Isomerization and dissociation in competition. The pentene ion story

Various isomeric C5H10 inverted left perpendicular (.+) ions are prepared by photoelectron photoion coincidence (PEPICO) in selected energies above their dissociation limit. All ions dissociate by losing either C2H4 or CH3.. The PEPICO time of flight distributions of the product ions provide information about translational energy release as well as the dissociation rates in the range of 10(4) to 2 x 10(6) s(-1). An unusual aspect of these reactions is that a number of the isomers dissociate via two-component decay kinetics. The fast component is faster than we can measure by PEPICO. The rate constant associated with the slow component is the same for all five isomers investigated. The results are analyzed in terms of a potential energy surface in which ions can dissociate directly or isomerize to the more stable branched C5H10 inverted left perpendicular (.+) isomers. Once they fall into the lowest energy wells, they can dissociate only via a slow component. The potential energy surface involves two dissociation and four isomerization rates. From the measured slow dissociation rate and various branching ratios (fast to slow, loss of C2H4 to CH3., etc.) it is possible to extract experimental values for all six rate constants which range from 10(5) to 10(11) s(-1). These rate constants as a function of the ion internal energy are then fitted with the RRKM theory. The three density of states associated with the three stable isomer wells are calculated with vibrational frequencies obtained from ab initio molecular orbital calculations. It is significant that the RRKM calculations are made with only six adjustable parameters. Since each k(E) curve requires two parameters [slope and magnitude], the ability to fit each rate constant with only one parameter is strong evidence for the validity of the proposed potential energy surface.