2-Ethynylpyridine dimers: IR spectroscopic and computational study Danijela Bakarić a, ⁎, Jens Spanget-Larsen b, ⁎ a Division of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička 54, 10001 Zagreb, Croatia b Department of Science and Environment, Roskilde University, Universitetsvej 1, P.O. Box 260, DK-4000 Roskilde, Denmark abstract article info Article history: Received 25 August 2017 Received in revised form 15 December 2017 Accepted 13 January 2018 Available online xxxx 2-ethynylpyridine (2-EP) presents a multifunctional system capable of participation in hydrogen-bonded com- plexes utilizing hydrogen bond donating (`C \\ H, Aryl−H) and hydrogen bond accepting functions (N-atom, C`C and pyridine π-systems). In this work, IR spectroscopy and theoretical calculations are used to study possi- ble 2-EP dimer structures as well as their distribution in an inert solvent such as tetrachloroethene. Experimen- tally, the `C \\ H stretching vibration of the 2-EP monomer absorbs close to 3300 cm −1 , whereas a broad band with maximum around 3215 cm −1 emerges as the concentration rises, indicating the formation of hydrogen- bonded complexes involving the `C \\ H moiety. The C`C stretching vibration of monomer 2-EP close to 2120 cm −1 is, using derivative spectroscopy, resolved from the signals of the dimer complexes with maximum around 2112 cm −1 . Quantum chemical calculations using the B3LYP + D3 model with counterpoise correction predict that the two most stable dimers are of the π-stacked variety, closely followed by dimers with intermolec- ular `C \\ H⋯N hydrogen bonding; the predicted red shifts of the `C \\ H stretching wavenumbers due to hydro- gen bonding are in the range 54–120 cm −1 . No species with obvious hydrogen bonding involving the C`C or pyridine π-systems as acceptors are predicted. Dimerization constant at 25 °C is estimated to be K 2 = 0.13 ± 0.01 mol −1 dm 3 . © 2018 Elsevier B.V. All rights reserved. Keywords: 2-ethynylpyridine Multifunctionality Hydrogen bonding IR spectroscopy Quantum chemical calculations 1. Introduction The simplest multifunctional molecule capable of forming weak hy- drogen bonds employing both hydrogen bond donating and accepting sites is ethynylbenzene (EB). With one hydrogen bond donating site (`C \\ H) and two hydrogen bond accepting sites (C`C and phenyl π systems), all of which are classified as rather weak, it can produce ver- satile interaction patterns. The resulting one is often highly unpredict- able and usually considered to be dictated by the distribution and strength of the available hydrogen bonding centers of the another pro- tagonist in a hydrogen-bonded complex [1–4]. In this context, the di- merization of EB attracted considerable interest [4–6]. The combination of IR–UV double resonance spectroscopy and high-level ab initio calculations demonstrated that the most stable EB⋯EB dimer is the one with an antiparallel π-stacked structure, i.e., the dimerization is primarily driven by π⋯π dispersion interactions between phenyl rings [5]. Furthermore, it is hypothesized to be the first π-stacked dimer without heteroatoms. The substitution of a particular C− or H–atom by a heteroatom in EB induces a change in the hydrogen bonding interaction pattern. For in- stance, the impact of H–atom substituents and their position on the overall disturbance of the hydrogen bonds interplay is demonstrated by the hydrogen-bonded complexes between fluoroethynylbenzenes and alcohols [7], and between ethynylbenzonitrile and water [8]. The impact of the replacement of a C–atom in the phenyl ring by a hetero- atom on the dimer formation is not reported so far. In this respect, 2- ethynylpyridine (2-EP) is an appropriate candidate. Unlike 3- and 4- ethynylpyridines, 2-EP is liquid at room temperature [9] implying that, as in EB case, it undergoes self-association but lacks higher order- ing at standard conditions. In addition, the N–atom is electronegative and is a stronger acceptor of the hydrogen bond than are the π systems on EB. The ability of 2-EP to make hydrogen-bonded complexes with compounds such as trimethyl phosphate and its analysis by vibrational spectroscopies is already reported [10,11]. We thus expect that the combination of IR spectroscopy and quantum chemical calculations is a promising strategy in the study of the formation of 2-EP dimers. In this work, the competition between different hydrogen bond do- nating and hydrogen bond accepting centers and the resulting dimer structures of 2-EP will be examined. IR spectroscopy is proven to be an indispensable tool in the exploration of the hydrogen-bonded sys- tems [12]. Even when weak hydrogen bonds are under study, it can dis- tinguish the signatures that originate due to free and hydrogen-bonded oscillators. As 2-EP can form multimers by employing simultaneously all hydrogen bond donating and hydrogen bond accepting sites, it is of im- portance to conduct the experiment in a concentration range in which only monomers and dimers can be assumed to exist. This strategy en- sures that the signals attributed to hydrogen-bonded oscillators are Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 195 (2018) 41–46 ⁎ Corresponding authors. E-mail addresses: dvojta@irb.hr (D. Bakarić), spanget@ruc.dk (J. Spanget-Larsen). https://doi.org/10.1016/j.saa.2018.01.046 1386-1425/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa