Not all short C–H···O contacts are hydrogen bonds: the prototypical example of contacts to C[double bond, length half m-dash]O⁺–H

Abstract

Intermolecular contacts between C–H groups and the charged moiety X[double bond, length half m-dash]O⁺–H show a pronounced negative directionality, i.e. short contacts are associated with strongly bent angles; this indicates that these interactions do not represent hydrogen bonds, although they are occasionally short with H⋯O distances down to 2.5 Å.

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