Chirality and spin density: Ab initio and density functional approaches

Abstract

The spin distribution in a stable nitroxide biradical that shows ferromagnetic interactions in the solid phase has been studied at three levels of theory: First, at the UHF level; then, including correlation effects in U M P~ calculations; and finally, the results are compared with the spin density obtained using the local density functional (LDF) approximation. It is shown that LDF spin densities are closer to U M P~ than to UHF predictions; the difference between the UHF and the ( U M P~, LDF) results points to a redistribution of the spin repartition between N and 0 due to electronic correlation. For planar conformations of the NO group, there is symmetric distribution ( D u ) of the spin density on the adamantane skeleton. For nonplanar nitroxides, the molecule is chiral ( C * ) , which results in a breakdown of the spin transmission on part of the adamantane cage. 0 1993 John Wiley & Sons, Inc.

Quantum chemical calculation of exchange interactions in supramolecularly arrangedN,N′-dioxy-2,6-diazaadamantane organic biradical

“…Earlier performed quantum‐chemical simulation of intermolecular interactions in the packing conditions of the TMAO crystal structure, using ab initio calculations by the UHF/4‐31G method, gave rather low positive J values . The exchange integrals for intermolecular spin–spin interactions were 0.700, 0.129, and 0.001 cm −1 in three spatial directions, theoretically confirming the possibility of the formation of three‐dimensional ferromagnetic networks in the TMAO crystal.…”

“…Table shows that calculations performed earlier in the relatively simple 4‐31G basis set poorly reproduce the experimental J value, which is determined from the experimental Curie–Weiss constant, Θ = 10 K, [2b] even at a rather high level of theory. The relatively large 6‐311++G(2d,2p) basis set very well reproduces the experimental data both in the case of the DFT method with the B3LYP functional and in the case of the CAS(2,2) method.…”

“…To reduce the processing time and to calculate the spin–spin interaction constants at a higher level of theory, the promesogenic substituents were replaced by methyl groups, since it is supposed that the cyclohexyl and methyl groups are very close by their chemical nature and by their influence to the nitroxyl radical group. It should be noted that the exchange interactions in N , N ′‐dioxy‐1,3,5,7‐tetramethyl‐2,6‐diazaadamantane (TMAO) have been studied earlier by quantum‐chemical methods . Since then, computational tools have been significantly improved, and significantly larger basis sets can be applied today as compared to the 4‐31G basis set used previously .…”

“…It should be noted that the exchange interactions in N , N ′‐dioxy‐1,3,5,7‐tetramethyl‐2,6‐diazaadamantane (TMAO) have been studied earlier by quantum‐chemical methods . Since then, computational tools have been significantly improved, and significantly larger basis sets can be applied today as compared to the 4‐31G basis set used previously . Using the density functional theory (DFT) method, geometry optimization of the molecular structures was performed in the Gaussian 09 program using the UB3LYP functional with a spin‐unrestricted determinant in the 6‐311++G(2d,2p) basis set.…”

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

“…It should be noted, that the indicated through‐bond paths are not equivalent in strength. As shown earlier, there are four W‐like planes that are favorable for the spin density distribution from the π‐orbital system of the nitroxyl groups to the central carbon atoms of the diazaadamantane cage and four unfavorable out‐of‐plane pathways. As a consequence, an excess of spin density is induced in one of the carbon atom where two favorable pathways from two different nitroxyl groups meet (C6 atom) and a relative lack of spin density in one of the carbon atoms where two unfavorable pathways meet (C4 atom in table).…”