Abstract Phytochemical investigation of Mimosa xanthocentra Mart. (Leguminosae: Mimosoideae), an herb comprising the diet of marsh deer and pampas deer in the Brazilian Pantanal, led to the isolation of flavones isovitexin-2″- O -α- l -rhamnopyranoside 1 and vitexin-2″- O -α- l -rhamnopyranoside 2 , and a mixture of flavonols avicularin 3a and reynoutrin 3b . At room temperature, compound 1 was observed in the 1 H and 13 C NMR spectra as a mixture of two rotamers in equilibrium in a DMSO- d 6 solution. Molecular modeling using MM2 calculations led to the first proposal of three-dimensional solution structure of both observed conformers. The energy barrier between both rotamers was evaluated theoretically by molecular modeling and experimentally by variable-temperature NMR studies and calculation of Δ G ‡ of rotation using Eyring equation. This is the first report on flavonoids from M. xanthocentra .