Do the molecules which form discotic liquid crystals have disc-like structures? The conformation of a simple model compound, 1,2-dihydroxydiacetylbenzene, determined from the NMR spectra of samples dissolved in liquid crystalline solvents

Many discotic mesogens are molecules with a central aromatic ring with adjacent alkylcarboxylate substituents. The simplest such molecule, 1,2-dihydroxydiacetylbenzene, which is not mesogenic, is studied by NMR spectroscopy as a solute in a nematic solvent. The spectra are analysed to give sets of residual dipolar couplings, D_ij , which are then used to test models for the conformation adopted by the acetate side groups. The conformations and geometry of an isolated molecule are calculated by the ab initio MP2/6-311G method and also by the DFT approach using the B3LYP functional with the 6-311++G** basis set. The quantum chemical calculations find that the minimum energy conformer has the acetate groups rotated in opposite directions out of the ring plane, and this kind of structure is also consistent with the NMR data.     

Structure and electronic properties of a benzene-water solution

Electronic properties of benzene in water were investigated by a sequential quantum mechanical/molecular dynamics approach. Emphasis was placed on the analysis of the structure, polarization effects, and ionization spectrum. By adopting a polarizable model for both benzene and water the structure of the benzene-water solution is in good agreement with data from first principles molecular dynamics. Further, strong evidence that water molecules acquire enhanced orientational order near the benzene molecule is found. Upon hydration, the quadrupole moment of benzene is not significantly changed in comparison with the gas-phase value. We are also reporting results for the dynamic polarizability of benzene in water. Our results indicate that the low energy behaviour of the dynamic polarizability of gas-phase and hydrated benzene is quite similar. Outer valence Green's function calculations for benzene in liquid water show a splitting of the gas-phase energy levels associated with the 1e(1g)(π), 2e(2g), and 2e(1u) orbitals upon hydration. Lifting of the orbitals degeneracy and redshift of the outer valence bands is related to symmetry breaking of the benzene structure in solution and polarization effects from the surrounding water molecules.     

One and two hydrogen molecules in the large cage of the structure II clathrate hydrate: quantum translation-rotation dynamics close to the cage wall

We have performed a rigorous theoretical study of the quantum translation-rotation (T-R) dynamics of one and two H2 and D2 molecules confined inside the large hexakaidecahedral (5(12)6(4)) cage of the sII clathrate hydrate. For a single encapsulated H2 and D2 molecule, accurate quantum five-dimensional calculations of the T-R energy levels and wave functions are performed that include explicitly, as fully coupled, all three translational and the two rotational degrees of freedom of the hydrogen molecule, while the cage is taken to be rigid. In addition, the ground-state properties, energetics, and spatial distribution of one and two p-H2 and o-D2 molecules in the large cage are calculated rigorously using the diffusion Monte Carlo method. These calculations reveal that the low-energy T-R dynamics of hydrogen molecules in the large cage are qualitatively different from that inside the small cage, studied by us recently. This is caused by the following: (i) The large cage has a cavity whose diameter is about twice that of the small cage for the hydrogen molecule. (ii) In the small cage, the potential energy surface (PES) for H2 is essentially flat in the central region, while in the large cage the PES has a prominent maximum at the cage center, whose height exceeds the T-R zero-point energy of H2/D2. As a result, the guest molecule is excluded from the central part of the large cage, its wave function localized around the off-center global minimum. Peculiar quantum dynamics of the hydrogen molecule squeezed between the central maximum and the cage wall manifests in the excited T-R states whose energies and wave functions differ greatly from those for the small cage. Moreover, they are sensitive to the variations in the hydrogen-bonding topology, which modulate the corrugation of the cage wall.     

