The muonium adduct to biacetyl – ab initio calculations and vibrational averaging

The room temperature muon‐electron hyperfine coupling constant of the 2‐muoxy‐3‐ketobut‐2‐yl radical formed by Mu addition to biacetyl (butane‐2,3‐dione) is the smallest thus far measured in any radical, with A\prime_μ\simeq 0.7 MHz (R.M. Macrae, C.J. Rhodes, K. Nishiyama and K. Nagamine, Chem. Phys. Lett, submitted). Calculations on this radical using the ROHF, UHF, and B/PW 91 formalisms are presented, with averaging performed over the C–O torsional mode using an expansion in free internal rotation eigenfunctions. UHF and B/PW 91 give similar predictions of the dependence of A\prime_μ on the torsional angle \gamma, but are strikingly different in their predictions of the torsional energetics. This difference is traced to spin contamination in the UHF wavefunction. UHF disagrees with both ROHF and B/PW 91 in its structural predictions, yielding excessive electron delocalisation into the central C–C bond. Vibrational corrections lead to a small |A\prime_μ| with a weak temperature‐dependence, and suggest that the radical may twist from the trans geometry of the parent compound into the energetically more favourable intramolecularly H‐bonded cis structure.