Effect of Intersystem Crossings on the Kinetics of Thermal Ion-Molecule Reactions: Ti + + O 2 , CO 2 , and N 2 O.

A selected-ion flow tube apparatus has been used to measure rate constants and product branching fractions of 2 Ti + reacting with O 2 , CO 2 , and N 2 O over the range of 200-600 K. Ti + + O 2 proceeds at near the Langevin capture rate constant of 6-7 × 10 -10 cm 3 s -1 at all temperatures to yield 4 TiO + + O. Reactions initiated on doublet or quartet surfaces are formally spin-allowed; however, the 50% of reactions initiated on sextet surfaces must undergo an intersystem crossing (ISC). Statistical theory is used to calculate the energy and angular momentum dependences of the specific rate constants for the competing isomerization and dissociation channels. This acts as an internal clock on the lifetime to ISC, setting an upper limit on the order of τ ISC < 1e -11 s. 2 Ti + + CO 2 produces 4 TiO + + CO less efficiently, with a rate constant fit as 5.5 ± 1.3 × 10 -11 ( T /300) -1.1 ± 0.2 cm 3 s -1 . The reaction is formally spin-prohibited, and statistical modeling shows that ISC, not a submerged transition state, is rate-limiting, occurring with a lifetime on the order of 10 -7 s. Ti + + N 2 O proceeds at near the capture rate constant. In this case, both Ti + ON 2 and Ti + N 2 O entrance channel complexes are formed and can interconvert over a barrier. The main product is >90% TiO + + N 2 , and the remainder is TiN + + NO. Both channels need to undergo ISC to form ground-state products but TiO + can be formed in an excited state exothermically. Therefore, kinetic information is obtained only for the TiN + channel, where ISC occurs with a lifetime on the order of 10 -9 s. Statistical modeling indicates that the dipole-preferred Ti + ON 2 complex is formed in ∼80% of collisions, and this value is reproduced using a capture model based on the generic ion-dipole + quadrupole long-range potential.