Tunneling splitting due to weak coupling between methyl rotators in acetylacetone

In acetylacetone the energy level matching spectrum of nuclear Zeeman and tunneling energies of CH₃ groups in the rotating frame of their proton spins demonstrates resonant transfers of population characteristic of a 100 KHz range tunneling splitting. However, in this material two A to E tunneling splittings, of 10.4 GHz and 1 GHz, were observed by inelastic neutron scattering. Therefore the small energy splitting must be a result of a weak torsion-torsion interaction, which has not been resolved in neutron spectroscopy. This small splitting is investigated through the dependencies of its energy level matching spectra on mixing time, tilt angle in the magnetic field, temperature, and the strength of the main dc field H₀. Upon heating, the level matching resonances begin to broaden at 10 K and average out to zero at 25 K. The spectrum does not depend on H⁰. These observations are explained by Zeeman-tunneling level matching transitions within manifolds of EEE-symmetry states and AEE-symmetry states, both of which are split by the interaction among three CH₃ groups. Based on resonances detected at v_z = nΔ, where n = 1/4, 1/3, 1/2, and 1, this splitting Δ is determined to be 116±2 KHz at 10 K.