Chemical absorption in amine aqueous solutions is a widespread technology for postcombustion carbon capture, and a large effort is ongoing to improve their performance. Characterization of the "reactant" and "product" solutions at the microscopic level is highly desirable for process optimization. Recently X-ray scattering experiments and "in situ" infrared spectroscopy have been applied to this aim, but a complete and convincing interpretation is missing. We present large-scale ab initio molecular dynamics simulations of monoethanolamine solutions at experimental concentration and temperature and analyze how structural and vibrational properties change after carbamate formation. An exhaustive account of the experimental data is obtained. Fingerprints of the reaction products and specific interactions are unravelled. Hydration effects are specific to each component of the solution and are essential for a correct assignment of the experimental data.