Fermi Resonance Effect Research Articles

Modeling Vibrational Sum Frequency Generation Spectra of Interfacial Water on a Gold Surface: The Role of the Fermi Resonance.

Studying the hydrogen bonding structure of H2O at the metal-water interface is a highly complex yet fascinating endeavor. The intricate interactions and diverse orientations of water molecules on metal surfaces with varying potentials pose a significant challenge in elucidating the coupling between O-H stretching and H-O-H bending modes. In this study, we employed DFT-MD simulation to explore how the orientation of interfacial water molecules changes with the applied potential on the Au(111) surface. Based on the surface-specific velocity-velocity correlation function (ssVVCF) formula, we calculated vibrational sum frequency generation (VSFG) spectra for the O-H stretches. We found that three assigned peaks (∼3300, ∼3450, and 3650 cm-1) shifted toward lower frequencies as the potential moved toward more negative values. Our results align remarkably well with experimental Raman spectroscopy data. Notably, our VSFG analysis revealed a significant change in the VSFG spectra of the hydrogen-bonded O-H groups (∼3300 cm-1), switching from a negative to a positive sign with decreasing potential. This alteration suggests a substantial change in the orientation of these low-frequency O-H groups owing to their increased interactions with the Au surface. In contrast, the orientations of both the high-frequency O-H groups (∼3450 cm-1) and the dangling O-H groups (∼3650 cm-1) remained unaffected by the applied potentials. Furthermore, our analysis of the decomposed vibrational density of states (VDOS) for the H-O-H bending mode uncovered the coupling between the H-O-H bending and O-H stretching vibrations, known as the Fermi resonance. Our work suggests that the H-O-H bending vibration becomes restricted when water molecules transition from the ″one-H-down″ to the ″two-H-down″ conformation, leading to a redshift in the O-H stretching vibration through the Fermi resonance. By constructing the VSFG and decomposed VDOS spectra, we gained valuable insights into the structural changes that Raman spectra alone cannot fully interpret. Specifically, our analysis revealed the critical role of the Fermi resonance effect in shaping the spectroscopic signature of interfacial water molecules on the Au(111) surface.

Relationships between conformational and vibrational features of tryptophan characteristic Raman markers

Tryptophan (Trp) residue provides characteristic vibrational markers to the middle wavenumber spectral region of the Raman spectra recorded from peptides and proteins. In this report, we were particularly interested in eight Trp Raman markers, referred to as Wi (i = 1,…,8). All responsible for pronounced Raman lines, these markers originate from indole moiety, a bicyclic conjugated segment involved in the Trp structure. Numerous investigations have previously attempted to relate the variations observed in the spectral features of these markers to the environmental changes of Trp residues. To emphasize the most important points we can mention (i) the variations in the Raman profile of W4 (∼1360 cm−1) and W5 (∼1340 cm−1), frequently observed as a doublet with variable intensity ratio. These two markers were thought to result from a Fermi-resonance effect between certain planar and nonplanar modes; (ii) the changes observed in the wavenumbers and relative intensities of W4, W7 (∼880 cm−1) and W8 (∼760 cm−1) were supposed to be related to the accessibility of Trp to surrounding water molecules; and (iii) the wavenumber fluctuations of W3 (∼1550 cm−1), taken as a Trp side chain orientational marker. However, some ambiguities still exist regarding the interpretation of these markers, needing further clarification. Herein, upon a joint experimental and theoretical analysis based on a multiconformational approach, attention was paid to the relationships between structural and vibrational features of three indole-containing compounds with increasing structural complexity, i.e., skatole (3–methylindole), tryptophan, and tripeptide Gly–Trp–Gly. This study clearly shows that the existing assignments given to certain Trp Raman markers should be reconsidered, especially those based on the Fermi-resonance origin of W4-W5 (∼1360–1340 cm−1) doublet, as well as the purely environmental dependence of W7 and W8 markers.

