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(mu)' similar or equal to 0.7 MHz [1]. 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(mu)' on the torsional angle gamma, but are strikingly different in their predictions of the torsional energetics. This difference is traced to a 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(mu)'\ 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.