CAH...F hydrogen bonds as the organising force in F-substituted a-phenyl cinnamic acid aggregates studied by the combination of FTIR spectroscopy and computations B. Tolnai, J.T. Kiss, K. Felföldi, I. Pálinkó * Department of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged H-6720, Hungary article info Article history: Received 25 August 2008 Received in revised form 17 October 2008 Accepted 21 October 2008 Available online 31 October 2008 Keywords: F-substituted E-2,3-diphenyl propenoic acid Hydrogen-bonded network CAH...F hydrogen bonds IR spectroscopy Molecular modelling abstract Various F-substituted E-2,3-diphenyl propenoic acid molecules were synthesised and their aggregation behaviour was studied by experimental (FT-IR spectroscopy) and computational (semiempirical and DFT) methods. Experimental approach embraced the identification of potential hydrogen bonding sites through finding the relevant IR bands and monitoring their shifts upon increasing the acid concentration and on going to the solid state. It was found that fluorine engaged in CAH...F hydrogen bonding easily, where the carbon atom could be of any kind available in the molecule (aromatic, aliphatic or olefinic). Shifts were found even in moderately concentrated solutions and in the solid state too. Hydrogen bond- ing sites could be assigned and relevant aggregate models could be built. Molecular modelling allowed obtaining good estimates for hydrogen bond lengths and angles and visualisation of the geometric arrangements even of extended networks also became feasible. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction It has been found previously that short-range ordering prevails in a-phenyl cinnamic acid (E- or Z-2,3-diphenyl propenoic acid) solutions (the organising force is strong OAH...O hydrogen bonds), while weak (aromatic) CAH...O hydrogen bonds are responsible for long-range ordering in the solid state [1]. If substituents are present in the molecules that are capable to act as hydrogen bond donors and acceptors (the oxygen of the methoxy group on the aromatic rings [2] or the fluorine in the CF 3 group in position 3 substituting the olefinic hydrogen [3]), they take part in forming extended aggregates, but only in the solid state. In this contribu- tion the scope of investigations is further extended, experimental and computational results concerning the aggregate-forming prop- erties of E-a-phenyl cinnamic acid molecules having one or two fluorine substituents on the phenyl rings and in some cases meth- oxy substituent as well are communicated. 2. Experimental and computational methods 2.1. Materials Six molecules have been synthesized and used for exploring all possible hydrogen bonding interactions in solution and the solid state too. They are the E isomers of 2-phenyl-3-(4 0 -F-phenyl), 2- (4 0 -F-phenyl)-3-(4 0 F-phenyl), 2-(2 0 -methoxyphenyl)-3-(4 0 -F-phe- nyl), 2-(4 0 -methoxyphenyl)-3-(4 0 -F-phenyl), 2-(4 0 -F-phenyl)-3-(2 0 - methoxyphenyl), 2-(4 0 -F-phenyl)-3-(4 0 -methoxyphenyl) propenoic acid molecules. The structural formulas are given in Fig. 1. The compounds were prepared with a slightly modified version of the Perkin condensation reaction [4]. The general method of syn- thesis is given as follows: benzaldehyde (or its fluorine- and/or methoxy-substituted derivative) was refluxed for an hour with phenylacetic acid (or its fluorine- and/or methoxy-substituted derivative) in the presence of acetic anhydride and triethyl amine. The workup procedure involved hydrolysis as well as several crys- tallisations from dichloromethane. Isomeric purity was checked with NMR spectroscopy and the proportion of the E isomer was found to be over 99. 7%. 2.2. IR spectroscopy of solutions and the solid acids In the experimental part of the work FT-IR spectroscopy (BIO- RAD FTS 65/896 spectrophotometer, 4000–400 cm 1 range) was the tool of structural investigation. For the solution-phase studies CCl 4 or CHCl 3 was chosen as the solvent and the 10 1 – 10 4 mol/dm 3 concentration range was investigated. For measure- ments in the solid state the KBr technique was applied (1.2 mg of the compound in 200 mg KBr). The FT-IR spectra were taken on a BIORAD FTS-65A/896 spec- trometer equipped with DTGS detector. Resolution was 2 cm 1 and usually 256 scans were collected for a spectrum. Spectra were 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.10.030 * Corresponding author. Tel.: +36 62 544 288; fax: +36 62 544 200. E-mail address: palinko@chem.u-szeged.hu (I. Pálinkó). Journal of Molecular Structure 924–926 (2009) 27–31 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc