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Spectral line surveys reveal rich molecular reservoirs in G331.512-0.103, a compact radio source in the center of an energetic molecular outflow. In this first work, we analyze the physical conditions of the source by means of CH₃OH and CH₃CN. The observations were performed with the APEX Telescope. Six different system configurations were defined to cover most of the band within (292-356) GHz; as a consequence, we detected a forest of lines toward the central core. A total of 70 lines of A/E-CH₃OH and A/E-CH₃CN were analyzed, including torsionally excited transitions of CH₃OH (ν_t=1). In a search for all the isotopologues, we identified transitions of ¹³CH₃OH. The physical conditions were derived considering collisional and radiative processes. We found common temperatures for each A and E symmetry of CH₃OH and CH₃CN; the derived column densities indicate an A/E equilibrated ratio for both tracers. The results reveal that CH₃CN and CH₃OH trace a hot and cold component with T_k∼141 K and T_k∼74 K, respectively. In agreement with previous ALMA observations, the models show that the emission region is compact ( 5′′ / . 5) with gas density n(H₂) = (0.7-1)x10⁷ cm^–3. The CH₃OH/CH₃CN abundance ratio and the evidences for prebiotic and complex organic molecules suggest a rich and active chemistry toward G331.512-0.103.