Soft X-ray emission, absorption, and resonant inelastic scattering experiments have been conducted at the nitrogen K-edge of urea and its derivatives in aqueous solutions and compared with density functional theory and time-dependent density functional theory calculations. This comprehensive study provides detailed information on the occupied and unoccupied molecular orbitals of urea, thiourea, acetamide, dimethylurea and biuret at valence levels. By identifying the electronic transitions that contribute to the experimental spectral features the energy gap between the highest occupied and the lowest unoccupied molecular orbital of each molecule is determined. Moreover, a theoretical approach is introduced to simulate resonant inelastic X-ray scattering spectra by adding an extra electron to the lowest unoccupied molecular orbital mimicking the real initial state of the core electron absorption before the subsequent relaxation process.
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