ABSTRACT: 4-Amino-3-penten-2-one (APO) is one of the interesting β-enaminones, involved in the N–H⋯O intramolecular hydrogen bond system assisted by resonance [1]. This special hydrogen bonding makes them to play the role of a good ligand to provide useful complexes. To identify the structure and stability of these complexes, the structure and intramolecular hydrogen bond strength of these ligands should be investigated. In this study, the molecular structure and vibrational spectra of 4-butylamino-3-penten-2-one (BPO) and 4-tert-butylamino-3-penten-2-one (tBPO) were investigated using density functional theory (DFT). The geometrical optimization and vibrational wavenumber calculations were performed at the B3LYP/6-311++G(d,p) level. The results were compared with the corresponding experimental infrared (IR) spectra. Fourier Transform IR spectra of BPO and tBPO were clearly assigned. The spectra were also compared with the simplest similar molecule, namely APO [2]. The hydrogen bond strength in all stable structures is estimated by considering the geometrical parameters obtained from the DFT calculations and also by calculating the hydrogen bond energies using Bader’s atoms in molecules (AIM) theory. According to our calculations, the N…O distance in BPO and tBPO is 2.664 and 2.666Å, respectively. It confirms that the hydrogen bond in these molecules are slightly stronger than that in APO (2.671 Å) [3]. The difference between the studied compounds and APO is the substitution of the –NH2 group by the –NHC4H9 group in APO. According to our study, two fac-tors are responsible for increasing the bond strength, i.e., steric effect and inductive effect. The results obtained from AIM and NBO calculations also confirm the above conclusion. Excellent correlations were achieved between geometrical pa-rameters (except N…O distance), AIM, NBO, γNH, νNH and δNH with the hydrogen bond strength, EHB. As an overall result, the hydrogen bond strength of the understudy molecules follows the below order: tBPO > BPO > APO.

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