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 obtain ed from the difference in gas-phase basicity (GB) of the neutral monoa mine, 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 kno wn basicity. The distance between charges and the Coulomb energy as a function of n can be accurately calculated; comparison to experimental ly measured values makes this an ideal system for evaluating the use o f proton transfer reactivity for obtaining electrostatic forces in mul tiply 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-diamino dodecane, this value is 19.7 kcal/mol. From estimates of the distance between charges obtained by molecular modeling, we calculate an averag e 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 f or small n-alkanes. These results show that the ion conformation is li near and that shielding the (CH2)(n) between two charges in the gas ph ase is negligible. We find the epsilon(r) does not significantly chang e with increasing distance between charges, providing experimental ver ification that the influence of a charge localized > 11 Angstrom away from the reaction center can be fully accounted for by the combined ef fects of Coulomb repulsion and intrinsic reactivity of that center.