Polarization-assisted hydrogen bonds

The hydrogen bonds (Scheme 15) in such compounds, of the type charge-assisted and also polarization-assisted HBs [92], are strong, reflecting in the distances between the donor and acceptor atoms. Scheme 16 shows a polarization-assisted hydrogen bond in a neutral phosphoramidate, for comparison. 

Scheme 15. Contribution of two factors in strengthening of hydrogen bonds (charge and polarization) in the proton-transfer compounds charge and polarization-assisted hydrogen bonds...

Scheme 16. A polarization-assisted hydrogen bond in a neutral phosphorus compound...

The hydrogen bond in a neutral phosphoramidate is of the type polarization-assisted hydrogen bond whereas, in the proton-transfer and phosphate compounds two factors help to strength of hydrogen bond polarization-assisted and charge-assisted. 

The nanotubes of 1, 3 and 4, all with apolar substituents, required more base (4, 2 and 2 equiv. per NDI, respectively) for complete dissociation than that needed for dissociation of 2 (1 equiv.), with a polar side chain. This may be a consequence of differences in solvation and/or creation of a more non-polar environment, which would raise the pKa in a manner that is reminiscent of carboxylic groups in enzyme active sites [28]. It is not clear whether removal of, on average, one proton per (COOH)2 link (which would still allow connection via a single, charge-assisted hydrogen bond) leads to the dissociation of the nanotubes or whether both protons need to be removed. In all cases, the nanotubes re-assembled when MSA was added to neutralise the base. 

An experimental and computational study describing a cascade transformation that breaks all three C-C bonds in a polarized alkyne moiety has been reported (Scheme 5)." Facile intermolecular Michael addition is followed by the relatively slow intramolecular steps. The slowest step corresponds to the 5-exo trig closure at the carbonyl-substituted alkene. This process is facilitated by the coupling of the intramolecular Michael addition with a concerted proton transfer along a resonance-assisted hydrogen bond path, which avoids the formation of an unfavourable zwitterionic intermediate. 

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