Matrix isolation infrared spectra of O-H ⋯ π Hydrogen bonded complexes of Acetic acid and Trifluoroacetic acid with Benzene

Abstract

Mid infrared spectra of two O–H ⋯π hydrogen-bonded binary complexes of acetic acid (AA) and trifluoroacetic acid (F3AA) with benzene (Bz) have been measured by isolating the complexes in an argon matrix at ∼8 K. In a matrix isolation condition, the O–H stretching fundamentals (ν O−H) of the carboxylic acid groups of the two molecules are observed to have almost the same value. However, the spectral red-shifts of ν O−H bands of the two acids on complexation with Bz are largely different, 90 and 150 cm−1 for AA and F3AA, respectively. Thus, the O–H bond weakening of the two acids upon binding with Bz in a non-interacting environment follows the sequence of their ionic dissociation tendencies (p K a ) in aqueous media. Furthermore, Δν O−H of the latter complex is the largest among the known π-hydrogen bonded binary complexes of prototypical O–H donors reported so far with respect to Bz as acceptor. It is also observed that the spectral shifts (Δν O−H) of phenol-Bz and carboxylic acid-Bz complexes show similar dependence on the acidity factor (p K a). Electronic structure theory has been used to suggest suitable geometries of the complexes that are consistent with the measured IR spectral changes. Calculation at MP2 /6-311 ++G (d, p) level predicts a T-shaped geometry for both AA-Bz and F3AA-Bz complexes, and the corresponding binding energies are 3.0 and 4.5 kcal /mol, respectively. Natural Bond Orbital (NBO) analysis has been performed to correlate the observed spectral behavior of the complexes with the electronic structure parameters. The spectral red-shifts of the probe ν O-H bands of carboxylic acid-benzene π-hydrogen bonded complexes in an argon matrix were found to correlate with their respective aqueous phase acidities (pK a), and are explained in terms of local charge transfer effects.