Rudolf Friedemann

Ten isomeric five-ring bent-core mesogens: The influence of the direction of the carboxyl connecting groups on the mesophase behaviour

In order to study the role of the direction of the connecting groups in bent-core mesogens we synthesized two series of ten possible achiral isomeric five-ring bent-core compounds in which all aromatic rings are connected by ester groups and each of which possesses the same length of the terminal chains (octyloxy or dodecyloxy, respectively). The structure of the isomers is distinguished by the direction of at least one ester group, only. The mesophase behaviour of the compounds has been studied by polarizing microscopy, differential scanning calorimetry, X-ray experiments and electro-optical measurements. We have found that in spite of the minor structural differences a variety of mesophases occur (SmCPA, Colrec, Colob) whereby the clearing temperatures vary from 121 to 193 °C (octyloxy isomers) and 112 to 189 °C (dodecyloxy isomers). Depending on the direction of the ester groups some of these isomers show interesting properties, such as field-induced inversion of chirality in SmCPA and columnar phases, the field-induced enhancement of the clearing temperature, a second-order phase transition Colob → SmCPA or the reversible field-induced phase transition Colob → SmCPF. The unexpectedly strong influence of the direction of the connecting groups is discussed on the base of theoretical calculations and molecular dynamics simulation on isolated molecules.

Unexpected liquid crystalline behaviour of three-ring bent-core mesogens: bis(4-subst.-phenyl) 2-methyl-iso-phthalates

Three-ring bent-core bis(4-subst.-phenyl) 2-methyl-iso-phthalates exhibiting nematic, SmA and SmC phases are reported. The occurring mesophases have been identified by their optical textures and X-ray diffraction measurements which give also geometrical structural parameters like layer spacing and molecular tilt. Quantum chemical calculations on single molecules and X-ray structure analysis in the crystalline state indicate wide opening angles (about 155°) of the molecular legs due to the lateral methyl group in position 2 of the central phenyl ring. However solid state NMR spectroscopy in the liquid crystalline phases finds stronger molecular bending (bending angle to be about 138° in the SmA and about 146° in the nematic phase). Dielectric and SHG measurements give evidence that in the SmA phase a polar structure can be induced by application of an electric field which disappears in the isotropic liquid phase. The electric field not only leads to a slight textural change even in the SmA phase but also polar-type electric current response (PS about 200 nC cm−2) is observed. This unusual electro-optical behaviour is discussed on the basis of the orientation of polar clusters formed by the bent molecules. In the paper we not only attempt to characterize the mesophases and to describe their physical properties, but we also show that these types of molecules represent the borderline between bent-shaped and calamitic liquid crystals.

DFT and MD studies on the influence of the orientation of ester linkage groups in banana-shaped mesogens

The influence of the direction of ester linkage groups on the structural and electronic properties of five-ring banana-shaped molecules with a central 1,3-phenylene unit has been investigated including hexyloxy and dodecyloxy terminal chains. DFT studies on the B3LYP/6-31G(d) level were performed on the conformational behaviour of the ten isomers in a systematic way. The one- and two-fold potential energy scans show that the flexibility of the wings significantly depends on the orientation of the carboxyl linkage groups. Moreover, the different directions of the carboxyl groups between the aromatic rings cause remarkable changes on the dipole moment and its components of the molecules. These findings are supported by investigations on the global charge pattern of the molecules calculated from electrostatic potential group charges. The bending angle alpha obtained from a simple model for the five-ring bent-core molecules is a characteristic structural parameter which can be correlated with experimental findings. Calculations on the bent-core molecules in an external electric dipole field related to the direction of their polar axis show remarkable effects with respect to the flexibility and polarity of the isomers. First molecular dynamics simulations on the banana-shaped molecules were carried out within the AMBER 7 package. The trajectories of relevant structural parameters support the findings of the DFT studies. The results concerning the structure and polarity revealed from the DFT and MD calculations of the ten isomers can be correlated with data from dielectric measurements and mesophase properties.

Ab initio and DFT calculations on the initial step in thiamin catalysis

Abstract The diphosphate ester (ThDP) of thiamin (vitamin B 1 ) is an important cofactor of enzymes within the carbohydrate metabolism. The initial reaction step shared by all ThDP-dependent enzymes is the deprotonation of the C2–H of the thiazolium ring. The replacement of the 4′-amino group by a hydroxyl one in the pyrimidine ring leads to the oxy-ThDP analogue which is known as an antagonist in thiamin catalysis. Ab initio and DFT calculations on the MP2/6-31G* and B3LYP/6-31G* level were performed to study the proton relay function in thiamin and oxythiamin systems. Both MP2 and B3LYP calculations show significant differences of the reaction coordinate of the ylide formation in the systems. Tautomers, protonated and deprotonated species of both systems show different trends regarding their stability. The influence of correlation effects on the results is discussed by comparison with the HF-SCF/6-31G* data. Frequency calculations on the B3LYP/6-31G* level were performed to characterize the minima and transition state structures, respectively.

Theoretical studies on the structure of chiral 1,2-diol systems. Monomers, dimers and monohydrates

Abstract The structures and stabilities of the isolated 1,2-propanediol (PDL1,2) monomers, dimers and monohydrates were investigated by the force field versions PIMM91 and GROMOS87 as well as the semiempirical PM3 method. The results on the PDL1,2 monomers are in agreement with ab initio data and gas phase experimental findings and show that the most stable conformers are stabilized by internal hydrogen bonding. The preferred structures of the dimers are characterized by the formation of additional intermolecular hydrogen bonds. Within the calculations dimers of PDL1,2 generated from the same (R-R) and different (R-S) enantiomers are taken into account. The aim of the investigations on the monohydrates of PDL1,2 was to study the function of water at the association of the 1,2-diol head groups in bilayers with a very simple model. Results on 1,3-propanediol are included for comparison. The different energetic and structural behaviours of both diol groups are discussed with respect to the formation of intramolecular and intermolecular hydrogen bonds.

