SYNTHESIS AND VARIABLE-TEMPERATURE FTIR STUDY OF 5 CHIRAL LIQUID-CRYSTALS INDUCED BY INTERMOLECULAR HYDROGEN-BONDING

Five new chiral liquid crystal systems induced by intermolecular hydrogen bonding between 4-[(S)-2-chloro-3-methyl]butyroyloxy-4'-stilbazole (MBSB, proton acceptor) and 4-alkoxybenzoic acids (nBA, proton donors) were prepared. Their liquid crystalline properties were investigated by DSC and polarized optical microscopy. Chiral nematic and chiral smectic phases were observed, and the thermal stability of one complex was studied through temperature dependent infrared spectroscopy.

SYNTHESIS AND PHASE-BEHAVIOR STUDIES OF 2 NEW CHIRAL LIQUID-CRYSTALS OF SCHIFF-BASE - THE INFLUENCE OF INTRAMOLECULAR HYDROGEN-BOND ON THE PHASE-BEHAVIOR

Two new chiral liquid crystals of schiff-base type have been synthesized. This series of compounds contain a-chloro acidic ester chain prepared from commercially available L-valine. Both of the compounds exhibit tilted smectic phases; their phase transitions were studied using DSC and polarized optical microscopy; the influence of intramolecular hydrogen bonds on the phase behavior was studied as well.

The effect of hydrogen bonding on the diffusion of water in n-alkanes and n-alcohols measured with a novel single microdroplet method.

While the Stokes-Einstein (SE) equation predicts that the diffusion coefficient of a solute will be inversely proportional to the viscosity of the solvent, this relation is commonly known to fail for solutes, which are the same size or smaller than the solvent. Multiple researchers have reported that for small solutes, the diffusion coefficient is inversely proportional to the viscosity to a fractional power, and that solutes actually diffuse faster than SE predicts. For other solvent systems, attractive solute-solvent interactions, such as hydrogen bonding, are known to retard the diffusion of a solute. Some researchers have interpreted the slower diffusion due to hydrogen bonding as resulting from the effective diffusion of a larger complex of a solute and solvent molecules. We have developed and used a novel micropipette technique, which can form and hold a single microdroplet of water while it dissolves in a diffusion controlled environment into the solvent. This method has been used to examine the diffusion of water in both n-alkanes and n-alcohols. It was found that the polar solute water, diffusing in a solvent with which it cannot hydrogen bond, closely resembles small nonpolar solutes such as xenon and krypton diffusing in n-alkanes, with diffusion coefficients ranging from 12.5x10(-5) cm(2)/s for water in n-pentane to 1.15x10(-5) cm(2)/s for water in hexadecane. Diffusion coefficients were found to be inversely proportional to viscosity to a fractional power, and diffusion coefficients were faster than SE predicts. For water diffusing in a solvent (n-alcohols) with which it can hydrogen bond, diffusion coefficient values ranged from 1.75x10(-5) cm(2)/s in n-methanol to 0.364x10(-5) cm(2)/s in n-octanol, and diffusion was slower than an alkane of corresponding viscosity. We find no evidence for solute-solvent complex diffusion. Rather, it is possible that the small solute water may be retarded by relatively longer residence times (compared to non-H-bonding solvents) as it moves through the liquid.

Vibrational spectra of α-amino acids in the zwitterionic state in aqueous solution and the solid state: DFT calculations and the influence of hydrogen bonding

The zwitterionic forms of the two simplest alpha-amino acids, glycine and l-alanine, in aqueous solution and the solid state have been modeled by DFT calculations. Calculations of the structures in the solid state, using PW91 or PBE functionals, are in good agreement with the reported crystal structures, and the vibrational spectra computed at the optimized geometries provide a good fit to the observed IR and Raman spectra in the solid state. DFT calculations of the structures and vibrational spectra of the zwitterions in aqueous solution at the B3-LYP/cc-pVDZ level were found to require both explicit and implicit solvation models. Explicit solvation was modeled by inclusion of five hydrogen-bonded water molecules attached to each of the five possible hydrogen-bonding sites in the zwitterion and the integration equation formalism polarizable continuum model (IEF-PCM) was employed, providing a satisfactory fit to observed IR and Raman spectra. Band assignments are reported in terms of potential-energy distributions, which differ in some respects to those previously reported for glycine and l-alanine.

