Molecular dynamics study of polarizability anisotropy relaxation in aromatic liquids and its connection with local structure

The collective polarizability anisotropy dynamics in a set of three aromatic liquids, benzene (Bz), hexafluorobenzene (HFB), and 1,3,5-trifluorobenzene (TFB), has been studied by molecular dynamics simulation. These liquids have very similar shapes, but different electrostatic interactions due to opposite polarities of C-H and C-F bonds, giving rise to different local intermolecular structures in the liquid phase. We have investigated how these structural arrangements affect polarizability anisotropy dynamics observed in optical Kerr-effect (OKE) spectroscopy. We have modeled the interaction-induced polarizability with the first-order dipole-induced dipole approximation, with the molecular polarizability distributed over the carbon sites. Local contributions to the librational OKE spectrum were computed separately for molecules participating in parallel or perpendicular relative orientations within the first coordination shell. We found that the relative locations of parallel and perpendicular librational bands of the OKE spectra are closely related to the corresponding pair energy distributions of the closest four neighbors of a given molecule, corresponding to a model of a harmonic oscillator in a cage of nearest neighbors. This model predicts higher librational frequencies for more attractive intermolecular interactions, which in all three liquids correspond to parallel local arrangements. On the diffusive orientational time scale, all three liquids exhibit slower relaxation of molecules in parallel arrangements, although the difference in relaxation rates is substantial only in TFB, which has the strongest tendency toward parallel stacking. The analysis of the collective polarizability relaxation was performed using two different approaches, the projection scheme (J. Chem. Phys. 1980, 72, 2801) and the theory developed by Steele (Mol. Phys. 1987, 61, 1031) for the second time derivatives applied to collective time correlations. Both approaches allow the decomposition of the OKE response into contributions from orientational relaxation and other dynamical processes. We find that they lead to different predictions on how the response depends on collective reorientation and processes arising from fluctuations in the interaction-induced polarizability. We discuss the reasons for these differences and the advantages and disadvantages of the two analysis schemes.     

Excited electronic states of the fluorobenzenes by variable angle electron impact spectroscopy

Electron-impact excitation spectra of benzene, fluorobenzene, o-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3,4-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene have been measured at impact energies of 50 eV and either 25 eV or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.6 eV. Three singlet → singlet transitions analogous to the 4.90, 6.20, and 6.95 eV transitions in benzene are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition that is most clearly observed in the hexafluorobenzene spectrum with a peak at 5.32 eV.     

The orbital interpretation of the photoelectron spectrum of benzene, 1,3,5-trifluorobenzene and hexafluorobenzene

An interpretation of certain features in the vibrational pattern of the 11.5 to 13.0 eV photoelectron spectrum of benzene in terms of a Jahn-Teller effect is presented. This supports Lindholm's orbital assignment of the ionization processes in this region. A change in the assignment of some of the higher systems in the photoelectron spectra of 1,3,5,-trifluorobenzene and hexafluorobenzene is proposed. Some discussion of Rydberg series found in these molecules is also given.     

Shape and electrostatic effects in optical Kerr effect spectroscopy of aromatic liquids

We present a detailed, temperature-dependent, optical Kerr effect (OKE) study of pyridine, pyridine-d(5), 2,4,6-trifluoropyridine, 2,4,6-trimethylpyridine, and 1,3,5-tris(trifluoromethyl)benzene. By combining these data with those for other aromatic liquids that we have studied previously (Loughnane, B. J.; Scodinu, A.; Fourkas J. T. J. Phys. Chem. B, 2006, 110, 5708), we are able to assess the relative importance of molecular shape and electrostatic forces in determining the form of the OKE reduced spectral density for such liquids.     

