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. Can. J. Chem. 64, 148 (1986). Two different crystalline complexes have been obtained from aqueous mixtures of glycylglycine (GlyGly) and methylmercury(II), and they were studied by vibrational spectroscopy and X-ray diffraction. In the first compound, a hydrogen atom of the protonated amino group of GlyGly is substituted by the CH3Hg+ cation, giving (CH3Hg)GlyGly: orthorhombic, P n~2~, a

(~-Ala). and (CH3Hg)(o~-Ala) were prepared by reacting CH3HgOH with glycine (HGly) and a1:nine (HAla). Crystals of (CH3Hg)(~~-Ala) are monoclinic, space group P 2 ' / c , u = 9.460(2), b = 8.794(2), c = 8.723(2) A, = 97.49(2)", Z = 4. The structure was refined on 935 MoKa reflections to R = 0.042. The complex results from displacement of an alanine NH3+ proton by the C H~H~+ ion, which is linearly bonded to the -NH2 group. An intramolecular Hg.. . O contact of 2.63 is also formed with a carboxylate oxygen. The Raman and infrared spectra of solid (CH3Hg)Gly and (CH3Hg)(~-Ala) are compared with those of the ligands. Raman spectra of aqueous solutions at different pH indicate that the NHz-bonded structure is retained in solution, although no complexation via the carboxylate occurs. MARIE-CLAUDE CORBEIL

Ultraviolet (UV)–visible and fluorescence spectra were obtained for complexes of ATR and TFA at different ratios and in four different solvents: hexane, chloroform, dichloromethane, and methanol. In the first three solvents, a large excess of TFA generates retinylic cations that absorb from 459 to 600 nm. Also, in CHCl3, Raman spectroscopy and fluorescence indicate that some aggregated species like ATR:(TFA)n, with λmax of ca. 470 nm, are present. In methanol, TFA protonates the solvent and it is CH3O+H2 which interacts with ATR so that only blue-shifted H-bonded ATR is present. From these results, it is shown that in the tautomeric equilibrium [Formula: see text], form (1) is always favored in the ground state whatever the solvent. In the excited state in hexane and in methanol, (1) is rapidly transformed into (2). In CH2Cl2 and CHCl3, this transformation is absent so that there is no energy dissipation, with the result that the retinal complexes become more unstable. Keywords: all-trans-retinal, fluorescence, H-bonds, trifluoroacetic acid, UV–vis spectroscopy.