C,O ATOMIC MOTION ASSOCIATED WITH SOLID-STATE PROTON-TRANSFER IN ENOLIC 1,3-DIKETONES

Mean-square displacement amplitudes for pairs of atoms (calculated from diffraction-derived anisotropic displacement parameters) are discussed in connection with solid-state proton transfer taking place in enolic 1,3-diketones. In contrast with the conclusions drawn from room-temperature diffraction data, prototropic tautomerism implies the presence of C-0 bonds within the enolic ring, which change by approximately 10 pm in concert with the proton jumps. The expected effect on the DELTA(C,O) values is only noticed in the low-temperature X-ray data for benzoylacetone and agrees with the fast tautomeric equilibrium detected by solid-state C-13 NMR. Room-temperature structures of several 1,3-diketones, however, fail to reveal this motion, because the expected effect on DELTA(C,O) lies within the experimental error.