The role of hydrogens in stabilizing organic ions

The changes in electron density that result from processes in which ions are formed were examined using both electron density difference maps and calculations of electron populations using Bader's theory of atoms in molecules. The processes include the loss of an electron from a neutral precursor forming a radical cation, the loss of an electron from a free radical forming a carbocation, the addition of a proton to ammonia, the addition of an electron to a free radical forming a carbanion, and the loss of a proton from a neutral precursor forming an anion. In the reactions forming cations, the new positive charge resides mainly at the hydrogens, and the heavy atoms generally gain electron density. The anion-forming reactions lead to negative charge being shared among atoms but, again, much of the charge appears at the hydrogens. sigma-pi polarization is an important feature of the charge distribution in all of the ions. There is an advantage to placing the charge at the periphery of an ion since this will minimize its electrostatic energy.