Tautomeric and conformational equilibrium of acenaphthenequinonemonooxime V. Enchev a, * , G. Ivanova a , A. Ugrinov a , G.D. Neykov b a Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad G Bonchev Street, 1113 Sofia, Bulgaria b Institute for Foreign Students, 1111 Sofia, Bulgaria Received 4 November 1998; received in revised form 5 January 1999; accepted 5 January 1999 Abstract The tautomeric and conformational equilibrium of acenaphthenequinonemonooxime was studied by liquid and solid state NMR spectroscopy and ab initio calculations. The compound studied exists in the solid state as a oxime tautomer. However, in solution both the oxime and the nitroso tautomeric forms are present. Ab initio 6-31G(d,p) calculations in agreement with the experimental data predict the oxime tautomer to be more stable. The calculations in CCl 4 and DMSO solutions (SCRF model) showed a decrease in the tautomerization barrier with increasing solvent polarity. It is also found that the oxime tautomer exists as two isomers. The calculated energy difference between the isomers is small and decreases upon increasing the solvent polarity.  1999 Elsevier Science B.V. All rights reserved. Keywords: Tautomerism; Barriers; NMR; Solvent; Ab initio MO calculations 1. Introduction Several studies have been dedicated to the nitroso- oxime tautomerism of cyclic organic compounds [1– 10]. It was shown that for some of them (nitroso- phenol, nitrosonaphtol and their derivatives) the tautomeric equilibria depend on the solvent polarity [5–7]. The existence of syn- and anti-isomers of the oxime tautomeric forms has also been discussed [8– 10]. The monooximes of cyclic polyketones form complexes with the transition metals [10–18]. Although the wide use of acenaphthenequinonemo- nooxime (AQM) in complex formation and for spec- trophotometric determination of ruthenium, rhodium, osmium, uranium, platinum, palladium and cobalt [11–17], its structure and possibility for isomerism have not been investigated until now. In continuance of our work on the structure of symmetric monoox- imes of cyclic 1,2,3-triones [19] we have studied the AQM. We now report the results from our structural inves- tigation on the tautomeric and conformational equili- bria of AQM by means of quantum chemical ab initio calculations and NMR spectroscopy. We present the assignment and parameter determination of the 1 H and 13 C NMR spectra of AQM in CCl 4 and DMSO-d 6 solutions, as well as the results from the solid state 13 C NMR spectroscopy. 2D NMR techniques were generally applied for the purpose of the spectral assignment. The solvent effect was taken into account in ab initio calculations by the Onsager reaction field method. Journal of Molecular Structure 508 (1999) 149–161 0022-2860/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0022-2860(99)00008-3 * Corresponding author. Fax: +359-2-700225. E-mail address: venelin@srv.orgchm.bas.bg (V. Enchev)