Calculated polar substituent effects on the stability of 3-substituted(X) bicyclo[1.1.1]pent-1-yl cations and 4-substituted(X) bicyclo[2.2.1]hept-1-yl cations: a DFT study

The effects of substituents on the stability of 3-substituted(X) bicyclo[1.1.1]pent-1-yl cations (3) and 4-substituted(X) bicyclo[2.2.1]hept-1-yl cations (4), for a set of substituents (X = H, NO₂, CN, NC, CF ₃, CHO, COOH, F, Cl, HO, NH₂, CH₃, SiH ₃, Si(CH₃)₃, Li, O^-, and NH ₃⁺) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6-311 + G(2d,p) and B3LYP/6-31 + G (d) levels of theory, respectively. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 3/4 and polar field/group electronegativity substituent constants (σ_F and σ_χ, respectively). The analysis reveals that the ΔE values for both systems are best described by a combination of both substituent constants. The result highlights the importance of the σ_χ dependency of charge delocalization in these systems.