Enhanced energy transfer within a microparticle

Enhanced energy transfer between donor-acceptor pairs within a single aerosol particle is demonstrated for the first time. The overall transfer within a particle 10 μm in diameter is found to exceed conventional dipole-dipole transfer by more than a factor of 10². A simple model indicates that the energy transfer is mediated by morphological resonances of the particle.     

Enhanced critical temperature in a dual-layered molecular superconductor

Single-crystal X-ray diffraction has shown that the high-critical-temperature (T(c)) phase of the filamentary molecular superconductor (BEDT-TTF)(2)Ag(CF(3))(4)(1,1,2-trichloroethane) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] contains layers of BEDT-TTF radical cations with alternating κ- and α'-type packing motifs. This molecule-based superconductor with dual BEDT-TTF packing motifs has a T(c) five times higher than that of its polymorph that contains only κ-type packing.     

Correlation regions within a localized molecular orbital approach

A hybrid scheme for the computation of reaction energies in large molecular systems is proposed. The approach is based on localized orbitals and allows for the treatment of different parts of a molecule at different computational levels. The localized orbitals are assigned to regions, and then different local correlation methods, such as local MP2 or local CCSD(T), can be applied to different regions. In contrast to previous hybrid schemes, the molecule does not have to be split into parts and, therefore, it is not necessary to saturate dangling bonds using link atoms. For fixed region sizes, the cost of the high-level calculation becomes independent of the molecular size, and it is demonstrated that O(1) scaling can be achieved. Illustrative applications are presented and the convergence of the results with respect to the size of the regions is investigated for reaction energies, barrier heights, and weakly bound complexes.     

Electron tomography shows molecular anchoring within a layer-by-layer film

The layer-by-layer self-assembly of thin films consisting of alternating layers of DNA and bis-urea nanoribbons prevents diffusion of the components within the film and allows the anchoring of biotinylated molecules through molecular recognition in a predetermined layer of the film. Electron tomography demonstrates with nanometer precision the location of gold-labeled streptavidin bound to the incorporated biotinylated molecules.     

Reaction rate for isomerization in a molecular beam

It is shown that in an ideal gas (molecular beam), at temperature T, consisting of molecules that can isomerize and whose molecular dynamics can be non-ergodic, the observed decay rate of the concentration can be greater than that. k_TST, predicted by transition state theory. The “observed” rate constant is then given by (Q/Q_≠⁽¹⁾)k_TST where Q_≠⁽¹⁾ is the contribution to the canonical partition function Q arising from only activated and irregular states. It is also shown that in the presence of strong collisions the long-time decay rate of the concentration provides no information about the intramolecular rate process, but at low collision rates it is sensitive to the non-ergodic dynamics.     

Controlling the rotation of a complexed guest within a molecular cage

A molecular cage-based complex, in which the complexed guest changes its binding geometry with respect to the macrocyclic host upon the addition and removal of K(+) ions, is reported.     

Enhanced hydrophobicity of fluorinated lipid bilayer: a molecular dynamics study

A molecular dynamics simulation of a partially fluorinated phospholipid bilayer has been carried out to understand the effects of fluorination of the hydrophobic chains on the structure and water permeability across the membrane. Fluorocarbon chains typically have an all-trans conformation, showing a highly ordered structure in the membrane core compared to ordinary hydrocarbon chains. The free energy profiles of water across the bilayers were successfully estimated by a revised cavity insertion Widom method. The fluorinated bilayer showed a higher free energy barrier than an ordinary nonfluorinated lipid bilayer by about 1.2 kcal/mol, suggesting a lower water permeability of the fluorinated bilayer membrane. A cavity distribution analysis elucidated the reduced free volume in the fluorinated membrane due to the neatly packed chains, which should account for the higher free energy barrier.     

Rapid screening of a receptor with molecular memory

[reaction: see text] Atropisomeric receptor 1 can change conformation and maintain the new conformation when heated and cooled in the presence of a guest molecule. This molecular memory can be used as a rapid method of screening potential guests. Heating atropisomeric diacid 1 with various hydrogen-bonding guests leads to a shift in the syn/anti ratio that could be easily monitored as it is stable at room temperature even in the absence of the guest molecules.     

