MEASUREMENT OF COULOMB ENERGY AND DIELECTRIC POLARIZABILITY OF GAS-PLATE DIPROTONATED DIAMINOALKANES

The coulomb energy and intrinsic dielectric polarizability (epsilon(r)) of a series of doubly protonated gas-phase diaminoalkanes ((H3N)-H-+(CH2)(n)NH3+, n = 7-10, 12) are reported. The Coulomb energy is obtained from the difference in gas-phase basicity (GB) of the neutral monoamine, CH3(CH2)(n)NH2, and the apparent gas-phase basicity (GB(app)) of the corresponding 1,n-diaminoalkane (M + H)(+) ion measured by proton transfer from (M + 2H)(2+) ions to neutral reference molecules of known basicity. The distance between charges and the Coulomb energy as a function of n can be accurately calculated; comparison to experimentally measured values makes this an ideal system for evaluating the use of proton transfer reactivity for obtaining electrostatic forces in multiply protonated ions. The (M + 2H)(2+) ion of 1,7-diaminoheptane, the smallest doubly protonated electrospray-generated ion ever reported, is found to have 32.0 kcal/mol of Coulomb energy; for the 1,12-diaminododecane, this value is 19.7 kcal/mol. From estimates of the distance between charges obtained by molecular modeling, we calculate an average epsilon(r) of 1.01 +/- 0.07 for these diaminoalkanes. This value is significantly below the bulk solution dielectric constant of 1.9-2.0 for small n-alkanes. These results show that the ion conformation is linear and that shielding the (CH2)(n) between two charges in the gas phase is negligible. We find the epsilon(r) does not significantly change with increasing distance between charges, providing experimental verification that the influence of a charge localized > 11 Angstrom away from the reaction center can be fully accounted for by the combined effects of Coulomb repulsion and intrinsic reactivity of that center.