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Intramolecular Hydrogen Bonding/Selfprotonation Processes Modulated by the Substituent Effect in Hydroxyl‐substituted Naphthoquinones
Author(s) -
Maldonado Teresita,
MartínezGonzález Eduardo,
Frontana Carlos
Publication year - 2016
Publication title -
electroanalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.574
H-Index - 128
eISSN - 1521-4109
pISSN - 1040-0397
DOI - 10.1002/elan.201600255
Subject(s) - intramolecular force , chemistry , substituent , deprotonation , hydrogen bond , radical , electrochemistry , photochemistry , adduct , molecule , ring (chemistry) , medicinal chemistry , stereochemistry , organic chemistry , ion , electrode
Abstract In this work, an electrochemical and spectroelectrochemical ESR study for a series of hydroxyl substituted 1,4‐naphthoquinones is presented. Results show that the electrochemical behaviour is dependent on the relative positions of these groups within the molecules. For compounds which have hydroxyl groups at the annellated benzene ring, intramolecular hydrogen bonding (a substituent field effect) determines the energy of reduction of the system. When hydroxyl functions are located at the C‐2 or C‐3 positions, a selfprotonation process occurs. The electrogenerated dianion or trianion radicals, derived from deprotonated quinones, show that intramolecular hydrogen bonding has a significant effect both in the spin density distribution and in the energy required for the formation of these radical species. The difference observed in the slope for Ep Ic vs. log v function for 2,3,5,8‐tetrahydroxy‐1,4‐naphthoquinone suggest that, for this compound, the proton transfer step does not occur as a single, but as a two‐step sequence, where an hydrogen bonded adduct could be present as a stable intermediate. These processes could help to explain the discrepancies observed earlier performing linear free energy relationships these compounds.