The relationship between the tyrosine residue 850–830 cm−1 Raman doublet intensity ratio and the aromatic side chain χ1 torsion angle

Tyrosine (Tyr) residue in a peptide chain is characterized by the presence of seven Raman markers, referred to as Yi (i = 1, …, 7), distributed over the middle wavenumber spectral region. Particularly, the changes observed in the relative intensity of Y5 and Y6 markers, appearing as a side by side doublet at ca. 850–830 cm−1, has received a great attention. Primarily assigned to a Fermi-resonance effect between phenol ring planar and nonplanar modes, former density functional theory calculations led us to affiliate the Y5–Y6 doublet to two distinct fundamental modes. Furthermore, despite the previous assumptions, it was evidenced that the reversal of the doublet intensity ratio cannot be solely explained by hydrogen bonding on the phenol hydroxyl group involved in Tyr. Herein, upon analyzing the observed and theoretical data collected from the cationic species of the tripeptide Gly-Tyr-Gly, the crucial effect of the aromatic side chain orientation, especially that of the χ1 torsion angle defined around the CαCβ bond, on the Tyr doublet intensity ratio has been evidenced.

Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect.

Intriguing vibrational features of solvated protonated methanol between 2400-3800 cm-1 are recorded by infrared predissociation spectroscopy. Positions of absorption bands corresponding to OH stretching modes are sensitive to changes in solvation environments, thus leading to changes in these vibrational features. Two anharmonic coupling mechanisms, Fermi resonance (FR) contributed by bending overtones and combination band (CB) associated with intermolecular stretching modes, are known to lead to band splitting of OH stretching fundamentals in solvated hydronium and ammonium. Theoretical analyses based on the ab initio anharmonic algorithm not only well reproduce the experimentally observed features but also elucidate the magnitudes of such couplings and the resulting interplay between these two mechanisms, which provide convincing assignments of the spectral patterns. Moreover, while the hydroxyl group plays the leading role in all the above-mentioned features, the role of the methyl group is also analyzed. Through the H/D isotope substitution, we identify overtones of the methyl-hydroxyl rocking modes and their participation in FR.

IR cryospectroscopic manifestation of complex formation between methoxyflurane and dimethyl ether in liquid Xe

The IR spectra of mixtures of methoxyflurane and dimethyl ether are studied with the help of FTIR cryospectroscopy in liquefied Xe. The region of CH3 and CH stretching vibrations is examined taking into account the Fermi resonance effect. Comparative analysis of the experimental data and results of ab initio calculations show that the Cl2CH group of methoxyflurane is involved in complex formation which is stabilized by H-bond between H atom of this group and O atom of dimethyl ether predominantly.

Surface-enhanced infrared absorption studies towards a new optical biosensor.

Reflectometric interference spectroscopy (RIfS), which is well-established in the visual regime, measures the optical thickness change of a sensitive layer caused, e.g., by binding an analyte. When operated in the mid-infrared range the sensor provides additional information via weak absorption spectra (fingerprints). The originally poor spectra are magnified by surface-enhanced infrared absorption (SEIRA). This is demonstrated using the broad complex fluid water band at 3300 cm−1, which is caused by superposition of symmetric, antisymmetric stretching vibration, and the first overtone of the bending vibration under the influence of H-bonds and Fermi resonance effect. The results are compared with a similar experiment performed with an ATR (attenuated total reflectance) set-up.

Hydrogen-Bonding-Induced Enhancement of Fermi Resonances: A Linear IR and Nonlinear 2D-IR Study of Aniline-d5

Hydrogen bonding of the amino group of aniline-d5 results in a huge enhancement of the NH2 bending overtone absorption strength, mainly attributed to the Fermi resonance effect. A quantitative analysis is presented, using a hybrid mode representation and encompassing experimental data on aniline with 0, 1, or 2 hydrogen bonds to dimethylsulfoxide (DMSO). Changes in enthalpy, hydrogen-bonding-induced frequency shifts, and the transition dipole moment increase of the local N-H stretching oscillator all demonstrate that the hydrogen bond is strongest in the single hydrogen-bonded complex. Linear IR overtone spectra show that the oscillator strength decreases upon hydrogen bonding for the N-H stretching overtones, which is opposite to the effect on the fundamental N-H stretching transitions. Polarization resolved 2D-IR spectra provide detailed information on the N-H stretching overtone manifold and support the relative orientations of the various IR transitions.

