The electrostatic potential surface (EPS) is calculated for molecular tweezers, clips, and bowls at different levels of theory (semiempirical AM1, ab initio HF/6-31G*, and density functional theory pBP/DN**). According to these calculations, the molecular electrostatic potential (MEP) on the concave side of the molecular tweezers and clips is suprisingly negative for hydrocarbons. This finding seems to be a general phenomenon in nonconjugated π-electron systems with concave-convex topology and it explains the receptor properties of the molecular tweezers and clips. Analogous calculations performed for the conjugated aromatic molecular bowls show different results. The DFT calculations predict that in these systems the more negative MEP lies on the concave side similar to the findings for the nonconjugated molecular tweezer-and clip-systems, whereas the AM1 calculation leads to the opposite result that the MEP is more negative on convex side of the bowl-systems.
Molecular tweezers and clips of type 1−3 substituted with OAc, OH, OCONHPh, OMe, OCH 2 COOR and OCH 2-CONHR groups in the central spacer units have been synthesized by modification, by standard methods, either of the known diacetoxy-substituted derivatives 1b, 2b and 3b, or of the correspondingly substituted bis-dienophiles 4b and 5b. The synthesis of the dimethoxy-diacetoxy-substituted tweezer 1d could be accomplished through pressure-induced repetitive Diels−Alder reactions between the bis-dienophile 4b and the newly prepared diene 6b and subsequent DDQ oxidation. The thermodynamic parameters (K a and ∆G) of complex formation between the new receptors and aromatic substrates such as DCNB 21, TCNB 22, TCNQ 24 and Kosower salt 25 and the maximum complexation-induced 1 H NMR shifts (∆δ max. ) were determined by 1 H NMR titration experiments. It was found that the presence of substituents OH, OAc and OCONHPh in the central spacer units of the tweezers and clips 1−3 favours complex formation, whereas that of the substituents OMe, OCH 2 COOR and OCH 2 -CONHR disfavours it. This finding can be explained in terms
A molecular clip made of two naphthalene side walls and bisphosphonate arms selectively binds to N‐alkylpyridinium ions in water. Through the unique potential of this clip to combine π–cation and electrostatic interactions with the hydrophobic effect, the host molecule is able to complex NAD+ (nicotinamide adenine dinucleotide; see picture) efficiently in aqueous solution.
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