NEUTRAL HYPERCONJUGATION CHALLENGED - NO SUPPORT FROM ONE-BOND CC COUPLING-CONSTANTS

A significant decrease in (1)J(C-ipso, CH2) coupling constants from a value typical of C(sp(2))-C(sp(3)) bonds (43.5 Hz or smaller) to the u nexpectedly low value of 36 Hz typical of C(sp(3))-C(sp(3)) bonds was reported by Lambert and Singer on passing from XC(6)H(4)CH(2)Sn(CH3)(3 ) to XC(6)H(4)CH(2)C(CH3)(3) (X = H, CH3, OCH3, NO2). This trend was i nvoked by these authors as the first evidence for neutral hyperconjuga tion in terms of coupling constants. However, the present measurements reveal that also in C6H5CH2C(CH3)(3) and p-O2NC6H4CH2C(CH3)(3) the (1 )J(C-ipso, CH2) coupling constant bears a value typical of this type o f bonding (43.8 and 42.6 Hz, respectively). Thus, at variance with Lam bert and Singer's observation, a slight increase in (1)J(CC) coupling constants occurs on passing from Sn to C derivatives. This contradicts not only their experimental data but also conclusions concerning hype rconjugation in the neutral ground state.