Vibrational Excitons in CH-Stretching Fundamental and Overtone Vibrational Circular Dichroism Spectra

A set of vibrational circular dichroism (VCD) spectra in the CH-stretching fundamental region for about twenty compounds belonging to the class of essential oils was empirically analyzed by the use of a sort of vibrational exciton mechanism, involving three centers. Through a general formula applicable to many coupled dipole oscillators, the rotational strengths of the previously identified vibrational excitons are evaluated. The results are then critically reviewed by the use of recent ab initio methodology, as applied to selected molecules of the original set. Further insight is gained by model calculations adding up the contribution of the coupling between electric dipole moments associated with normal mode behavior and that of the polarizability from polarizable groups. The former part is responsible for the excitonic behavior of the VCD spectra. For the same selected molecules we have also investigated whether some excitonic behavior is taking place in the second overtone region, and have concluded that this is not the case

Molecular dynamics simulation of aqueous solutions of 26-unit segments of p(NIPAAm) and of p(NIPAAm) "doped" with amino acid based comonomers

We have performed 75-ns molecular dynamics (MD) simulations of aqueous solutions of a 26-unit NIPAAm oligomer at two temperatures, 302 and 315 K, below and above the experimentally determined lower critical solution temperature (LCST) of p(NIPAAm). We have been able to show that at 315 K the oligomer assumes a compact form, while it keeps a more extended form at 302 K. A similar behavior has been demonstrated for a similar NIPAAm oligomer, where two units had been substituted by methacryloyl-l-valine (MAVA) comonomers, one of them being charged and one neutral. For another analogous oligomer, where the same units had been substituted by methacryloyl-l-leucine (MALEU) comonomers, no transition from the extended to the more compact conformation has been found within the same simulation time. Statistical analysis of the trajectories indicates that this transition is related to the dynamics of the oligomer backbone, and to the formation of intramolecular hydrogen bonds and water-bridges between distant units of the solute. In the MAVA case, we have also evidenced an important role of the neutral MAVA comonomer in stabilizing the compact coiled structure. In the MALEU case, the corresponding comonomer is not equally efficacious and, possibly, is even hindering the readjustment of the oligomer backbone. Finally the self-diffusion coefficient of water molecules surrounding the oligomers at the two temperatures for selected relevant times is observed to characteristically depend on the distance from the solute molecules

On the use of an appropriate TdT-mediated dUTP-biotin nick end labeling assay to identify apoptotic cells.

Apoptosis is an essential cellular mechanism involved in many processes such as embryogenesis, metamorphosis, and tissue homeostasis. DNA fragmentation is one of the key markers of this form of cell death. DNA fragmentation is executed by endogenous endonucleases such as caspase-activated DNase (CAD) in caspase-dependent apoptosis. The TUNEL (TdT-mediated dUTP-biotin nick end labeling) technique is the most widely used method to identify apoptotic cells in a tissue or culture and to assess drug toxicity. It is based on the detection of 3'-OH termini that are labeled with dUTP by the terminal deoxynucleotidyl transferase. Although the test is very reliable and sensitive in caspase-dependent apoptosis, it is completely useless when cell death is mediated by pathways involving DNA degradation that generates 3'-P ends as in the LEI/L-DNase II pathway. Here, we propose a modification in the TUNEL protocol consisting of a dephosphorylation step prior to the TUNEL labeling. This allows the detection of both types of DNA breaks induced during apoptosis caspase-dependent and independent pathways, avoiding underestimating the cell death induced by the treatment of interest

A comparative study of overtone CH- stretching vibrational circular dichroism spectra of fenchone and camphor

Near-infrared vibrational circular dichroism (VCD) spectra in the region 1500-850 nm are recorded for both enantiomers of camphor and fenchone using a home-built dispersive instrument. Two CH-stretching overtone regions and two CH-stretching/HCH-bending combination regions are investigated. The near-infrared VCD data are examined and compared with the corresponding previously published data of (3R)-(+)-methylcyclopentanone and (3R)-(+)-methylcyclohexanone. The absorption spectra of both the fundamental and overtone regions are also considered, and the fundamental CH-stretching VCD data are taken into account, where possible. An empirical assignment of the spectra is first considered, with special regard to the second CHstretching overtone region. This region is then investigated by a heuristic approach based on DFT calculations, using the hybrid functional B3LYP at the 6-31G** basis set level. Subsequently rotational strengths have been evaluated by use of a classical model based on the contribution of polarizable groups to the optical activity. The same models are also used to interpret the VCD spectra between 2000 and 3000 wavenumbers for (3R)-(+)-methylcyclopentanone-d4

