Urea and methylurea dipole-bound anions
Résumé
We report experimental (Rydberg Electron Transfer) and theoretical results on dipole-bound anions of urea, deuterated urea, 1,1- and 1,3-dimethylureas and tetramethylurea. For the 1,1-dimethylurea and tetramethylurea molecules, which possess only one low-lying energy conformer, the experimental excess electron energies are found to be in good agreement with neutral structure quantum chemistry calculations and semi-empirical model calculations of the corresponding dipole-bound anions. For 1,3-dimethylurea two low-lying energy conformers (trans-trans and cis-trans) should contribute to the dipole-bound anion formation behaviour but cannot fully account for it. The large-amplitude motion, associated with the anti/syn conformations of urea molecules, has been studied by performing experiments on the deuterated species and by calculating the potential energy surfaces, for both the neutral and the dipole-bound anion, along the corresponding coordinate. Even if this low-frequency mode is very likely to be involved in the anomalous anion formation behaviour, a full understanding of this process could only come from a dynamic theory, beyond the Born-Oppenheimer approximation, of both the electron and large-amplitude motions which possess similar characteristic frequencies.