Influence Of Molecular Flexibility Research Articles

The influence of molecular flexibility on the mesogenic behavior of a new homologous series based on azo-azomethine: Synthesis, characterization, photoisomerization and DFT study

Through condensation of 2-hydroxy-4-n-alkoxybenzaldehyde (n = 2 to 8, 10, 12, 14, 16, 18) with (E)-4-((2,4-dimethylphenyl)diazenyl) benzene-1,3-diamine, new homologous series of rod shaped calamite liquid crystals based on azo-azomethine have been prepared. All synthesized compounds were characterized by FT-IR, 1H NMR, and 13C NMR spectroscopy. Use Differential Scanning Calorimeter (DSC) and Polarizing Optical Microscope (POM) to check the liquid crystal properties of all synthetic compounds. In the homologous series, all the compounds are mesogenic; some of them are in the nematic phase; some of them appear in the smectic A phase, and some appear in the nematic and smectic A phases. UV–vis pectroscopy was used to study the photoisomerization in chloroform. DFT (Density functional theory) level calculations and complete geometric optimization of all synthetic compounds are performed using B3LYP/6-31+G (d, p) basis sets.

On-Surface Synthesis of BN-Substituted Heteroaromatic Networks.

We report on the bottom-up fabrication of BN-substituted heteroaromatic networks achieved by surface-assisted polymerization and subsequent cyclodehydrogenation of specifically designed BN-substituted precursor monomers based on a borazine core structural element. To get insight into the cyclodehydrogenation pathway and the influence of molecular flexibility on network quality, two closely related precursor monomers with different degrees of internal cyclodehydrogenation have been employed. Scanning tunneling microscopy shows that, for both monomers, surface-assisted cyclodehydrogenation allows for complete monomer cyclization and the formation of covalently interlinked BN-substituted polyaromatic hydrocarbon networks on the Ag(111) surface. In agreement with experimental observations, density functional theory calculations reveal a significantly lower energy barrier for the cyclodehydrogenation of the conformationally more rigid precursor monomer, which is also reflected in a higher degree of long-range order of the obtained heteroaromatic network. Our proof-of-concept study will allow for the fabrication of atomically precise substitution patterns within BNC heterostructures.

Biaxial nematics composed of flexible molecules: a molecular field theory

The key requirement for the formation of a biaxial nematic phase is clearly biaxiality in the shape or interactions of the constituent molecules. However, in addition to this feature mesogenic molecules are invariably flexible so that their anisotropy and biaxiality can change. The orientational order of the system is also coupled to the conformational distribution and this coupling may stabilise the biaxial nematic phase. Here a molecular field theory of nematics composed of flexible biaxial mesogenic molecules is assembled from its essential elements. Of special significance is the use of the variational approach employed by de Gennes to formulate a molecular field theory for uniaxial nematics. This has the distinct advantage that it avoids the use of a pair potential of unknown reliability. Instead it requires the identification of the dominant orientational order parameters for the phases involved. Invariants constructed from these and an interaction supertensor then give the free energy for the system. This powerful variational approach has been employed for the biaxial nematic phase composed of flexible molecules taking into account the relationship between the interaction supertensors for the numerous conformers. The way now seems to be open to explore the influence of molecular flexibility on the stability of the biaxial nematic phase for a molecularly realistic model.

Influence of Molecular Flexibility and Polar Surface Area Metrics on Oral Bioavailability in the Rat

The relationship of rotatable bond count (N(rot)) and polar surface area (PSA) with oral bioavailability in rats was examined for 434 Pharmacia compounds and compared with an earlier report from Veber et al. (J. Med. Chem. 2002, 45, 2615). N(rot) and PSA were calculated with QikProp or Cerius2. The resulting correlations depended on the calculation method and the therapeutic class within the data superset. These results underscore that such generalizations must be used with caution.

Influence of molecular flexibility on DNA radiosensitivity: a simulation study.

Radiation damage in DNA is caused mainly by hydroxyl radicals which are generated by ionizing radiation in water and removing hydrogen atoms from the DNA chain. This damage affects certain nucleotide sequences more than others due to differences in the local structure of the DNA chains. This sequence dependence has been analyzed experimentally and calculated theoretically for a rigid DNA model. In this paper we take into account the flexibility of the DNA chain and show how it modifies the strand breakage probabilities. We use a simple harmonic model for DNA flexibility which permits the study of a long (68 base pair) fragment with modest computational effort. The essential influence of flexibility is an increased breakage probability towards the ends of the fragment, which can also be identified in the experimental data.

A molecular dynamics study of hydrocarbons adsorbed in silicalite

A molecular dynamics study reproducing the vibrational spectra of a series of flexible hydrocarbon molecules adsorbed in silicalite yields very satisfactory structural and diffusional data. The form selectivity appears clearly when the molecules are compared. The influence of molecular flexibility on the calculated diffusional data is not obvious. The influence of adsorption on the molecular geometry is not very important.

Comment on the influence of molecular flexibility on molecular packing in Langmuir monolayers

Comment on the influence of molecular flexibility on molecular packing in Langmuir monolayers

The influence of molecular flexibility on the transfer of vibrational energy in hydrocarbons

The marked efficiency of certain hydrocarbons in promoting the interconversion of translational and vibrational energy during collisions with ethylene has been further studied by ultrasonic methods. In order to elucidate the detailed influence of molecular structure, molecules with approximately the same mass have been compared. Including other published results, the average number of collisionsZeftrequired for each act of interconversion at 298°K is as follows: added gasZeπ.n-hexane 30iso-hexane 40cyclo-hexane 100 sulphur hexafluoride 120 added gasZeff.n-hexane 45iso-hexane 55neo-pentane 120 ethylene self collisions 2500cyclo-pentane 3300 A gradation of efficiency is apparent in going from then-hexane tocyclo-pentane. This scale of efficiencies in the colliding molecules might be attributed to possible overlaps of vibration frequencies, or to molecular flexibility smearing quantum restrictions and thus favouring transfer during a collision. At present, the balance of evidence strongly support the importance of molecular flexibility in promoting the interconversion of translational and vibrational energy. The effect of dipoles in facilitating transfer in the case of vibrations active in the infra-red is also discussed. Further extension of experimental results on the dispersion of sound in pure ethylene at different temperatures supportsT-⅓ the relation developed by Landau & Teller and by Schwartz, Slawsky & Herzfeld.

The Influence of Molecular Flexibility on the Intrinsic Viscosity, Sedimentation, and Diffusion of High Polymers

Expressions for the mean square separation of chain ends and modifications of the formula for an ideal coil are discussed. On the basis of these and of the hydrodynamic theory of intrinsic viscosity, an interpretation of the modified Staudinger rule is offered. It relates the exponent a of the molecular weight to a flexibility parameter p of the chain in a given solvent, varying between zero and one (Eq. (4), (5)). Recent data on polystyrene and on cellulose nitrate are analyzed in greater detail. By means of the frictional ratio f/f0, the sedimentation constant s and the diffusion constant D, respectively, are connected with the degree of polymerization in terms of p (Eq. (9)). The limiting dependence of sedimentation and diffusion rate upon molecular weight for a straight chain and an ideal coil is also found in this manner. A comparison shows satisfactory agreement between values for p found from intrinsic viscosity and those determined from sedimentation or diffusion rates, for certain cellulose esters and starch derivatives. Effects of solvent and of inhomogeneity in respect to molecular weight are discussed briefly.