Chiral Polyalkylthiophenes for Organic Light Emitting Diodes

Chiral polyalkylthiophenes are noncentrosymmetric organic materials which can be used both in second harmonic-generation devices and in polarized light emitting diodes. In this work we present the synthesis and the characterization of a polyalkylthiophene with a chiral center very close to the conjugated backbone: poly(3-[(S)-2-methylbutyl]thiophene) (PMBT). Circular dichroism (CD) measurements have been carried out to ascertain the chirality of these materials. The CD spectra show intense signals both in mixed solvents and in the solid state. The strong Cotton effect can be associated to a highly ordered aggregated phase whose nature is still under investigation. We also present the photo and electroluminescence characterization of single layer light emitting diode (LED) with the following configuration: ITO (Tin Indium Oxide)/PMBT/Al

Influence of surfactants on the structure of titanium oxide gels : experiments and simulations

We report here on experimental and numerical studies of the influence of surfactants on mineral gel synthesis. The modification of the gel structure when the ratios water-precursor and water-surfactant vary is brought to the fore by fractal dimension measures. A property of {\em polydispersity of the initial hydrolysis} is proposed to explain these results, and is successfuly tested through numerical experiments of three dimensional chemically limited aggregation.Comment: 12 pages, 4 Postscript figures, uses RevTe

Experimental and calculated circular dichroism spectra of monoaza[5]helicenes

Circular dichroism (CD) spectra have been measured in the range of 400–200 nm on CH3OH solutions of both enantiomers for the almost complete series of monoaza[5]helicenes, namely the molecules where the hetero N atom occupies positions 1, 3, 4, 5, 6, and 7, respectively (the 2 isomer is missing due to difficulties in the synthesis). CD spectra recorded at controlled room temperature allow one to define precise racemization rates, that are nicely interpreted on the basis of DFT molecular orbital calculations. Time-dependent DFT methods provide us with calculated CD and UV spectra, that are compared with the corresponding experimental data. We discuss the role of the N atom in determining the height of the racemization barrier and in shaping the appearance of the CD spectra

Universal Static and Dynamic Properties of the Structural Transition in Pb(Zn1/3Nb2/3)O3

The relaxors Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PZN) and Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) have very similar properties based on the dielectric response around the critical temperature $T_{c}$ (defined by the structural transition under the application of an electric field). It has been widely believed that these materials are quite different below $T_{c}$ with the unit cell of PMN remaining cubic while in PZN the low temperature unit cell is rhombohedral in shape. However, this has been clarified by recent high-energy x-ray studies which have shown that PZN is rhombohedral only in the skin while the shape of the unit cell in the bulk is nearly cubic. In this study we have performed both neutron elastic and inelastic scattering to show that the temperature dependence of both the diffuse and phonon scattering in PZN and PMN is very similar. Both compounds show a nearly identical recovery of the soft optic mode and a broadening of the acoustic mode below $T_{c}$. The diffuse scattering in PZN is suggestive of an onset at the high temperature Burns temperature similar to that in PMN. In contrast to PMN, we observe a broadening of the Bragg peaks in both the longitudinal and transverse directions below $T_{c}$. We reconcile this additional broadening, not observed in PMN, in terms of structural inhomogeneity in PZN. Based on the strong similarities between PMN and PZN, we suggest that both materials belong to the same universality class and discuss the relaxor transition in terms of the three-dimensional Heisenberg model with cubic anisotropy in a random field.Comment: 11 pages, 10 figures. Updated version after helpful referee comment

Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules

Using molecular dynamics computer simulations, we investigate the dynamics of the rotational degrees of freedom in a supercooled system composed of rigid, diatomic molecules. The interaction between the molecules is given by the sum of interaction-site potentials of the Lennard-Jones type. In agreement with mode-coupling theory (MCT), we find that the relaxation times of the orientational time correlation functions C_1^(s), C_2^(s) and C_1 show at low temperatures a power-law with the same critical temperature T_c, and which is also identical to the critical temperature for the translational degrees of freedom. In contrast to MCT we find, however, that for these correlators the time-temperature superposition principle does not hold well and that also the critical exponent gamma depends on the correlator. We also study the temperature dependence of the rotational diffusion constant D_r and demonstrate that at high temperatures D_r is proportional to the translational diffusion constant D and that when the system starts to become supercooled the former shows an Arrhenius behavior whereas the latter exhibits a power-law dependence. We discuss the origin for the difference in the temperature dependence of D (or the relaxation times of C_l^(s) and D_r. Finally we present results which show that at low temperatures 180 degree flips of the molecule are an important component of the relaxation dynamics for the orientational degrees of freedom.Comment: 17 pages of RevTex, 12 figure

Double excitation transitions in Mn2+ -doped alkali halides

4 págs.; 4 figs.It is shown in this work that the room-temperature excitation spectra of as-grown crystals of LiF, NaF, NaCl, KCl, and KBr doped with Mn2+ reveal the existence of double excitation peaks. These peaks are related to the formation of precipitated phases containing Mn2+ in the alkali halide lattice. In the case of LiF:Mn2+ the position of the [A14(G), E4(G)] and T14(G) peaks suggests an anomalously small Mn2+- F- distance for the precipitated phase. © 1983 The American Physical Society.Financial support by the Comision Asesora para la Investigacion Cientifica y Tecnica, is gratefully acknowledged.Peer Reviewe