Ab Initio Study of Ullman's Nitroxide Biradicals. Exchange Coupling versus Structural Characteristics Analysis

Abstract

The exchange coupling of Ullman's nitroxide biradicals, bis[2,2'-( 1-oxy-3-oxido-4,4,5,5-tetramethyldihydro-lH-imidazolyl)] (1) and bis[2,2'-( l-oxy-4,4,5,5-tetramethyldihydro-1H-imidazolyl)] (2), has been determined by performing ab initio calculations followed by a specific configuration interaction on model compounds where the methyl groups have been substituted by hydrogen atoms, C1 and C2, respectively. The analysis of the dependence of the coupling on the conformation angle between each nitroxide moiety shows that even in the perpendicular structure the singlet state is found below the triplet. The calculated singlet-triplet gaps for the experimental value of the twist angle, -364 cm-l for C1 and -175 cm-' for C2, are in very good agreement with recent experimental results for 1 and 2.

Intramolecular Spin Alignment in Photomagnetic Molecular Devices: A Theoretical Study

“…In all calculations concerning iminonitroxide derivatives (Td 1 and Dd 1), methyl substituents (R 1 ) were replaced with hydrogen atoms. This simplification is widely applied [58,59] as it does not significantly affect either the electronic or the structural overall properties of the systems. No such structural simplification was applied when modeling verdazyl derivatives Td 2 and Dd 2.…”

Magnetic coupling in bis-nitronylnitroxide radicals: The role of aromatic bridges

“…These are indeed computationally more expensive than DFT-based approaches, but have the advantage of a more solid conceptual basis and, beside the singlet-triplet energy gap, allow the computation of the eigenfunctions of the two spin states. [22][23][24][25][26][27][28][29] It has also been found that the Malrieu's difference dedicated configuration interaction ͑DDCI͒, with its selective choice of single and double excited determinants to be included in the CI space, is the post-Hartree-Fock method providing the best agreement with experimental results. 29 Using the DDCI scheme, in a previous paper we have studied the singlet-triplet energy gap in polyene-spaced bisnitronyl nitroxide diradicals 30 with the aim of investigating, in systems with variable spin-spin distance and different delocalization, the effects of the localization onto the separate fragments of the diradical molecular orbitals ͑MOs͒ forming the basis set for the CI computations.…”

“…The basic theory underlying the magnetic exchange interaction in diradicals was formulated several decades ago 22,24,28,32 and has been recently reviewed. 10 Diradical systems are characterized by two unpaired electrons placed in nearly degenerate MOs g and u corresponding to the highest occupied molecular orbital ͑HOMO͒ and HOMO-1.…”

Density Functional and Multireference Perturbation Theory Calculations of Bis-nitronyl Nitroxide Biradical

“…8 Castell et al reported ab initio calculations of bis-nitronyl nitroxide where methyl groups were substituted by hydrogen atoms, showing that the open-shell singlet state is 1.04 kcal/mol lower in energy than the triplet state. 9 Recently, theoretical calculations by Ali et al showed that the dihedral angle between two nitroxide units was calculated to be 78 o , and the singlet-triplet (S-T) energy gap was 0.188 kcal/mol at the UB3LYP/6-311+G(d,p)//ROHF/6-31G(d,p) level.…”