Ring Inversion Dynamics of Encapsulated Cyclohexane

O'Leary, Brendan M.; Grotzfeld, and Robert M.; Rebek, Julius

doi:10.1021/ja9714898

Cited by 63 publications

(34 citation statements)

References 16 publications

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“…The other NMR spectra were recorded with a Varian 300 MHz. [D 12 ]Mesitylene was purchased from Cambridge Isotope Laboratories Inc. Andover, MA and was referenced to δ H = 2.11 (Ar-CH 3 ) with respect to the resonance of (CH 3 ) 4 Si. High resolution mass spectra were recorded with an Agilent ESI-TOF mass spectrometer.…”

Section: Methodsmentioning

confidence: 99%

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Tertiary Amide Rotation in a Nanoscale Host

et al. 2007

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A 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

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Section: Methodsmentioning

confidence: 99%

“…Ring inversion of cyclohexane hosted in a different capsule was studied, but only a small effect on the inversion rate of the cyclohexane ring was observed. [4] In the present study we investigate the rotational barriers of encapsulated tertiary amides and compare them with the barriers of the free molecules in solution.…”

Section: Introductionmentioning

confidence: 99%

Tertiary Amide Rotation in a Nanoscale Host

et al. 2007

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“…[6,7] The well-defined sizes and shapes of the cavities impose limitations on the motions of the guests, both internal, such as ring inversion [8,9] or external, such as molecular translation and tumbling. The symmetry of the capsule reduces the capacity to six bits from the eight expected.…”

Section: Methodsmentioning

confidence: 99%

“…[2] Metal-assembled bowlshaped molecules that are structural analogues of calixarenes, such as metallacalix [3]arenes, [3] metallacalix [4]arenes, [4] and others [5,6] have attracted considerable attention. [8] This pocket served as a reaction container for a highly stereoselective [2þ2] photodimerization of olefins. [8] This pocket served as a reaction container for a highly stereoselective [2þ2] photodimerization of olefins.…”

Section: Modular-assembled Bowls and Crownsmentioning

confidence: 99%

Isomeric Constellations of Encapsulation Complexes Store Information on the Nanometer Scale

Shivanyuk¹,

Rebek²

2003

Angew Chem Int Ed

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“…This value is unknown and, if calculations of protein interiors are any indication, it is likely to remain so. The partners are frozen into a close association that restricts diffusion and resists conformational changes [14] such as inversion/rotation and tumbling. [9] In conclusion, isolation of a guest can be accomplished in solution through formation of ªmolecule-within-moleculeº complexes, for example, with covalently bonded structures such as carcerands [10] and cryptophanes, [11] in reversibly formed capsules held together with hydrogen bonds, [12] or with metal ± ligand interactions.…”

Section: Isolation Of An Acid/base Complex In Solutionmentioning

confidence: 99%

Isolation of an Acid/Base Complex in Solution Puts the Brakes on Nitrogen Inversion

2001

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Acid/base interactions pervade chemistry and dominate molecular recognition in biology. The isolation of individual acid/base complexes can be achieved in the gas phase at low pressures [1] and in the solid or glassy states within inert matrices. In solution, rapid diffusion permits the frequent exchange of acid/base partners and the characterization of individual complexes requires the fastest of spectroscopic techniques. We have recently introduced a synthetic host that features an inwardly directed carboxy group on its concave surface, [2] and we apply it here to the study of isolated acid/ base interactions. The results augur well for the observation and characterization of short-lived intermediates in these receptacles.The effects are demonstrated with the amine base N-ethyl-N-methyl-N-isopropylamine [1, Eq. (1)]. This base features one of the simplest amines with a stereogenic nitrogen atom and its behavior has been closely examined in solution. [3,4] The nitrogen atom inverts rapidly at room temperature, a motion that can be frozen out at low temperatures. The activation barrier for inversion/rotationÐthe process that converts the molecule into its mirror imageÐis approximately 7.5 kcal mol À1 at 160 K in CBrF 3 . Extrapolated to room temperature, the rate of racemization is about 10 7 s À1 .The same amine shows starkly different behavior within the racemic receptacle molecule 2 (Figure 1). [2] The inversion/ rotation of 1 within 2 is slow on the NMR timescale at room temperature, and even at 323 K. A portion of the NMR spectrum of 1 in 2 is shown and highlights the consequences of amine complexation: large upfield shifts, emergence of coupling details, an overall expansion of the spectrum, and the doubling of its resonances. The last feature reflects the modest (about 2:1) diastereoselectivity of the racemic receptacle for the amine enantiomers.The guest exchange rates into and out of 2 are slow on the NMR timescale and separate signals are seen for free and bound guest. [5] Steric barriersÐmechanical bondsÐisolate the amine from the bulk solution and costly conformational changes (including the rupture of hydrogen bonds) are necessary to allow the passage of molecules into and out of the cavity. The rate of this process for 1, as measured by an exchange spectroscopy (EXSY) experiment, is 2 s À1 at room temperature and corresponds to a barrier of about 17 kcal mol À1 . However, EXSY experiments on the bound guest failed to show exchange between diastereomeric complexes, even at 323 K. Accordingly, the lowest energy path for interconversion of diastereomers of the complex involves dissociation to the free components in solution and then their recombination.What causes the large energetic barrier to the process of Equation (2): the interconversion of the diastereomeric complexes of 1 within 2? The attractive forces between the acid and base must contribute but they cannot be a large factor. The model (racemic) acid 3 was used as a model for estimating the effects of acid/base attraction. The carboxy groups ...

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Ring Inversion Dynamics of Encapsulated Cyclohexane

Cited by 63 publications

References 16 publications

Tertiary Amide Rotation in a Nanoscale Host

Tertiary Amide Rotation in a Nanoscale Host

Isomeric Constellations of Encapsulation Complexes Store Information on the Nanometer Scale

Isolation of an Acid/Base Complex in Solution Puts the Brakes on Nitrogen Inversion

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