Vibrational Exciton Research Articles

Resonance dynamical intermolecular interaction in the crystals of pure and binary mixture n-paraffins

In the present paper, we report temperature dependent FTIR spectra studies of Davydov splitting value for CH 2 rocking vibrations of pure crystalline n-paraffins C n H 2 n+2 ( n is the number of carbon atoms) and some isomorphically substituted binary mixtures of n-paraffins C 22H 46:C 24H 50. Temperature dependencies of Davydov splitting value have been shown to be characterized by the amount of irregularities (sharp decreasing), which corresponds to the phase transitions into the high-temperature (hexagonal) state for pure n-paraffins or different rotator crystalline states for the mixtures. Statistic and dynamic models have been proposed, which provides an adequate description of the observed effect. In the framework of these models, two different mechanisms are responsible for the temperature behavior of the vibrational mode splitting value. Besides the thermal expansion of crystals at heating, the quenching of vibrational excitons on the orientational defects of different nature takes place, accompanied with the breakage of the crystal lattice translational symmetry. The creation of such defects is resulted from the excitation of librational and rotational molecular degrees of freedom at the crystal polymorphic transitions into different rotary crystalline states. The manifestation of the resonance dynamical intermolecular interaction in the spectra of intramolecular vibrations in these crystals has been theoretically analyzed in terms of stochastic equations, taking into consideration the above mentioned phase transition. We have obtained the explicit expression for the theoretically predicted dependence of Davydov splitting value on temperature. The absorption bands, corresponding to Davydov splitting components, have been shown to approach rapidly each other at the transition to the high-temperature (hexagonal) phase. Computer simulation of such dependence has been performed for some aliphatic compounds. Good agreement between the experimental and computer simulation results has been obtained. The theoretical approach developed in the present paper for the resonance dynamical intermolecular interaction near such transitions from the three-dimensional to one-dimensional phase of crystalline n-paraffins has a general character and can be applied to the description of some specific features observed in the vibrational spectra of rotary crystals.

Extent of Delocalization of Vibrational Modes in Liquids as a Result of Competition between Diagonal Disorder and Off-Diagonal Coupling

The changes in the extent of delocalization of vibrational modes in liquids arising from the competition between diagonal disorder and off-diagonal coupling in the vibrational exciton picture are examined theoretically. Calculations of vibrational modes and polarized Raman spectra are carried out on a model liquid system, in which spherical particles interact with each other by the Lennard-Jones and dipole−dipole interactions, and a one-dimensional oscillator (representing a vibrational degree of freedom) with a transition dipole and a Raman tensor is buried in each particle. The diagonal disorder is assumed to be in a Gaussian form, and the transition dipole coupling is introduced as the off-diagonal coupling in the vibrational Hamiltonian. The magnitude of the Raman noncoincidence effect (NCE), which is the phenomenon that the vibrational frequency positions of the isotropic and anisotropic components of a Raman band do not coincide and is often used as an experimental measure of the delocalization of vibrational modes, is calculated. In addition, two theoretical measures of the delocalization of vibrational modes are introduced, and their changes with the thermodynamic state of the liquid, the diagonal disorder, and the strength of the off-diagonal coupling are examined. It is shown that the extent of delocalization of vibrational modes is correlated with the “NCE detectability”, defined as the ratio of the magnitude of the NCE and the isotropic or anisotropic Raman bandwidth. This result indicates that the extent of delocalization of vibrational modes in a vibrational band that shows the NCE may be estimated from the observed spectral profile of the band. The size of the space in which vibrational spectra can probe structural formation is also discussed.

Raman noncoincidence effect: A spectroscopic manifestation of the intermolecular vibrational coupling in dipolar molecular liquids

Abstract This lecture addresses the analysis of the noncoincidence effect (NCE), a spectroscopic manifestation of the intermolecular coupling in molecular liquids. The vibrational bandshapes of molecular groups like C=O (strongly active in the IR spectrum) in dipolar liquids exhibit this phenomenon at a rather large extent. It will be shown that the vibrational exciton approach, developed under the assumption of the transition dipole coupling (TDC) mechanism, predicts how the orientational structure of the molecular liquid determines the magnitude and sign of the NCE. Specifically, it predicts that in simple molecular liquids, solely structured by dipolar forces, the NCE is large and positive, whereas when liquid structures are dominated by non-dipolar forces (as those present in H-bonded liquids), this scenario dramatically changes and IR-active modes may give rise to negative NCEs. This lecture is intended to offer a general overview of NCEs observed in dipolar (simple and structured) liquids in different thermodynamic conditions and of the theoretical and simulation results that assisted in their interpretation.

