Characterization of the t-Butyl Radical and Its Elusive Anion

The t-butyl radical and its anion are studied theoretically using state-of-the-art quantum mechanical methods including coupled cluster theory with full single, double, and triple excitations (CCSDT) and CCSDT with perturbative quadruple excitations [CCSDT(Q)], in concert with large correlation-consistent cc-pVXZ and aug-cc-pVXZ (X = D, T, Q, 5) basis sets. The relative energies are extrapolated to the complete basis set limit (CBS). The lowest energy structure of the t-butyl radical has a nonplanar carbon backbone with overall C3v symmetry. Low-lying C3h and Cs symmetry transition states, for pyramidal inversion and methyl group rotation, respectively, between equivalent C3v minima are investigated. The barriers for these interconversions are both less than 1 kcal mol(-1), but the corresponding barriers on the anion potential energy surface are more pronounced. Using the focal point analysis technique, we obtain a value of -0.48 kcal mol(-1) for the t-butyl radical adiabatic electron affinity at the CCSDT(Q)/CBS level of theory, where the negative sign indicates that the formation of the t-butyl anion is adiabatically unfavorable. We show that the electron affinity, whose sign has been the subject of debate, is very sensitive to both the basis set and the correlation treatment, and previous experimental and theoretical estimates of its value bracket the value computed herein. Our results indicate that the t-butyl anion is classically metastable with a vertical detachment energy of over 10 kcal mol(-1) to reach the neutral potential energy surface. However, the inclusion of the zero-point vibrational effects seems to favor its nonexistence.