Elastic constants of uniaxial nematic liquid crystals: A comparison between theory and experiment

Using the unified molecular theory developed in our earlier paper (1992, Phys. Rev. A, 45, 974) we study in detail the influence of molecular interactions on the fundamental elastic properties of uniaxial nematic liquid crystals composed of molecules of cylindrical symmetry. The expressions for the elastic moduli associated with 'splay', 'twist' and 'bend' modes of deformations are written in terms of order parameters characterizing the nature and amount of ordering in the phase and the structural parameters which involve the generalized spherical harmonic coefficients of the direct pair correlation function of an effective isotropic liquid. Numerical calculations are done for a model system, the molecules of which have prolate ellipsoid of revolution symmetry and interact via a pair potential having both repulsive and attractive parts. The repulsive interaction is represented by a repulsion between hard ellipsoids of revolution. The attractive potential is represented by the dispersion and electrostatic interactions. Results for the elastic constants are reported for a range of molecular length-width ratio, temperature, density and molecular parameters and are compared with the experimental values of p-azoxyanisole (PAA) and 4'-n-octyloxy-4-cyanobiphenyl (8OCB). It is found that the inclusion of electrostatic interactions reduces the values of the ratios K₂/K₁ and K₃/K₁. The absolute values of the elastic constants and their ratios are in good agreement with the experimental and computer simulation values. The temperature dependence of the elastic constants and their ratios is studied. It is observed that the twist elastic constant has a weak temperature dependence but a pronounced influence is observed on the bend moduli. We also observed a pronounced increase in the values of the twist and bend elastic constants on approaching the nematic-smectic A transition temperature.     

Raman scattering in crystalline oligothiophenes: a comparison between density functional theory and bond polarizability model

Raman intensity of intramolecular and lattice modes of crystalline alpha-bithiophene (alpha-2T) are investigated within density functional theory using a nonlinear response formalism. First, comparison between the calculated Raman spectrum and the experimental data allows the assignment of the main Raman lines over the whole frequency range. Then, a bond polarizability (BP) model, limited to first neighbors, is built. We show that, although the BP model cannot reproduce the changes of dielectric susceptibility under individual atomic displacements, it is accurate enough to reproduce the profile of the unpolarized nonresonant Raman spectrum of alpha-2T powder. Finally, the BP model, fitted on our first-principles results on alpha-2T, is applied with success to the alpha-quaterthiophene polymorph phases and alpha-sexithiophene, demonstrating on practical examples that first-principles and BP approaches are powerful complementary tools to calculate the nonresonant Raman spectrum of alpha-2T and make reasonable predictions on larger oligothiophenes.     

Fifth-order Raman spectroscopy of liquid benzene: experiment and theory

The heterodyned fifth-order Raman response of liquid benzene has been measured and characterized by exploiting the passive-phase stabilization of diffractive optics. This result builds on our previous work with liquid carbon disulfide and extends the spectroscopy to a new liquid for the first time. The all-parallel and Dutch Cross polarization tensor elements are presented for both the experimental results and a finite-field molecular dynamics simulation. The overall response characteristics are similar to those of liquid carbon disulfide: a complete lack of signal along the pump delay, an elongated signal along the probe delay, and a short-lived signal along the time diagonal. Of particular interest is the change in phase between the nuclear and electronic response along the probe delay and diagonal which is not seen in CS2. Good agreement is achieved between the experiment and the finite-field molecular dynamics simulation. The measurement of the low-frequency Raman two-time delay correlation function indicates the intermolecular modes of liquid benzene to be primarily homogeneously broadened and that the liquid loses its nuclear rephasing ability within 300 fs. This rapid loss of nuclear correlations indicates a lack of modal character in the low-frequency motions of liquid benzene. This result is a validation of the general nature of the technique and represents an important step forward with respect to the use of nonlinear spectroscopy to directly access information on the anharmonic motions of liquids.     

Identification of the point defects in diamond as measured by Raman spectroscopy: comparison between experiment and computation

Raman spectroscopy and molecular dynamics (MD) simulations are used to identify the vibrational spectrum of simple point defects in diamond. Two local mode frequencies are found in ion irradiated diamond. The first (with an energy of 185 meV) is clearly identified as arising from the vacancy defect, whereas a mode at 202 meV is demonstrated to be due to the [1 0 0] split interstitial.     