Quantum chemical studies on the conformational behaviour of substituted banana‐shaped mesogens with a central 1,3‐phenylene unit

Ab initio and DFT calculations on the HF/STO‐3G and B3LYP/6‐31G(d) level were performed on the conformational behaviour of isolated banana‐shaped molecules of 1,3‐phenylene bis[4‐(4‐n‐hexyloxyphenyliminomethyl)benzoate] systems (P‐6‐O‐PIMB). The influence of small substituents in both the central phenyl ring and the external phenyl rings on the shape, polarity and flexibility of these molecules was investigated by one‐ and two‐fold relaxed potential energy scans in a systematic way. The effect of substituents on the global polarity of banana‐shaped mesogens was analysed by the magnitude and direction of the dipole moment and its components in relation to the long axis of the molecules. Moreover, a simple model for the calculation of the bending angle was tested for banana‐shaped molecules with a central 1,3‐phenylene unit. The findings for the isolated banana‐shaped molecules are correlated with solid state X‐ray and liquid crystalline state NMR results. Banana‐shaped molecules with both hexyloxy (P‐6‐O‐PIMB) and hexyl (P‐6‐PIMB) terminal chains are included to study the effect of substituents in the external phenyl rings on the flexibility of these chains. An attempt will be made to correlate the results with experimental findings on banana‐shaped mesogens.

Theoretical studies on the decarboxylation reaction in thiamin catalysis

The conformational behavior and the stability of thiazolium and thiamin diphosphate (ThDP) adducts formed by the C2 addition of the substrates pyruvate and glyoxylate to the corresponding thiamin systems are investigated within the force-field version PIMM90 as well as the semiempirical AM1 and PM3 methods. Moreover, the reaction coordinate of the decarboxylation process of the adducts with respect to the C2α(SINGLEBOND)COO− bond are calculated by PM3 and AM1. The calculations on the key intermediates of the Breslow mechanism are performed in order to study the steric aspects of both substrate adducts that show a different pathway in the catalytic cycle. The alternative structural findings for the decarboxylation products are compared with first 6–31C* studies on the corresponding thiazolium model systems. Especially, the PM3 calculations show that the elimination of CO2 is favored if the arrangement of the carboxylate group is nearly perpendicular to the plane of the thiazolium ring. These results support the least-motion maximum-overlap mechanism in the enzymatic decarboxylation reaction, proposed by Kluger. The most stable conformers of the ThDP adducts and its decarboxylation products are characterized by V-like structures and the formation of a significant intramolecular hydrogen bonding under participation of the 4′-aminopyrimidine ring.

Comparative dielectric measurements on isomeric bent-core mesogens

The physical behaviour, especially the dielectric properties, of two series of isomeric compounds formed by banana-shaped molecules is reported. The feature of all liquid crystalline substances investigated is the formation of a ferroelectric short range order. In one layer of the SmCPA phase the spontaneous polarisation shows in one direction and in the following in the opposite resulting in an antiferroelectric behaviour. It is demonstrated that the molecular origin of the polarization is connected with the perpendicular dipole moments. From the two relaxation modes seen the first results from ferroelectric units and the second from the reorientation of the molecules about the long axes. The collective character of both motions is demonstrated. The same mechanisms appear in the columnar phase, but they behave in a different way due to the different structure. Strong changes of the dielectric properties are observed at the transition from the isotropic in the liquid crystalline phase, but astonishingly not between both mesophases.

Theoretical studies on the structure of 1,3-diol systems: Part 1. Monomers, dimers and monohydrates

Abstract The structures and stabilities of the isolated 1,3-propanediol (PDL) monomers and dimers, and their monohydrates were investigated by the force field versions MMP2 and PIMM90, the semiempirical AM1 and PM3 methods, and the point charge model LEP. The calculations on the monomers were performed in order to test the applicability of these methods for diol systems. Especially the PIMM90, AM1 and PM3 results on PDL show (in agreement with ab initio data and experimental findings) that intramolecular hydrogen bonding is a significant structural element in such systems. The most stable structures of the dimers are characterized by a remarkable retention of the intramolecular hydrogen bonds and the formation of up to three additional intermolecular ones. The aim of the calculations on the monohydrates of PDL and (PDL) 2 was to simulate the function of water at the aggregation of diol head groups in bilayers in a very simple way. Within the PIMM90, PM3 and AM1 methods a different arrangement and function of the water molecule in the formation of most stable monohydrate structures results. From the studies on the PDL systems it follows that intramolecular hydrogen bonding is overestimated by the AM1 method in comparison with intermolecular hydrogen bonding. Especially the calculations on systems with intennolecular interactions show that the PIMM90 findings are in better agreement with the PM3 results than with the AM1 ones.

GROMOS-MD SIMULATIONS ON 1,2-DIOL WATER CLUSTERS

Abstract The structure, stability and dynamics of clusters of amphiphilic 1,2-octanediol, phenyl and cyclohexyl substituted 1,2-ethanediols as well as 1,2-hexadecanediol in the gas phase and aqueous solution were investigated. For the molecular dynamics simulations the program package gromos 87 was used. Such diols are suitable as model systems for the study of the structural formation in amphotropic liquid crystals and bilayers. First results of MD simulations on clusters up to 64 diol molecules are presented. Especially, the role of hydrogen bonding as well as the function of water molecules in the process of association of the hydrophilic head groups were studied in more detail. Moreover, the influence of modifications in the hydrophobic part on the molecular aggregation was investigated. For the visualization of the MD results a graphics tool was created on workstations.