Characteristics of the two-dimensional infrared spectroscopy of helices from approximate simulations and analytic models

Abstract The characteristics of the two-dimensional infrared (2D-IR) rephasing, non-rephasing, absorptive and dispersive spectra of model helical polypeptides in the amide-I region are examined by simulation in the frequency domain, based on a vibrational exciton Hamiltonian matrix approach. Analytical models and solutions of the exciton band structure, the density of states, and the allowed IR transitions for the simplest cases are presented and compared with simulations. The through space and through bond vibrational anharmonic coupling energies between pair-wise amide units are evaluated by using ab initio density functional theory calculations on glycine di-, tri- and tetrapeptides, and by using the transition charge–transition charge interactions including charge-fluxes. A contour diagram at 5° interval of the nearest neighbor coupling energies for the right-handed helices in the region of −25° to −70° for both φ and ψ dihedral angles is provided. The static diagonal disorder is introduced into the Hamiltonian diagonal frequency through statistical models and is used to account for localizations of the excitations and the inhomogeneous broadening associated with the conformational fluctuations and local interactions with solvent. The role of the anharmonicity in constructing the 2D-IR spectra is described qualitatively, and the manifestations of the given anharmonicity in the 2D-IR spectra under various line broadening conditions are examined. The absorptive and dispersive 2D-IR spectra under several experimental polarization conditions have been examined, and spectral features for the 3 10 -and α-helices have been illustrated.

Optical properties of molecular ice particles from a microscopic model

The vibrational exciton approach is used for the first time to calculate indices of refraction for molecular ice particles (NH 3, 12CO 2, 13CO 2, and different mixtures) in the region of strong infrared absorption bands. This model provides a microscopic explanation for the characteristic patterns found in the optical data and thus leads to propensity rules for the occurrence of pronounced shape effects in infrared spectra of molecular particles. Compared with experimental data, the calculated indices of refraction do not suffer from deficiencies due to the lack of experimental information.

Isotope effects on vibrational excitons in carbon dioxide particles

The vibrational exciton approach is used to answer a long-standing question concerning the infrared spectra of CO 2 particles: What causes the characteristic structure of the infrared absorption bands? The CO 2 particles which are generated in a collisional cooling cell at 78 K consist of many thousands of molecules. Isotopically pure ( 12 CO 2 or 13 CO 2 ) particles as well as isotopically mixed ( 12 CO 2/ 13 CO 2 ) particles are investigated. Combining the experimental observations with exciton calculations, the band structure in the infrared spectra can be traced back to the shape of the particles.

Linear and nonlinear infrared signatures of local α- and 310-helical structures in alanine polypeptides

Vibrational exciton Hamiltonians for the amide I and amide A modes of both the α- and 310-helical conformations of a fifteen unit polyalanine oligomer CH3–CO(Ala)15–NHCH3 are constructed using density-functional calculations for smaller model peptides. Energy levels as well as the transition dipoles of all singly and doubly excited-state manifolds are calculated. A variety of C13-substituted isotopic derivatives are examined with respect to their ability to reveal differences in local secondary structures in two-dimensional infrared spectra in the amide I region. Amide mode anharmonicities are predicted to be valid indicators of secondary helical structures.

Verification of the vibrational exciton approach for CO2 and N2O nanoparticles

It is investigated how far the vibrational exciton approximation is suitable to describe the characteristic band shapes in the infrared spectra of CO2 and N2O nanoparticles. The particles typically contain between 50 and 104 molecules and have spatial dimensions between 1 and 10 nm. The accuracy of the exciton approach is estimated by comparison with experimental data and quantum chemical calculations for small clusters. The spectral changes due to different particle shapes and particle sizes are investigated with respect to the estimated accuracy. This includes the determination of a typical effective range for the dipole–dipole coupling.

