Preferential Formation of CºC-H...p(CºC) Interactions in the Solid State

This paper probes the propensity for terminal alkynes to form C=C-H ... pi interactions in the solid state, by rationalization of the structural properties of molecular solids in which C-C-H is the only recognized hydrogen bond donor group, but in which there are different possible pi systems that may serve as the acceptor for C=C-H ... pi interactions. These pi systems are the C=C-H group itself and the arene ring pi system. Thus, in the crystal structures of 1,4-diethynylbenzene (redetermined here) and 1,3,5-triethynylbenzene (reported for the first time here) there is competition between the C=C-H groups and the arene rings as accepters for C=C-H ... pi interactions. It is found that both structures contain chains of C=C-H ... pi(C=C) interactions-zig-zag chains in the case of 1,4-diethynylbenzene and helical chains in the case of 1,3,5-triethynylbenzene. Neither structure contains any C=C-H ... pi(arene) interactions. Cooperativity in the formation of the chains of C-C-H ... pi(C=C) interactions is strongly implicated (although not directly proven) by the results reported here. The conclusions derived here concerning the preferences for the formation of C-C-H ... pi interactions have important implications with regard to the recognition and utilization of weak intermolecular interactions in the structural design of molecular solids. (C) 1997 Academic Press.