Comment on the Possible Presence of Bubbles Inside Self‐Assembled Molecular Cages

Michl, Josef

doi:10.1002/9783527632817.ch6

Cited by 1 publication

(2 citation statements)

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“…Even though the computations later turned out to be incorrect due to programming and input errors, they played a heuristic role by hinting at a possible explanation of the odd observed Eyring parameters. The value Δ S ⧧ = ∼60 cal·mol –1 ·K –1 roughly agreed with expectations for the excluded volume entropy of a spherical bubble with a volume equal to that estimated for the internal cavity in 3(3) , and encouraged us to propose that at equilibrium a solvent vapor bubble is present. , …”

Section: Discussionsupporting

confidence: 75%

“…First, we illustrate the challenges involved in cage structure determination by taking a closer look at several cage structures that we have assigned ourselves based on evidence that appeared reasonably convincing, and propose corrections. Second, we ask whether in solution, cages large enough to accommodate several solvent molecules as guests are actually filled with solvent and/or counterions or contain a solvent vapor bubble instead . In view of the small sizes of such cavities, at equilibrium a saturated solvent vapor bubble would often contain only very few solvent molecules or none, and would represent a void.…”

Section: Introductionmentioning

confidence: 99%

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Challenges in the Structure Determination of Self-Assembled Metallacages: What Do Cage Cavities Contain, Internal Vapor Bubbles or Solvent and/or Counterions?

Givelet¹,

Dron²,

Wen³

et al. 2016

J. Am. Chem. Soc.

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Proving the structures of charged metallacages obtained by metal ion coordination-driven solution self-assembly is challenging, and the common use of routine NMR spectroscopy and mass spectrometry is unreliable. Carefully determined diffusion coefficients from diffusion-ordered proton magnetic resonance (DOSY NMR) for six cages of widely differing sizes lead us to propose a structural reassignment of two molecular cages from a previously favored trimer to a pentamer or hexamer, and another from a trimer to a much higher oligomer, possibly an intriguing tetradecamer. In the former case, strong support for the reassignment to a larger cage is provided by an observation of a slow reversible transformation of the initially formed cage into a smaller but spectrally very similar one upon dilution. In the latter case, freeze-fracture transmission electron micrographs demonstrate that at least some of the solutions are colloidal, and high-resolution electron transmission and atomic force microscopy images are compatible with a tetradecamer but not a trimer. Comparison of solute partial molar volumes deduced from measurement of solution density with volumes anticipated from molecular models argues strongly against the presence of large voids (solvent vapor bubbles) in cages dissolved in nitromethane. The presence of bubbles was previously proposed in an attempt to account for the bilinear nature of the Eyring plot of the rate constant for pyridine ligand edge exchange reaction in one of the cages and for the unusual activation parameters in the high-temperature regime. An alternative interpretation is proposed now.

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Section: Discussionsupporting

confidence: 75%

Section: Introductionmentioning

confidence: 99%