Journal of Chemical Crystallography, Vol. 31, No. 2, February 2001 ( C  2002) Molecular distortions in crystalline 2-phenylethynylbenzoic acid Lee D. Hall, (1) Guy Orpen, (1) * Melanie Pilkington, (2) and John. D. Wallis (2) * Received December 18, 2000 2-Phenylethynylbenzoic acid shows a short intramolecular 1,5 C O···spC contact with no significant in-plane distortion of bonding geometry at the sp C atom. However, there are angular distortions at the second sp C atom and the adjacent ipso C atom of the benzene ring, most likely because of optimization of the packing arrangement. Crystal data: C 15 H 10 O 2 , Mr = 222.23, a = 6.1890(10), b = 9.791(2), c = 18.849(4) ˚ A, β = 97.14(2) ◦ , mono- clinic, P2 1 /n, Z = 4. KEY WORDS: Intramolecular interactions; alkyne; angular distortion. Introduction Intramolecular interactions between electron-deficient alkyne groups and the oxygen atoms of nitro, methoxy, and carboxylic acid groups have been interpreted as representing the initial stages of nucleophilic attack on the triple bond. Thus, alkyne 1 shows a trans distortion about the triple bond in response to the close approach of a nitro oxygen atom (O···C: 2.642(2) ˚ A). 1 The triple bond is bent by 6.8(1) ◦ at each carbon atom in such a way that the carbon atom involved in the interaction deviates from the vector drawn between its two bonded neigh- boring atoms toward the oxygen atom. A similar effect is seen in 2-amino-2 ′ -nitrotolane (2), 2 which contains another 1,5 O···spC interaction. Analysis of the calculated total electron density for this molecule, according to Bader’s theory (1) School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom. (2) Department of Chemistry and Physics, The Nottingham Trent Uni- versity, Clifton Lane, Nottingham NG11 8NS, United Kingdom. * To whom correspondence should be addressed. of atoms in molecules, 3 indicates that there is a bond path between the nitro oxygen atom and the alkyne carbon atom. This may be interpreted as a weak attractive interaction, although Cioslowski has pointed out that such a feature can occur between sterically compressed groups too. 4 In the 8-methoxynaphthyl-1-ethyne derivatives 3–6, 5 there are 1,5 peri MeO···C C contacts in the range 2.593(2)–2.645(1) ˚ A, with trans bendings of the alkyne groups and angles at sp C atoms in the range 169.0(3)–175.4(2) ◦ . Structural studies on the dicarboxylic acid (7) 6 showed it to have an unusual solid-state conformation with a pair of symmetry related cis 1,5 spC···O C interactions of length 2.775(2) ˚ A. The triple bond deviates from linearity by 5.9(2) ◦ at each alkyne carbon atom. Furthermore, analysis of the total electron density derived from careful X-ray diffraction measurements indicated a bond path 7 between the O and C atoms involved in each short contact. 8 We now report the results of X-ray diffraction studies at 200 K on 8, the monocarboxylic acid analogue of 7, to determine the interaction of a single carboxylic acid group with the alkyne bond. 97 1074-1542/02/0200-0097/0 C  2002 Plenum Publishing Corporation