Deuterium isotope effect on 1H and 13C chemical shifts of intramolecularly hydrogen bonded perylenequinones
The primary and secondary deuterium isotope effects on proton and carbon chemical shifts were measured for a number of natural pigments and their derivatives, which contain the perylenequinone system and present a phenol–quinone tautomerism, cercosporin 1, isocercosporin 2, phleichrome 3, isophleichrome 4, cladochrome E 5, elsinochromes 6–8, hypocrellin 9, noranhydrocercosporin 10 and noranhydrophleichrome 11. Deuterium isotope effects on protons were also measured for 1,4-dihydroxyanthraquinone 12, methyl 4-(1,4-dihydroxy-9,10-dioxo-9,10-dihydroanthracen-2-yl)butanoate 13, N-acetyldaunomycin 15, daunomycinone 16, naphthazarin 17 and a number of intramolecularly hydrogen-bonded enols from β-diketones, β-ketoesters and o-hydroxyacyl aromatic compounds 19–24. The primary isotope effects Δδ(1H,2H) on OH proton shift and 1H chemical shifts of OH groups are correlated and can be used to estimate the strength of the hydrogen bonds in solution. The secondary isotope effects on proton and carbon nuclei are transmitted along the whole extended conjugated perylenequinone system. Long-range effects over eleven bonds and over seven bonds were observed in compounds 1–11 and in 12–18.
The perturbation of the equilibrium due to the presence of deuterium was considered and calculations were performed in order to evaluate the amount of this contribution to the isotope effect. The variation with temperature of primary and secondary effects, from 25 °C down to –70 °C, was studied for compounds 1, 3, 6, 12 and 17, and for acetylacetone 22 and benzoylacetone 23. Parameters from a number of X-ray analyses, for example the O · · · O distances (as suggested by Gilli et al., J. Am. Chem. Soc., 1991, 113, 4917), gave evidence of a substantial parallelism between the liquid and the solid phase. The strength of the hydrogen bond in perylenequinones depends on the planarity of the two naphthalene units rather than on the distortion of the polycyclic ring.