Elucidating chain-length-dependent hydrogen-bond network reorganization and stimulated Raman response in aqueous carboxylic acids.

Understanding hydrogen-bond (HB) reorganization in aqueous carboxylic acids is essential for linking solvation structure to vibrational response. In this work, aqueous formic acid (FA), acetic acid (HAc), and propionic acid (PA) were studied by spontaneous Raman spectroscopy, excess Raman spectroscopy (ERS), two-dimensional correlation spectroscopy (2D-COS), density functional theory (DFT) calculations, and stimulated Raman measurements. The OH stretching region shows chain-length dependence in the extent and pathway of HB reorganization. FA exhibits continuous redistribution, HAc varies less over most compositions, whereas PA shows the largest transfer toward weakly HB and heterogeneous environments, with the weak/strong OH area ratio increasing from about 1.1 to about 7.0. ERS and 2D-COS show that these changes cannot be described by linear superposition or band attenuation, but arise from composition-dependent, asynchronous restructuring of the HB network. DFT analysis relates these trends to redistribution among hydration-dominated, partially hydrated, and acid-associated motifs, and indicates that alkyl-chain extension reduces molecular symmetry, increases conformational flexibility, broadens the distribution of hydration motifs, and enhances Raman activity in the CH stretching region. Under nanosecond stimulated Raman conditions, this structural evolution drives switching of the stimulated output from the OH region to the CH region. The CH response appears at acid fractions of about 0.4 in FA, 0.3 in HAc, and 0.2 in PA, and becomes dominant at about 0.7, 0.6, and 0.5, respectively. These results establish a chain-length-dependent structure-response relationship linking non-ideal solvation reorganization to stimulated vibrational gain redistribution across the OH and CH manifolds.