Exciplexes with Ionic Dopants: Stability, Structure, and Experimental Relevance of M⁺(²P)⁴He_<i>n</i> (M = Sr, Ba)

M⁺(²P)⁴He_<i>n</i> species, possibly involved in the post ²P ← ²S excitation dynamics of Sr⁺ and Ba⁺ in cold ⁴He gas or droplets, are studied employing both high level <i>ab initio</i> calculations to determine the potential energy curves (PEC) and diffusion Monte Carlo (DMC) to obtain information on their ground state structure and energetics. PEC for the excited M⁺(²P)He dimers were obtained using MRCI calculations with extended basis sets. Potential energy surfaces (PES) for M⁺(²P)­He_<i>n</i> were built with the DIM model including spin–orbit coupling via a perturbative procedure. DMC simulations indicated several exciplexes (<i>n</i> > 2) to be stable against He dissociation whatever the ion state, a finding that is at variance with what was previously suggested for Ba⁺(²P_1/2) due to the repulsive nature of the interaction potential obtained in [Phys. Rev. A 2004, 69, 042505]. Our results, instead, support the suggestion made in [J. Chem. Phys. 2012, 137, 051102] for the existence of Ba⁺(²P_1/2)­He_<i>n</i> exciplexes emitted following the excitation of the barium cation solvated into He droplets. In the ²P_1/2 state, the Ba ion also shows a peculiar behavior as a function of <i>n</i> with respect to the location and binding strength of the attached He atoms compared to Sr⁺. Although the latter forms the usual equatorial He ring, Ba⁺ deviates from this geometry for 1 ≤ <i>n</i> ≤ 4, with the radial distribution functions strongly depending on the number of solvent atoms. Finally, a putative species is proposed to explain the emission band at 523 nm that follows D1 or D2 excitation of Ba⁺ in superfluid bulk helium

Interpreting “Acidity” as a Global Property Controlling Comonomer Reactivity in Olefin Polymerization

A possible rationale for the different catalytic behaviors of systems based on <i>rac</i>-(ethylenebis­(1-indenyl))zirconium dichloride (<i>rac</i>-EBIZrCl₂), <i>rac</i>-(ethylenebis­(1-indenyl))­hafnium dichloride (<i>rac</i>-EBIHfCl₂), and <i>rac</i>-(isopropylidenebis­(1-indenyl))zirconium dichloride (<i>rac</i>-iPrBIZrCl₂) toward ethene–styrene copolymerization has been sought by studying related active systems. For this purpose, the metallocene ion pairs (IPs) <i>rac</i>-EBIZrMeMeB­(C₆F₅)₃, <i>rac</i>-EBIHfMeMeB­(C₆F₅)₃, and <i>rac</i>-iPrBIZrMeMeB­(C₆F₅)₃ have been synthesized and their structures in solution explored with ROESY and pulsed gradient NMR spectroscopy. The energetics of dynamical processes relevant for catalysis that can be used as indicators of the cation acidity have been studied with variable-temperature NMR experiments and density functional theory (DFT). NMR experiments successfully provided IP structural details in solution and also indicated the presence of an intricate dynamic behavior for all the IPs. DFT results, instead, indicated quantitatively how changing the metal and/or the ancillary ligand bridge influences the energetics of the active species and modifies the reaction energy profile. The theoretical results also drew attention to the fact that finding a rationale for the ligand influence on the catalytic behavior of <i>rac</i>-EBIZrCl₂/MAO and <i>rac</i>-iPrBIZrCl₂/MAO in ethene–styrene copolymerization requires not only considering the steric effects but also determining how subtle changes in the ligand sphere affect the capability of the metal center to accept electrons from the counteranion or the olefins

Tuning Knotted Copolyelectrolyte Conformations via Solution Properties

We used Langevin dynamics simulations to study coarse-grained knotted copolyelectrolytes, composed by a neutral and a charged segment, in solutions of different salt concentrations, valency, and solvent screening power. We show that the facile variation of these parameters allows for tuning the length and position of the knotted region, which in turn controls the overall metric properties. Specifically, adding either monovalent or divalent ions causes the knot to swell at the expense of the copolyelectrolyte overall size. However, the knot typically straddles the charged–neutral interface in the presence of monovalent counterions, whereas it is attracted on the charged segment with divalent ones. Notably, similar modulations of knot size and position can also be achieved by varying the dielectric constant of the solvent. Our results demonstrate the feasibility of harnessing the solution-mediated balance of electrostatics and conformational entropy toward a facile external tuning of the conformational properties of knotted polymers

Modulating Antimicrobial Activity by Synthesis: Dendritic Copolymers Based on Nonquaternized 2-(Dimethylamino)ethyl Methacrylate by Cu-Mediated ATRP

The synthesis of novel star-like heteroarms polymers A­(BC)_<i>n</i> containing m-PEG (block A), methylmethacrylate (MMA), and nonquaternized 2-(dimethylamino)­ethyl methacrylate (DMAEMA) (blocks BC) is here reported. We demonstrated that copolymer films with comparable amounts of DMAEMA have antimicrobial properties strongly depending on the topological structure (i.e., the number of arms) of the composing copolymers. We interpret the highest antimicrobial activity of A­(BC)₂ with respect to A­(BC)₄ and linear copolymers (respectively, A­(BC)₂ ≥ A­(BC)₄ > A­(BC)) as probably due to the formation of strong hydrogen bonds between close amino-ammonium groups in the A­(BC)₂ film. Strong hydrogen bonds seem to be somewhat disfavored in the case of the linear species by the difference in both polymer architecture and film morphology compared with the A­(BC)₂ and A­(BC)₄ architectures

Dicopper(II) Mozobil^TM: a dinuclear receptor for the pyrophosphate anion in aqueous solution

<p>In this work, we investigated the dicopper(II) complex of Mozobil^TM as a potential receptor for anions in MeOH and MeOH/water mixture. The results were compared with those obtained for the mononuclear complex, copper(II) benzyl-cyclam as a model system. Experimental investigations were also supported by computational studies on both the dinuclear and the mononuclear species. Among the investigated anionic guests, the dicopper complex showed an outstanding affinity for the pyrophosphate anion, leading to the formation of a stable adduct in MeOH solution. Our computational studies strongly suggested a 2:2 stoichiometry for the adduct. At least in MeOH, this dimeric structure, with two pyrophosphate anions sandwiched between two dicopper Mozobil^TM units, is stabilized by H-bonding interactions between the receptor and pyrophosphate. In MeOH:water mixture, beside the strong competition of solvent, the affinity toward the anion was preserved. We exploited this property to develop a novel indicator displacement assay (IDA) for pyrophosphate in aqueous solution, using Pyrocatechol Violet (PV) as the indicator.</p