Infrared-driven dynamics and scattering mechanisms of NO radicals with propane and butane: impacts of pseudo Jahn–Teller effects

Abstract

The topology of multidimensional potential energy surfaces defines the bimolecular collision outcomes of open–shell radicals with molecular partners. Understanding these surfaces is crucial for predicting the inelastic scattering and chemical transformations of increasingly complex radical–molecule collisions. To characterize the inelastic scattering mechanisms of nitric oxide (NO) radicals with large alkanes, we generated the collision complexes comprised of NO with propane or n-butane. The infrared action spectroscopy and infrared-driven dynamics of NO–propane and NO–(n-butane) collision complexes in the CH stretch region were recorded, while also comparing the results to the analogous experiments carried out for NO–CH₄ and NO–ethane. The infrared spectroscopy is analyzed using rovibrational simulations to characterize the transition bands and to determine the vibrational predissociation lifetimes of NO–propane and NO–(n-butane). Due to pseudo Jahn–Teller dynamics, the NO–propane and NO–(n-butane) decay mechanisms from IR activation appear similar to those for NO–ethane previously reported from this laboratory (J. P. Davis et al. Faraday Discuss., 2024, 251, 262–278). Furthermore, the NO (X²Π, v′′ = 0, J′′, F_n, Λ) product state distributions from NO–alkane fragmentation reveal a strong electron-spin polarization and a propensity for NO products to rotate in the plane of the π* molecular orbital, yielding mechanistic insights into the inelastic scattering outcomes. We hypothesize that a geometric phase may be present, impacting the relative population distributions, in addition to the accessible pathway timescales.