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ABSTRACT:The molecular structure and intramolecular hydrogen bond energy of 32 conformers of 4-methylamino-3-penten-2-one were investigated at MP2 and B3LYP levels of theory using the standard 6 -31G** basis set and AIM analyses. Furthermore, calculations for all the possible conformations of 4-methylamino-3-penten-2-one in water solution were also carried out at B3LYP/6 -31G** level of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the ketoamine conformers of this compound are more stable than the other conformers (i.e., enolimine and ketoimine). This stability is mainly due to the formation of a strong NOH⅐⅐⅐O intramolecular hydrogen bond, which is assisted by -electrons resonance. Hydrogen bond energies for all conformers of 4-methylamino-3-penten-2-one were obtained from the related rotamers method. The nature of intramolecular hydrogen bond existing within 4-methylamino-3-penten-2-one has been investigated by means of the Bader theory of atoms in molecules, which is based on topological properties of the electron density. The results of these calculations support the results which obtained by related rotamers method.