Dipolar Coupling Theory Research Articles

Temperature- and pressure-dependent infrared spectroscopy of 1-butyl-3-methylimidazolium trifluoromethanesulfonate: A dipolar coupling theory analysis

Continued growth and development of ionic liquids requires a thorough understanding of how cation and anion molecular structure defines the liquid structure of the materials as well as the various properties that make them technologically useful. Infrared spectroscopy is frequently used to assess molecular-level interactions among the cations and anions of ionic liquids because the intramolecular vibrational modes of the ions are sensitive to the local potential energy environments in which they reside. Thus, different interaction modes among the ions may lead to different spectroscopic signatures in the vibrational spectra. Charge organization present in ionic liquids, such as 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([C4mim]CF3SO3), is frequently modeled in terms of a quasicrystalline structure. Highly structured quasilattices enable the dynamic coupling of vibrationally-induced dipole moments to produce optical dispersion and transverse optical-longitudinal optical (TO-LO) splitting of vibrational modes of the ionic liquid. According to dipolar coupling theory, the degree of TO-LO splitting is predicted to have a linear dependence on the number density of the ionic liquid. Both temperature and pressure will affect the number density of the ionic liquid and, therefore, the amount of TO-LO splitting for this mode. Therefore, we test these relationships through temperature- and pressure-dependent FT-IR spectroscopic studies of [C4mim]CF3SO3, focusing on the totally symmetric SO stretching mode for the anion, νs(SO3). Increased temperature decreases the amount of TO-LO splitting for νs(SO3), whereas elevated pressure is found to increase the amount of band splitting. In both cases, the experimental observations follow the general predictions of dipolar coupling theory, thereby supporting the quasilattice model for this ionic liquid.

Liquid Structure of Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids Assessed by FT-IR Spectroscopy.

Ionic liquids are a fertile and active area of research, in part, due to the unique properties these solvents offer over traditional molecular solvents. Because these properties are rooted in the fundamental ion-ion interactions that govern their liquid structure, there is a strong motivation to characterize the liquid structure of ionic liquids. Infrared spectroscopy is a standard analytical tool for assessing liquid structures, for the intramolecular vibrational modes of the ions composing the materials are often quite sensitive to their local potential energy environment. In this work, we demonstrate that the band asymmetry for the νa(SNS) anion mode of N(Tf)2--based ionic liquids originates from the dynamic coupling of vibrationally induced dipole moments of anions across a quasilattice. The magnitude of TO-LO splitting is linearly correlated with the number densities of the ionic liquids; an observation that is in accord with the predictions of dipolar coupling theory. Dipole moment derivatives of νa(SNS) calculated from dipolar coupling theory, (∂μ/∂q)DCT, are lower than those obtained from independent measurements of (∂μ/∂q). The most likely explanation for this disparity is that although ionic liquids possess sufficient long-range structure to support TO-LO splitting of infrared-active modes, there is enough orientational and translational disorder in the quasilattice to partially disrupt the coupling of vibrationally induced dipole moments across the quasilattice. This will result in diminished amounts of TO-LO splitting than would be expected if the ionic liquid were a perfect crystal at 0 K. Impacts of cation molecular structure and the formation of a binary solution on the liquid structure are also explored.

Existence of optical phonons in the room temperature ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate

