The low-frequency fundamentals together with the high-frequency modes, responsible for hydrogen bonding, are important for the understanding of the hydrogen bond strength of the hydrogen-bonded systems. Acetic acid forms several dimers under specific experimental conditions and the vibrational frequencies have been assigned to the low- and high-frequency modes of such dimers. The low-frequency and high-frequency modes of six acetic acid dimers were analyzed to correlate their intensities with the hydrogen-bond strengths/binding energies using density functional theory (DFT). Two different functionals (B3LYP and M06) were used for such studies using aug-cc-pVdz basis set of atoms and explicit anharmonicity corrections were incorporated. The cyclic structure of acetic acid was found to be the most stable one with the anharmonicity-corrected binding enthalpy of -15.3 kcal/mol. It is very close to experimental value of the cyclic dimer. The low-frequency in-plane bending modes and the corresponding intensities of these dimers linearly correlated with their hydrogen-bond/binding energies. It indicates the importance of such modes (together with the high-frequency O-H stretching modes) in the relative stabilities of such dimers.

Role of low-frequency modes in the relative stabilities of acetic acid dimers: A density functional study including anharmonicity effect

247th National Meeting (2014)

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