| Abstract |
Electron transfer controlled hydrogen bonding was studied for a series of nitrobenzene derivative radical anions, working as large guest anions, and substituted ureas, including dihomooxacalix[4]arene bidentate urea derivatives, in order to estimate binding constants (K-b) for the hydrogen-bonding process. Results showed enhanced K-b values for the interaction with phenyl-substituted bidentate urea, which is significantly larger than for the remaining compounds, e.g., in the case of 4-methoxynitrobenzene a 28-fold larger K-b value was obtained for the urea bearing a phenyl (K-b similar to 6888) vs tert-butyl (K-b similar to 247) moieties. The respective nucleophilic and electrophilic characters of the participant anion radical and urea hosts were parametrized with global and local electrodonating (omega(-)) and electroaccepting (omega(+)) powers, derived from DFT calculations. omega(-) data were useful for describing trends in structure-activity relationships when comparing nitrobenzene radical anions. However, omega(+) for the host urea structures lead to unreliable explanations of the experimental data. For the latter case, local descriptors omega(+)(k)(r) were estimated for the atoms within the urea region in the hosts [Sigma(k)omega(+)(k)(r)]. By compiling all the theoretical and experimental data, a K-b-predictive contour plot was built considering omega(-) for the studied anion radicals and Sigma(k)omega(+)(k)(r) which affords good estimations. |