The fundamental photophysics of conjugated oligomer herringbone aggregates

The photophysical properties of defect-free herringbone aggregates of π-conjugated oligomers are investigated theoretically using a two-particle basis set consisting of vibronic excitons and coupled vibronic–vibrational excitons. Incorporation of periodic boundary conditions allows the treatment of aggregates containing up to 1000 molecules. The vibrational distortion fields for the optically allowed excitons, including those responsible for the upper and lower Davydov components, are evaluated. The herringbone lattice supports both vibrationally dressed, heavy excitons as well as nearly free, light excitons. The former are responsible for the b-polarized absorption origin as well as two ac-polarized peaks slightly higher in energy. The strongly blueshifted main absorption peak is due to an exciton which travels with almost no nuclear distortion. The main absorption features are studied as a function of aggregate size and exciton bandwidth. The vibronic replicas in the aggregate emission spectrum are found to be strongly dependent on a destructive interference between one and two particle emissions. The primarily ac polarized replica intensities initially decrease with the number of molecules comprising the aggregate, N, converging to a nonzero value in the large N limit. By contrast, the b-polarized 0–0 line intensity increases linearly with N, eventually dominating the rest of the vibronic progression when N surpasses approximately 10. Beyond this size the aggregate radiative decay rate, γagg, scales linearly with N, eventually driving the quantum yield to unity when γagg surpasses the nonradiative decay rate. The relative magnitude of the 0–0 emission line versus the rest of the progression generally increases with increasing excitonic interactions. The sum of the (dimensionless) replica intensities diminishes from 1−exp(−λ2) in the weak excitonic coupling regime to approximately zero in the strong coupling regime. By contrast, the 0–0 line intensity scales as N throughout, increasing by a factor of exp(λ2) in going from the weak to strong excitonic coupling regimes.

Inter-peptide interaction and delocalization of amide I vibrational excitons in myoglobin and flavodoxin

The amide I vibrational normal-mode analyses of two globular proteins, myoglobin and flavodoxin, were carried out and the spatial delocalization and phase correlation of each normal mode was studied. The Hessian matrix in the restricted space constructed by the N peptides was obtained by using both ab initio quantum chemistry calculation results and empirical transition dipole coupling model. For both of the two proteins, the amide I normal modes are fairly delocalized and on average about 15% of the polypeptides in myoglobin and flavodoxin are participated in the formation of each amide I normal mode. The analysis of vibrational phase correlation of myoglobin reveals that the high-frequency amide I normal modes are mostly (symmetric) A-mode like, whereas the low-frequency ones are (antisymmetric) E-mode like. For flavodoxin, among the three distinct bands at 1653, 1636, and 1628 cm−1, the high-frequency modes are found to be fairly localized in the four α helices, the low-frequency modes around 1628 cm−1 are strictly localized in the five β sheets, and the midfrequency modes around 1636 cm−1 are delocalized in both the α helices and β sheets. Effects of the diagonal disorder on the amide I band of both proteins were studied. We found that the sharp peak at 1628 cm−1 in the resolution enhanced amide I infrared spectrum of flavodoxin is a direct signature that the through-space interactions between two different β sheets exist.

Dynamics of molecules and phase transitions in the crystals of pure and binary mixtures of n-paraffins

In the present paper, we report on temperature dependent FT-IR spectra studies of the value of Davydov splitting for CH 2 rocking vibrations of pure crystalline n-paraffins C n H 2 n+2 (with odd number n of carbon atoms) and some isomorphic substituted binary mixtures of n-paraffins C 22/C 24. It was shown that the temperature dependencies of Davydov splitting value are characterized by the number of irregularities (sharp decreasing), which correspond to phase transitions into the high-temperature (hexagonal) state for pure n-paraffins or different rotator crystalline states for the mixtures. A statistic and dynamic model is proposed which provides adequate description of the observed effect. In the framework of this model, two different mechanisms are responsible for the temperature behavior of the splitting value of vibrational modes. In addition to thermal expansion of crystals under heating, the damping of vibrational excitons on orientational defects of different nature takes place breaking the translational symmetry of the crystal lattice. Genesis of such defects is connected with excitation of librational and molecular rotational degrees of freedom at the crystal polymorphic transitions into different rotator crystalline states. Theoretical analysis of the effect of resonance dynamical intermolecular interaction on the spectra of intramolecular vibrations of the crystals was performed in terms of stochastic equations with account for mentioned phase transitions. Computer simulation of such dependence was performed for pure n-paraffin compounds. Good agreement between the experimental and computer simulation results was obtained.

