABSTRACT FROM AUTHOR]