GAS-PHASE HETEROAROMATIC SUBSTITUTION .13. A QUANTITATIVE APPLICATION OF THE CURVE-CROSSING REACTIVITY MODEL TO HETEROAROMATIC SUBSTITUTION

Gas-phase kinetic data concerning the electrophilic attack of free, unsolvated cations, such as CH3+, C2H5+, i-C3H7+, t-C4H9+, C6H5+, CF3+, (CH3)2F+, (CH3)3Si+, CH3CO+, and C6H5CO+, toward simple five-membered heteroarenes have been embodied into Klopman's charge and frontier orbital control reactivity model. The origin of the deviations from a linear correlation between the positional selectivity of some of the above electrophiles and their SCF STO-3G calculated LUMO energy has been investigated. The influence of the computational level used for estimating the LUMO eigenvalues of the ionic reactants has been assessed by increasing the basis set dimensions from the STO-3G to the 6-31G*. Similar positional selectivity vs LUMO energy correlations have been obtained at both computational levels, revealing no significant difference at the two levels of theory employed. Direct evaluation of the activation parameters governing the gas-phase electrophilic attack on pyrrole by some representative ionic reactants, such as i-C3H7+, t-C4H9+, CF3+, and (CH3)2F+, has been derived from their Arrhenius plots, measured within the temperature interval 30-140-degrees-C. The curve-crossing reactivity model has been quantitatively applied to interpret the experimental kinetic results. A satisfying linear relationship between the intramolecular selectivity of the electrophiles considered and their G values has been obtained, the G parameter being a function of the vertical ionization potential IP(v) of the heteroarene and of the vertical electron affinity EA(v) of the electrophile. The influence of the G parameter in determining the nature of the transition states and the relative height of the activation barriers involved in the attack of the ionic electrophile on the alpha and beta-carbons of pyrroles has been discussed and compared with the effects of Klopman's HOMO (donor)-LUMO (acceptor) energy gap on the corresponding positional selectivity. A close relationship between the G parameter and the HOMO-LUMO gap, both reflecting the properties of the unperturbed donor-acceptor pair, has been verified.