Reaction pathways for the addition of ethylene, 1, to butadiene radical cation, 2, involving H-shifts have been investigated at the coupled cluster UCCSD(T)/DZP//UMP2(fc)/DZP + ZPE level of theory. Activation energies are relatively low for [1,2]- (10.0 kcal mol(-1), TS-4/20) and [1,5]-hydrogen shifts (7.7 kcal mol(-1), TS-4/26) but are relatively high for [1,4]- (33.8 kcal mol(-1), TS-4/14) and [1,3]-H shifts (e.g. 42.2 kcal mol(-1), TS-12/13; 57.2 kcal mol(-1); TS-16/21). Several rearrangement reactions have been found to occur below the energy limit of separated 1 + 2. The cyclopentenyl cation, [C5H7](+), 18, experimentally observed as reaction product of the butadiene radical cation, 2, and ethylene, 1, in the gas phase may origin from various reaction pathways. The following reaction sequence has been identified as the lowest in energy path from 1 + 2 to 18 with all relative energies (Delta E degrees) of transition structures below that of 1 + 2: (a) ethylene adds to the butadiene radical cation to form an open-chain distonic intermediate, 4, that undergoes a [1,5]-H shift to the 1,4-hexadiene radical cation, 26; (b) intramolecular [2 + 1] cycloaddition to methyl-cyclopenta-1,3-diyl intermediates, 22 and 24, which can interconvert through a bicyclo[2.1.0]pentane radical cation, 23; (c) [1,2]-H shift of 24 to the 3-methyl cyclopentene radical cation, 16; (d) methyl radical loss to give cyclopenten-3-yl cation, 18. Along this reaction pathway, Delta H-298 changes by -18.1 kcal mol(-1) (Delta G(298) by -16.0 kcal mol(-1)) and only transition structures low in energy (Delta H-298 is below that of 1 + 2; max. Delta G(298)double dagger = 10.4 kcal mol(-1) for [1,5]-H shift relative to 1 + 2) are involved. Ethylene, 1, can also add to 2, simultaneously accepting a transferred hydrogen to give a 1,3-hexadiene radical cation. Back dissociation of the latter into 1 + 2 is favored over methyl radical loss.