In the present work, molecular structures and intramolecular hydrogen bonding -IHB- of 3-methyl-4-amino-3-penten-2-one, 3-MeAPO, 3-methyl-4-methylamino-3-penten-2-one, 3-Me-MeAPO, and the 3-methyl-4-phenylamino-3-penten-2-one, 3-Me-PhAPO have been investigated by means of density functional theory -DFT- calculations, at the B3LYP/6-311++G** level, and experimental investigations including NMR, IR, Raman, UV spectroscopy, and X-ray crystallographic methods. The mentioned results were compared to those of 4-amino-3-penten-2-one, APO. DFT calculations suggest a relatively strong intramolecular hydrogen bond in 3-MeAPO and its NMe and N-Ph substituted derivatives with N⋯O distances of 2.597, 2.584, and 2.581 Å, and calculated hydrogen bond energies of about 10.9, 13.6, and 14.9 kcal/mol in 3-MeAPO, 3-Me-MeAPO, and 3-Me-PhAPO, respectively. While, the calculated N⋯O distances and hydrogen bond energies, of APO, 2.669 Å and 7.67 kcal/mol respectively, suggest a medium IHB strength. According to the NBO results, the steric effect is the dominant factor for changes of the IHBs. The calculated geometrical parameters of 3-MeAPO are in excellent agreement with the X-ray results. UV-Vis spectra of 3-MeAPO, 3-Me-PhAPO, and 3-Me-MeAPO were recorded in ethanol supporting the results mentioned above. According to the calculated results and deuterium isotope effects on 13C chemical shifts, 3-MeAPO exists as a keto-amine. Comparison of the observed and theoretical IR and Raman spectra and the spectral behavior upon deuteration were used for assignment of the vibrational spectra of 3-MeAPO. The assigned normal modes were compared to those of APO. The theoretical results were in good agreement with the experimental data. The vibrational frequencies also confirmed that the IHB strength in 3-MeAPO is stronger than that in APO.

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