169,989 research outputs found
Insights into the formation of chiral second sphere coordination complexes with aromatic tris amines: combined single crystal X-ray crystallography and molecular modeling analyses
No full textInsights into the formation of chiral second sphere coordination complexes with aromatic tris amines: combined single crystal X-ray crystallography and molecular modeling analyses
Antonino Famulari, Massimo Cametti and Stefano Valdo Meille (Politecnico di Milano), and Javier Martí-Rujas (Center for Nano Science and Technology@Polimi, Istituto Italiano di Tecnologia)
Control over the formation of non-centrosymmetric chiral materials is highly desirable due to their potential applications in areas such as ferroelectricity, piezo- and pyroelectricity, and second harmonic generation.[1] The development of a reliable approach to induce the formation of a chiral material from achiral molecules remains a great challenge in the field of molecular chemistry.[2] Furthermore, the understanding of the driving forces behind those aggregations is a prerequisite for the design and construction of chiral molecular arrays. C3-symmetrical tripodal molecules have emerged as attractive organic frameworks for the construction of chiral coordination compounds.[3] In the present contribution we report about a family of isostructural, chiral supramolecular networks obtained in the solid state by exploiting second sphere coordination interactions in the self-assembly of 2 achiral tris amines with tetrahalometallate and halide ions. Quantum-Mechanical calculations (including the usage of approaches specific for crystalline solid phases) provided important insights into the intramolecular and packing interactions which determine chirality, pointing to a direct effect of the methyl groups of the central benzene ring of the tris amines. [4]
[1] (a) P. A. Maggard, C. L. Stern and K. R. Poeppelmeier, J. Am. Chem. Soc., 2001, 123, 7742–7743; (b) M. Liu, L. Zhang and T. Wang, Chem. Rev., 2015, DOI: 10.1021/ Q5 cr500671p. [2] P. S. Halasyamani and K. R. Poeppelmeyer, Chem. Mater., 1998, 10, 2753–2769. [3] Z. Dai and J. W. Canary, New J. Chem., 2007, 31, 1708–1718. [4] H. Yu, L. Li, J. Gao, J. Tong, W. Zheng, M. Cametti, A. Famulari, S.V. Meille, F. Guo and J. Martí-Rujas Journal Article Dalton Trans., 2016,44, 15960-15965. DOI: 10.1039/C5DT02387D
Dielectric properties of soft-particles in aqueous solutions
No full textIn this review, we present a summary of the recent theoretical approaches developed in the last few years examining the dielectric and conductometric properties of soft colloidal particles in aqueous solution. Whereas the electrokinetic properties, concerning the movement of soft particles (electrophoretic mobility), have been extensively investigated and rather sophisticated theoretical models have been developed, much less attention has been addressed to the electrical polarization mechanisms induced in these systems by an external electric field, even though the rich phenomenology they present and the chances they offer in the study of their structure and functionality deserve much more interest. We analyze the recent progress achieved in the dielectric and conductometric properties of this kind of colloidal particle aqueous suspensions in the light of the electrokinetic standard model and we discuss in some detail how the structure of the polymer layer might alter the dielectric response of the system, seen as a highly heterogeneous colloidal system. In particular, the limits and the benefits of the soft particle electrokinetic formalism in the understanding the electrostatic features of these systems will be illustrated
Low-frequency electrical conductivity of poly(phenylacetylene)/iodine in tetrahydrofuran solution: effects of iodine concentration
No full textThe electrical conductivity of poly(phenylacetylene)-iodine in tetrahydrofuran solution has been measured at the frequency of 10 kHz, in the temperature range from -15 to 40 degrees C as a function of iodine concentration. The data have been analysed on the basis of the counterion fluctuation model developed for highly charged polyelectrolyte aqueous solutions. The dependence of the electrical conductivity on the iodine concentration suggests that the conduction mechanism is governed by the interaction of the polymer with iodine molecules, resulting in a charge-transfer complex in solution, the effectiveness of which is maximum at some well-defined concentrations corresponding to six polymer units for each I-5(-) ion
RADIOWAVE DIELECTRIC-PROPERTIES OF XANTHAN IN AQUEOUS-SOLUTIONS
No full textThe dielectric and conductometric properties of aqueous solutions of xanthan, an ionic polysaccharide consisting of a linear cellulosic backbone with three-sugar side chains, have been extensively investigated over two frequency ranges, from 1 kHz to 10 MHz and from 1 MHz to 1 GHz by means of frequency domain dielectric spectroscopy, at different polyion concentrations and at two different temperatures, above room temperature, in the range where an order-disorder transition occurs. The dielectric spectra have been analyzed on the basis of two contiguous dielectric dispersions described by a Cole-Cole relaxation function, and the parameters have been discussed on the basis of currently accepted dielectric relaxation theories of polyelectrolyte solutions. For the majority of the polyion concentrations employed, a semidilute regime occurs and the two observed dielectric relaxation are ascribed to counterion fluctuations on two different scale lengths, the first associated to the polymer contour length and the second one to an average distance between charged groups of adjacent polyions. Moreover, these measurements suggests that a conformational transition occurs at temperatures above 40 degrees C with the consequent formation of an extended three-dimensional network. This transition results in a particular behavior of the dielectric relaxation on the polymer concentration
