Computational study of organic radicals of technological interest in molecule-based magnetism

Abstract

A new magnetic crystal composed of an anthranol/anthroxyl unit has been synthesized and characterized.[1] The crystal, which is composed by anthroxyl radicals, appears to undergo a magnetic phase transition when it reaches a temperature of ca. 100 K. The anthranol/anthroxyl pairs found in the 100 K and 200 K structures (see Figure I) characterized by X-rays, show large differences in the position of the central hydrogen between the two oxygens in each pair. While in the structure at 100 K the hydrogen is bonded to the oxygen of the anthranol molecule, in the structure at 200 K the hydrogen seems to be delocalized between the two oxygens of the pair, thus forming two semi-radicals. Upon phase transition, a proton coupled electron transfer (PCET) reaction takes place.This work performs a computational study on the PCET reaction taking place inside the crystal as well as the topological changes leading the magnetic transition that occurs at ca. 100 K. The study of the reaction is carried out in the gas phase through different anthranol/anthroxyl pairs extracted from the X-Ray crystal data. A study of the energies, the topology and the differences between the anthranol/anthroxyl pairs in the gas phase, as well as a search of the transition state for a better understanding of the reaction in the crystal is carried out.The energy barrier for the proton coupled electron transfer (PCET) in gas phase has found to be 11.11 kcal mol-1. It is believed that accounting for the anthranol/anthroxyl solid state environment would lead to a decrease of this barrier. For a better characterization of the magnetic behavior of the system, a study of the distribution of spin density on both structures Figure I (a) and (b), is performed, in which the semi-radical nature of the 200 K structure can be observed. To carry out the magnetic analysis, different pairs of anthranol/anthroxyl dimers extracted from the two X-ray diffraction structures, at 100 K and 200 K, will be used. The magnetic coupling interaction of these dimers is calculated to be able to build the magnetic topology of the crystal. Once known, magnetic models are used to calculate the magnetic susceptibility of both structures and compare them with the value obtained experimentally. The results obtained suggest that the structure at 100 K behaves diamagnetically. At 200 K, however, a dynamic behavior is observed since the structure found experimentally could be a statistical average of a set of structures belonging to the reaction path. Finally, we have investigated whether electron-donor or withdrawing substituents can give rise to new anthranol/anthroxyl pairs with enhanced proton coupled electron transfer capabilities.