This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys. Cite this: DOI: 10.1039/c3cp53399a Effect of aromatic ring fluorination on CHp interactions: rotational spectrum and structure of the fluorobenzeneacetylene weakly bound dimer† Nathan W. Ulrich, a Tabitha S. Songer, a Rebecca A. Peebles, a Sean A. Peebles,* a Nathan A. Seifert, b Cristo ´bal Pe ´rez b and Brooks H. Pate b The rotational spectra for the normal isotopic species and for six 13 C singly substituted isotopologues (in natural abundance) of the fluorobenzeneacetylene (C 6 H 5 FHCCH) weakly bound dimer have been measured in the 6.5–18.5 GHz region using chirped-pulse Fourier-transform microwave spectroscopy. The HCCH molecule interacts with the fluorobenzene via a CHp contact and is determined to lie almost over the center of, and approximately perpendicular to, the aromatic ring, with an Hp distance (perpendicular distance from the H atom to the ring plane) of around 2.492(47) Å; a slight tilt of HCCH towards the para carbon atom of the fluorobenzene is evident. Binding energies of this complex and related benzene and fluorobenzene dimers obtained from the pseudodiatomic approximation are compared and indicate that fluorobenzeneacetylene lies among the more weakly bound of the complexes exhibiting some type of CHp interaction. Introduction CHp interactions are among the weakest of hydrogen bonds, but their influence has been linked to many physical, chemical and biological phenomena. 1 This includes supramolecular chemistry, molecular conformations, 2,3 and even discrimination of enantio- mers (the selective formation of an optical isomer was first suggested in 1990 4 and reviewed more recently by Nishio). 2 A database study in 1998 concluded that CHp interactions are present in 35% of organic crystals and Hp distances tend to follow the increasing acidity of the CH group, 5 while the presence of these contacts in studies of proteins has suggested such interactions may play a role in overall protein stability. 6,7 Much computational work upon CHp interactions has focused on the importance of contributions from the various forces involved (electrostatic, dispersion, charge transfer) 8,9 and examined the effects of substituents on the aromatic ring, 10,11 while applica- tions of Bader’s atoms in molecules (AIM) theory appear to support the hydrogen bond like nature of this interaction. 12 The relatively acidic proton in acetylene makes this molecule a good probe for investigating CHp interactions, and the prototype system, benzene(BZ)HCCH has been studied extensively by theoretical and experimental techniques. 13–15 X-ray studies of the 1 : 1 cocrystal of benzene with acetylene 16 revealed a time- averaged position of each HCCH molecule between parallel benzene rings with its axis perpendicular to the aromatic planes. These HCCHaromatic interactions should be distinguished from the interactions between aromatic p systems and alkyl hydrogen atoms, since acetylene is often described as containing an ‘‘activated’’ CH bond, which is more acidic than those of, for instance, ethylene or methane (pK a values: 17 25 (acetylene), 45 (ethylene), 59 (methane)). The low pK a of acetylene indicates that the dominant forces in the interaction with benzene (or fluorobenzene) are likely to be electrostatic in nature, in contrast to aromatic complexes of the less acidic hydrocarbons such as ethylene and methane, where dispersion interactions are found to dominate. 15 The pK a of fluoroform (B25.5) 18 is very similar to that of acetylene, making for a potentially interesting comparison between CHp interactions in fluoroform and acetylene complexes. Experimental gas-phase studies of the rotational spectra of weakly bound clusters can provide information on the directionality of hydrogen bonds, which can be more difficult to classify from crystallographic data as a result of competing interactions with nearby fragments. In contrast to solid-state studies, gas-phase investigations of CHp interactions allow structural characteriza- tion unperturbed by solvent or crystal packing effects, providing benchmarks for comparison and classification of interactions in larger systems, such as proteins. However, relatively few benzene complexes with H donors have been explored by high resolution microwave spectroscopy: complexes of benzene with HF, 19 HCl, 20 HBr, 21 HCF 3 , 22,23 and with itself, 24,25 have all been a Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, IL 61920, USA. E-mail: sapeebles@eiu.edu; Tel: +1(217)581-2679 b Department of Chemistry and Biochemistry, University of Virginia, McCormick Road, PO Box 400319, Charlottesville, VA 22904, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53399a Received 9th August 2013, Accepted 9th September 2013 DOI: 10.1039/c3cp53399a www.rsc.org/pccp PCCP PAPER Published on 24 September 2013. Downloaded by University of Virginia on 25/09/2013 18:26:11. View Article Online View Journal