Exploring short intramolecular interactions in alkylaromatic substrates

From proteins and peptides to semiconducting polymers, aliphatic chains on aromatic groups are recurring motifs in macromolecules from very diverse application fields. Fields in which molecular folding and packing determine the macroscopic physical properties that make such advanced materials appealing in the first place. Within each macromolecule, the intrinsic structure of each unit defines how it interacts with its neighbours, ultimately opening up or denying certain backbone conformations. This eventually also determines how macromolecules interact with each other. This account deals specifically with the conformational problem of many common alkylaromatic units, examining the features of an intramolecular interaction involving a side chain with as few as three methylene groups. A set of 23 model compounds featuring an intramolecular interaction between an aliphatic X-H (X = C, N, O, and S) bond and an aromatic ring was considered. Quantitative computational analysis was made possible, thanks to complete basis set extrapolated CCSD(T) calculations and NCI topological analysis, the latter of which revealed an elaborate network of dispersive and steric interactions leading to somewhat unintuitive and unexpected results, such as the higher energetic stability of certain twisted conformational isomers over those with extended side chains. Vicinal covalent effects from polarizing groups and various heteroatoms, along with the occurrence of non-dispersive phenomena, were also investigated. The conclusions drawn from the investigation include a comprehensive set of guidelines intended to aid in the prediction of the most stable conformation for this class of building blocks. Our findings affect a variety of different research fields, including the tailoring of functional materials for organic electronics and photovoltaics, with insights into a rational treatment of conformational disorder, and the study of protein- and peptide-folding preferences, putting an emphasis on peculiar interactions between the backbone and aromatic residues

Pyrazolium- versus imidazolium-based ionic liquids: Structure, dynamics and physicochemical properties

Ionic liquids (ILs) composed of two different pyrazolium cations with dicyanamide and bis(trifluoromethanesulfonyl)imide anions have been synthesized and characterized by NMR, Kamlet-Taft solvatochromic parameters, conductivity and rheological measurements, as well as ab initio calculations. Density functional calculations for the two pyrazolium cations, 1-butyl-2- methylpyrazolium [bmpz] and 1-butyl-2,3,5-trimethylpyrazolium [bm 3pz], provide a full picture of their conformational states. Homo- and heteronuclear NOE show aggregation motives sensitive to steric hindrance and the anions' nature. Self-diffusion coefficients D for the anion and the cation have been measured by pulsed field gradient spin-echo NMR (PGSE-NMR). The ionic diffusivity is influenced by their chemical structure and steric hindrance, giving the order Dcation > Danion for all of the examined compounds. The measured ion diffusion coefficients, viscosities, and ionic conductivity follow the Vogel-Fulcher-Tammann (VFT) equation for the temperature dependencies, and the best-fit parameters have been determined. Solvatochromic parameters indicate an increased ion association upon going from bis(trifluoromethanesulfonyl)imide to dicyanamide-based pyrazolium salts, as well as specific hydrogen bond donor capability of H atoms on the pyrazolium ring. All of these physical properties are compared to those of an analogous series of imidazolium-based ILs

Paramagnetic species in catalysis research: A unified approach towards (the role of EPR in) heterogeneous, homogeneous and enzyme catalysis

Paramagnetic (open-shell) systems, including transition metal ions, radical intermediates and defect centres, are often involved in catalytic transformations. Despite the prevalence of such species in catalysis, there are relatively few studies devoted to their characterisation, compared to their diamagnetic counterparts. Electron Paramagnetic Resonance (EPR) is an ideal technique perfectly suited to characterise such reaction centres, providing valuable insights into the molecular and supramolecular structure, the electronic structure, the dynamics and even the concentration of the paramagnetic systems under investigation. Furthermore, as EPR is such a versatile technique, samples can be measured as liquids, solids (frozen solutions and powders) and single crystals, making it ideal for studies in heterogeneous, homogeneous and enzyme catalysis. Coupled with the higher resolving power of the pulsed, higher frequency and hyperfine techniques, unsurpassed detail on the structure of these catalytic centres can be obtained. In this Chapter, we provide an overview to demonstrate how advanced EPR methods can be successfully exploited in the study of open-shell paramagnetic reaction centres in heterogeneous, homogeneous and enzymatic catalysts, including heme-based enzymes for use in biocatalysts, polymerisation based catalysts, supported microporous heterogeneous catalytic centres to homogeneous metal complexes for small molecule actions

Testing Consumer Rationality using Perfect Graphs and Oriented Discs

Given a consumer data-set, the axioms of revealed preference proffer a binary test for rational behaviour. A natural (non-binary) measure of the degree of rationality exhibited by the consumer is the minimum number of data points whose removal induces a rationalisable data-set.We study the computational complexity of the resultant consumer rationality problem in this paper. This problem is, in the worst case, equivalent (in terms of approximation) to the directed feedback vertex set problem. Our main result is to obtain an exact threshold on the number of commodities that separates easy cases and hard cases. Specifically, for two-commodity markets the consumer rationality problem is polynomial time solvable; we prove this via a reduction to the vertex cover problem on perfect graphs. For three-commodity markets, however, the problem is NP-complete; we prove thisusing a reduction from planar 3-SAT that is based upon oriented-disc drawings

Turbulent flow at 190 m height above London during 2006-2008: A climatology and the applicability of similarity theory

Flow and turbulence above urban terrain is more complex than above rural terrain, due to the different momentum and heat transfer characteristics that are affected by the presence of buildings (e.g. pressure variations around buildings). The applicability of similarity theory (as developed over rural terrain) is tested using observations of flow from a sonic anemometer located at 190.3 m height in London, U.K. using about 6500 h of data. Turbulence statistics—dimensionless wind speed and temperature, standard deviations and correlation coefficients for momentum and heat transfer—were analysed in three ways. First, turbulence statistics were plotted as a function only of a local stability parameter z/Λ (where Λ is the local Obukhov length and z is the height above ground); the σ_i/u_* values (i = u, v, w) for neutral conditions are 2.3, 1.85 and 1.35 respectively, similar to canonical values. Second, analysis of urban mixed-layer formulations during daytime convective conditions over London was undertaken, showing that atmospheric turbulence at high altitude over large cities might not behave dissimilarly from that over rural terrain. Third, correlation coefficients for heat and momentum were analyzed with respect to local stability. The results give confidence in using the framework of local similarity for turbulence measured over London, and perhaps other cities. However, the following caveats for our data are worth noting: (i) the terrain is reasonably flat, (ii) building heights vary little over a large area, and (iii) the sensor height is above the mean roughness sublayer depth