Insights into the formation of chiral second sphere coordination complexes with aromatic tris amines: combined single crystal X-ray crystallography and molecular modeling analyses
No full textControl over the formation of non-centrosymmetric chiral materials is highly desirable due to their potential applications in areas such as ferroelectricity, piezo- and pyroelectricity, and second harmonic generation.[1] Unfortunately, the development of a reliable approach to induce the formation of a chiral material from achiral molecules remains a great challenge in the field of molecular chemistry.[2] Furthermore, the understanding of the driving forces behind those aggregations is a prerequisite for the design and construction of chiral molecular arrays. C3-symmetrical tripodal molecules have emerged as attractive organic frameworks for the construction of chiral coordination compounds.[3] In the present contribution we report about a family of isostructural, chiral supramolecular networks obtained in the solid state by exploiting second sphere coordination interactions in the self-assembly of achiral tris amines L1 and L2 with tetrahalometallate and halide ions. Quantum-Mechanical calculations (including the usage of approaches specific for crystalline solid phases) provided important insights into the intramolecular and packing interactions which determine chirality, pointing to a direct effect of the methyl groups of the central benzene ring of the tris amines. [4]
References
[1] (a) P. A. Maggard, C. L. Stern and K. R. Poeppelmeier, J. Am. Chem. Soc., 2001, 123, 7742–7743; (b) M. Liu, L. Zhang and T. Wang, Chem. Rev., 2015, DOI: 10.1021/ Q5 cr500671p.
[2] P. S. Halasyamani and K. R. Poeppelmeyer, Chem. Mater., 1998, 10, 2753–2769.
[3] Z. Dai and J. W. Canary, New J. Chem., 2007, 31, 1708–1718.
[4] H. Yu, L. Li, J. Gao, J. Tong, W. Zheng, M. Cametti, A. Famulari, S.V. Meille, F. Guo and J. Martí-Rujas Journal Article Dalton Trans., 2015,44, 15960-15965. DOI: 10.1039/C5DT02387D, Pape
Counterion condensation in Xanthan aqueous solutions in the semidilute and concentrated regime
No full textThe dielectric and conductometric properties of aqueous solutions of Xanthan have been investigated from 1 kHz to 1 GHz by means of frequency domain dielectric spectroscopy, at different polyion concentrations from 0.5 to 10 mg/ml, covering both the semidilute and concentrated regimes. The dielectric spectra have been analysed, in these concentration regions, on the basis of a fixed counterion condensation model, considering that a fraction of counterions will remain in the neighbouring of each charged group of the polyion and that this fraction does not change with the degree of ionization and polymer concentration. This assumption, which differs from that given by the Manning model for charged polyelectrolyte solutions, defines the overall dielectric and conductivity behaviour of the system over the whole temperature interval and polymer concentration investigated
Power law behavior in the frequency dependence of the electrical conductivity of poly(L-glutamic acid) aqueous solutions.
No full textDielectric Relaxation Spectroscopy of Lysozyme Aqueous Solutions: Analysis of the delta-Dispersion and the Contribution of the Hydration Water
No full textThe dielectric properties of lysozyme aqueous solutions have been investigated over a wide frequency range, from 1 MHz to 50 GHz, where different polarization mechanisms, at a molecular level, manifest. The dielectric relaxation spectra show a multimodal structure, reflecting the complexity of the protein water interactions, made even more intricate with the increase of the protein concentration. The deconvolution of the spectra into their different components is not unambiguous and is generally a delicate process which requires caution. We have analyzed the whole relaxation region, on the basis of the sum of simple Debye-type relaxation functions, considering three main contributions. Particular attention has been payed to the delta-dispersion, intermediate between the beta-dispersion (rotational dynamics of the protein) and the gamma-dispersion (orientational polarization of the water molecules). This intermediate contribution to the dielectric spectrum is attributed to the orientational polarization of water molecules in the immediate vicinity of the protein surface (hydration water). Our measurements clearly demonstrate that, at least at high protein concentrations, the delta-dispersion has a bimodal structure associated with two kinds of hydration water, i.e., tightly bound and loosely bound hydration water. In the concentration range investigated, the existence of a three-modal delta-dispersion, as recently suggested, is not supported, on the basis of statistical tests, by the analysis of the dielectric relaxations we have performed and a bimodal dispersion is accurate enough to describe the experimental data. The amount of the hydration water has been evaluated both from the dielectric parameters associated with the delta-dispersion and from the decrement of the loss peak of the gamma-dispersion. The relative weight of tightly bound and loosely bound hydration water is briefly discussed
Molecular Dynamics in Sodium poly(L-glutamate) Aqueous Solutions Analyzed by means of the Stretched Exponential Decay of the Williams-Watts Function
No full text- …
