The strong aromatic hydrogen bonding in crystalline propargylammonium tetraphenylborate

Crystalline propargylammonium tetraphenylborate contains strong N+-H and moderate C=C-H hydrogen bond donors, but no conventional accepters. Therefore, the donors can only form T-type hydrogen bonds with the phenyl groups of the anion, leading to a dense system of aromatic hydrogen bonds which involves also the ethynyl groups. For the latter, this is the first documented case of C=C-H ... Ph hydrogen bonds in an ionic compound; the infrared absorption spectrum shows very similar characteristics as previously observed for C=C-H ... Ph interactions in uncharged systems. The distances of the donor H-atoms to the aromatic centroids are in the range 2.08-2.37 Angstrom for N+-H and 2.55-2.77 Angstrom for C=C-H donors. (C) 1997 Elsevier Science B.V.