Cite this: CrystEngComm, 2013, 15, 5403 Received 26th February 2013, Accepted 9th May 2013 H/F isosteric substitution to attest different equi- energetic molecular conformations in crystals3{ DOI: 10.1039/c3ce40697k www.rsc.org/crystengcomm Amol G. Dikundwar, a Ch. Venkateswarlu, b R. N. Chandrakala, b Srinivasan Chandrasekaran* b and Tayur N. Guru Row* a The sequential replacement of aromatic H-atoms by F-atoms in 1,6-bis(phenylcarbonate) hexa-2,4-diyne allows access to its possible iso-energetic ‘‘syn’’, ‘‘gauche’’ and ‘‘anti’’ conformations. The crystallization of organic molecules depends on several factors with the molecular conformation being the key component. With the X-ray crystallographic technique providing an unambiguous determination of 3D structure in the solid state, conformational analysis has become facile in molecular crystals. The specific conformation of a molecule in a crystal results from a delicate balance of intra and intermolecular forces such as dipole–dipole interactions, steric (van der Waals) interactions and hydrogen bonding. 1,2 While the interactions within a molecule ensure conformational locking, the intermolecular interactions provide tools for crystal engineering, exploiting conformational flexibility in the molecule. In the literature, there are several examples where different molecular conformations of a compound have been obtained by allowing the molecules to assemble in a different crystallization milieu. Such an ‘experimental conformational scan’ has been conventionally achieved by conformational polymorph- ism 3 and cocrystallization experiments. 4 The variability in the conformation of a molecule may also be brought about by decorating the surface of a molecule with substituents providing both electronic and steric influences. In this article, we explore the utilization of the isosteric (equi-volume) properties of a hydrogen atom and a fluorine atom in constituting different equi-energetic molecular conformations of the conformationally flexible mole- cule, 1,6-bis(phenylcarbonate) hexa-2,4-diyne, 1 (Scheme 1). The repertoire of a molecule in adopting a specific molecular conformation decides the efficiency of the intermolecular packing in the solid state. 5 The basic rules governing molecular packing indicate that molecules with a centre of symmetry prefer to acquire an inversion centre in the crystal lattice leading to crystal structures with centrosymmetric space groups. 6 In a centrosym- metric conformation with the molecular dipole moment being zero, the resulting crystal structure is nearly always centrosym- metric. 7 However, Brock and Dunitz 8 demonstrated that centro- symmetric molecules can even adopt noncentrosymmetric conformations and pack with a higher crystallographic symmetry at the expense of noncovalent intermolecular interactions. 9 When present in a non-centric conformation, molecules can arrange so as to cancel the intermolecular dipoles resulting in a centrosym- metric organization or they can even afford a polar crystal packing. 7 Interestingly, in the literature, there have been arguments that a particular conformation is inherent in a molecule, which is mainly determined by its covalent connectivity with the intermolecular interactions being just a consequence of the crystal packing. 10 On the contrary, there are several reports explaining the stabilization of unusual molecular conformations through molecular aggregation in the solid, liquid and gas states. 11 This issue takes prime importance in cases where the molecular conformation observed in the crystal structure deviates significantly from the ideal conformation calculated for the gas phase or in solution. 12 The non-availability of any strong H-bonding pairs makes 1 a suitable candidate to examine the effect of fluorination on the resulting molecular conformations in the solid state. 13 In its a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, Karnataka, India. E-mail: ssctng@sscu.iisc.ernet.in; Fax: +91-080-23601310; Tel: +91-080-22932796 b Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India. E-mail: scn@orgchem.iisc.ernet.in; Fax: +91-80-23600529; Tel: +91-80-22932404 3 In memory of Professor A. Srikrishna (1955–2013) { Electronic supplementary information (ESI) available: detailed synthetic proce- dures and characterization data for compounds 1–6; CIFs, ORTEPs and packing diagrams of compounds 1–6 and CSD search details. CCDC numbers 759062 and 922272–922275. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce40697k Scheme 1 Chemical structures of compounds 1–6 with a general scheme for their synthesis. CrystEngComm COMMUNICATION This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15, 5403–5406 | 5403 Published on 10 May 2013. Downloaded by Indian Institute of Science on 07/02/2014 07:27:12. View Article Online View Journal | View Issue