Guided ion-beam and theoretical studies of the reaction of Os+ (6D) with O2: Adiabatic and nonadiabatic behavior

Abstract

The kinetic-energy dependence of the Os+ + O2 reaction is examined using guided ion-beam mass spectrometry. The cross section for OsO+ formation from ground state Os+ (6D) is unusual, exhibiting two endothermic features. The kinetic energy dependence for OsO+ formation is analyzed to determine D0(Os +O) = 4.96 ± 0.02 eV, with the higher energy feature having a threshold 1.36 ± 0.11 eV higher in energy. This bond energy is roughly consistent with previous values determined by bracketing measurements. Formation of OsO2+ is also observed with a pressure dependent cross section, establishing that it is formed in an exothermic reaction of OsO + with O2. The nature of the bonding for OsO+ and OsO2+ is discussed and analyzed primarily using theoretical calculations at the B3LYP/def2-TZVPPD level of theory. The ground state of OsO+ is identified as either 6Σ+ or 4Π, with the latter favored once estimates of spin-orbit splitting are included. Bond energies for ground state OsO+ are calculated at this level as well as BHLYP, BLYP, BP86, and CCSD(T,full) levels along with using the Stuttgart-Dresden (SDD) and Hay-Wadt (HW+) basis sets on osmium with a 6-311+G(3df) basis on oxygen. BLYP and BP86 theoretical bond energies are higher than the experimental value, whereas B3LYP and CCSD(T,full) values are lower, and BHLYP values are much too low. Potential energy surfaces for the reaction of Os+ with O2 are also calculated at the B3LYP/def2-TZVPPD level of theory and reveal that ground state Os+ (6D) inserts into O2 by forming a Os+(O 2) (4B2) complex which can then couple with additional surfaces to form ground state OsO2+ ( 2B1). Several explanations for the unusual dual endothermic features are explored, with no unambiguous explanation being evident. As such, this heavy metal system provides a very interesting experimental phenomenon of both adiabatic and nonadiabatic behavior.