Speciation 98: Abstracts

The energies of different conformations of primary and secondary hydroxamic acids: -RCONR'OH; R = CH₃, C₂H₅; R₁ = CH₃ were calculated at the MP2 level using a 6 - 311++G* basis set which places both diffuse and polarization functions on atoms. The results show that for both aceto and N-methyl acetohydroxamic acids the most stable forms are the E anticlinal followed by the Z synperiplanar. Similar calculations were carried out on corresponding hydrates which resulted in preferential stabilization of the Z conformer. Finally, calculations were carried out using a spherical cavity approximation to simulate solvent effects thereby comparing dichloromethane and water as solvents. The E isomer was preferentially stabilized in CH₂Cl₂ whilst the Z isomer was the more stable conformer in water. Rotational barriers were calculated and compared with experimental (NMR) values.

Metal complexation utilizing the normal O,O mode of hydroxamate-metal coordination requires initial formation of the Z conformer and overcoming any E-rotational barrier. The effects of this barrier are also reflected in kinetic parameters obtained from the sequestration of iron by hydroxamic acids from the polynuclear iron complex, [Fe₁₁O₆(OH)₆(O₂CPh)₁₅]. It is hoped to extend these studies to -aminohydroxamic acids e.g. glycine hydroxamic acid which is capable of acting as both an O,O coordinator and an N,N coordinator to different metals.

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