Carbon-13 NMR spectra of polybromoalkanes and polychlorobromoalkanes. Structural increments of halogens in polyhalogenated groups

The ¹³C NMR spectra of 48 polychlorobromoalkanes have been studied. Unlike the ¹³C signals of chlorine-containing groups (38–105 ppm), those of bromine-containing fragments, with the exception of CBr₂ (60–70 ppm), appear in a rather narrow range (25–50 ppm) and are shifted to higher field in relation to similar chlorine-containing groups. The spin–spin coupling constants in similar bromine- and chlorine-containing groups practically coinciEN. Calculation of the chemical shifts for the polyhaloalkanes under study according to the additivity scheme, as previously observed for polychloroalkanes, renders values which are in considerable discord with experimental values (up to –32 ppm for CBr₃). These discrepancies may be compensated for by corrections for the binary interaction of halogen atoms by grouping the halogen-containing fragments according to the geminal, vicinal, 1,3-, 1,3,5- and 1,2,3-arrangement of halogen atoms, and by introducing an increment for the position of the halogen at the secondary atom. It is established that as compared to 1-monohaloalkanes: (a) in the case of the geminal arrangement of halogen atoms the α- and γ-effects diminish (Δ α from –3.2 to –8 ppm; Δγ = 2.6 ppm), while the β-effect increases slightly (from 0 to 1.2 ppm); (b) in the case of a vicinal arrangement both the α- and β-effects diminish (by about –3.5 ppm) and the γ-effect remains constant, as if the vicinal system of the halogens was topologically insulated; (c) for the 1,3- and 1,3,5-arrangement of halogens their mutual influence is weak (about –0.5 ppm for each halogen atom in the α- and γ-positions); (d) the 1,2,3 system (serial arrangement of halogen atoms) is the sum of two vicinal fragments and hardly deviates from the additivity scheme; (e) the arrangement of a halogen at the secondary C atom enhances the α-effect (Δα = 2.8 and 1.0 for methyl and methylene, respectively, in the case of Cl, and 3.5 and 3.7 ppm in the case of Br); the variation of the β-effect has a different sign in relation to CH₃ and CH₂ groups (+1.2 and –1.7 for Cl, and +2.5 and –1.0 for Br). More distant effects of halogens (δ and ?) were not considered. The determined increments (Δα, Δβ and Δγ) for the α-, β- and γ-effects of chlorine and bromine atoms allow the prediction of the 13^C chemical shifts in polyhaloalkanes with an accuracy up to ±1.5 ppm. Some deviations of up to ±5 ppm may be connected with the influence of a three particle interaction of halogen atoms, which was taken into account only in the case of a geminal arrangement of halogen atoms.     

Ion condensation on charged patterned surfaces

We study ion condensation on a patterned surface with stripes of alternating charge. The competition between adsorbed ion-ion and adsorbed ion-surface interactions leads to the formation of different strongly correlated structures of condensed ions in the low-temperature limit (LTL). We consider two types of arrangements which have lowest energy in the LTL: (1) ions adsorbed onto the stripe center lines and (2) arrays of dipoles at the interfaces between charged domains. We determine the preferred arrangement as a function of surface charge density, the chemical potential of the ions in the surrounding medium, and the geometric parameters of the system. We determine the conditions for the appearance of more complex ionic patterns by considering simple perturbations of the stripe-centered and dipolar array structures.     

Molecular simulation study of peptide amphiphile self-assembly

We study the self-assembly of peptide amphiphile (PA) molecules, which is governed by hydrophobic interactions between alkyl tails and a network of hydrogen bonds between peptide blocks. We demonstrate that the interplay between these two interactions results in the formation of assemblies of different morphology, in particular, single beta-sheets connected laterally by hydrogen bonds, stacks of parallel beta-sheets, spherical micelles, micelles with beta-sheets in the corona, and long cylindrical fibers. We characterize the size distribution of the aggregates as a function of the molecular interactions. Our results suggest that the formation of nanofibers of peptide amphiphiles obeys an open association model, which resembles living polymerization.     

Low-radii transitions in co-assembled cationic-anionic cylindrical aggregates

We investigate the formation of charged patterns on the surface of cylindrical micelles from co-assembled cationic and anionic amphiphiles. The competition between the net incompatibility chi (which arises from the different chemical nature of oppositely charged molecules) and electrostatic interactions (which prevent macroscopic segregation) results in the formation of surface domains. We employ Monte Carlo simulations to study the domains at thermal equilibrium. Our results extend previous work by studying the effect of the Bjerrum length l(B) at different values of the cylinder's radius R and chi and analyze how it affects the transition between helical, ring, and isotropic patterns. A critical surface in the space (l(B), R, chi) separating these three phases is found, and we show how it corresponds to a first-order phase transition. This confirms that the Bjerrum length l(B) is a significant parameter in the control of the helical-ring transition; the ring pattern is strongly associated with short-range forces, whereas the helical pattern develops from dominant long-range electrostatic interactions.