Molecular Ordering and Structure of Quasi-spherical Solutes by Liquid Crystal NMR and Monte Carlo Simulations: The Case of Norbornadiene

NMR Diffusiometry Spectroscopy, a Novel Technique for Monitoring the Micro-Modifications in Bitumen Ageing

In the past three decades, several conventional methods have been employed for characterizing the bitumen ageing phenomenon, such as rheological testing, ultraviolet testing, gel permeation chromatography (GPC), gas chromatography (GC), atomic force microscopy (AFM), X-ray scattering, and Fourier transform infrared spectroscopy (FTIR). Nevertheless, these techniques can provide only limited observations of the structural micro-modifications occurring during bitumen ageing. In this study, Fourier transform nuclear magnetic resonance self-diffusion coefficient (FT-NMR-SDC) spectroscopy, as a novel method, was employed to investigate and compare the microstructural changes between virgin bitumen (pristine bitumen) and aged bitumen. The virgin bitumen was aged artificially using two standard ageing tests: Rolling Thin-Film Oven Test (RTFOT) and Pressure Ageing Vessel (PAV). For a comprehensive comparison and an assessment of the validity of this method, the generated samples were studied using various methods: rheological test, atomic force microscopy, and optical microscopy. Significant differences were obtained between the structure and ageing patterns of virgin and aged bitumen. The results indicate that the modification of maltenes to asphaltenes is responsible for the ageing character. When compared with the other methods’ findings, FT-NMR-SDC observations confirm that the asphaltene content increases during ageing processes.

Analysis of Mechanical Performance of Bitumen Modified with Waste Plastic and Rubber Additives by Rheology and Self Diffusion NMR Experiments

In this study, the mechanical and physico-chemical properties of a new kind of modified bitumen are presented. The bituminous binders have been modified in order to understand the effect on the structural properties of several compounds such as a Polymer elastomer as Styrene Butadiene Rubber (SBR), Polymer thermoplastic polypropylene (PP) and a waste plastic (Waste PP). Laboratory tests have been focused on the characterization of bitumen modified with single product and their binary combinations compared with pristine binder as a reference. Characterization has been conducted by using conventional as well as advanced methods on bitumens. Fundamental rheological tests, based on dynamic shear rheometer in the temperature range from -30 °C to +160 °C have been performed and the structure of a bitumens and modified bitumens has been analysed by the mobility of the oily maltene by self-diffusion Pulsed field gradient spin-echo (PGSE) FT-NMR experiments.

A titanium tetrachloride-based effective methodology for the synthesis of dipeptides

A series of dipeptide systems have been easily achieved through a TiCl4-assisted condensation reaction. The reaction of N-protected amino acids with amino acid methyl esters in pyridine and in the presence of TiCl4 furnished the corresponding dipeptides with high yields and diastereoselectivity. The reaction was successfully applied to amino acids protected on the α-amino function with different protecting groups. The adopted experimental conditions allowed preserving both the protecting groups on the α-amino function and on the side chain functionalities. Furthermore, the preservation of the stereochemical integrity at the amino acid chiral centres has been verified.

A combined LX-NMR and molecular dynamics investigation of the bulk and local structure of ionic liquid crystals

The unique power of NMR spectroscopy in anisotropic media (LX-NMR) as a tool to obtain local and bulk structural information, combined with the effectiveness of molecular dynamics simulations at the atomistic level, shows very attractive potentialities for the study of interesting, even though still poorly understood, materials such as Ionic Liquid Crystals (ILCs). In this work, we focused our attention, in particular, on the orientational ordering of two mesophases: 1-dodecyl-3-methylimidazolium chloride, [C12C1im]Cl, and 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C12C1im][BF4]. Both ILCs were studied by a 2H NMR direct investigation of the molecules forming the phases, suitably deuterated, and by 1H NMR spectroscopy, using the small rigid probe-solutes 1,4-dichlorobenzene (DCB), dissolved in [C12C1im][BF4] and [C12C1im]Cl, and 1,4-dibromobenzene (DBB) dissolved in [C12C1im][BF4], to probe the local, internal structure and organization of the mesophases. The experimental results were then compared with the predictions, by atomistic MD simulations, of the structure of the smectic phase of the two salts, at two selected temperatures, containing a single DCB molecule as a probe. The MD simulations show that the DCB solute is distributed only within the hydrophobic layers of the ILC. Orientational order parameters of the imidazolium cations and of the DCB molecule were obtained and compared with the experiments, showing a general good agreement and allowing a deeper understanding of the microscopic structure of the systems.

Enantiotopic discrimination in the NMR spectrum of prochiral solutes in chiral liquid crystals

Theory and applications to stereo- and bio-chemistry of the discrimination of enantiotopic elements observed in the NMR spectra of prochiral solutes dissolved in chiral liquid crystals are comprehensively discussed.

Domain cooperativity in multidomain proteins: what can we learn from molecular alignment in anisotropic media?

