DFT Study of Nitroxide Radicals. 1. Effects of solvent on structural and electronic characteristics of 4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl

摘要

Imidazoline-based nitroxide radicals are often used as spin probes for mediumacidity and polarity in different systems. In this work, using the densityfunctional theory (DFT) approach, we have studied how physico-chemicalcharacteristics (geometry, atomic charges and electron spin densitydistribution) of pH-sensitive spin label4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl (ATI) depend on protonationand aqueous surroundings. Our calculations demonstrate that ATI protonationshould occur at the nitrogen atom of the imidazoline ring rather than at theamino group. Protonation of ATI leads to a decrease in a spin density on thenitrogen atom of the nitroxide fragment >N-O. For simulation of ATI hydrationeffects, we have constructed a water shell around a spin label molecule bymeans of gradual (step-by-step) surrounding of ATI with water molecules (n =2-41). Calculated spin density on the nitrogen atom of the nitroxide fragmentincreased with an extension of a water shell around ATI. Both protonation andhydration of ATI caused certain changes in calculated geometric parameters(bond lengths and valence angles). Investigating how structural and energyparameters of a system ATI-(H2O)n depend on a number of surrounding watermolecules, we came to the conclusion that a hydrogen-bonded cluster of n≥ 41 water molecules could be considered as an appropriate model forsimulation of ATI hydration effects.