Substituent R-effects on the core–electron excitation spectra of hydrogen-bonded carboxylic-acid (R–COOH) clusters: Comparison between acetic-acid and formic-acid clusters

Core-excitation spectra observed for small acetic-acid (AcA) clusters have been interpreted with density functional theory calculations. By comparison of O1s(CO/OH)→π∗(CO) bands between formic-acid (FA) and AcA clusters, larger band-shifts were identified in AcA clusters. Substituent R-effects on these bands are examined in terms of geometrical parameters and acid–base properties of constituent carboxylic-acids (R–COOH). Since small carboxylic-acid clusters comprise a centrosymmetric dimer-unit with resonance-assisted hydrogen-bonds (RAHBs), cooperative π-electron delocalization characterized by π-conjugated CO/C–O bond equalization plays an important role in intermolecular interactions. The larger band-shifts for AcA clusters result from increased π-electron delocalization in the constituent molecules relative to FA clusters.