Structure and vibrational assignment of the enol form of α-bromo-acetylacetone
- سال انتشار: 1388
- محل انتشار: دوازدهمین سمینار شیمی فیزیک ایران
- کد COI اختصاصی: ISPTC12_162
- زبان مقاله: انگلیسی
- تعداد مشاهده: 986
نویسندگان
Chemistry Department, Islamic Azad University, Shahroud Branch, Shahroud, Iran Chemistry
Department, Ferdowsi University of Mashhad, Mashhad, Iran
Chemistry Department, Alzahra University, Tehran, Iran
Department, Ferdowsi University of Mashhad, Mashhad, Iran
چکیده
It is well known that the cis-enol form of α-diketones is characterized by a strong intramolecular hydrogen bond, IHB, [1,2]. The simplest members of this class of compounds are malonaldehyde (MA) and acetylacetone (AA), which have been the subject of many theoretical and experimental studies [3,4]. This hydrogen bond formation leads to an enhancement of the resonance conjugation of the p-electrons, which causes a marked tendency forequalization of the bond orders of the valence bonds in the resulting six-member chelating ring. Therefore, it seems that any parameter that affects the electron density of the chelating ring will change the hydrogen bond strength. The strength of such a bridge decreases when an electron withdrawing group replaces the hydrogen atom in position α or replaces the hydrogen atoms of the methyl groups in position β [5,6]. The electron-withdrawin groups tend to reduce the hydrogen bond strength by increasing the positive charges on the carbonyl oxygen atom [3-6]. Finally, electron withdrawing groups that are able to conjugate with the enol double bond increase the stability of the enol and strengthen its hydrogen bonding [3,7]. The complexity of these substituents effects is apparent when the sequence of enol contents on going from Cl to Br and to I is considered. A chloro substituent usually increases the enol content, but this becomes smaller for Br and I because of the predominant steric effects. According to the NMR proton chemical shifts, the trend in IHB strength is BrAA > AA > α-chloroacetylacetone,ClAA [8]. Therefore, studying the vibrational spectra and theoretical structure of BrAA and comparing the structural and vibrational properties with those of AA and ClAA may explain the discrepancies.کلیدواژه ها
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