Highlights on the reversible nonpolar-to-polar P3121-P31 phase transition at low temperature in NaLa(SO4)2·H2O: mechanism and piezoelectric properties.

We report the existence in NaLa(SO4)2·H2O of a displacive phase transition under 200 K from the nonpolar P3121 to the polar P31 space group. This phase transition was predicted by density functional theory based calculations and experimentally confirmed from infrared spectroscopy and X-ray diffraction. The A2 polar irreducible representation is the primary order parameter. The structural water and hydrogen bonding are the mechanism driving the phase transition. The piezoelectric properties of this new P31 phase have been investigated by first principles based calculations. The highest piezoelectric-strain constants in the zero Kelvin limit are predicted for the d12 and d41 elements with values about 3.4 pC N-1. This compound could be interesting as piezoelectric actuator for cryogenic applications.

FT-IR and NMR structural markers for thiazole-based γ-peptide foldamers.

Nuclear magnetic resonance (NMR) spectroscopy has been established as a potent method for the determination of foldamer structures in solution. However, the NMR techniques could be limited by averaging, so additional experimental techniques are often needed to fully endorse the folding properties of a sequence. We have recently demonstrated that oligo-γ-peptides composed of 4-amino(methyl)-1,3-thiazole-5-carboxylic acids (ATCs) adopt an original helical fold stabilized by hydrogen bonds forming C9 pseudocycles. The main objective of the present work is to reinvestigate the folding of ATC oligomer 1 in order to identify reliable FT-IR and NMR structural markers that are of value for tracking the degree of organization of ATC-based peptides.

Computation of the infrared active modes in single-walled boron nitride nanotube bundles.

In this work, the infrared active modes are computed for homogeneous bundles of single-walled boron nitride nanotubes (BBNNTs), using the so-called spectral moments method. The dependence of the wavenumber on these modes in terms of diameters, lengths, and numbers of tubes, is investigated. To this end, use is made of a Lennard-Jones potential for describing the van der Waals interactions between tubes in a bundle. We find that, for a finite homogeneous bundle, additional modes appear as a specific signature. Finally, these results are useful for the interpretation of the experimental infrared spectra of BBNNTs.

Lattice dynamics of oligo(phenylenethienylene)s: a far-infrared and inelastic neutron scattering study.

New promising oligo(phenylenethienylene)s have been synthesized to realize suitable materials for improving electronic transport properties, particularly in organic field effect transistors (OFETs). Far-infrared and incoherent neutron scattering measurements have been performed to assign their phonon modes. The assignment of the main low-frequency phonon modes of these materials has been performed experimentally by using a filiation procedure. Assuming a small frequency dispersion of the high-frequency modes, the main intramolecular phonon modes of a model oligomer have been assigned by using first-principles calculations on its isolated molecule. These assignments constitute the preliminary work for a better understanding of these new promising materials in electronic and opto-electronics applications.

Correlations between structure and far-infrared active modes in polythiophenes.

We have investigated the experimental X-ray and far-infrared responses of three polythiophenes synthesized from a thiophene, alpha-bithiophene, and alpha-quaterthiophene monomer. The X-ray data show that the crystallinity of the different polythiophene samples depends on the synthesis conditions. An excellent correlation between the crystallinity of polythiophenes and their far-infrared signatures is demonstrated. In addition, the assignment of the far-infrared phonon modes in polythiophenes is given by using both an experimental filiation procedure and first-principles calculations. In particular, the ring libration inside the polymeric chain, directly involved in the electron-phonon coupling, is assigned.

NEXAFS study of multi-walled carbon nanotubes functionalization with sulfonated poly(ether ether ketone) chains.

Functionalization of multi-walled carbon nanotubes (MWNTs) surface by sulfonated poly (ether ether ketone) SPEEK chains using a direct attachment reaction was investigated. A two step method was performed. MWNTs were oxidized by a nitric acid treatment to generate carboxyl groups on their surface. The grafting reaction of sulfonated groups of SPEEK with carboxyl groups present on the surface of oxidized MWNTs readily proceeds by using hexane diamine as an interlinking molecule. Transmission electron microscopy (TEM) shows that tubes are wrapped by polymer chains. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) at the C K-edge, O K-edge, and N K-edge and X-ray photoelectron spectroscopy (XPS) were used to give evidence of covalent functionalization of MWNTs by SPEEK macromolecules.