IR vibrational assignments for 1,3,5-triamine-2,4,6-trinitrobenzene (TATB) based on the temperature-dependent frequency shifts

IR spectra of TATB at different temperature were investigated for vibrational assignments. The results showed that the hydrogen bonding interaction of TATB was significantly weakened with increasing the temperature. Based on the frequency shifts of the hydrogen bonding interaction to several vibrational modes, 12 absorbance bands in the region 600–1700cm−1 were assigned to corresponding vibrational modes. In addition, Fermi resonance effect between NO2 and NH2 scissors was observed, and the enhancement of Fermi resonance effect caused the intensity of v3 (at 1545cm−1) to grow during heating.

Anharmonic interactions and infrared bandshape of the hydrogen bond vibration of potassium hydrogen (deuterium) maleate crystals

The spectral profiles of the hydrogen bond vibrations in the potassium hydrogen maleate (KHM) crystal and its deuterated analogue (KDM) were studied theoretically taking into account the Fermi resonance (FR) effect and strong coupling between the hydrogen bond vibration and lattice phonons. In addition, ab initio calculations of the frequencies and intensities of the H(D)-maleate ion for both C 2v and C s symmetry were made. It was found that C s symmetry has to be used and that practically all intense bands have a contribution from the OH vibration. It was shown that, taking into account all the factors: ab initio results, FR and anharmonicity of vibrations, the experimental spectra for both KHM and KDM crystals can be explained adequately. Anharmonic constants were obtained as a result of fitting theoretical spectra to the experimental ones.

Furan derivatives of substituted phenylthiourea: spectral studies, semi-empirical quantum-chemical calculations and X-ray structure analyses

Fifty new derivatives of 1-(furan-2-carbonyl)- and 1-(furan-3-carbonyl)-3-phenyl substituted thiourea have been synthesised and identified. Intramolecular hydrogen bonds were investigated in detail, using IR spectroscopy. The three-level Fermi resonance effect in the IR spectra was analysed after deconvolution and band separation. Semi-empirical quantum-chemical calculations (AM1 and PM3) support the results of the IR spectroscopic studies. X-ray single crystal diffraction analyses of four selected compounds, namely 1-(furan-3-carbonyl)-3-(2-trifluoromethyl-phenyl)-thiourea ( 1e), 1-(2-methyl-furan-3-carbonyl)-3-(2-trifluoromethyl-phenyl)-thiourea ( 2e), 1-(2,6-dichloro-phenyl)-3-(2-methyl-furan-3-carbonyl)-thiourea ( 2n) and 1-(4-methoxyphenyl)-3-(3-methyl-2-furan-carbonyl)-thiourea ( 3e), corroborated the molecular and crystal structure of these compounds. Relatively strong intramolecular hydrogen bonds of the N–H⋯O C type as well as intermolecular two-centred and bifurcated three-centred hydrogen bonds were observed, confirming the results of the IR spectral study and the semi-empirical quantum-chemical calculations. A variety of intermolecular interactions, yielding the supramolecular architectures in the four crystalline compounds, are discussed in detail.

Index auteurs 2003

A high-pressure Raman study of β-BaB2O4 reveals the occurrence of four pressure-induced phase transitions near 48, 70, 80 and 96 kbar, respectively. Above 96 kbar, β-BaB2O4 becomes amorphous and the transition is irreversible. A Raman quartet originating from Davydov splitting and the pressure-scanned Fermi resonance effect is observed.

Theory of the profiles of hydrogen stretching infrared bands of hydrogen-bonded solids: Fermi resonance effect and strong coupling between the high-frequency hydrogen stretching vibration and low-frequency phonons

A theory is developed to describe the profiles of hydrogen stretching infrared bands of hydrogen-bonded solids taking into account the anharmonic coupling between the high-frequency stretching vibration, ν(XH), and low-frequency lattice phonons as well as a Fermi resonance between states involving the ν(XH) stretching and an overtone or combination of some internal modes. The theory was constructed in the framework of the extended molecular exciton Davydov approach.

