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 (CH, Aryl-H) and hydrogen bond accepting functions (N-atom, CC and pyridine π-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 CH 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 CH moiety. The CC 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 intermolecular CH⋯N hydrogen bonding; the predicted red shifts of the CH stretching wavenumbers due to hydrogen bonding are in the range 54-120 cm^-1. No species with obvious hydrogen bonding involving the CC or pyridine π-systems as acceptors are predicted. Dimerization constant at 25 °C is estimated to be K₂ = 0.13 ± 0.01 mol^-1 dm³.