Butyl cations and their gas-phase dissociation chemistry: Uniting experiments with theory

The results of extensive mass spectrometric experiments have been interpreted with the aid of recent high-level ab initio theoretical calculations. The dissociative ionization of species containing formal n-C(4)H(9) and i-C(4)H(9) groups leads only to mixtures of s-C(4)H(9)(+) and t-C(4)H(9)(+), irrespective of the precursors' internal energy. Experimental distinction between s-C4H9+ and t-C(4)H(9)(+) ions rests upon the much larger kinetic energy release observed when s-C(4)H(9)(+) are collisionally excited by O(2) to lose CH(3)(.) (500 meV) relative to t-C(4)H(9)(+) (90 meV). This distinction is lost when He is the target gas. Target gas behavior was shown to moat strongly depend on its polarizability, and it is proposed that, in contrast to vertical excitation in the case of collision with He, small polarizable targets allow structural change during the collisional excitation. The C(3)H(6)(.+) fragment ions produced from collisionally activated C(4)H(9)(+) ions only have the [propene](.+) structure. The C(3)H(5)(+) fragment ion resulting from the metastable loss of CH(4), a process common to all C(4)H(9)(+) isomers, has the 2-propenyl cation structure, not allyl as earlier concluded. Metastable C(4)H(9)(+) ions also lose C(2)H(4) yielding the nonclassical ethyl cation. The structure leading to this dissociation is proposed to be a proton-bridged tetramethylene ion, shown by theory to occupy a potential energy well.