Sequence dependence of methylation rate enhancement in meta-phenyleneethynylene foldamers

The methylation rates of a meta-phenyleneethynylene (mPE) oligomer with a terminally-attached 4-dimethylaminopyridine (DMAP) residue are reported for a series of linear and branched methyl sulfonates; these data show that selective methylation is enhanced by locating the DMAP unit at the midpoint of the sequence, allowing the foldamer cavity to function as a reactive sieve.     

Reactions between or within molecular crystals

Reactions that occur within or between molecular crystals, in particular those reactions that are activated by mechanical methods, are reviewed. The focus is on processes (whether intrasolid or intersolid) that are controlled primarily by supramolecular bonding, such as template cycloadditions, formation of inclusion compounds, reactions between molecular crystals by the reassembling of noncovalent bonds, and the formation of complexes and coordination compounds. It is proposed that solvent-free mechanochemical methods, for example, cogrinding, milling, and kneading, represent viable "green" routes for the preparation of novel molecular and supramolecular solids.     

Noncovalent interactions within a synthetic receptor can reinforce guest binding

Structural and thermodynamic data are presented on the binding properties of anion receptors containing two covalently linked cyclopeptide subunits that bind sulfate and iodide anions with micromolar affinity in aqueous solution. A synchrotron X-ray crystal structure of the sulfate complex of one receptor revealed that the anion is bound between the peptide rings of the biscyclopeptide. Intimate intramolecular contacts between the nonpolar surfaces of the proline rings of the individual receptor moieties in the complex suggest that hydrophobic interactions within the receptor that do not directly involve the guest contribute to complex stability. This finding is supported by a microcalorimetric analysis of the solvent dependence of complex stability, which showed that increasing the water content of the solvent has only a weak influence on the Gibbs energy of binding. Hence, the increasing amount of energy required for desolvating the binding partners in solutions containing more water is almost compensated by the increasingly favorable hydrophobic interactions. Further observations that suggest that guest-induced intra-receptor interactions contribute to guest binding are (i) anion binding of a monomeric cyclopeptide lacking the covalent linkage between the two rings leads to the formation of 2:1 complexes; (ii) in the crystal structure of the 2:1 iodide complex of this monotopic receptor, a similar arrangement of the two cyclopeptide rings has been found as in the sulfate complex of the biscyclopeptide; (iii) complex formation of the monomeric cyclopeptide in aqueous solution is highly cooperative with a large stability constant corresponding to the formation of the 2:1 complexes from relatively instable 1:1 complexes; (iv) the monomeric cyclopeptide forms only 1:1 anion complexes in DMSO where hydrophobic interactions do not take place; and (v) introducing polar hydroxy groups on the proline rings of the monomeric cyclopeptide disrupts cooperativity causing the formation of only 1:1 complexes even in aqueous solution. Taken together these observations demonstrate that, in addition to direct receptor-substrate interactions, noncovalent interactions between the two subunits of such biscyclopeptides contribute significantly to anion complex stability. Reinforcement of molecular recognition through intra-receptor interactions should be an attractive new strategy to boost host-guest affinities.     

Study of a hydrogen-bombardment process for molecular cross-linking within thin films

A low-energy hydrogen bombardment method, without using any chemical additives, has been designed for fine tuning both physical and chemical properties of molecular thin films through selectively cleaving C-H bonds and keeping other bonds intact. In the hydrogen bombardment process, carbon radicals are generated during collisions between C-H bonds and hydrogen molecules carrying ～10 eV kinetic energy. These carbon radicals induce cross-linking of neighboring molecular chains. In this work, we focus on the effect of hydrogen bombardment on dotriacontane (C(32)H(66)) thin films as growing on native SiO(2) surfaces. After the hydrogen bombardment, XPS results indirectly explain that cross-linking has occurred among C(32)H(66) molecules, where the major chemical elements have been preserved even though the bombarded thin film is washed by organic solution such as hexane. AFM results show the height of the perpendicular phase in the thin film decreases due to the bombardment. Intriguingly, Young's modulus of the bombarded thin films can be increased up to ～6.5 GPa, about five times of elasticity of the virgin films. The surface roughness of the thin films can be kept as smooth as the virgin film surface after thorough bombardment. Therefore, the hydrogen bombardment method shows a great potential in the modification of morphological, mechanical, and tribological properties of organic thin films for a broad range of applications, especially in an aggressive environment.     

