Ken D. Shimizu

Application of the Freundlich adsorption isotherm in the characterization of molecularly imprinted polymers

The binding isotherm for a polymer molecularly imprinted with ethyl adenine-9-acetate was observed to obey the common Freundlich isotherm. To test the generality of the Freundlich isotherm with respect to molecularly imprinted polymers (MIPs), a survey of systems from the literature was conducted, revealing that the Freundlich isotherm gives a good mathematical approximation of the binding characteristics for non-covalently imprinted polymers. The utility of the Freundlich isotherm in the calculation of binding parameters, as well as its limitations and implication of an exponential distribution of binding sites in imprinted polymers have been discussed.

Measurement of the continuous distribution of binding sites in molecularly imprinted polymers

Reported is the first affinity spectrum (AS) [number of binding sites (N) vs. association constant (K)] for a non-covalently imprinted polymer. The AS method yields the distribution of sites over a continuous range of binding constants and characterizes the heterogeneity present in imprinted polymers better than current methodologies. To demonstrate the generality of the AS method, the distributions for three different imprinted polymers (two of which were taken from the literature) were calculated from their respective binding isotherms. The shapes of the distribution curves were different yet consistent with the respective covalent or non-covalent imprinting mechanisms. Finally, the binding parameters derived from the AS method were compared with those determined by the more common Scatchard analysis and were in general agreement.

High‐Throughput Strategies for the Discovery of Catalysts

Are there more efficient ways to discover new catalysts? Classical efforts, while very successful, have been typically quite linear in design. In contrast, early efforts using diversity-based techniques such as parallel synthesis and screening (right) to rapidly identify and optimize new catalysts are beginning to yield some promising results.

Molecularly imprinted polymer sensor arrays.

The sensor array format has proved an effective method of transforming sensors of modest selectivity into highly selective and discriminating sensors. The primary challenge in developing new sensor arrays is collecting together a sufficient number of recognition elements that possess different binding affinities for the analytes of interest. In this regard, the use of molecularly imprinted polymers (MIPs) as the recognition elements in sensor arrays has a number of unique advantages. MIPs can be rapidly and inexpensively prepared with different selectivities and tuned with different selectivity patterns via the choice of templates in the imprinting process. The array format also helps compensate for the low selectivities and high cross-reactivities of MIP sensors. These attractive qualities of MIP sensor arrays have been demonstrated in recent examples, which have established the viability and generality of the approach. In particular, the versatility of the imprinting process enables MIP sensor arrays to be tailored to specific analytes. MIP sensor arrays have also shown surprisingly broad utility, as even analytes that were not used as templates in the imprinting process can be effectively discriminated.

A rigid molecular balance for measuring face-to-face arene-arene interactions.

A new molecular balance was developed to measure face-to-face arene-arene interactions. The balance adopts distinct folded and unfolded conformations due to restricted rotation about a C aryl-N imide bond. In the folded conformer, the rigid bicyclic framework enforces an offset face-to-face geometry to the exclusion of edge-to-face geometries, which was verified by X-ray crystallography. Measurement of the folded to unfolded ratio yields accurate values for the arene-arene interaction in a range of different solvents.

The first ‘two-over/two-under’(2O/2U) 2D weave structure assembled from Hg-containing 1D coordination polymer chains

Self-assembly of HgI2 with a semirigid ditopic ‘Z’ type ligand affords the zigzag chains that interweave into a clothlike 2D network in a ‘two-over/two-under’ (2O/2U) fashion.

Additivity of Substituent Effects in Aromatic Stacking Interactions

The goal of this study was to experimentally test the additivity of the electrostatic substituent effects (SEs) for the aromatic stacking interaction. The additivity of the SEs was assessed using a small molecule model system that could adopt an offset face-to-face aromatic stacking geometry. The intramolecular interactions of these molecular torsional balances were quantitatively measured via the changes in a folded/unfolded conformational equilibrium. Five different types of substituents were examined (CH3, OCH3, Cl, CN, and NO2) that ranged from electron-donating to electron-withdrawing. The strength of the intramolecular stacking interactions was measured for 21 substituted aromatic stacking balances and 21 control balances in chloroform solution. The observed stability trends were consistent with additive SEs. Specifically, additive SE models could predict SEs with an accuracy from ±0.01 to ±0.02 kcal/mol. The additive SEs were consistent with Wheeler and Houks direct SE model. However, the indirect or polarization SE model cannot be ruled out as it shows similar levels of additivity for two to three substituent systems, which were the number of substituents in our model system. SE additivity also has practical utility as the SEs can be accurately predicted. This should aid in the rational design and optimization of systems that utilize aromatic stacking interactions.

Do deuteriums form stronger CH-π interactions?

The D/H isotope effect for the CH-π interaction was studied experimentally and computationally. First, a series of molecular balances that are very sensitive to changes in the strength of the CH-π interactions in solution were designed. Balances with deuterated and non-deuterated alkyl groups were synthesized, and their intramolecular CH-π interactions were compared. The geometries of their intramolecular CH-π and CD-π interactions were characterized in the solid state by X-ray analysis, and the strength of each interaction was characterized in solution by the folded/unfolded ratio as measured by (1)H NMR spectra. Second, the relative strengths of the CH-π and CD-π interactions were also assessed computationally using dispersion-corrected DFT (PDE-D2/6-31+G*). No significant differencee was observed in either the experimental or theoretical studies, indicating that the D/H isotope effect for the CH-π interaction is either very small or nonexistent.

Preparation of cationic cobaltocenium polymers and block copolymers by “living” ring-opening metathesis polymerization

We report a robust synthetic strategy to prepare high molecular weight side-chain cationic cobaltocenium-containing polymers and block copolymers via ring-opening metathesis polymerization. All polymerizations were extremely fast with high yields (∼100%) under open air conditions at room temperature and followed a living and controlled manner.

A Molecular Balance for Measuring Aliphatic CH−π Interactions

A series of conformationally flexible bicyclic N-arylimides were employed as molecular balances to study the weak aliphatic CH-π interaction between alkyl and arene groups. The formation of intramolecular CH-π interactions in the folded conformers was characterized by X-ray crystallography. The strengths of the interactions were characterized in CDCl(3) by the changes in the folded/unfolded ratios, as measured by (1)H NMR. The CH-π interaction between a methyl group and an aromatic surface was ∼1.0 kcal/mol and was easily disrupted or masked by conformational entropy and repulsive steric interactions.