OH overtone bands and potential energy curves of the hydrogen-bonded formic and acetic acids

The absorption spectra of formic and acetic acids and acetic acid-d in pure liquid and of their carbon tetrachloride solutions were measured in the 3850–15400 cm⁻¹ region. The higher overtone bands of the hydrogen-bonded OH stretching vibrational mode for the chain polymer of formic acid (in the pure liquid) and for the cyclic dimer of formic acid (in the carbon tetrachloride solution) or acetic acid were observed at 5450 ± 200, 6000 ± 150, 7850 ± 150, 9700 ± 200, and possibly 12000 ± 300 cm⁻¹ and at 5350 ± 200, 5900 ± 150, 7750 ± 150, 9500 ± 200, and 11300 ± 300 cm⁻¹, respectively, having broad half-widths (500–600 cm⁻¹) and submaxima as in the gaseous state. The 5450 and 7850 cm⁻¹ bands of the chain polymer and the 5350, 7750 and 11300 cm⁻¹ bands of the cyclic dimers were first observed by the present study. With the aid of the observed frequencies, we determined the asymmetrical or symmetrical double minimum potential energy curve with a Gaussian barrier of 0.82 ∼ 0.88 eV or 0.80 ∼ 0.86 eV, respectively.