DFT Calculations of Some Important Radicals Used in the Nitroxide-Mediated Polymerization and Their HOMO‒LUMO, Natural Bond Orbital, and Molecular Electrostatic Potential Comparative Analysis

Nitroxide radicals are important organic compounds, the stability of which is governed by a degree of delocalization of an unpaired electron and steric hindrances in their molecules. The most stable radicals can be identified by theoretical methods. The B3PW91/6-31G + (d, p) density functional theory calculation for the 2,2,5,5-tetramethylpyrrolidine-N-oxyl nitroxide, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, (2,2,8,8-tetramethyl-4-oxoazocan-1-yl)oxidanyl, 1,1,3,3-tetramethylisoindolin-N-oxyl nitroxide, 2,2,10,10-tetraethyl-isoindolin-N-oxyl, di-tert-buthyl nitroxide, 2,2,5,5-tetramethyl-4-phenyl-3-azahexane-N-oxyl, and N-(2-methylpropyl)-N-(1-diethyl-phosphono-2,2-dimethylpropyl)-N-oxyl nitroxide radicals has been made. The FTIR and UV–Vis vibrational frequencies have been calculated by the same method. It is shown that the shortest N‒O bond length (1.2684 Å) is observed in the 1,1,3,3-tetramethylisoindolin-N-oxyl nitroxide radical. It has been found that the most stable of the investigated radicals is 2,2,10,10-tetraethyl-isoindolin-N-oxyl, which is indicated by the low EHOMO‒LUMO value for this substance. For all the examined nitroxides, the electron density, electrostatic potential surfaces, and natural bond orbital data have been obtained. The natural bond orbital analysis has confirmed the electron density delocalization within the nitroxide radicals.

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