Abstract
Investigation on solvent-induced polymorphism in X-ray structures of 2-hydroxy-1,4-naphthoquinone (Lawsone) 1, is carried out. In protic methanol, 1 crystallizes in monoclinic space group P21/c (1a) comprising of 2D hydrogen bonded network via cyclic dimers. In aprotic solvent such as acetone on the other hand, 1 exhibits orthorhombic space group Pna 21 (1b) and emerges with 1D catemeric chain. Solvent-induced topological isomerism of cyclic dimers and helical catemeric chains arising from (i) bifurcated intra- and inter molecular hydrogen bondings viz. OH⋯OC interactions between C(2) hydroxyl and C(1), C(4) carbonyls, (ii) CH⋯O interactions viz. C(3)H⋯O(1)C(1) have been discussed. A signal for radical in 1 at g=2.0058 is signatured by EPR spectrum and it's oxime derivative viz. 2-hydroxy-4-naphthoquinone-1-oxime 2, in solid state shows biradical and monoradical formation with aggregation of dimer and monomer due to non-covalent hydrogen bonds. Zero field split parameters for 2 are estimated to be D=215G, Ex=13G, Ey=47G at 298K. A half field signal at 77K indicates triplet ground state. Frozen glass EPR of 2 resolves as regioregular dimeric–monomeric species showing hyperfine interactions with 1-oximino nitrogen in dimer A¯(14N)=15.5G].
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