Many proteins have modular design with multiple globular domains connected via flexible linkers. As a simple model of such system, we study a tandem construct consisting of two identical SH3 domains and a variable-length Gly/Ser linker. When the linker is short, this construct represents a dumbbell-shaped molecule with limited amount of domain-domain mobility. Due to its elongated shape, this molecule efficiently aligns in steric alignment media. As the length of the linker increases, the two domains become effectively uncoupled and begin to behave as independent entities. Consequently, their degree of alignment drops, approaching that found in the (near-spherical) isolated SH3 domains. To model the dependence of alignment parameters on the length of the interdomain linker, we have generated in silico a series of conformational ensembles representing SH3 tandems with different linker length. These ensembles were subsequently used as input for alignment prediction software PALES. The predicted alignment tensors were compared with the results of experimental measurements using a series of tandem-SH3 samples in PEG/hexanol alignment media. This comparison broadly confirmed the expected trends. At the same time, it has been found that the isolated SH3 domain aligns much stronger than expected. This finding can be attributed to complex morphology of the PEG/hexanol media and/or to weak site-specific interactions between the protein and the media. In the latter case, there are strong indications that electrostatic interactions may play a role. The fact that PEG/hexanol does not behave as a simple steric media should serve as a caution for studies that use PALES as a quantitative prediction tool (especially for disordered proteins). Further progress in this area depends on our ability to accurately model the anisotropic media and its site-specific interactions with protein molecules. Once this ability is improved, it should be possible to use the alignment parameters as a measure of domain-domain cooperativity, thus identifying the situations where two domains transiently interact with each other or become coupled through a partially structured linker.

Experimental assessment of the vibration-reorientation contribution to liquid crystal NMR dipolar couplings: the case of tetramethylallene dissolved in a nematic mesophase

In the present paper, the peculiar orientational behavior, studied by liquid crystal NMR (LXNMR) spectroscopy, of the D(2d) symmetry quasi-spherical molecule of tetramethylallene (TMA) dissolved in the nematic solvent I52 is exploited to attempt a quantitative experimental assessment of the correlation between molecular vibrations and overall rotations in weakly oriented molecules. The analysis of the very small D(HH) and (1)D((13)C-H) dipolar couplings, available from the natural abundance LXNMR spectra of TMA at different temperatures, allows for a derivation leading (by making a few approximations) to the quantification of the vibration-reorientation (also called nonrigid) contribution affecting the observed direct (1)D((13)C-H) dipolar coupling. The obtained results show that, under the particular conditions of the studied system (very weak orientational ordering of a highly symmetric molecule), this contribution is particularly important, in order to reproduce the whole value of the "observed" dipolar coupling. This issue is discussed and commented on at length in the work, also, by making reference to the analogy with perfectly symmetric molecules (such as methane and analogues) dissolved in liquid crystalline phases.

Orientational mechanisms in liquid crystalline systems. 1. A reaction field analytical description of the interaction between the electric quadrupole moment of a probe solute and the electric field gradient of a nematic solvent

In the present paper, the fundamental problem of calculating the electric field gradient (EFG) experienced by a highly idealized solute, represented by a general point quadrupole immersed in an anisotropic uniaxial medium, has been tackled. Following a generalized reaction field approach (based upon the original ideas and the "mean-field philosophy" due to Kirkwood and Onsager) in the linear response approximation, a closed analytical expression of the EFG has been derived (to the best of our knowledge, for the first time). The obtained expression is particularly simple and elegant, also thanks to the oversimplifying approximation that the virtual cavity containing the solute is assumed to be perfectly spherical. This compact and manageable formula, obtained by a rigorous mathematical derivation (unlike other mean-field phenomenological models previously suggested in literature) can be useful to investigate and better understand a likely orientational mechanism, partly responsible for the ordering of small solutes dissolved in nematic mesophases, based on the interaction between the electric quadrupole of the solute and the electric field gradient of the anisotropic uniaxial medium (in the next paper of this issue, the formulation obtained in this work is widely tested on a variety of uniaxial and biaxial solutes dissolved in different nematic solvents).

Orientational mechanisms in liquid crystalline systems. 2. The contribution to solute ordering from the reaction field interaction between the solute electric quadrupole moment and the solvent electric field gradient

In the previous paper of this issue, [Celebre, G.; Ionescu, A. J. Phys. Chem. B doi: 10.1021/jp907310g], following a generalized reaction field approach in the linear response approximation, we were successful in obtaining an analytical compact expression for the mean-field anisotropic orientational potential U(Q-EFG) theoretically experienced by a highly idealized nonionic and apolar solute, considered as a point quadrupole immersed in a uniaxial polarizable continuum medium (model of a nematic solvent comprised of dipolar mesogenic molecules). The term U(Q-EFG) describes the electrostatic interaction between the electric quadrupole of the solute and the electric field gradient induced at the solute by the surrounding medium polarized by the distribution of electric charges representing the quadrupolar solute itself. In the present paper, the obtained potential has been considered as an additional orientational interaction contributing to the solute ordering, besides the well-recognized and very effective "short-range" (size-and-shape-dictated) mechanisms. Since in our theory the solvent is characterized by its dielectric tensor, the model has been widely tested by taking as references the experimental order parameters of several uniaxial and biaxial different small rigid probe molecules (H(2), N(2), acetylene, allene, propyne, benzene, hexafluorobenzene, 1,4-difluorobenzene, and norbornadiene) dissolved in the nematic solvents ZLI1132 (Deltaepsilon >> 0) and EBBA (Deltaepsilon < 0); moreover, the order parameters of the same solutes in the so-called nematic "magic mixture" (45 wt % EBBA + 55 wt % ZLI1132), where the short-range orientational effects are commonly believed to be very dominant, have been conventionally assumed as reference of the absence of electrostatic orientational effects. The experimental order parameters of the treated solutes, obtained in the past by liquid crystal NMR and available from literature, have been then compared with those theoretically predicted by our theoretical approach in order to obtain useful hints about two basic points, (a) the real physical nature of the interactions (other than the "size-and-shape") involved in the orientational mechanisms and (b) the conceptual effectiveness of the suggested mean-field approach in describing this kind of phenomena. Successes and failures of the approach in the predictions are discussed at length, along with their possible reasons and implications.