STRUCTURE, BARRIERS FOR INTERNAL-ROTATION, VIBRATIONAL FREQUENCIES, AND THERMODYNAMIC FUNCTIONS OF THE SEC-BUTYL RADICAL - AN ABINITIO STUDY

The potential energy surface of sec-butyl free radical was studied by ab initio molecular orbital techniques using the GAUSSIAN 86 system of programs at the UHF/6-31G* level of theory. Six conformational minima in three enantiomeric pairs, generated by combinations of rotations about single bonds and inversion at the radical site, were located by analytical methods. The less stable pairs of conformers are 0.45 and 1.03 kcal/mol higher in energy on the Born-Oppenheimer potential energy surface than are the minima. Barriers to interconversion of the equilibrium structures by rotation/inversion were located by coordinate driving. Vibrational frequencies for each of the conformers, as well as moments of inertia for overall and internal rotations, are reported. Calculated heat capacities, entropies, enthalpies, and free-energy functions are tabulated as a function of temperature for the Boltzmann-averaged equilibrium mixture. Adopting a reference value of ΔH°f,298 = 16.0 kcal/mol, ΔH°f,T, ΔG°f,T, and log Kf values as a function of temperature in the range 0-1500 K are reported. © 1990 American Chemical Society.