Spectroscopic study of nitrogen doping of multi-walled carbon nanotubes.

Combined spatially resolved electron-energy loss spectroscopy (EELS) and high resolution near-edge X-ray absorption fine structure (NEXAFS) spectroscopy have been used to investigate the nitrogen doping of multi-walled carbon nanotubes (N-MWNT). EELS indicates that most of the tubes are nitrogen-doped. NEXAFS spectroscopy reveals pyridine-like and nitrile N structures. High resolution NEXAFS experiments show that the main nitrogen concentration originates from a high amount of molecular N2 encapsulated into only a small quantity of tubes.

Infrared spectroscopy of single-walled carbon nanotubes.

By using the spectral moments method, we calculate the infrared spectra of chiral and achiral single-walled carbon nanotubes (SWCNTs) of different diameters and lengths. We show that the number of the infrared modes, their frequencies, and intensities depend on the length and chirality of the nanotubes. Furthermore, the dependence of the infrared spectrum as a function of the size of the SWCNT bundle is analyzed. These predictions are useful to interpret the experimental infrared spectra of SWCNTs.

Far- and mid-infrared of crystalline 2,2'-bithiophene: ab initio analysis and comparison with infrared response.

Infrared intramolecular vibrations and lattice modes in the crystalline phase of 2,2'-bithiophene (2T) are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. For the first time, the far- and mid-infrared responses have been calculated from the Gamma-point modes and the Born effective charge tensors of the 2T crystalline phase. The relative good agreement between the calculated and experimental infrared spectra allows us to assign the origin of the main features of the experimental spectra, which is of particular interest in the far-infrared domain. These assignments are useful for understanding all the properties of the 2T crystalline phase in which phonon-phonon and electron-phonon interactions play an important role.

Polymorphism of crystalline alpha-quaterthiophene and alpha-sexithiophene: ab initio analysis and comparison with inelastic neutron scattering response.

Phonons in the alpha-quaterthiophene (4T) and alpha-sexithiophene (6T) polymorph phases are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. The simulation of inelastic neutron scattering spectra (INS) on the LT and HT polymorph phases of 4T and 6T enable the corresponding spectral signatures of these materials to be identified. In particular, there are two fingerprints: (i) the low-frequency vibrational modes (frequencies lower than 200 cm(-1)) and (ii) the vibrational modes in the 600-900 cm(-1) frequency range. The good agreement with the INS experimental data allows us to assign unambiguously the origin of all features (first-order and high-order processes) of these spectra and to predict that the LT phase is the phase measured experimentally both on the 4T and 6T materials. Moreover, the broad background in the 600-1400 cm(-1) frequency range and the well-defined features which appear around 940 cm(-1) in the calculated INS spectra of 4T/HT and 6T/HT are assigned to multiphonon contributions. This multiphonon contribution at 940 cm(-1), which is absent in the 4T/LT and 6T/LT INS spectra, also constitutes a fingerprint of the HT phases. Finally, the calculated dispersion curves of the two polymorph phases of 4T and 6T are given.

Organic reagent interaction with hair spatially characterized by infrared microspectroscopy using synchrotron radiation.

The penetration of chemical reagents through human hair after bleaching has been spatially characterized using infrared microspectroscopy (IMS) with a synchrotron source. Chemical imaging of hair cross sections before and after bleaching was achieved with high contrast, using the peptide and lipid mid-infrared vibrational bands which are characteristic of hair. The ability to make images using functional groups as a contrast mechanism can be applied to studies of other chemical groups, if present, in the structure of the hair. As an example we show how the penetration of an organic active reagent in the hair structure can be quantified with a spatial resolution of few microns. These results demonstrate that synchrotron IMS is a powerful tool for characterizing chemical interactions of hair samples with specific cosmetic materials.