An experimental value for the B(1u) C-H stretch mode in benzene

We here present experimental infrared spectra on two (C(6)H(6))(C(6)D(6)) benzene dimer isomers in the gas phase. The spectra show that the two benzene molecules in the dimer are symmetrically inequivalent and have distinct IR signatures. One of the two molecules is in a site of low symmetry, which leads to the IR activation of fundamental modes that are IR forbidden by symmetry in the monomer. In the spectra, all four fundamental C-H stretch modes of benzene are observed. Modes in the dimer are shifted up to 3 cm(-1) to the red, compared to the modes that are known for the monomer. For the nu(13) B(1u) C-H stretch fundamental mode of benzene, a first experimental value of 3015(+2) (-5) cm(-1) is determined, in excellent agreement with anharmonic frequency calculations presented here.     

Computer simulation of the supercritical carbon dioxide fluid (II) internal energy and structure of carbon dioxide system containing one benzene molecule

Using potential models based on ab initio quantum chemical calculations, we study a supercritical CO₂ fluid containing one benzene molecule using Monte Carlo simulations. First, molecular average internal energy is calculated for the whole system and for the first solvation shell of the benzene molecule. This analysis shows that the CO₂ molecules in the first solvation shell have a large energetic stabilization owing to the shape of the solute. In addition to the stabilization, the solute-solvent interactions in the first solvation shell show large fluctuations for both the in-plane and out-of-plane parts. Secondly, an orientational distribution function is defined to investigate the CO₂ fluid structure. This function indicates that the CO₂---CO₂ intermolecular configuration has a large dependence on the temperature of the system for both the whole system, and for the first solvation shell of the solute. Moreover, the benzene molecule is confirmed to control the mutual arrangement between neighboring CO₂ molecules.     

Conformational energy landscapes of liquid crystal molecules

The conformational energy landscape of the prototypical nematic liquid crystal 4-pentyl-4cyanobiphenyl (5CB) is studied using first principles computer modelling. It is found that the most favourable conformation occurs when the two constituent phenyl rings are inclined at an angle of 31 with respect to each other. Also, the orientation of the alkyl chain is found to have an important influence on the ring-ring torsional potential. We fit the energy surface of these coupled torsions to yield an accurate intramolecular potential for use in empirical modelling. To test the strength of the coupling between the alkyl tail and the phenyl rings and the cyano group, we also calculate potentials for the relative orientation of the phenyl rings in biphenyl and cyanobiphenyl (0CB). Our calculations are performed using density functional theory using pseudo-potentials and the generalized gradient approximation to exchange and correlation. The molecular electronic wavefunction is expanded in terms of a plane wave basis set. We compare our results with recent NMR and Gaussian-based quantum chemistry calculations where available.     

Reactivity and binding energies of transition metal halide ions with benzene

We have generated novel halogen-ligated transition metal ions MX(n)+ (M = Sc, Ti, V, and Fe, X = Cl, Br and I, n = 1-3). We have explored their reactions with benzene, a typical aromatic hydrocarbon. Attachment of one benzene molecule is usually rapid, whereas attachment of a second benzene molecule is generally much slower. The kinetics were analyzed to estimate binding energies, modeling the attachment reaction as a radiative association process. In all cases the Standard Hydrocarbon semiquantitative estimation approach was employed, and in some cases the more accurate variational transition state (VTST) kinetic modeling approach was also applied. Density functional (DFT) quantum calculations were also performed to give computed binding energies for some of the complexes. Taking previously determined binding energies for halogen-ligated alkaline-earth ions as benchmarks, it is concluded that binding of the first benzene molecule to the transition-metal species is strongly enhanced by specific chemical interactions, while binding of the second benzene molecule is more nearly electrostatic. The binding energies are not strongly dependent on the identity of the transition metal ion, and the metal-ion dependences can be rationalized in terms of valence-orbital occupations of the metals. The binding energies are nearly independent of the identity of the halogen ligands.     