Simulations of two-dimensional femtosecond infrared photon echoes of glycine dipeptide

The multidimensional optical response of the amide I band of glycine dipeptide is calculated using a vibrational–exciton model, treating each peptide bond as a localized anharmonic vibration. The 2D photon echo signal is obtained by solving the non-linear exciton equations. Comparison of different models of spectral broadening (homogeneous and diagonal and off-diagonal static disorder) shows completely different 2D signals even when the 1D infrared spectra are very similar. The phase of the 2D signal may be used to distinguish between overtone and collective types of two-exciton states. Vanishing of the 2D signal along certain directions can be attributed to the variation of the phase. Copyright © 2000 John Wiley & Sons, Ltd.

Migrating electronic excitations in π-electron-containing polymers

The results of the investigations of spectral properties and excitation energy transfer in π-electron-containing polymers carried out in Kyiv University are briefly reviewed. The peculiarities of fluorescence, delayed fluorescence, singlet and triplet exciton propagation, mechanisms of elementary acts of singlet and triplet energy transfer are examined and analyzed. Some proofs of vibrational excitons in π-electron-containing polymers are presented.

Vibron-mediated electronic relaxation in crystalline chlorine

Luminescence decay of thin solid films of molecular chlorine exhibit distinctly non-exponential temporal behavior following pulsed ultraviolet laser excitation. The temporal characteristics are dependent on sample temperature and laser fluence in the range 11–80 K. This behavior is attributed to internal conversion from the first excited A′( 3Π 2u) electronic state to the ground state, which is accelerated by phonons and localized vibrational excitons (vibrons). This unusual mechanism of electronic relaxation is consistent with a recent theoretical model for the mechanism of explosive desorption of cryogenic films.

High-resolution PIRSS using a tunable infrared diode laser: the Davydov doublet of the antiferroelectric (2 × 1) monolayer CO adsorbed on NaCl(001)

The temperature dependence of the Davydov doublet of the antiferroelectric (2 × 1) monolayer CONaCl(001) has been studied between 11 and 25 K by means of polarization infrared surface spectroscopy (PIRSS) using a tunable lead salt diode laser with a resolution better than 5 × 10 −3cm −1. At 11 K the linewidth of the symmetric vibrational exciton, polarized perpendicular to the surface, was determined to be 0.035 cm −1, the linewidth of the antisymmetric vibrational exciton, polarized parallel to the surface, to be 0.025 cm −1, the sharpest spectral line of an adsorbate observed. To obtain phase relaxation times T 2 in the low-temperature region, we fitted the parameters of the exchange dephasing model to the experimental data. A satisfactory fit for both the frequency and the linewidth is obtained at temperatures below 25 K. Above 25 K the antisymmetric vibrational exciton becomes extremely broad and finally disappears, and a successful fit was obtained only for the linewidth of the symmetric vibrational exciton. It is shown how the vibrational energy relaxation time T 1 may be obtained by varying the spectral power of the laser light.

An amorphous monolayer: Infrared spectroscopic and theoretical studies of SO2 on NaCl (100)

At temperatures between 100 and 110 K, exposing the (100) face of NaCl to unsaturated SO2 gas yields a stable adlayer. Infrared spectra of adsorbed SO2 contain complex resonances near the origins of the molecular symmetric and asymmetric stretching vibrations. On photometric grounds the absolute coverage of the surface is found to be one molecule per exposed Na+Cl− ion pair. The spectra of this monolayer consist of several sharp lines overlapping one broad feature for each molecular mode. By comparison to vibrational excitons in simpler systems, the coexistence of crystalline and amorphous adlayer structures is strongly indicated. This partial ordering is pressure dependent, and develops spontaneously on a time scale of minutes. The disordered component, in contrast, is never in equilibrium with the gas phase. Computational simulations have detailed the microscopic basis of this behavior. Accurate ab initio models of the SO2 molecule and NaCl(100) surface were used in a Monte Carlo simulation of the experimental conditions. At both half and full coverage, an amorphous two-dimensional condensate developed. This is minimally consistent with the polarized infrared photometry. Seemingly equilibrated Monte Carlo runs retained some memory of the initial molecular configuration, again consistent with hysteresis observed in the spectroscopic experiments. No structural order developed in the simulations, but the energetic state of affairs was clarified: the potential driving adsorption and condensation is deep in comparison to available thermal energy, but relatively insensitive to molecular orientation.

