UNEXPECTED HYDROGEN-BONDING IN THE CRYSTAL-STRUCTURE OF (4-CHLOROPHENYL)PROPIOLIC ACID - ROLE OF C-H O HYDROGEN-BONDS IN DETERMINING O-H O NETWORKS

There is a very strong tendency for carboxylic acids to form centrosymmetric, hydrogen-bonded dimers in the solid state, and recent studies have attempted to use this dimeric building block in the crystal engineering of molecular solids. However, hydrogen-bonded molecules of (4-chlorophenyl)propiolic acid, p-ClC6H4C≡CCO2H, do not pack in the crystal according to the predicted dimer motif but, in an unusual manner for this category of molecule, in the catemer arrangement. Crystals of this compound are triclinic, space group [formula omitted], Z = 2, a = 6.120 (4) Å, b = 17.323 (13) Å, c = 3.944 (2) Å, α = 90.47 (6)°, β = 92.70 (5)°, γ = 102.26 (6)°, R = 0.065, Rw = 0.071, with 831 nonzero reflections. The catemer arrangement observed in this case might have been anticipated for some other carboxylic acids, but it is unexpected here and could arise due to the inability of the molecules to form C-H•••O hydrogen bonds. This study shows that weak C-H•••O hydrogen bonds need to be considered as important contributors in the formation of hydrogen-bond patterns and that their presence or absence could well determine the manner of networking of stronger O-H•••O hydrogen bonds in molecular crystals. Consequently, an ability to predict hydrogen bond patterns could be improved by a better understanding of C-H•••O interactions. © 1990, American Chemical Society. All rights reserved.