11,057 research outputs found
Single Molecule Solvation and Its Effects on Tautomeric Equilibria in a Self-Assembled Capsule
No full textA self-assembled cylindrical capsule provides a nanoscale environment that affects keto−enol equilibria. The equilibrium constants for encapsulated β-ketoesters show values that differ by an order of magnitude from that of the free tautomers in solution. For complexes with a single, large encapsulated guest, the inner surfaces of the capsule and the seam of the hydrogen bonds influence the equilibrium between the encapsulated keto and enol forms. For complexes of smaller β-ketoesters, the coencapsulated solvent influences the equilibria. The solvent reduces the space available and affects the positioning of the ester in the capsul
Helical Folding of Alkanes in a Self-Assembled, Cylindrical Capsule
No full textThe reversible encapsulation of a series of normal alkane guests in a cylindrical host was studied
by NMR methods. For small hydrocarbons such as n-pentane or n-hexane, two guests enter the host, and
they move freely within. With n-heptane no encapsulation takes place. For longer alkanes such as n-decane,
a single guest enters and the aromatic walls of the host are seen to twist to avoid empty spaces and
increase favorable interactions with the hydrocarbon. The best guest (n-undecane) adopts a conformation
with minimal gauche interactions. The longest alkane accommodated, n-tetradecane, adopts a helical
conformation to fit in the cavity, a shape that maximizes CH/ð interactions with the aromatic walls of the
receptor. These reciprocal conformational changes are discussed in terms of optimal host/guest interaction
Coencapsulation of Large and Small Hydrocarbons
No full textCombinations of small, gaseous hydrocarbons and sizable aromatics, e.g. methane and anthracene, are directly observed in a cylindrical capsule by NMR
Encapsulation Induces Helical Folding of Alkanes
No full textThumbnail image of graphical abstract
Doing the twist: Incorporation of long hydrocarbons during the self-assembly of a cylindrical capsule containing an elongated cavity lined with eight benzene rings and additional π surfaces induces helical conformations of the chains so that they are fully encapsulated within the cavity (see picture)
Isotope Effects on Non-Covalent Host-Guest Interactions in a Cylindrical Molecular Capsule
No full textAsymmetric Environments in Encapsulation Complexes
No full textSymmetrical, self-assembled capsules capable of surrounding two guests offer a new approach to enantioselection through coencapsulation: when one guest is chiral, the space remaining is also chiral. This notion is explored within a cylindrical capsule. The dimensions of the capsule select appropriately sized combinations of guests, the shape of the capsule prevents tumbling of rigid molecules, and the chemical surface of the capsule orients polar functions within. Chiral carboxylic acids such as mandelic acid and α-Br-butyric acid are identified as promising compounds for this purpose, but diastereoselection is modest (<25% de)
Individual Solvent/Solute Interactions through Social Isomerism
No full textReversible coencapsulation of a solute molecule and a single solvent molecule takes place in solution at ambient temperature. Two isomeric complexes are formed (social isomers), and their relative energies are assessed by NMR methods. Intermolecular interactions between 3 aromatic solutes and 15 common solvents are evaluated
Isotopomer Encapsulation in a Cylindrical Molecular Capsule: A Probe for Understanding Noncovalent Isotope Effects on a Molecular Level
No full textIsotope effects for the encapsulation of deuterated versus nondeuterated guests are determined. This system involves the use of social isomers, and the origin of the isotope effect is localized to methyl groups interacting with aromatic rings
Tertiary Amide Rotation in a Nanoscale Host
No full textA self-assembled cylindrical capsule provides a nanoscale environment that affects the rotational barriers of tertiary amides. Measurements of the activation energies for the rotations and behaviors of the amides inside the capsule were determined by using 1H NMR spectroscopic methods in deuterated mesitylene solution. For amides 3-8, rotation rates can decrease or increase in the capsule by up to an order of magnitude from those of the free amides in solution depending on the structure of the amides. The acceleration/deceleration of the rotation results from selective destabilization/stabilization of the ground state or the transition state. In the case of compound 10, the rotation generates two isomers that are equimolar in solution but inside the capsule only one of them is observed. Accordingly, the rotation rate is slowed by several orders of magnitude inside the capsule. In the case of amide 8, a competition experiment indicates that the acceleration of the rotation inside the capsule is due to destabilization of the ground state. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007
- …