Spectral properties and kinetics of the alkaline hydrolysis of chromium, tricarbonyl[3-[( η6-aryl)methylene]- Z-1(3H)-isobenzofuranones

The infrared and 13C NMR spectra and the kinetics of alkaline hydrolysis of chromium, tricarbonyl[3-[( η 6-aryl)methylene]- Z-1(3H)-isobenzofuranones] have been studied in comparison with those of 3-arylmethylene- Z-1(3H)-isobenzofuranones. The 13C NMR spectroscopy has been found to be the most suitable tool for recognition of the site of complexation in the 3-arylmethylene- Z-1(3H)-isobenzofuranone system. A statistically significant correlation has been found between the arithmetic means of the wave numbers of symmetric and asymmetric stretching vibrations of Cr(CO) 3 group and σ p constants of the substituents. The large slope of this linear dependence suggests an important role of the mesomeric mechanism in the transmission of electronic effects to the Cr(CO) 3 moiety. The strong electron-withdrawing effect of the Cr(CO) 3 group causes a significant decrease of the polarity of the phthalide CO bond. The reaction rates of the alkaline hydrolytic ring-opening reaction dramatically increase due to the effect of Cr(CO) 3 group. The absorption bands of the carbonyl stretching vibrations are markedly split due to Fermi resonance effect. After mathematical correction for Fermi resonance the wave numbers of the unperturbed fundamental CO stretching vibration and the log k values of alkaline hydrolysis exhibit significant linear correlations mutually, as well as with Hammett σ substituent constants. Charton's transmissive factors γ(PhCr(CO) 3) have been determined for tricarbonylchromiumphenyl system using the parameters of the linear correlations. The efficiency of the substituent effect transmission is high in CCl 4, because a back-donation effect of the carbonyl ligands and decreases in CHCl 3 due to the hydrogen bonding between carbonyl groups and solvent molecules. The preparation of some novel chromium, tricarbonyl[3-[( η 6-aryl)methylene]- Z-1(3H)- isobenzofuranones] is described.

Hydrogen bonds and hydrogen-bonded systems in Mannich bases of 2,2′-biphenol: an FTIR study of the proton polarizability and Fermi resonance effects as a function of temperature

5,5′-Dibromo-3-diethylaminomethyl-2,2′-biphenol ( 1) and 5,5′-dibromo-3,3′-bis(diethylaminomethyl)-2,2′-biphenol ( 2) were synthesized and studied by FTIR and 1H NMR spectroscopy. In dichloromethane solution, 1 forms a hydrogen-bonded system with large proton polarizability due to collective tunnelling of protons in the two intramolecular hydrogen bonds. In acetonitrile the system is polarized to a greater extent. If this system is protonated, the HO⋯HO⋯bond is broken and hence the collective system destroyed. The FTIR spectrum of protonated 2 shows an intense continuum indicating a collective hydrogen-bonded system with large proton polarizability due to collective proton motion. The butyronitrile solution of 1 shows an increase of the Fermi resonance bands with decreasing temperature (above the glass transition). The hydrogen bonds become stronger. Furthermore, the proton polarizability increases. With the transition in the glass state the continuum remains unchanged since the local fields are frozen. Only the Fermi resonance effect becomes slightly stronger. With the butyronitrile solution of 2 the IR continuum caused by the polarizable OH⋯N ⇌ O −⋯H +N bonds increases above the glass transition with decreasing temperature. The equilibrium is shifted in favour of the polar structure. In the glass state no change of the continuum and thus of the proton polarizability is observed, since the local fields are frozen.

High-pressure Raman study of the β-BaB2O4 crystal and pressure-induced phase transitions

Abstract A high-pressure Raman study of β-BaB 2 O 4 reveals the occurrence of four pressure-induced phase transitions near 48, 70, 80 and 96 kbar, respectively. Above 96 kbar, β-BaB 2 O 4 becomes amorphous and the transition is irreversible. A Raman quartet originating from Davydov splitting and the pressure-scanned Fermi resonance effect is observed.

