Kinetics for the Reactions of Ar + , O 2 + , and NO + with Isoprene (2-Methyl-1,3-butadiene) as a Function of Temperature (300-500 K).

Rate constants and product branching fractions were measured for reactions of Ar + , O 2 + , and NO + with isoprene (2-methyl-1,3-butadiene C 5 H 8 ) as a function of temperature. The rate constants are large (∼2 × 10 -9 cm 3 s -1 ) and increase with temperature, exceeding the ion-dipole/induced dipole capture rate. Adding a hard sphere term to the collision rate provides a more useful upper limit and predicts the positive temperature dependences. Previous kinetic energy-dependent rate constants show a similar trend. NO + reacts only by non-dissociative charge transfer. The more energetic O 2 + reaction has products formed through both non-dissociative and dissociative charge transfer, or possibly through an H atom transfer. The very energetic Ar + has essentially only dissociative products; assumption of statistical behavior in the dissociation reasonably reproduces the product branching fractions.