Phenol-benzene complexation dynamics: quantum chemistry calculation, molecular dynamics simulations, and two dimensional IR spectroscopy

Molecular dynamics (MD) simulations and quantum mechanical electronic structure calculations are used to investigate the nature and dynamics of the phenol-benzene complex in the mixed solvent, benzene/CCl4. Under thermal equilibrium conditions, the complexes are continuously dissociating and forming. The MD simulations are used to calculate the experimental observables related to the phenol hydroxyl stretching mode, i.e., the two dimensional infrared vibrational echo spectrum as a function of time, which directly displays the formation and dissociation of the complex through the growth of off-diagonal peaks, and the linear absorption spectrum, which displays two hydroxyl stretch peaks, one for the complex and one for the free phenol. The results of the simulations are compared to previously reported experimental data and are found to be in quite reasonable agreement. The electronic structure calculations show that the complex is T shaped. The classical potential used for the phenol-benzene interaction in the MD simulations is in good accord with the highest level of the electronic structure calculations. A variety of other features is extracted from the simulations including the relationship between the structure and the projection of the electric field on the hydroxyl group. The fluctuating electric field is used to determine the hydroxyl stretch frequency-frequency correlation function (FFCF). The simulations are also used to examine the number distribution of benzene and CCl4 molecules in the first solvent shell around the phenol. It is found that the distribution is not that of the solvent mole fraction of benzene. There are substantial probabilities of finding a phenol in either a pure benzene environment or a pure CCl4 environment. A conjecture is made that relates the FFCF to the local number of benzene molecules in phenol's first solvent shell.     

Molecular dynamics study of the tautomeric equilibrium in the 4-nitro- and 2,4,6-trichloro derivatives of 2-(N,N-dialkyloaminomethyl)phenol

 Molecular dynamics thermodynamic integration (MDTI) method and quantum chemical calculations at the density functional theory B3LYP 6-31+(d,p) level, which included the Tomasi model of the solvent reaction field, were applied to study the tautomeric equilibrium of Mannich base in methanol solution. The values obtained for the free-energy difference are in good agreement with experimental data. However, the results from quantum mechanical calculations were not as good as the results of MDTI simulations owing to inappropriate treatment of intermolecular hydrogen bonds between the solute molecule and the first shell of solvent molecules in the Tomasi model of the solvent reaction field. The radial distribution functions between solute atoms and solvent atoms confirmed the formation of hydrogen bonds between the solute molecule and surrounding methanol molecules and indicated that the zwitterionic form is associated more with an organized solvent structure at the level of the first solvation shell than is the molecular form. Received: 26 April 2002 / Accepted: 9 September 2002 / Published online: 31 March 2003     

Adsorption structures of benzene on a Si(5 5 12)-2x1 surface: a combined scanning tunneling microscopy and theoretical study

In the first ever attempt to study the adsorption of organic molecules on high-index Si surfaces, we investigated the adsorption of benzene on Si(5 5 12)-(2x1) by using variable-low-temperature scanning tunneling microscopy and density-functional theory (DFT) calculations. Several distinct adsorption structures of the benzene molecule were found. In one structure, the benzene molecule binds to two adatoms between the dimers of D3 and D2 units in a tilted butterfly configuration. This structure is produced by the formation of di-sigma bonds with the substrate and of two C[Double Bond]C double bonds in the benzene molecule. In another structure, the molecule adsorbs on honeycomb chains with a low adsorption energy because of strain effects. Our DFT calculations predict that the adsorption energies of benzene are 1.03-1.20 eV on the adatoms and 0.22 eV on the honeycomb chains.     

Molecule‐specific determination of atomic polarizabilities with the polarizable atomic multipole model

Recently, many polarizable force fields have been devised to describe induction effects between molecules. In popular polarizable models based on induced dipole moments, atomic polarizabilities are the essential parameters and should be derived carefully. Here, we present a parameterization scheme for atomic polarizabilities using a minimization target function containing both molecular and atomic information. The main idea is to adopt reference data only from quantum chemical calculations, to perform atomic polarizability parameterizations even when relevant experimental data are scarce as in the case of electronically excited molecules. Specifically, our scheme assigns the atomic polarizabilities of any given molecule in such a way that its molecular polarizability tensor is well reproduced. We show that our scheme successfully works for various molecules in mimicking dipole responses not only in ground states but also in valence excited states. The electrostatic potential around a molecule with an externally perturbing nearby charge also exhibits a near‐quantitative agreement with the reference data from quantum chemical calculations. The limitation of the model with isotropic atoms is also discussed to examine the scope of its applicability. © 2012 Wiley Periodicals, Inc.     

Polymer-Metal Nanoparticle Complexes for Improving the Performance of Liquid Crystal Displays

Summary: Here we applied metal nanoparticles as a dopant of liquid crystals. Since liquid crystal molecules are self-assembled, it is not so easy to disperse metal nanoparticles in liquid crystal media. We first prepared metal nanoparticles protected by liquid crystal molecules by reduction of metal ions in the presence of liquid crystal molecules. This liquid crystal molecule-protected metal nanoparticles can be easily dispersed in liquid crystal media to fabricate liquid crystal sol containing metal nanoparticles. A simple liquid crystal molecule, 4′-pentylbiphenyl-4-carbonitrile (abbreviated as 5CB) was used in the present experiments at first. 5CB sol containing metal nanoparticles could construct novel twisted nematic liquid crystal devices (TN-LCDs), which revealed the electrooptic properties depending on the kind of metal of nanoparticles. During the experiments we discovered that 5CB-protected metal nanoparticles could move in liquid crystal media by applying the voltage. This phenomenon is inconvenient for liquid crystal displays, especially those driven by a matrix of thin-film transistors (TFTs). In order to avoid this phenomenon, we prepared polymer-protected metal nanoparticles and applied them to liquid crystal devices, which provided good performance as the devices, i.e., low driving voltage, rapid response at low temperature, and so on.     

Molecular dynamics simulations applied to electric field induced second harmonic generation in dipolar chromophore solutions

Electric field induced second harmonic generation (EFISH) is an important experimental technique in extracting the first hyperpolarizability of an organic chromophore molecule. Such experiments are carried out in solutions with chromophore molecules dissolved in some common solvents. A known fact is that the first hyperpolarizabilities extracted from EFISH experiments are subject to the use of local field factors. In this work, we apply simulations to study the EFISH properties of chromophore solutions. By combining quantum chemistry calculations with the results derived from molecular dynamics simulations, we show how macroscopic EFISH properties can be modeled, using 4-(dimethylamino)-4'-nitroazobenzene dissolved in chloroform as a demonstration case. The focus of the study is on deriving accurate local field factors. We find that the local field approach applies very well to dipolar solutions, such as the one studied here, but that the local field factors derived are much smaller than the commonly used Onsager or Lorentz local field factors. Our study indicates that many of the reported first hyperpolarizabilities for dipolar molecules from EFISH experiments are most probably underestimated because the Onsager/Lorentz approach, commonly used in extracting the molecular first hyperpolarizability, neglects the effects of the shapes of dipolar chromophore molecules on the local field factors.     

Contribution to understanding of the molecular dynamics in liquids

The dielectric relaxation spectroscopy is used for studying the orientational molecular dynamics in the isotropic (I) and nematic (N) phases of two mesogenic liquids composed of the molecules of similar structure and length, but of an essentially different polarity: n-heptylcyanobiphenyl, C(7)H(15)PhPhCN, 7CB (molecular dipole moment mu approximately 5D) and 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene, C(6)H(13)CyHxPhNCS, 6CHBT (mu approximately 2.5D); advantageously, the temperatures of the I-N phase transition for the two compounds are very close to each other (T(NI) = 316.6 +/- 0.2 K). It is shown that regardless of the differences in polarity of 7CB and 6CHBT molecules and their abilities in dipolar aggregation, the values and temperature dependences of the relaxation time (corresponding to the rotational diffusion of the molecules around their short axis) are very close to each other, in both the isotropic and nematic phases of the liquids studied. Therefore, the data show that the dielectric relaxation processes occurring in dipolar liquids in the isotropic and nematic states lead through the rotational diffusion of individual molecules and the diffusion seems to be not influenced by the intermolecular interactions.     

