Methyl tunnelling in acetylacetone

Infrared Spectroscopy of the Intramolecular Hydrogen Bond in Acethylacetone: A Computational Approach

The intramolecular hydrogen bond in the enol-acethylacetone (ACAC) is investigated by performing reduced-dimensional quantum calculations. To analyze the shared proton vibrations, two sets of coordinates were employed: normal mode coordinates describing the motion in the vicinity of the most stable configuration, and internal coordinates accounting for the double minimum proton motion. It is proved that the extreme broadness of the OH-stretch band in ACAC is a consequence of the coexistence of two enol-ACAC structures: the global minimum and the transition state for rotation of the distal methyl group. Further, a ground-state tunneling splitting of 116 cm(-1) is found, and it is shown that the inclusion of the kinematic coupling is mandatory when treating large-amplitude proton motion. In the OH-stretch direction a splitting of 853 cm(-1) was predicted.

Bi-exponential nuclear spin-lattice relaxation in solid hexafluoroacetylacetone

The reorientation of CF3 rotors in solid hexafluoroacetylacetone has been studied using pulsed NMR techniques. The temperature dependence of both fluorine (19F) and hydrogen (1H) nuclear spin-lattice relaxation has been measured using saturation-recovery pulse sequences. Following saturation of one of the spin reservoirs, the evolution with time of the longitudinal magnetisations associated with both nuclei are observed to be strongly bi-exponential. A theory is presented for the spin-lattice relaxation which is mediated by the heteronuclear dipole-dipole interaction between 19F and 1H nuclei. The experimental results are in good quantitative agreement with this relaxation theory. The spectral density functions have been evaluated using the known molecular structure and they are consistent with the experimental data. From this analysis we conclude that the relaxation is dominated by an intramolecular 19F-1H dipolar interaction. The spectral density function J(0)(omega H-omega F) makes an important contribution to the relaxation theory with the result that the motional spectrum is conveniently sampled at the difference frequency between 1H and 19F Larmor precessions. Two physically distinguishable CF3 rotors have been identified in this investigation, their dynamics are characterised by activation energies of 7.5 +/- 0.4 and 13.3 +/- 0.8 kJ mol-1, respectively.