Effect of core substituents on the intramolecular exchange interaction in N,N′‐dioxy‐2,6‐diazaadamantane biradical: DFT studies

Abstract

Density-functional theory calculations of a series of organic biradicals on the basis of the N,N 0dioxy-2,6-diazaadamantane core with different substituents at carbon atoms adjacent to the nitroxyl groups have been performed by the UB3LYP/6-31111G(2d,2p) method. Using the breaking symmetry approach, the values of the exchange interaction parameter, J, between the radical centers are calculated. It is shown that the intramolecular exchange interaction for the most part is ferromagnetic in nature, but the J parameter gradually decreases, changing its sign to antiferromagnetic interaction for the last substituent in the following sequence:The calculations at the UHSEH1PBE/6-31111G (2d,2p) level with the most of substituents show nearly the same variation sequence for the J parameter. It is concluded that spin polarization effects in the diazaadamantane cage and a direct through-space antiferromagnetic exchange interaction between the nitroxyl groups are the main mechanisms contributing to the exchange interaction parameter value in the studied series of compounds. The exchange coupling constant, J, depends on the electronic effects and geometry of the substituents, as well as on their specific interactions with the nitroxyl radical groups. K E Y W O R D S broken symmetry approach, DFT, 2,6-diazaadamantane, exchange interaction, organic biradicals, open-shell systems | I N TR ODU C TI ONStable and persistent organic radicals have drawn much attention in recent years in connection with the design and preparation of purely organic magnetic materials exhibiting ferromagnetic, ferrimagnetic, or superparamagnetic properties. [1,2] Commonly, exchange interactions between freeradical centers which are present in this kind of materials cause these properties. First examples of purely organic ferromagnetic compounds were prepared in the early 1990s. [3,4] Since then, significant progress has been made in the area of purely organic magnets, including the report of dithiadiazolyl radicals showing remanent magnetization below 36 K. [5] So far, this is the highest Curie temperature recorded for organic radicals. An interesting case of combining the ferromagnetic exchange interaction between paramagnetic centers with second-order nonlinear optical responses is found theoretically in bis-TEMPO biradicals coupled through fluorescent protein chromophores. [6,7] Stable organic radicals are also of special interest in connection with the development of quantum computing [8][9][10] and molecular spintronics. [11] For example, information processing based on weakly exchange-coupled multispin systems is one of the promising directions in quantum computation technologies. [12] The design of stable multiradical organic materials, potentially applicable as quantum bits, can be carefully controlled by the use of modern quantum-chemical approaches. [13] Since Int J Quantum Chem. 2018;118:e25568.