Reactions of $O2+$ with OH-isoprene adduct isomers: Exothermicity, product ions, and rate constants

The ion-molecule reaction $C5H8OH+O2+→C5H8OH++O2$ has been investigated in order to develop detection scheme for the OH-isoprene adduct radicals using the chemical ionization mass spectrometry (CIMS) method. Ab initio molecular orbital calculations have been employed to determine the structures and energies of the product ions from the charge transfer reactions of $O2+$ with the OH-isoprene adduct radicals. Geometry optimizations of the OH-isoprene adduct cations were performed with density function theory (DFT)-B3LYP calculations and the energies were computed with the single-point calculation using the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations [CCSD(T)]. At the CCSD(T)/6-311G^** level of theory with corrected zero-point-energy (ZPE), the ionization potentials of the four OH-isoprene adduct isomers with OH additions to C1- to C4-positions (isomers I to IV) are 6.9, 5.6, 6.0, and 7.1 eV, respectively. The ion-molecule collision rate constants between $O2+$ and the OH-isoprene adduct radicals were estimated using the average-dipole-orientation (ADO) theory, along with our calculated the permanent dipole moment and polarizability for the OH-isoprene adduct isomers. We also report direct measurements of the ion-molecule reaction rate constant using the CIMS method, which are in good agreement with the predicted values using the ADO theory.