tert-Butyl peroxy radical: ground and first excited state energetics and fundamental frequencies

Alkylperoxy radicals (RO2˙) are key intermediates in combustion and atmospheric oxidation processes. As such, reliable detection and monitoring of these radicals can provide a wealth of information about the underlying chemistry. The tert-butyl peroxy radical is the archetypal tertiary peroxy radical, yet its vibrational spectroscopy is largely unexplored. To aid in future experimental investigations, we have performed high-level theoretical studies of the fundamental vibrational frequencies of the ground- and first excited states. A conformer search on both electronic surfaces reveals single minimum-energy structures. We predict an Ã2A' ← X[combining tilde]2A'' adiabatic excitation energy of 7738 cm-1via focal point analysis, approximating the CCSDT(Q)/CBS level of theory. This excitation energy agrees to within 17 cm-1 of the most accurate experimental measurement. We compute CCSD(T) fundamental vibrational frequencies via second-order vibrational perturbation theory (VPT2), using a hybrid force field in which the quadratic (cubic/quartic) force constants are evaluated with the ANO1 (ANO0) basis set. Anharmonic resonance polyads are treated with the VPT2 + K effective Hamiltonian approach. Among the predicted fundamental frequencies, the ground state O-O stretch, excited state O-O stretch, and excited state C-O-O bend fundamentals are predicted at 1138, 959, and 490 cm-1, respectively. Basis set sensitivity is found to be particularly great for the O-O stretches, similar to what has already been noted in smaller, unbranched peroxy radicals. Exempting these O-O stretches, agreement with the available experimental fundamentals is generally good (±10 cm-1).