syn- and anti-Isomers and Fermi Resonance of Acetic Acid in Acetonitrile Revealed by 2D IR.

Acetic acid exists as syn- and anti-isomers in a low-temperature rare-gas matrix. However, in aprotic acetonitrile, its IR spectrum in the C═O stretch region exhibits a multiple-band feature, which is not clearly understood by conventional FTIR spectroscopy. In this regard, two-dimensional infrared (2D IR) spectroscopy is applied to understand the origin more deeply. Polarization-dependent 2D IR studies reveal that the spectral shape is primarily influenced by the coexistence of syn- and anti-isomers, along with Fermi resonance between the C═O stretch and the CH3 rocking overtone in each isomer. Numerical 2D IR simulations using a local-mode Hamiltonian model, which incorporates Fermi resonance but does not assume either weak or strong coupling limits, accurately reproduce the experimental results. The simulations determine the relative populations of the syn- and anti-isomers at 22 °C to be 0.935 and 0.065, respectively. Additionally, the simulations quantify the Fermi coupling of -13.5 cm-1 for the syn-isomer and -10.5 cm-1 for the anti-isomer, as well as the energy levels of the associated vibrational states. A time-dependent 2D IR study of deuterated acetic acid suggests that the two isomeric forms do not exchange on the time scale of 10 ps. Besides, aggregation causes an additional peak at concentrations above 20 mM.