Heats of Formation of t-Butyl Peroxy Radical and t-Butyl Diazyl Ion: RRKM vs SSACM Rate Theories in Systems with Kinetic and Competitive Shifts

Abstract

The dissociations of energy-selected di-t-butyl peroxide and di-t-butyl diazene ions have been studied by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. Di-t-butyl peroxide ions dissociate via two parallel channels: (1) methyl loss at a 0 K onset (E0) of 9.58 +/- 0.04 eV followed by a sequential dissociation of the daughter ion to produce C4H9O+ and acetone; and (2) the dominant dissociation channel, producing t-butyl ion and t-butyl peroxy radical at an E0 of 9.758 +/- 0.020 eV. Di-t-butyl diazene ions dissociate through three parallel channels: (1) a rearrangement to form isobutene ion; (2) C-N bond cleavage with the charge staying on the t-butyl diazyl species (E0 = 8.069 +/- 0.050 eV); and (3) C-N bond cleavage with the charge instead on the t-butyl (E0 = 8.122 +/- 0.050 eV); the coproduct for this latter channel is a weakly, or possibly unbound, N2...t-butyl structure. Both the peroxide and diazene ion dissociations produce metastable daughters, and the dissociation rates are modeled with two rate theories: the Rice-Ramsperger-Kassel-Marcus (RRKM) theory and a simplified version of the statistical adiabatic channel model (SSACM). Due to a large kinetic shift, RRKM incorrectly models the peroxide ion rate curve. Using SSACM, the heat of formation of t-butyl peroxy radical is determined to be DeltaH0Kdegrees = - 81.1 +/- 3.9 kJ mol-1, and, using B3LYP/6-311++G(d,p) thermal energy, DeltaH298Kdegrees = - 109.7 +/- 3.9 kJ mol-1. Due to a competitive shift of the higher energy channel onsets, RRKM also incorrectly models the diazene rate curves. The 298 K heat of formation of the t-butyl diazyl ion, which is bound by 14 kJ mol-1, is determined to be 701.2 +/- 5.9 kJ mol-1.