The technologically important properties of room temperature ionic liquids (RTILs) are fundamentally linked to the ion-ion interactions present among the constituent ions. These ion-ion interactions in one RTIL (1-ethyl-3-methylimidazolium trifluoromethanesulfonate, [C(2)mim]CF(3)SO(3)) are characterized with transmission FTIR spectroscopy and polarized attenuated total reflection (ATR) FTIR spectroscopy. A quasilattice model is determined to be the best framework for understanding the ionic interactions. A novel spectroscopic approach is proposed to characterize the degree of order that is present in the quasilattice by comparing the dipole moment derivative calculated from two independent spectroscopic measurements: (1) the TO-LO splitting of a vibrational mode using dipolar coupling theory and (2) the optical constants of the material derived from polarized ATR experiments. In principle, dipole moment derivatives calculated from dipolar coupling theory should be similar to those calculated from the optical constants if the quasilattice of the RTIL is highly structured. However, a significant disparity for the two calculations is noted for [C(2)mim]CF(3)SO(3), indicating that the quasilattice of [C(2)mim]CF(3)SO(3) is somewhat disorganized. The potential ability to spectroscopically characterize the structure of the quasilattice, which governs the long-range ion-ion interactions in a RTIL, is a major step forward in understanding the interrelationship between the molecular-level interactions among the constituent ions of an ionic liquid and the important physical properties of the RTIL.

Material-specific mapping of metal/semiconductor/dielectric nanosystems at 10 nm resolution by backscattering near-field optical microscopy

We report that three main constituents of nanosystems—metals, semiconductors, and dielectrics—can be categorically distinguished by their specific optical near-field contrast at 633 nm wavelength. The decisive property is the local dielectric constant as we show by calculations based on dipolar coupling theory. Experiments with Au/Si/PS(polystyrene) nanostructures using an apertureless scattering-type near-field optical microscope yield optical images at 10 nm resolution, with clear material contrast close to predicted levels.

Morphology Dependent Anomalous Frequency Shifts of Infrared Parallel Bands of Polymer Crystals: Spectral Changes Induced by Isotopic Dilution in H-D Cocrystals of Poly(oxymethylene) as Evidence of a Transition Dipolar Coupling Mechanism

Morphology dependent anomalously large frequency shifts of infrared parallel absorption bands found in some crystalline polymers, including poly(oxymethylene) and poly(ethylene oxide), have been accounted for by the strong transition dipolar interactions among the molecules constructing one crystallite domain of the lamella-type. 15 In order to get conclusive evidence supporting this mechanism, infrared spectra of lamella-type mixed crystals of normal and deuterated paraformaldehydes (PFA and PFA-d 2 , linear oligomers of poly(oxymethylene)) were investigated. The A 2 parallel bands due to both PFA and PFA-d 2 were found to exhibit remarkable changes in both peak position and profile with variation in the PFA/PFA-d 2 composition, whereas the El perpendicular bands remained unchanged. The main feature of the observed spectral changes was interpreted in terms of the reduction of the transition dipolar interactions in the infrared-active zone-center modes (due to the in-phase oscillation of the transition dipoles) caused by the frequency mismatch between the normal and deuterated molecules. The simplest spectral change was found for the A 2 (4) band of PFA-d 2 which shifted from 850 cm -1 (in pure PFA-d 2 under the highest transition dipolar interactions) to 833 cm -1 (highly diluted with PFA). The observed spectral change in the 860-830 cm -1 range was reproduced fairly well by a Monte Carlo simulation based on the transition dipolar coupling theory. The more complicated spectral change seen for the A 2 -(4) of PFA and A 2 (3) of PFA-d 2 was accounted for by the vibrational coupling between the two isotopic species generated in the mixed crystals.

Polymorphic structures and molecular vibrations of linear oligomers of polyoxymethylene studied by polarized infrared and Raman spectra measured on single crystals

Abstract Molecular vibrations of linear oligomers of polyoxymethylene (POM) having acetoxyl groups at the chain ends, CH 3 CO 2 (CH 2 ,O) m COCH 3 (POMA cm ) with m = 10 and 11, were investigated by means of polarized infrared and Raman spectra taken on single-crystal specimens. From the spectral data obtained for solution-grown crystals of POMAc10 consisting of the orthorhombic POM (o-POM) sublattice, the dispersion curves for the ν 6 through ν 9 branches of the (2/1) helix of the POM molecule were obtained. POMAc10 crystals grown from the melt gave rise to spectra characteristic of the (9/5) helices adapted to the trigonal POM (t-POM) sublattice, indicating that the trigonal packing of the (9/5) helices is more stable than the orthorhombic arrangement of the (2/1) helices at a high-temperature range. Using the spectral data of a POMAc11 crystal consisting of the t-POM sublattice, combined with those of melt-grown POMAc10, the ν 6 through ν 9 dispersion curves of t-POM were obtained. For these oligomers it was found that the frequencies of the infrared-active zone-center modes having the transition dipoles parallel to the chain axis exhibited remarkable shifts depending on the inclination of the molecules to the lamella face of molecular layers, as predicted from the transition dipolar coupling theory.

Polarized Raman and infrared studies of single crystals of orthorhombic modification of polyoxymethylene and its linear oligomer: Crystal structures and vibrational assignments

AbstractIn order to make vibrational assignments of the metastable orthorhombic modification of polyoxymethylene (o‐POM), polarized Raman and infrared spectra of a single crystal of deca(oxymethylene) diacetate, CH3COO(CH2O)10COCH3 (POMAc10), a linear oligomer of POM, were measured and compared with previous results obtained using a micron‐sized single crystal of o‐POM. On the basis of the orientation of the o‐POM sublattice in the POMAc10 crystal, assignments of the zone‐centre modes of o‐POM was established and the unit cell orientation in the micron‐sized o‐POM single crystal was decided. The morphology‐dependent frequency shifts of the parallel infrared bands (B1 modes) in the POMAc10 crystal, which has a specific morphological structure different from the lamellar crystal of high‐molecular‐weight o‐POM, were investigated on the basis of the transition dipolar coupling theory.

Vibrational relaxation of proton acceptor in H-bonded complexes

Abstract The isotropic Raman band-widths of the ν CN stretching mode and ν 2 (A 1 ) mode of benzonitrile in the H-bonded complex with various phenols have been studied as a function of concentration and temperature. The ν CN and ν 12 band of free benzonitrile has been analyzed in terms of the isolated binary collision model. The ν 12 band of the complexed benzonitrile has been analysed in terms of the coupling of the oscillators and the dipolar coupling theory. The changes in the spectral profiles produced by the sub- stituent in phenols can be understood in terms of the dipolar coupling theory. It was shown that both the strength of the H-bond and the viscosity of the solution are important factors for the band broadening in H-bonded complexes.

Interaction-induced contribution to changes in the infrared spectrum of hydrogen-bonded complexes

An attempt to include interaction-induced effects into the study of absorption in H-bonded complexes is presented. The IR band shapes of the ν s(OH) mode in phenol, 2,4,6-trimethylphenol, 2,6-dichloro-4-nitrophenol, and dimethylamine with nitriles or pyridine derivatives have been studied in terms of the dipolar coupling theory.

Crystal vibration of low temperature phase β-barium metaborate and its multipolar coupling

The Raman and i.r. spectra of the newly synthesized second harmonic generating crystal β-barium metaborate in the low temperature phase are presented with a complete assignment of the external and internal vibrational modes. Emphasis is put on the discussion of the site group effect, the isotope effect and the dynamic coupling between the external and the internal motion of the B 3O 3− 6 ring and the Ba 2+ ion. Force constants are also calculated with the prediction that there is one internal vibrational mode of the B 3O 3− 6 ring at a frequency of 177 cm −1 which is of the order of the frequencies of the lattice modes. Multipolar coupling between the B 3O 3− 6 rings in this crystal is also analyzed with the generalization of a dipolar coupling theory.

Dipolar coupling and molecular vibrations in crystals. V. Correlation of microscopic and macroscopic theory

The application of dipolar coupling theory to the computation of dipole derivatives is reviewed from both a microscopic and macroscopic basis. It is shown that the two methods are essentially equivalent. In addition, two frequently used approximations to the complete dipolar coupling formalism are discussed. The results indicate that the assumption of a Lorentz field is not unreasonable for a number of crystals.