The study of secondary effects in vibrational and hydrogen bonding properties of 2- and 3-ethynylpyridine and ethynylbenzene by IR spectroscopy

•If in hydrogen bond donor, ν̃ (CC) is lowered by 9 cm−1.•If in hydrogen bond acceptor, ν̃ (CC) is raised by 3 cm−1.•The calculated frequency shifts are also in opposite directions and are rationalised by the changes in the electronic charge distribution.

Weak hydrogen bonds formed by 2- and 3-ethynylpyridine and ethynylbenzene with trimethylphosphate and phenol were characterized by IR spectroscopy and DFT calculations (B3LYP/6-311++G(d, p)). The structure and stability of ethynylpyridines and ethynylbenzene in the gas phase and in the complexes with trimethylphosphate and phenol are discussed in terms of geometry and electronic charge redistribution. Anharmonic effects are taken into account when calculating vibrational wavenumbers of these systems what lead to partial improvement of agreement with experiment. The changes in the electronic charge distribution are behind the frequency shifts of the CC stretching in opposite direction depending on the role the ethyne molecule has in a hydrogen bonded complex (Δν̃=+9 cm−1 in trimethylphosphate complexes, Δν̃=-3 cm−1 in phenol complexes). The association constants were determined by keeping the concentrations of proton donors approximately constant and low enough to avoid self-association and the proton acceptors were present in excess. The values obtained for the association constants and enthalpy changes in C2Cl4 (for trimethylphosphate complexes K ≈ 0.5–1.0 mol−1 dm3 and -ΔrH⦵≈6–8kJmol-1, for phenol complexes K ≈ 20–40 mol−1 dm3-ΔrH⦵≈17–22kJmol-1) are in good agreement with literature data.

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