Hydrogen bonding and collective proton modes in clusters and periodic layers of squaric acid: A density functional study

Hydrogen bonding in clusters and extended layers of squaric acid molecules has been investigated by density functional computations. Equilibrium geometries, harmonic vibrational frequencies, and energy barriers for proton transfer along hydrogen bonds have been determined using the Car-Parrinello method. The results provide crucial parameters for a first principles modeling of the potential energy surface, and highlight the role of collective modes in the low-energy proton dynamics. The importance of quantum effects in condensed squaric acid systems has been investigated, and shown to be negligible for the lowest-energy collective proton modes. This information provides a quantitative basis for improved atomistic models of the order-disorder and displacive transitions undergone by squaric acid crystals as a function of temperature and pressure. (C) 2001 American Institute of Physics.

Platinum(II) phosphine and orotate complexes with aminopyridine co-ligands, and their molecular recognition via hydrogen bonding

The new complexes [Pt(dppp)(py)(2)][OTf](2), 1, [Pt(dppp)(2-ap)(2)][OTf](2), 2, [(dppp)Pt(mu -OH){mu -NH(C5H3N)NH2}Pt(dppp)][OTf](2), 3 (py=pyridine, 2-ap=2-aminopyridine, NH(C5H3N)NH2=2,6-diaminopyridine anion, dppp = 1,3-bis(diphenylphosphino)propane, OTf=O3SCF3) have been prepared via reactions between [Pt(dppp)(OTf)(2)] and pyridine, 2-aminopyridine or 2,6-diaminopyridine (2,6-dap) respectively. The amines exhibit a range of co-ordination modes. Pyridine and 2-aminopyridine co-ordinate to platinum through endo-nitrogen atoms in complexes 1 and 2, the latter existing as a pair of rotomers due to the steric hindrance introduced by the 2-substituent. However, 2,6-diaminopyridine co-ordinates to platinum through the exo-nitrogen of one amino group, to give the unusual mu -amido complex 3. Reaction of the known orotate chelate complex [Pt(PEt3)(2)(N,O-HL)] [HL=orotate, the dianion of 2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid (orotic acid)] with 2,6-dap gave [Pt(PEt3)(2)(2,6-dap)(N-HL)] 4, which contains an unconventional monodentate orotate ligand. In this co-ordination mode the orotate retains an ADA hydrogen bonding site and was found to co-crystallise with 2,6-dap via complementary ADA:DAD triple hydrogen bonds to give [Pt(PEt3)(2)(N-HL)(2,6-dap)].2,6-dap, 5. Complex 5 exhibits a helical chain structure of associated [1+1] adducts in the solid state.

Effect of weak hydrogen bonding and included solvent on the crystal structure of the square-planar complex trans-Pt{PPh2(C16H15)}(2)Cl-2

Crystallisation of the square-planar complex trans-Pt{PPh2(C16H15)}(2)Cl-2 from dichloromethane-diethyl ether (1:1) affords two different solvates; trans-Pt{PPh2(C16H15)}(2)Cl-2. CH2Cl2 1 and trans-Pt{PPh2(C16H15)}(2)Cl-2. Et2O 2; the CH2Cl2 forms H-bonding interactions with the complex whereas the Et2O participates only in weak van der Waals interactions; these differences arise from the different hydrogen-bonding characteristics of each solvent.

