Effect of Intersystem Crossings on the Kinetics of Thermal Ion–Molecule Reactions: Ti⁺ + O₂, CO₂, and N₂O

Abstract

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.

Activation of Methane by Zr+: A Deep-Dive into the Potential Surface via Pressure- and Temperature-Dependent Kinetics with Statistical Modeling

“…The rate constant for collision with the helium buffer gas was determined from the Langevin rate constant of helium with the ionic complex (i.e., ∼[He­(cm –3 )] × 5.5 × 10 –10 s –1 ). The rate constant for an intersystem crossing was defined by an energy- and angular momentum-independent value, treated as an adjustable parameter; this approximation has been used previously to successfully model ion–molecule kinetics. ,− The reaction path terminated when a dissociation occurred or the intermediate was stabilized. Tunneling through transition state barriers was considered using an Eckart barrier treatment but found to be unimportant in the current system.…”

“…Incorporating ISC into the statistical analysist has been shown to work well for other systems. 12 This detailed fundamental understanding of the methane activation mechanism will provide additional value as a model system for other hydrocarbon reactions.…”

“…We show for the title reaction that we can also determine the degree that an association complex is activated and how fast the ISC occurs. Incorporating ISC into the statistical analysist has been shown to work well for other systems . This detailed fundamental understanding of the methane activation mechanism will provide additional value as a model system for other hydrocarbon reactions.…”

Kinetics of O3 with Ca+ and Its Higher Oxides CaOn+ (n = 1–3) and Updates to a Model of Meteoric Calcium in the Mesosphere and Lower Thermosphere

“…The stochastic statistical modeling method applied here, which is based on the work of Troe and co-workers, − has been described in detail elsewhere. , For each reaction, a reaction coordinate defined by only the stationary points is determined using quantum chemical calculations. The specific rate constants k ( E , J ), where E is internal energy and J is angular momentum, are calculated for each isomerization or dissociation along the reaction coordinate.…”

Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

“…Another interesting observation is that the last oxygen reload step has a near-zero activation barrier for an anion, 49,104 when using N 2 O, generally smaller (certainly not bigger) than neutral and cationic metal oxide units. 18,34,[108][109][110][111] For practical applications, it has to be seen if this holds true for O 2 . In addition, given the enhanced electron affinities of ligated metal oxide anions, more detailed studies on the electronic structure of the ground and more excited states are necessary for both ligated and diatomic metal oxides.…”