Temperature-dependent fermi resonance studies and lineshape simulation by random perturbation model
TL;DR: The lineshapes of Fermi diads of thioacetamide and methyl thiourea have been measured at different temperatures in the 120-300 K range as mentioned in this paper.
Abstract: The lineshapes of Fermi diads of thioacetamide and methyl thiourea have been measured at different temperatures in the 120–300 K range. For computation of the lineshape, the dynamic part of the Fermi resonance interaction has been considered as a random perturbation by Redfield's theory. The optical Bloch equations are set up including the Redfield relaxation terms. The steady state solution of these equations lead to the absorption spectrum. The computed lineshape of a Fermi diad is shown to be a superposition of two Lorentzians and an interaction term. The linewidth of the Lorentzians are controlled by the Redfield and 1/ T 2 relaxation terms. The comparison of the computed curves with the observed spectra demonstrates the importance of the dynamic part of the interaction on the lineshape.
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TL;DR: In this paper, the bandwidths of the two components of the Fermi dyad were measured in the liquid and gaseous phase of 12C16O2 with an exact FermI resonance ν1=2ν2 in the high density fluid phase.
Abstract: Isotropic Raman studies of density effects on the lower Fermi doublet ν1, 2ν2 of the 12C16O2 and 13C16O2 compressed molecules have been made in the gaseous and liquid phases along the saturation line up to the triple point temperature. After a brief bibliography review of the unperturbed frequency ordering for the 12C16O2 case, the problem of the determination of unperturbed levels from our experimental data is treated for both isotopic species. Comparison between 12C and the 13C frequency shifts due to the density effect leads to the conclusion that the order of the ν1, 2ν2 levels is reversed in the liquid and gaseous phase of 12C16O2 with an exact Fermi resonance ν1=2ν2 in the high‐density fluid phase. For the first time, bandwidths of the two components of the Fermi dyad are measured in the liquid.
40 citations
TL;DR: In this article, it was shown that the Fermi resonance interaction between the SiOH bending fundamental and a doubly excited vibration gives rise to a mixed-state doublet around 800 cm−1.
Abstract: The band at about 4550 cm−1 in the near-infrared (IR) spectra of OH-containing high-surface area silicas is known to resolve into a doublet under certain experimental conditions. The interpretation of this by Tsyganenko [Zh. Fiz. Khim. (Moscow) 56 (1982) 2330], which has received no recognition so far, is advocated and is elaborated as follows: (1) there is Fermi resonance interaction between the SiOH bending fundamental and a doubly excited vibration giving rise to a mixed-state doublet around 800 cm−1, (2) the Fermi doublet propagates in the near IR in combinations with the OH stretching vibration (≈3750 cm−1). From a review of available IR transmission, inelastic neutron scattering (INS), and Raman scattering experimental data and model densities of vibrational states it is inferred that the doubly excited state causing Fermi resonance is likely to be either the first overtone of the bridging-oxygen rocking mode (400 cm−1 in the density of vibrational states) or the binary summation of the bridging-oxygen symmetric stretching in bent Si–O–SiOH units (the D1 Raman band near 490 cm−1, in the interpretation of Mulder et al.) with the A′ component of the Si–OH wagging mode (expected at about 300 cm−1). In the near-IR spectra of OH-containing vitreous silica and natural opals, the irregular shape of the band near 4500 cm−1, as argued, has a similar origin.
33 citations
TL;DR: It is shown, 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 the scheme provides an accurate and computationally convenient approach to account for them.
Abstract: 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.
22 citations
TL;DR: In this paper, the authors study the IR spectra of thiourea and N -methylthioura at various temperatures down to 120 K and confirm the earlier finding that replacement of a hydrogen atom of the amide group by a methyl group leads to hydrogen bonding and the change of lineshape on cooling shows that cooling leads to stronger hydrogen bonding but does not affect the methyl torsional frequency.
3 citations
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TL;DR: In this article, the qualitative explanation to explain an unusual feature in the infrared absorption spectrum of m -toluidine has been expressed in quantitative terms and several examples of the effect are described and compared with theoretical predictions.
136 citations
TL;DR: In this article, the asymmetric CH stretching bands of the methyl groups of a number of alkane systems have been examined as a function of temperature, and the authors derived expressions for the splittings and widths in terms of parameters which give the coupling of the various types of motion.
Abstract: The asymmetric CH stretching bands of the methyl groups of a number of alkane systems have been examined as a function of temperature. At low temperatures, two bands are seen for a CH3 group. These bands collapse into one as the temperature is raised. Examination of crystals of alkanes with both odd and even numbers of carbon atoms, alkanes in urea clathrates, and deuterium substituted alkanes allows the separation of intramolecular and intermolecular contributions to the band splittings and widths. Model Hamiltonians that express the stretching energy levels as a function of both the methyl torsional coordinates and the external coordinates of the alkanes are set up. These are used in the Redfield equations to derive expressions for the splittings and widths in terms of parameters which give the coupling of the various types of motion. The splittings are found to be proportional to the average of cos 5ϑ, where ϑ is the torsional angle, and a contribution to the width is found to be due to fluctuations in this quantity. Comparison of the equations with the observed widths shows that the relaxation times of the torsional motion, which are greater than 0.2 but less than about 20 ps, are fast enough to yield the motionally narrowed limit for the spectrum as required in the derivation of the Redfield equations.
113 citations
TL;DR: In this paper, an interpretation of the experimental data in terms of Fermi resonances is reported, and a theoretical treatment has been worked out and formulas are presented which allow the calculation of both intensities and frequencies in terms with anharmonic coupling constants.
Abstract: The analysis of the low temperature vibrational spectrum of N‐methyl acetamide recorded in our laboratory, has shown new features in the amide I–amide II and in the N–H stretching regions. In this paper an interpretation of the experimental data in terms of Fermi resonances is reported. For this purpose a theoretical treatment has been worked out and formulas are presented which allow the calculation of both intensities and frequencies in terms of anharmonic coupling constants. A satisfactory agreement between calculated and experimental data has been found.
38 citations
TL;DR: In this article, a quantum-mechanical treatment of the dynamics of relaxation of a two-level mode coupled to a thermal bath is presented, where the Zwanzig-Mori projection operator formalism is employed to derive exact equations of motion for the correlation function and the excess population.
Abstract: A quantum-mechanical treatment of the dynamics of relaxation of a twolevel mode coupled to a thermal bath is presented. The Zwanzig-Mori projection-operator formalism is employed to derive exact equations of motion for the correlation function and the excess population , where and are the coordinate and number operators associated with the two-level mode. In the van Hove weak-coupling limit it is shown that |G ν|2 and Δn decay exponentially with time constants τ and τ′, respectively. Explicit expressions for τ and τ′ are determined and the relationship between them clearly established. The application of the model to the specific problem of the effects of vibrational relaxation on isotropic Raman spectral lineshapes in dense media is emphasized.
36 citations
TL;DR: In this article, a series of high frequency hot bands for the 1660 cm−1 band of urea has been attributed to H-bonding and the values of anharmonic constants χ3a and γ3 aa and the hydrogen bond stretching frequency νa are reported.
11 citations