Gold nanoparticles electrooxidation: comparison of theory and experiment

The article presents the findings of microscopic and electrochemical studies of size-dependent gold particles electrooxidation. Gold particles were immobilized on the surface of carbon-containing screen-printed electrodes. The experiment demonstrated that the transition from macroparticles to nanoparticles caused a shift of the maximum current potential of gold oxidation into the area with more negative potentials. A decrease in particle size resulted in an increase in the electrochemical activity of metal. A positive correlation between experimental and calculated curves confirms a mathematical model (2) and correctness of the calculations. Measured parameters of voltammograms, in particular, maximum current potential, can be used to describe the electrochemical activity and energy properties of nanoparticles.     

Vibrational relaxation and line widths in liquids: a comparison of theory and experiment

Published experimental data on the Raman line widths of some a₁ bands of methyl iodide and benzene are compared with theoretical predictions of relaxation theories which assume either rapid or slow time scales for intermolecular fluctuations. The observed variations with density and with mole fraction are found to fit the rapid motion theory with an additional temperature independent core contribution.     

Non-equilibrium vibrational kinetics of CO pumped by vibrationally excited nitrogen molecules: A comparison between theory and experiment

Temporal evolution of CO vibrational distribution pumped by vibrationally excited nitrogen molecules has been experimentally determined by side-on infrared emission spectroscopy. Cooled and uncooled radiofrequency N₂ discharges have been utilized to pump vibrational energy in N₂, which then does excite CO in the post-discharge regime. CO₂ formed during the contact time has been detected by a mass-spectrometric technique. The experimental results show the different stages of the relaxation of CO, merely the pumping, the redistribution and the loss of vibrational quanta introduced by N₂. The results have been rationalized by means of a theoretical model which qualitatively reproduces the experimental relaxation of CO distributions as well as the production of CO₂.     

Coalescence of a bubble at a fluid-fluid interface: comparison of theory and experiment

Coalescence times for air bubbles rising through hexadecane to an air-hexadecane interface are measured and compared with an analysis based upon our previous extension of continuum mechanics to the nanoscale [J.C. Slattery, E.-S. Oh, K. Fu, Chem. Eng. Sci. 59 (2004) 4621-4635] with the assumption of retarded dispersion forces. The relation between the retarded and non-retarded Hamaker constants proposed by G?rner and Pich [J. Aerosol Sci. 20 (7) (1989) 735-747] is tested for the first time.     

Luminescent lifetimes near multiple interfaces: A quantitative comparison of theory and experiment

Results are presented for the classical theory of an emitting dipole located between two parallel mirrors. The calculated lifetime variations are in quantitative agreement with experiment for the silver/dielectric/Eu³⁺/air system.     

Single-molecule surface-enhanced Raman spectroscopy of crystal violet isotopologues: theory and experiment

Single-molecule surface-enhanced Raman spectroscopy (SMSERS) of crystal violet (CV) has been reported since 1997, yet others have offered alternative explanations that do not necessarily imply SMSERS. Recently, the isotopologue approach, a statistically significant method to establish SMSERS, has been implemented for members of the rhodamine dye family. We provide the first demonstration of SMSERS of a triphenylmethane dye using the isotopologue approach. Two isotopologues of CV are employed to create chemically identical yet vibrationally distinct probe molecules. Experimental spectra were compared extensively with computational simulations to assign changes in mode frequencies upon deuteration. More than 90 silver nanoparticle clusters dosed with a 50:50 mixture of CV isotopologues were spectroscopically characterized, and the vibrational signature of only deuterated or undeuterated CV was observed 79 times, demonstrating that the isotopologue approach for proving SMSERS is applicable to both the CV and the rhodamine systems. The use of CV, a minimally fluorescent dye, allowed direct evaluation of enhancement factors (EF), which are reported herein. Through experiment and theory, we show that molecular electronic resonance Raman (RR) and surface-enhanced Raman effects combine synergistically in SMSERS. Excluding RR effects, the EF(SERS) is ～10(9). Variations and relationships between substrate morphology and optical properties are further characterized by correlated SMSERS-localized surface plasmon resonance (LSPR)-high-resolution transmission electron microscopy (HRTEM) studies. We did not observe SMSERS from individual nanoparticles; further, SMSERS-supporting dimers are heterodimers of two disparately sized particles, with no subnanometer gaps. We present the largest collection to date of HRTEM images of SMSERS-supporting nanoparticle assemblies.     

