COMBINATION OF CP MAS NMR-SPECTROSCOPY AND X-RAY CRYSTALLOGRAPHY - STRUCTURE AND DYNAMICS IN MOLECULAR-CRYSTALS OF HYDROGEN, LITHIUM, SODIUM, RUBIDIUM, AND CESIUM PENICILLIN-V/

C-13 CP/MAS NMR spectra of the free acid of phenoxymethyl penicillin ( penicillin V) and of its lithium, sodium, rubidium, and cesium salts ( C16H17N2O5SM, M = H, Li . H2O, Na, Rb, and Cs) have been studied at a range of temperatures between 180 and 400 K. The C-13 CP/MAS NMR spect ra at ambient temperature indicate that the structures have one, two, one, four, and one molecule(s), respectively, in the crystallographic asymmetric unit. This is confirmed by the known crystal structure of t he free acid and the single-crystal X-ray diffraction studies reported here of the various salts. The variable-temperature C-13 CP/MAS NMR s pectra indicate that the phenyl rings of all the molecules perform 180 degrees flips about their local C-2 axes, with measured or extrapolat ed rate constants in the range 5 x 10(-12) to 8 x 10(9) s(-1) in the t emperature regime 200-380 K, and spanning 11 orders of magnitude at am bient temperature. The atomic displacement parameters of the X-ray str uctural model suggest that these rings also undergo significant in-pla ne librations. The Arrhenius activation parameters for the ring flip m otions have been determined, by analysis of exchange and dipolar broad ened NMR line shapes and with 1-D (and 2-D) magnetization transfer exp eriments, for the free acid and all four salts. The activation barrier s have been interpreted in terms of the degree of entrapment of the ph enyl groups and the nature of the packing interactions in the differen t crystal structures.