Spectroscopic study of conformational dynamics of 1,1′-binaphthyl in a jet

Analysis of the fluorescence excitation spectrum of 1,1′-binaphthyl in a jet shows that in the origin region there are four transitions, in an energy range of 150 cm−1, which strongly overlap through the occurrence of strong progressions in a torsional mode. The four transitions can be described in terms of excitations to levels of an interchange-equivalent naphthalene dimer in a trans and cis configuration. The dimer splittings are 50 and 63 cm−1 for 1,1′-binaphthyl-h14 and 50 and 57 cm−1 for 1,1′-binaphthyl-d14. The Franck–Condon envelope of the librational progressions indicates that the potential energy surface for states of A and B symmetry are different, which implies that the interplanar angle in each of the four excited states is different. From the intensities in the A and B progressions it is further deduced that for the origin transitions the transition moment in each subunit is oriented in the naphthalene plane at an angle of ∼50° with respect to the long axis. From the Franck–Condon envelope in the librational progressions it is further deduced that the change in torsional angle on optical excitation to the B-potential energy surface is ∼12° to the trans and 6° to the cis side. From this a maximum (state dependent) barrier of 150 cm−1 is calculated. It is further shown that, near the intense false origin, numerous modes appear which are induced through vibrational mixing. This Fermi-resonance effect is most obvious in perdeutero binaphthyl, where the vibronically induced intensity in the region of the most intense false origin is smeared out over many modes. The emission spectrum of the molecule can be interpreted by assuming that in 1,1′-binaphthyl very efficient vibrational relaxation occurs through conversion of vibrational into librational energy with molecular rotations providing the matching energy. It is also suggested that, after vibronic excitation, vibrational relaxation occurs into the energetically most stable structure. Finally, the observed red shift of the binaphthyl fluorescence in solution must be due to solvent stabilization of the excited molecule in a structure which is more planar than the one observed in the isolated molecule.

Fermi resonance effect and vibrational intensity in the weak charge-transfer complexes

Infrared and Raman spectra of naphthalene, tetrachlorophthalic anhydride, tetrabromophthalic anhydride and their respective weak charge-transfer complexes were recorded in solids. The spectral intensities of both donor and acceptor molecules were found to alter drastically upon complexation. The observed intensity changes might be attributed to the removal and formation of the Fermi resonance effect in the complexes.

Vibrational absorption spectrum of solid CO in the first harmonic region. Two-phonon transition

Infrared absorption spectrum of CO crystal at low temperature, recorded in the Δυ = 2 region, shows a strong Δυ = 2 zero phonon line, a broad phonon side band and a small line which is attributed to the simultaneous vibrational transitions between two molecules. The absorption coefficient and line frequency of this two-phonon line is calculated by a theory taking account both the induced dipole moment between molecular couples and the Fermi resonance between the ν = 2 level and the two-phonon level. It is shown that most of the two-phonon line intensity comes from the Fermi resonance effect. Good agreement with experiment is obtained. The linewidth is given by the convolution product of two 0 → 1 line shapes.

Fermi resonance and coupling effect in the NH 2-group of thioacetamide

In the i.r. spectra of 1:1 and 2:1 complexes of thioacetamide with proton acceptors, the NH 2 symmetric stretching vibration splits into a doublet, due to Fermi resonance (FR) with the first overtone of the NH 2 bending mode. FR coupling coefficients and the distance of the unperturbed levels (Δ 0) have been determined. Δ 0 seems to be larger for 2:1 complexes. A level inversion of the FR bands, due to the change in the strength of the H-bonding to the solvent, has been detected. FR interaction has been characterized by FR displacement parameter (Δ F = Δ-Δ 0) where Δ stands for the distance between the FR bands.

Study of Fermi resonance in ethylene carbonate by the method of isotopic substitution

The i.r. spectra of ethylene carbonate, its 13C-carbonyl and d 4-labelled compounds of the liquid phase and the chloroform and carbon tetrachloride solutions have been investigated. Isotopic substitution as well as solvent variation methods based on frequency shift and change in the relative integrated band intensities of the doublet carbonyl absorption confirm the action of Fermi resonance effect. As the normal and 13C-ethylene carbonates have a doublet carbonyl absorption, the d 4-compound has only single absorption. Assignment of the origin of the perturbing vibrational mode in the carbonyl region is discussed in view of the previous work based on frequency shift due to solvent variations only.