Normal modes of redox-active tyrosine: conformation dependence and comparison to experiment

Redox-active tyrosine residues play important roles in long-distance electron reactions in enzymes such as prostaglandin H synthase, ribonucleotide reductase, and photosystem II (PSII). Spectroscopic characterization of tyrosyl radicals in these systems provides a powerful experimental probe into the role of the enzyme in mediation of long-range electron transfer processes. Interpretation of such data, however, relies critically on first establishing a spectroscopic fingerprint of isotopically labeled tyrosinate and tyrosyl radicals in nonenzymatic environments. In this report, FT-IR results obtained from tyrosinate, tyrosyl radical (produced by ultraviolet photolysis of polycrystalline tyrosinate), and their isotopologues at 77 K are presented. Assignment of peaks and isotope shifts is aided by density-functional B3LYP/6-311++G(3df,2p)//B3LYP/6-31++G(d,p) calculations of tyrosine and tyrosyl radical in several different charge and protonation states. In addition, characterization of the potential energy surfaces of tyrosinate and tyrosyl radical as a function of the backbone and ring torsion angles provides detailed insight into the sensitivity of the vibrational frequencies to conformational changes. These results provide a detailed spectroscopic interpretation, which will elucidate the structures of redox-active tyrosine residues in complex protein environments. Specific application of these data is made to enzymatic systems.     

Controlling the electrophoretic mobility of single-walled carbon nanotubes: a comparison of theory and experiment

The electrophoretic mobilities of single-walled carbon nanotubes (SWNTs) in agarose gels subjected to negatively charged covalent functionalization and noncovalent anionic surfactant adsorption are compared using a simplified hydrodynamic model. Net charges are calculated on the basis of estimated friction coefficients for cylindrical rodlike particles. The effects of functionalization with negatively charged 4-hydroxybenzene diazonium and anionic sodium cholate are quantified and compared with model predictions. The adsorption of Na+ counterions into the nonionic surfactant layer adsorbed on SWNTs (Triton-X-405) is shown to induce a positive charge and reverse the mobility under select conditions. This effect has not been identified or quantified for nanoparticle systems and may be important in the processing of these systems.     

Thermolysis of alkyl sulfoxides and derivatives: a comparison of experiment and theory.

Gas-phase activation data were obtained for model sulfoxide elimination reactions. The activation enthalpy for methyl 3-phenylpropyl sulfoxide is 32.9 +/- 0.9 kcal/mol. Elimination by methyl vinyl sulfoxide to form acetylene has an enthalpic barrier of 41.6 +/- 0.8 kcal/mol and that of 3-phenylpropyl methanesulfinate to form hydrocinnamaldehyde is 34.6 +/- 0.6 kcal/mol. Calculations at the MP2/6-311+G(3df,2p)//MP2/6-31G(d,p) level for simplified models of these reactions provide barriers of 32.3, 40.3, and 32.7 kcal/mol, respectively. A series of other compounds are examined computationally, and it is shown that the substituent effects on the sulfoxide elimination reaction are much more straightforward to interpret if DeltaH data are available in addition to the usually determined DeltaH++. The activation enthalpy of the reverse addition reaction is also subject to structural variation and can usually be rationalized on the basis of nucleophilicity of the sulfur or polarity matching between the sulfenic acid and olefin derivative.     

Rotational distributions following van der Waals molecule dissociation: comparison between experiment and theory for benzene-Ar

The translational energy release distribution for dissociation of benzene-Ar has been measured and, in combination with the 6(1)(0) rotational contour of the benzene product observed in emission, used to determine the rotational J,K distribution of 0(0) benzene products formed during dissociation from 6(1). Significant angular momentum is transferred to benzene on dissociation. The 0(0) rotational distribution peaks at J=31 and is skewed to low K:Javerage=27, (K)average=10.3. The average angle between the total angular momentum vector and the unique rotational axis is determined to be 68 degrees. This indicates that benzene is formed tumbling about in-plane axes rather than in a frisbeelike motion, consistent with Ar "pushing off" benzene from an off-center position above or below the plane. The J distribution is very well reproduced by angular momentum model calculations based on an equivalent rotor approach [A. J. McCaffery, M. A. Osborne, R. J. Marsh, W. D. Lawrance, and E. R. Waclawik, J. Chem. Phys. 121, 1694 (2004)], indicating that angular momentum constraints control the partitioning of energy between translation and rotation. Calculations for p-difluorobenzene-Ar suggest that the equivalent rotor model can provide a reasonable prediction of both J and K distributions in prolate (or near prolate) tops when dissociation leads to excitation about the unique, in-plane axis. Calculations for s-tetrazine-Ar require a small maximum impact parameter to reproduce the comparatively low J values seen for the s-tetrazine product. The three sets of calculations show that the maximum impact parameter is not necessarily equal to the bond length of the equivalent rotor and must be treated as a variable parameter. The success of the equivalent rotor calculations argues that angular momentum constraints control the partitioning between rotation and translation of the products.     