Synthesis of a molecular receptor containing two recognition sites

We report the synthesis and preliminary binding properties of a molecular receptor containing two distinct binding sites; a cyclophane and a crown ether.     

Hula-twist within the confinement of molecular crystals

E/Z-photoisomerizations within molecular crystals are varied. Existing cases are summarized. They require crystal lattices that allow for long-range molecular movements in the nontopotactic solid-state mechanism. Reactivity and directionality can be foreseen on the basis of the crystal packing. The reacting crystal changes continuously by phase rebuilding, phase transformation and disintegration. Two possibilities for the chemical mechanism exist: (1) highly space-demanding (cooperative) double-bond rotations; and (2) space-conserving hula-twist (HT) motions while the substituents move within their planes and only one C-H unit undergoes out-of-plane translocation. If internal rotation cannot be reasonably modeled within the crystal lattice, HT remains the only choice, as in the case of trans-1,2-dibenzoylethene. Direct experimental proof is still lacking because the differences in the conformational outcome could not be assessed in the studied examples. Density functional theory calculations of cis-1,2-dibenzoylethene revealed very low differences in energy content of the helical (s-cis, s-cis)- and the almost orthogonal (s-cis, s-trans)-cis-conformers. The almost orthogonal (s-cis, s-trans)-cis-conformer that is found in the pure crystal is very similar to the calculated counterpart. It is suggested that more favorable initial conformers might be obtained by proper vinylic substitution. The stereochemical outcome of highly space-demanding thermal vinylic-bond rotations followed by cyclizations of conjugated bisallenes to give bismethylene cyclobutenes excludes the alternative HT mechanism (double-bond isomerization) in the present cases. But space-conserving HT might be a mechanistic alternative in less-substituted cases under photoexcitation. The stereochemical consequences are discussed.     

Encapsulation of guests within a gated molecular basket: thermodynamics and selectivity

Molecular basket 1 has been designed to contain a set of aromatic gates, each with rotational mobility restricted via intramolecular hydrogen bonding. This structural, yet dynamic, feature of the host has been revealed to permit the formation of a transient enclosed space capable of containing haloalkanes, whose size/shape, electronic and entropic attributes contributed to the thermodynamics of binding. Markedly, the basket is capable of mediating the trafficking of a broad range of molecules.     

Removal of organic pollutants from water via molecular inclusion within a cavitand

The molecular inclusion chemistry of cavitands provides a useful way for the removal of organic pollutants from water. A wide range of lipophilic organic compounds, present at trace level in water, are efficiently extracted by cavitand1, which can be easily reactivated and reused.     

Algorithm to infer the structures of molecular formulas within a reaction pathway

MECHEM is a computer aid for elucidation of reaction pathways that was developed over the last 5 years. The program searches systematically and comprehensively for the simplest multistep reaction pathways (or mechanisms) that are consistent with the experimental constraints formulated by the experimentalist, any ad hoc assumptions, and the program's internal theory. Previous articles have reported the basic pathway-generation algorithm and another algorithm that tests the structural soundness of individual steps. This article introduces an algorithm to solve another basic problem: Given a multistep pathway containing a mixture of molecular structures and formulas, assign possible structures to the formulas while obeying (and exploiting) the constraint imposed by the pathway steps. With this new algorithm, MECHEM is now approaching competence as an interactive tool for elucidating some catalytic reaction pathways, which is the current chemical focus. © 1994 by John Wiley & Sons, Inc.