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

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 C C stretching in opposite direction depending on the role the ethyne molecule has in a hydrogen bonded complex (Delta(nu) over tilde = +9 cm(-1) in trimethylphosphate complexes, Delta(nu) over tilde = -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 approximate to 0.5-1.0 mol(-1) dm(3) and -Delta H-r(circle minus) approximate to 6-8 kJ mol(-1), for phenol complexes K approximate to 20-40 mol(-1) dm(3) -Delta H-r(circle minus) approximate to 17-22 kJ mol(-1)) are in good agreement with literature data. (C) 2014 Elsevier B.V. All rights reserved.