Dynamics of the D+ + H2 and H+ + D2 reactions: a detailed comparison between theory and experiment

An extensive set of experimental measurements on the dynamics of the H(+) + D(2) and D(+) + H(2) ion-molecule reactions is compared with the results of quantum mechanical (QM), quasiclassical trajectory (QCT), and statistical quasiclassical trajectory (SQCT) calculations. The dynamical observables considered include specific rate coefficients as a function of the translational energy, E(T), thermal rate coefficients in the 100-500 K temperature range. In addition, kinetic energy spectra (KES) of the D(+) ions reactively scattered in H(+) + D(2) collisions are also presented for translational energies between 0.4 eV and 2.0 eV. For the two reactions, the best global agreement between experiment and theory over the whole energy range corresponds to the QCT calculations using a gaussian binning (GB) procedure, which gives more weight to trajectories whose product vibrational action is closer to the actual integer QM values. The QM calculations also perform well, although somewhat worse over the more limited range of translational energies where they are available (E(T) < 0.6 eV and E(T) < 0.2 eV for the H(+) + D(2) and D(+) + H(2) reactions, respectively). The worst agreement is obtained with the SQCT method, which is only adequate for low translational energies. The comparison between theory and experiment also suggests that the most reliable rate coefficient measurements are those obtained with the merged beams technique. It is worth noting that none of the theoretical approaches can account satisfactorily for the experimental specific rate coefficients of H(+) + D(2) for E(T)≤ 0.2 eV although there is a considerable scatter in the existing measurements. On the whole, the best agreement with the experimental laboratory KES is obtained with the simulations carried out using the state resolved differential cross sections (DCSs) calculated with the QCT-GB method, which seems to account for most of the observed features. In contrast, the simulations with the SQCT data predict kinetic energy spectra (KES) considerably cooler than those experimentally determined.     

Microwave spectra of the Xe-N2 van der Waals complex: a comparison of experiment and theory

Rotational transitions for the Xe-N2 complex were measured in the frequency region from 4 to 18 GHz using a pulsed-nozzle Fourier-transform microwave spectrometer. Twelve (four) a-type transitions were recorded for the 132Xe-14N2 and 129Xe-14N2 (131Xe-15N)) isotopomers. In addition, the nuclear quadrupole hyperfine structures due to the presence of the 14N (nuclear-spin quantum number I=1) and 131Xe (I=32) nuclei were detected and analyzed. Two ab initio potential-energy surfaces were calculated at the coupled-cluster level of theory with single, double, and pertubatively included triple excitations. Dunning's augmented correlation-consistent polarized valence triple-zeta basis set was used for the nitrogen atoms. For the first surface, a well-tempered basis set with additional polarization functions was used for the Xe atom; for the second surface, a newly developed augmented correlation-consistent polarized valence quintuple-zeta basis set employing small-core relativistic pseudopotentials was used for the Xe atom. The basis sets were supplemented with bond functions for the van der Waals bond. The counterpoise correction was applied to reduce the basis-set superposition error. The resulting two surfaces both have a single minimum at a T-shaped geometry, with well depths of 122.4 and 119.3 cm(-1), respectively. Bound-state energies supported by the potential-energy surface were determined. The quality of the ab initio potential-energy surfaces was evaluated by comparison of the experimental transition frequencies and rotational and centrifugal distortion constants with those derived from the bound-state energies. A scaled potential-energy surface was obtained which has excellent agreement with the experimental data.     

Twisting power of bridged binaphthol derivatives: comparison of theory and experiment

The twisting ability of a novel series of bridged binaphthol derivatives with substituents of various lengths and chemical nature in the 6,6-positions, recently synthesized and used as dopants in nematic solvents, is investigated with the help of the model based upon surface- helicity tensors. Structures of the low energy conformers of these compounds have been generated by molecular mechanics calculations. Their orientational behaviour and the coupling between anisotropy in the alignment and molecular chirality, which are at the origin of the helical twisting power, are analysed on the basis of the anisometry and the chirality of the shape.     

On the comparison of the predictions of lifshitz theory with experiment: Van der Wads forces between mica plates

The behavior of the experimental force-separation curve for two crossed cylinders of mica is compared to theoretical force-separation curves for two flat parallel plates. The theoretical force is computed using a dielectric function for mica, which contains different ir and uv frequencies from that employed previously by Richmond and Ninham. A theoretical method is described for determining the variation in the force-power law with distance of separation.