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 of acetic-acid clusters are simulated by DFT calculations. ► Substituent R-effects of (R-COOH)2 on the intermolecular interactions are examined. ► A centrosymmetric dimer unit made of resonance-assisted hydrogen bonds (RAHBs). ► π-Electron delocalization cooperates with hydrogen-bonding interaction in RAHBs. ► Larger core-to-π∗ band-shifts of clusters support the superiority of π-delocalization.