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 01s(CO/OH)->pi*(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 carboxylicacids (R-COOH). Since small carboxylic-acid clusters comprise a centrosymmetric dimer-unit with resonance-assisted hydrogen-bonds (RAHBs), cooperative pi-electron delocalization characterized by pi-conjugated C=O/C-O bond equalization plays an important role in intermolecular interactions. The larger band-shifts for AcA clusters result from increased pi-electron delocalization in the constituent molecules relative to FA clusters. (C) 2012 Elsevier B.V. All rights reserved.