Does tautomeric equilibrium exist in ortho-nitrosonaphthols? Galya Ivanova, Venelin Enchev * Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad G Bonchev str, 1113 So®a, Bulgaria Received 2 October 2000; in ®nal form 9 January 2001 Abstract The structure and conformational equilibrium of the monooximes of 1,2-naphthoquinone were studied by solid and liquid state NMR spectroscopy and non-empirical quantum-chemical calculations. According to the experimental data and the ab initio HF/6-31G** and MP4SDTQ)/6-31G**//6-31G** levels) calculations the compounds studied exist in the gas phase and in solution as oxime tautomers only. The relative stabilities of the above compounds in chloroform and dimethylsulfoxide solution are calculated within the polarizable continuum model. Solvent eects are found to change the relative stability of the syn- and anti-isomers of 1,2-naphthoquinone-2-oxime. The presence of syn- and anti- oxime isomers of 1,2-naphthoquinone-2-oxime and two rotameric forms of syn-1,2-naphthoquinone-1-oxime in solu- tion is proved by NMR spectroscopy. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: NMR; Solvent; Ab initio calculations; Oxime; Nitroso 1. Introduction The nitroso±oxime tautomerism of 1-nitroso- 2-naphthol and 2-nitroso-1-naphthol has been treated in several papers [1±8]. By means of UV [1], IR [2], and NMR [3±5] spectroscopy it has been shown that 1-nitroso-2-naphthol exists in solution in oxime form only. According to X-ray data [6] in the solid state the same compound exists as an syn- oxime tautomer. However, the existence of both tautomeric forms of 2-nitroso-1-naphthol in chlo- roform [4] and dimethylsulfoxide adapted with 5% dioxane or acetic acid [3,4] has been suggested, based on 1 H NMR data. To our knowledge there are only two theoretical studies on the nitroso± oxime tautomerism of two ortho isomers of the nitrosonaphthol [7,8]. Pilipenko and Savranskii [7] have reported that the p-electron energies of quinone monooxime tautomers of 1-nitroso-2- naphthol and 2-nitroso-1-naphthol, calculated by means of the semiempirical PPP method, are lower than those of their respective nitroso tautomers. A more systematic theoretical study concerning the tautomerism of ortho-nitrosonaphthols has been reported by Krzan et al. [8]. Using HF/6-31G ab initio calculations the authors have shown that the nitroso tautomers of 1-nitroso-2-naphthol and 2-nitroso-1-naphthol are more stable than the ox- ime forms by 0.5 and 0.01 kcal/mol, respectively. However, these results are in contradiction with the experimental data published before [1±6]. The aim of the present study is the reinvesti- gation of the structure and relative stabilities of the tautomeric forms of 1-nitroso-2-naphthol and 2- nitroso-1-naphthol in the gas phase, solution non- polar and polar solvents) and solid state by a Chemical Physics 264 2001) 235±244 www.elsevier.nl/locate/chemphys * Corresponding author. Fax: +359-2-700225. E-mail address: venelin@orgchm.bas.bg V. Enchev). 0301-0104/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0301-010401)00245-2