Orotate complexes of rhodium(I) and iridium(I): effect of coligand, counter ion, and solvent of crystallisation on association via complementary hydrogen bonding

The new complexes [NEt3H][M(HL)(cod)] (M = Rh 1 or Ir 2; H3L = 2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid, erotic acid; cod = cycloocta-1,5-diene) have been prepared by the reaction between [M2Cl2(cod)(2)] and erotic acid in dichloromethane in the presence of Ag2O and NEt3. They crystallise as dichloromethane adducts 1 . CH2Cl2 and 2 . CH2Cl2 from dichloromethane-hexane solutions. These isomorphous structures contain doubly hydrogen-bonded dimers, with additional hydrogen bonding to NEt3H+ cations and bridging CH2Cl2 molecules to form tapes. The use of (NBu4OH)-O-n instead of NEt3 gave the related complex [NBu4n][Rh(HL)(cod)] 1' which has an innocent cation not capable of forming strong hydrogen bonds and in contrast to 1 exists as discrete doubly hydrogen-bonded dimers. Complex 1' cocrystallises with 2,6-diaminopyridine (dap) via complementary triple hydrogen bonds to give [NBu4n][Rh(HL)(cod)]. dap . CH2Cl2 3. Complex 3 exhibits an extended sheet structure of associated [2 + 2] units, with layers of NBu4n, cations separating the sheets. These structural data together with those reported previously for platinum orotate complexes suggest that the steric requirements of the other ligands co-ordinated to the metal are important in influencing their hydrogen-bonding abilities. The solvent of crystallisation, the hydrogen-bonding propensity of the coligand and the nature of the counter ion also determine the type of association in the solid state.

Rate acceleration of the Baylis-Hillman reaction in polar solvents (water and formamide). Dominant role of hydrogen bonding, not hydrophobic effects, is implicated

A substantial acceleration of the Baylis-Hillman reaction between cyclohexenone and benzaldehyde has been observed when the reaction is conducted in water. Several different amine catalysts were tested, and as with reactions conducted in the absence of solvent, 3-hydroxyquinuclidine was found to be the optimum catalyst in terms of rate. The reaction has been extended to other aldehyde electrophiles including pivaldehyde. Attempts to extend this work to acrylates was only partially successful as rapid hydrolysis of methyl and ethyl acrylates occurred under the base-catalyzed and water-promoted conditions. However, tert-butyl acrylates were sufficiently stable to couple with relatively reactive electrophiles. Further studies on the use of polar solvents revealed that formamide also provided significant acceleration and the use of 5 equiv of formamide (optimum amount) gave faster rates than reactions conducted in water. Using formamide, further acceleration was achieved in the presence of Yb(OTf)(3) (5 mol %). The scope of the new conditions was tested with a range of Michael acceptors and benzaldehyde and with a range of electrophiles and ethyl acrylate. The origin of the rate acceleration is discussed.

Exploring molecular recognition pathways within a family of gelators with different hydrogen bonding motifs

We report the synthesis of a family of gelators in which alkyl chains are connected to the amino groups of L-lysine methyl ester using a range of different hydrogen bonding linking groups (carbamate, amide, urea, thiourea and diacylhydrazine) using simple synthetic methodology based on isocyanate or acid chloride chemistry. The ability of these compounds to gelate organic solvents such as toluene or cyclohexane can be directly related to the ability of the linking group to form intermolecular hydrogen bonds. In general terms, the ability to structure solvents can be considered as: thiourea <carbamate <amide <urea similar to diacylhydrazine. This process has been confirmed by thermal measurements, scanning electron microscopy (SEM) and infrared and circular dichroism spectroscopies. By deprotecting the methyl ester group, we have demonstrated that a balance between hydrophobic and hydrophilic groups is essential-if the system has too much hydrophilicity (e. g., diacylhydrazine, urea) it will not form gels due to low solubility in the organic media. However, the less effective gelators based on amide and carbamate linkages are enhanced by converting the methyl ester to a carboxylic acid. Furthermore, subsequent mixing of the acid with a second component (diaminododecane) further enhances the ability to form networks, and, in the case of the amide, generates a two-component gel, which can immobilise a wide range of solvents of industrial interest including petrol and diesel (fuel oils), olive oil and sunflower oil (renewable food oils) and ethyl laurate, isopropyl myristate and isopropyl palmitate (oils used in pharmaceutical formulation). The gels are all thermoreversible, and may therefore be useful in controlled release/formulation applications.