2-Ethynylpyridine dimers: IR spectroscopic and computational study

2-ethynylpyridine (2-EP) presents a multifunctional system capable of participation in hydrogen-bonded complexes utilizing hydrogen bond donating (≡C−H, Aryl−H) and hydrogen bond accepting functions (N-atom, C≡C and pyridine pi-systems). In this work, IR spectroscopy and theoretical calculations are used to study possible 2-EP dimer structures as well as their distribution in an inert solvent such as tetrachloroethene. Experimentally, the ≡C–H stretching vibration of the 2-EPmonomer absorbs close to 3300 cm⁻¹, whereas a broad band withmaximum 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^–1is, 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 pi-stacked variety, closely followed by dimers with intermolecular ≡C–H···N hydrogen bonding; the predicted red shifts of the ≡C–H stretching wavenumbers due to hydrogen bonding are in the range 54 – 120 cm^–1. No species with obvious hydrogen bonding involving the C≡C or pyridine pi-systems as acceptors are predicted.  Dimerization constant at 25 °C is estimated to be K₂  =  0.13 ± 0.01 mol^–1 dm³.