Structure and binding energy of anion-pi and cation-pi complexes: a comparison of MP2, RI-MP2, DFT, and DF-DFT methods

Several complexes of benzene with cations, hexafluorobenzene with anions, 1,3,5-trifluorobenzene with cations and anions, and s-triazine with cations and anions have been evaluated and compared at the MP2 and resolution of the identity MP2 (RI-MP2) levels. The RI-MP2 method is considerably faster than the MP2 and the interaction energies and equilibrium distances are almost identical for both methods. A similar result is found when comparing DFT and density fitting DFT (DF-DFT) levels. Therefore RI-MP2 and DF-DFT methods are well suited for the study of ion-pi interactions.     

Why does the intermolecular dynamics of liquid biphenyl so closely resemble that of liquid benzene? Molecular dynamics simulation of the optical-Kerr-effect spectra

The combination of optical-Kerr-effect (OKE) spectroscopy and molecular dynamics simulations has provided us with a newfound ability to delve into the librational dynamics of liquids, revealing, in the process, some surprising commonalities among aromatic liquids. Benzene and biphenyl, for example, have remarkably similar OKE spectra despite marked differences in their shapes, sizes, and moments of inertia--and even more chemically distinct aromatics tend to have noticeable similarities in their spectra. We explore this universality by using a molecular dynamics simulation to investigate the librational dynamics of molten biphenyl and to predict its OKE spectrum, comparing the results with our previous calculations for liquid benzene. We suggest that the impressive level of quantitative agreement between these two liquids is largely a reflection of the fact that librations in these and other aromatic liquids act as torsional oscillations with oscillator frequencies selected from the liquid's librational bands. Since these bands are centered about the librational Einstein frequencies, the quantitative similarities between the liquids are essentially reflections of the near identities of their Einstein frequencies. Why then are the Einstein frequencies themselves so insensitive to molecular details? We show that, for nearly planar molecules, mean-square torques and moments of inertia tend to scale with molecular dimensions in much the same way. We demonstrate that this near cancellation provides both a quantitative explanation of the close relationship between benzene and biphenyl and a more general perspective on the similarities seen in the ultrafast dynamics of aromatic liquids.     

Structures of some fluorinated benzenes determined by ab initio computation

The structures of fluorobenzene, 1,3-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3-trifluorobenzene, pentafluorobenzene, and hexafluorobenzene have been determined by the ab initio gradient method with a 4-21 basis set. Approximate corrections have been developed to yield r_e and r₀ structures so that comparison can be made with the available experimental information. The C? F bond distance decreases in a uniform manner with increasing fluorination of the ring, either on adjacent or on meta sites. The ring structure also shows systematic deformation, with a decrease of about 0.01 Å in the C? C bond length adjacent to the point of substitution and an increase of about 2.5? in the ring angle at the substitution site in those cases where the requirement of ring closure permits.     

Nanostructural organization and anion effects in the optical Kerr effect spectra of binary ionic liquid mixtures

This article reports a study of the effect of anions on the optical Kerr effect (OKE) spectra of binary ionic liquid mixtures with one mixture comprising the 3-methyl-1-pentylimidazolium ([C 5mim] (+)) cation and the anions PF 6 (-) and CF 3CO 2 (-) (TFA (-)), and another mixture comprising the [C 5mim] (+) cation and the anions Br (-) and bis(trifluomethanesulfonyl)imide (NTf 2 (-)). The spectra were obtained by the use of optical heterodyne-detected Raman-induced Kerr Effect Spectroscopy at 295 K. The OKE spectra of the mixtures are compared with the calculated mole-fraction weighted sum of the normalized OKE spectra of the neat liquids. The OKE spectra are nearly additive for [C 5mim]Br/[C 5mim][NTf 2] mixtures, but nonadditive for [C 5mim][PF 6]/[C 5mim][TFA] mixtures. In the case of the equimolar [C 5mim][PF 6]/[C 5mim][TFA] mixture, the nonadditivity is such that the experimental OKE spectrum is narrower than the calculated OKE spectrum. The additivity or nonadditivity of OKE spectra for IL mixtures can be explained by assuming ionic liquids are nanostructurally organized into nonpolar regions and ionic networks. The ionic networks in mixtures will be characterized by "random co-networks" for anions that are nearly the same in size (PF 6 (-) and TFA (-)) and by "block co-networks" for anions that differ greatly in size (Br (-) and NTf 2 (-)).     

Nanostructural organization and anion effects on the temperature dependence of the optical Kerr effect spectra of ionic liquids

The intermolecular spectra of three imidazolium ionic liquids were studied as a function of temperature by the use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids comprise the 1,3-pentylmethylimidazolium cation ([C(5)mim]+), and the anions, bromide (Br-), hexafluorophosphate (PF(6)-), and bis(trifluoromethanesulfonyl)imide (NTf(2)-). Whereas the optical Kerr effect (OKE) spectrum of [C(5)mim][NTf(2)] is temperature-dependent, the OKE spectra of [C(5)mim]Br and [C(5)mim][PF6] are temperature-independent. These results are surprising in light of the fact that the bulk densities of these room temperature ionic liquids (RTILs) are temperature-dependent. The temperature independence of the OKE spectra and the temperature dependence of the bulk density in [C(5)mim]Br and [C(5)mim][PF(6)] suggest that there are inhomogeneities in the densities of these liquids. The existence of density inhomogeneities is consistent with recent molecular dynamics simulations that show RTILs to be nanostructurally organized with nonpolar regions arising from clustering of the alkyl chains and ionic networks arising from charge ordering of the anions and imidazolium rings of the cations. Differences in the temperature dependences of the OKE spectra are rationalized on the basis of the degree of charge ordering in the polar regions of the RTILs.     

Intermolecular vibrations and fast relaxations in supercooled ionic liquids

Short-time dynamics of ionic liquids has been investigated by low-frequency Raman spectroscopy (4 < ω < 100 cm(-1)) within the supercooled liquid range. Raman spectra are reported for ionic liquids with the same anion, bis(trifluoromethylsulfonyl)imide, and different cations: 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-butyl-1-methylpiperidinium, trimethylbutylammonium, and tributylmethylammonium. It is shown that low-frequency Raman spectroscopy provides similar results as optical Kerr effect (OKE) spectroscopy, which has been used to study intermolecular vibrations in ionic liquids. The comparison of ionic liquids containing aromatic and non-aromatic cations identifies the characteristic feature in Raman spectra usually assigned to librational motion of the imidazolium ring. The strength of the fast relaxations (quasi-elastic scattering, QES) and the intermolecular vibrational contribution (boson peak) of ionic liquids with non-aromatic cations are significantly lower than imidazolium ionic liquids. A correlation length assigned to the boson peak vibrations was estimated from the frequency of the maximum of the boson peak and experimental data of sound velocity. The correlation length related to the boson peak (～19 A?) does not change with the length of the alkyl chain in imidazolium cations, in contrast to the position of the first-sharp diffraction peak observed in neutron and X-ray scattering measurements of ionic liquids. The rate of change of the QES intensity in the supercooled liquid range is compared with data of excess entropy, free volume, and mean-squared displacement recently reported for ionic liquids. The temperature dependence of the QES intensity in ionic liquids illustrates relationships between short-time dynamics and long-time structural relaxation that have been proposed for glass-forming liquids.     

Additivity in the optical kerr effect spectra of binary ionic liquid mixtures: implications for nanostructural organization

Low-frequency spectra of binary room-temperature ionic liquid (RTIL) mixtures of 1-pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and 1-pentyl-3-methylimidazolium bromide in the 0-250 cm(-1) region were studied as a function of mole fraction at 295 K. The spectra were obtained by use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The spectra of these binary mixtures are well described by the weighted sums of the spectra for the neat RTILs. This surprising result implies that the intermolecular modes giving rise to the spectra of the neat liquids must also produce the spectra of the mixtures. Additivity of the OKE spectra can be explained by a model in which locally ordered domains are assumed to exist in the neat liquid with the structures of these locally ordered domains preserved upon mixing. Recently published molecular dynamics simulations show that RTILs are nanostructurally organized with ionic networks and nonpolar regions. If ionic networks also exist in the mixture, the additivity of the OKE spectra implies that there are "blocks" along the network of the mixture that are ordered in the same way as in the neat liquids. These "block co-networks" would have a nanostructural organization resembling that of a block copolymer.     

Additivity of ring distortions in halogen-substituted aromatics: a gas-phase electron diffraction and computational study

The gas-phase structures of 1,2,3-trifluorobenzene, 1,3,5-trifluorobenzene, 2,6-difluoropyridine, and 2,6-dichloropyridine have been determined using electron diffraction and computational methods. The accuracy afforded by modern experimental methods has allowed subtle trends to be identified in the geometrical parameters of the rings. Comparisons have also been made between experimental and theoretical structures. Although the effects of multiple fluorine substituents on a benzene ring are shown to be more or less additive, distortions caused by replacement of a ring carbon atom by nitrogen and by halogen substituents are not.     

An endogenous multiple scattering method for the calculation of the elastic scattering cross sections of electron shape resonances in polyatomic molecu

A procedure is described for obtaining the scattering potential for elastic electron—molecule scattering within the one-electron overlapping sphere multiple scattering Xα method. The method has been used to calculate the total elastic electron scattering cross sections for the nitrogen, ethylene and 1,3,5-trifluorobenzene molecules, which compare well with experimental data.     

A generalized formulation of ion-π electron interactions: role of the nonelectrostatic component and probe of the potential parameter transferability

The intermolecular potentials for hexafluorobenzene (HFBz) and 1,3,5-trifluorobenzene (TFBz) interacting with alkali (M(+); M = Li, Na, K, Rb, Cs) and halogen (X(-); X = F, Cl, Br, I) ions are provided as a combination of electrostatic and nonelectrostatic terms. The ion-HFBz and ion-TFBz electrostatic components are formulated as a sum of Coulombic potentials associated with the interactions between the ion charge and point charges on the molecular frame, whose distributions are consistent with the permanent quadrupole moment of HFBz and TFBz, respectively. The corresponding nonelectrostatic components are represented as a sum of effective potential functions, each one having a specific physical meaning, related to ion-molecular bond pair interactions. In the present paper, we test the transferability of the ion-bond potential parameters. Moreover, the powerfulness of the model is analyzed by comparing predicted binding energies and equilibrium geometries for the family of M(+)-HFBz, X(-)-HFBz, M(+)-TFBz, and X(-)-TFBz systems with available ab initio results.     

Structural features of 1,3,5-trifluorobenzene radical anion,as studied by optically detected ESR spectroscopy and quantum chemistry

The ESR spectrum of 1,3,5-trifluorobenzene radical anion was obtained for the first time using optically detected ESR spectroscopy. Experimental data and results of a quantum chemical study of the potential energy surface (density functional theory B3LYP and BHHLYP methods) are indicative of fast interconversions between nonplanar conformations of this radical anion.     

One-photon mass-analyzed threshold ionization spectroscopy of 1,3,5-trifluorobenzene: the Jahn-Teller effect and vibrational analysis for the molecular cation in the ground electronic state

One-photon mass-analyzed threshold ionization spectrum of 1,3,5-trifluorobenzene was obtained by using vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The Jahn-Teller parameters for the e' modes (nu(8)-nu(14)) of 1,3,5-C(6)H(3)F(3)(+) in the ground electronic state needed for spectral analysis were taken from the density functional theory results initially and were upgraded through fits to the experimental results. Excellent agreement was achieved between the experimental and calculated Jahn-Teller energy levels. Assignments of the Jahn-Teller inactive modes were accomplished by referring to the calculated frequencies and the selection rule. The ionization energy of 1,3,5-trifluorobenzene determined from the position of the 0-0 band was 9.6359+/-0.0006 eV.     

Microstructural analysis of poly(vinylidene fluoride) using benzene derivative pyrolysis products

Poly(vinylidene fluoride) (PVDF) was pyrolyzed, and the pyrolysis products were analyzed using gas chromatography/mass spectrometry (GC/MS) to develop a method for identification of the microstructures of head-to-tail (H-T), tail-to-tail (T-T), and head-to-head-to-tail-to-tail (H-H-T-T) sequences. Key pyrolysis products to determine the relative degrees of the microstructures were benzene derivatives. 1,4-Difluorobenzene, 1,2,4-trifluorobenzene, and 1,3,5-trifluorobenzene were major pyrolysis products as benzene derivatives. 1,3,5-Trifluorobenzene and 1,2,4-trifluorobenzene were formed from two HF-eliminated PVDF by 1,6-HF elimination and 1,6-rearrangement, while 1,4-difluorobenzene was generated from two HF-eliminated PVDF by 1,6-H₂ elimination and 1,6-rearrangement. 1,3,5-Trifluorobenzene was generated from the H-T sequence, whereas 1,4-difluorobenzene was formed from the T-T one. 1,2,4-Trifluorobenzene can be formed from the H-H-T-T sequence. Relative component ratios of the H-T, T-T, and H-H-T-T sequences of PVDFs can be estimated by comparing relative abundances of the benzene derivative pyrolysis products.     

Glassiness of thermotropic liquid crystals across the isotropic-nematic transition

The orientational dynamics of thermotropic liquid crystals across the isotropic-nematic phase transition have traditionally been investigated at long times or low frequencies using frequency domain measurements. The situation has now changed significantly with the recent report of a series of interesting transient optical Kerr effect (OKE) experiments that probed orientational relaxation of a number of calamitic liquid crystals (which consist of rod-like molecules) directly in the time domain, over a wide time window ranging from subpicoseconds to tens of microseconds. The most intriguing revelation is that the decay of the OKE signal at short to intermediate times (from a few tens of picoseconds to several hundred nanoseconds) follows multiple temporal power laws. Another remarkable feature that has emerged from these OKE measurements is the similarity in the orientational relaxation behavior between the isotropic phase of calamitic liquid crystals near the isotropic-nematic transition and supercooled molecular liquids, notwithstanding their largely different macroscopic states. In this article, we present an overview of the understanding that has emerged from recent computational and theoretical studies of calamitic liquid crystals across the isotropic-nematic transition. Topics discussed include (a) single-particle as well as collective orientational dynamics at a short-to-intermediate time window, (b) heterogeneous dynamics in orientational degrees of freedom diagnosed by a non-Gaussian parameter, (c) fragility, and (d) temperature-dependent exploration of underlying energy landscapes as calamitic liquid crystals settle into increasingly ordered mesophases upon cooling from the high-temperature isotropic phase. A comparison of our results with those of supercooled molecular liquids reveals an array of analogous features in these two important classes of soft matter systems. We further find that the onset of growth of the orientational order in the parent nematic phase induces translational order, resulting in smectic-like layers in the potential energy minima of calamitic systems if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. We discuss implications of this startling observation. We also discuss recent results on the orientational dynamics of discotic liquid crystals that are found to be rather similar to those of calamitic liquid crystals.     

Measuring acetic acid dimer modes by ultrafast time-domain Raman spectroscopy

Acetic acid is capable of forming strong multiple hydrogen bonds and therefore different dimeric H-bonded structures in neat liquid phase and in solutions. The low frequency Raman spectra of acetic acid (neat, in aqueous solution and as a function of temperature) were obtained by ultrafast time and polarization resolved optical Kerr effect (OKE) measurements. Isotropic OKE measurements clearly reveal a specific totally symmetric mode related to the dimeric structure H-bond stretching mode. The effects of isotope substitution, water dilution and temperature on this mode were investigated. These results together with anisotropic OKE measurements and density functional theory calculations for a number of possible dimers provide strong evidence for the cyclic dimer structure being the main structure in liquid phase persisting down to acetic acid concentrations of 10 M. Some information about the dimer structure and concentration dependence was inferred.