Authors : Al-Ameen Abubakar Mohammed

Volume/Issue : Volume 11 - 2026, Issue 4 - April

Google Scholar : https://tinyurl.com/yj77pmcp

Scribd : https://tinyurl.com/mwc8hj82

DOI : https://doi.org/10.38124/ijisrt/26apr538

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Abstract : The strength of the intramolecular hydrogen bond (IHB) in β-diketones is a critical determinant of their chemical behaviour and biological activity. Despite extensive Hammett studies on simple β-diketones, a quantitative linear free-energy relationship (LFER) analysis of asymmetrically multi-substituted dibenzoylmethane (DBM) derivatives – where both aryl rings bear substituents with differing electronic characters – remains unexplored. Twelve asymmetrically substituted DBM derivatives were synthesised via Claisen condensation. One aryl ring (Ring A) maintained a constant dimethoxy or trimethoxy pattern while the other (Ring B) was varied with electron-donating to strongly electronwithdrawing substituents. Compounds were characterised by ¹H and ¹³C NMR spectroscopy, and chemical shifts were correlated with Hammett substituent constants (σ) using linear regression. The ¹³C chemical shift of the Ring B carbonyl (δC=Oᴮ) exhibited an excellent local correlation with Ring B substituents (ρ = −4.54, R² = 0.764, p < 0.0002), validating Hammett additivity. The enolic proton shift (δOH) showed moderate correlation with overall substituent character (ρ = −0.33, R² = 0.483, p = 0.012). Critically, the Ring A carbonyl (δC=Oᴬ) showed no correlation with Ring A substituents but responded systematically to Ring B electronic variation, providing direct NMR evidence for long-range through-bond electronic communication across the β-diketone bridge. This work quantifies substituent effects on IHB strength in DBM derivatives and demonstrates the scaffold's capacity for asymmetric, sign-inverted long-range electronic communication, with direct implications for rational design of functional organic materials and bioactive compounds.

Keywords : β-Diketone; Dibenzoylmethane; Hammett Equation; Linear Free-Energy Relationship; Intramolecular Hydrogen Bond; Resonance-Assisted Hydrogen Bond.

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