The monolayer CO adsorbed on MgO(100) detected by polarization infrared spectroscopy

The adsorption of carbon monoxide on the MgO(100) surface, prepared in situ by cleaving a single crystal in ultra high vacuum, has been studied at temperatures between 32 and 56 K using polarization Fourier transform infrared spectroscopy. Above 45 K a single IR absorption at 2150.5 cm −1 is detected with p-polarization only which indicates a dynamic CO dipole perpendicular to the surface. Below 45 K the spectra of the monolayer are completely different. Using p-polarization the main IR absorption is detected at 2151.2 cm −1. Two additional features are observed at 2137.2 and at 2132.5 cm −1, only the latter is also observed with s-polarization. The spectrum indicates an ordered monolayer with two energetically different adsorption sites per unit cell. A c(4 × 2) symmetry is inferred in concert with helium atom scattering [Gerlach et al., Surf. Sci. 331–333 (1995) 1490]; from LEED [Audibert et al., Surf. Sci. 273(1992) L467; Panella et al., J. Phys. Paris 4 (1994) 905] a (4 × 2) symmetry was derived. The p-polarized absorption at 2151.2 cm −1 is assigned to a dynamic CO dipole perpendicular to the surface, the doublet is attributed to the in-phase and the out-of-phase vibrational exciton of two tilted CO molecules at energetically equivalent sites. Their projections on the surface are oriented antiparallel, the dipole of the in-phase exciton at 2137.2 cm −1 is perpendicular to the surface and has no azimuthal component, the dipole of the out-of-phase exciton at 2132.5 cm −1 is parallel to the surface. A weak prevalence of one domain is inferred from the spectra.

Temperature-induced changes in protein heat capacity

In this paper, we describe the heat-capacity changes under heating as a process defined by phonon–vibrational exciton interactions. Following Davidov, we describe an α-helix as three noninteracting linear molecular crystals. Further development is achieved by solving the Boltzman's transport equation in the second quantization approximation for the quasi-particles: phonons and vibrational excitons. Heat capacity is defined in terms of the distribution function changes with temperature. We found out that heat absorption is realized through the transfer energy from the molecular vibrations to the internal vibrational degrees of freedom in the physiological temperature range. © 1993 John Wiley & Sons, Inc.

Phase transition in crystalline benzil : an infrared study of vibrational excitons.

The molecular crystal of benzil, [C 6 H 5 CO] 2, is known to undergo a phase transition at T c = 84 K. The phase transition is from a high temperature trigonal phase with space group D 4 3 (P3 121) to a low temperature monoclinic phase with space group C 3 2 (C 2). This paper reports a study of the exciton structure of the infrared bands of benzil as a function of temperature in the vicinity of T c = 84 K. The benzil molecule belongs to the C 2 molecular point group. Group theoretical analysis of the exciton structure of infrared bands predicts two components for molecular B modes and one component for molecular A modes in the high temperature phase. Below T c all the internal modes of benzil are expected to split into two components. Our experimental results show that the A molecular modes are resolved in a doublet structure in the low temperature phase whereas only one component is observed above T c. The doublet structure of infrared bands is studied as a function of temperature in the vicinity of T c. These splittings of crystal states in the low temperature phase are found to be described by a ¦T c - T¦ β law. The temperature dependence of the doublet structure of internal B modes is also studied below and above T c.

Effect of orientational and substitutional disorders on the N 2 vibrational exciton in N 2/CO mixed crystals

High resolution Raman spectroscopy was performed on the N 2 vibron bands over a large concentration range at various temperatures in N 2/CO mixed crystals. The Davydov splitting appears for N 2 concentration above 0.2 and linearly increases up to the pure crystal limit. The Raman linewidth exhibits a maximum for a N 2 concentration of 0.15 and decreases towards a residual value in the low concentration limit. Two types of disorder occur in this molecular alloy, namely a substitutional disorder between CO and N 2 molecules and a frozen-in orientational disorder for the asymmetric CO molecules. A Hamiltonian including correlated diagonal and off-diagonal disorder terms was suggested to describe the vibron band. The first moments of the Raman intensity were calculated as a function of the concentration. The first moment accounts for the linear dependence of the Davydov splitting in terms of the concentration. The second moment predicts the residual bandwidth in the low N 2 concentration limit which is proved to result from the orientational disorder of the CO molecules.