A computational study of organic polyradicals stabilized by chromium atoms

Density functional theory has been used to investigate the properties of organic high spin molecules. The M05/cc-pVDZ calculations predict a septet ground state for the 2,3,6,7,10,11-hexahydro-1,4,5,8,9,12-hexaoxocoronene-2,3,6,7,10,11-hexayl radical (coronene-6O). The computations show further that the formation of intermolecular carbon-carbon bonds yields a singlet ground state for the dimer rather than a possible tridectet state as expected from the monomer's multiplicity. A benzene molecule placed between coronene-6O molecules leads to the desired high-spin cluster, but the overall stability of the cluster is low. A chromium atom inserted between two peripheral C(6) rings of coronene-6O yields a sandwich structure with the expected tridectet ground state and a binding energy which is 15 times larger than the corresponding tridectet dimer stabilized by a benzene molecule. The presented DFT calculations suggest that a chromium atom can effectively link organic polyradicals to larger magnetic units.     

Mesogenic behaviour of (4-alkoxybenzylideneamino)bromobenzene, (4-alkoxybenzylideneamino)benzenethiols, and thiolatonickel complexes of the thiols

The synthesis, characterization and mesogenic properties of Schiff base compounds arising from the reaction of 4-alkoxybenzaldehydes with 4-aminothiophenol or 4-bromoaniline are described. Whereas the Schiff base thiol with two benzene rings in the molecule, HSC₆H₄NC(H)C₆H₄OC₁₆H₃₃ (2), is non-mesogenic, the bromo analogue, BrC₆H₄NC(H)C₆H₄OC₁₆H₃₃ (3), is mesogenic. The introduction of a third benzene ring into the molecular architecture of 2 and 3 produced thiol- and bromo-Schiff base compounds, HSC₆H₄NC(H)C₆H₄OC(O)C₆H₄OC₁₆H₃₃ and BrC₆H₄NC(H)C₆H₄OC(O)C₆H₄OC₁₆H₃₃, respectively, that are both mesogenic. The thiol compounds react with nickelocene to form [(η ⁵-C₅H₅)Ni(μ ₂-SC₆H₄NC(H)C₆H₄OC₁₆H₃₃)]₂ and [(η ⁵-C₅H₅)Ni(μ ₂-SC₆H₄NC(H)C₆H₄OC(O)-C₆H₄OC₁₆H₃₃)]₂, but the nickel complexes are not mesogenic.     

Electrostatic properties of cyano-containing mesogens

We analyse the electrostatic properties of a set of cyano-containing mesogen molecules with different rigid cores and variable alkyl chain lengths, computing the molecular charge distributions. Using the simple prototype benzonitrile, we analyse the reliability of the quantum chemical methods used to estimate the electrostatic dipole moments of polar conjugated molecules. We show that the electronic properties of the long mesogenic molecules can be treated by combining HF geometry optimization procedures with single point MP2 calculations. We compare the results of these computations with the available experimental phase transition data of mesogens and discuss some examples of how the non-trivial mesomorphic behaviour, which is usually observed in these cyano compounds, can be qualitatively explained by the molecular electrostatic interaction potential.     

Electrostatic properties of cyano-containing mesogens

We analyse the electrostatic properties of a set of cyano-containing mesogen molecules with different rigid cores and variable alkyl chain lengths, computing the molecular charge distributions. Using the simple prototype benzonitrile, we analyse the reliability of the quantum chemical methods used to estimate the electrostatic dipole moments of polar conjugated molecules. We show that the electronic properties of the long mesogenic molecules can be treated by combining HF geometry optimization procedures with single point MP2 calculations. We compare the results of these computations with the available experimental phase transition data of mesogens and discuss some examples of how the non-trivial mesomorphic behaviour, which is usually observed in these cyano compounds, can be qualitatively explained by the molecular electrostatic interaction potential.     

Classical and quantum studies of the photodissociation of a HX (X=Cl,F) molecule adsorbed on ice

The photodissociation dynamics of a HX (X = Cl,F) molecule adsorbed on a hexagonal ice surface at T = 0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures.