Molecular Tailoring and Prediction of Strongly Ferromagnetically Coupled Trimethylenemethane-Based Nitroxide Diradicals (original) (raw)
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The Journal of Physical Chemistry A, 2009
We predict extremely large and pos. intramol. magnetic exchange coupling consts. (J) for coupled diradicals constructed from nitronyl nitroxide (NN) and oxoverdazyl (o-VER). These radicals have the general formula o-VER(N)-nC-NN where nC represents an olefinic spacer with n = 0,2,4,6, and 8. Species like o-VER(C)-nC-NN have neg. coupling consts. The atoms in the parentheses show the point of attachment of the coupler to the verdazyl moiety. Both the N-linked series and C-linked series have comparable stability. The triplet mol. geometries were optimized by the d. functional (UB3LYP) method using the 6-311 g(d,p) basis set. This was followed by single-point UB3LYP calcns. using 6-311++g(3df,3pd) basis. To calc. J, single-point broken-symmetry computations were performed on the optimized triplet geometries and using the same basis set. The N-linked diradicals coupled through conjugated polyenes are topol. different. These are found to have coupling consts. of the order of 1000 cm-1, whereas the C-linked diradicals show coupling consts. of the order of -100 cm-1. In general, for both cases, the abs. magnitude of the coupling const. decreases with the increase in the length of the spacer.
Journal of Physical Chemistry A, 2005
The magnetic properties of the monoradicals 2-(4-phenyl acetylene)-4,4,5,5-tetramethyl-4,5-dihydro-1Himidozolyl-oxyl (1) and 2-(4-phenyl acetylene)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide (2) and the diradicals 2,2′-(1,2-ethynediyldi-4,1-phenylene)bis [4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyloxyl] (3), 2,2′-(1,2-ethynediyldi-4,1 3,1-phenylene)bis [4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyl-oxyl] (4), and 2,2′-(1,2-ethynediyldi-4,1 3,1-phenylene)bis[4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide] (5) are investigated by ab initio quantum chemical methods. The rule of spin alternation in the unrestricted Hartree-Fock (UHF) method clearly shows that the radical sites are antiferromagnetically coupled in 3 and ferromagnetically coupled in 4 and 5, which is consistent with a previous experiment. The molecular geometries are optimized at Hartree-Fock levels. This is followed by single-point calculations using the density functional (UB3LYP) treatment and the multiconfigurational complete active space self-consistent field (CASSCF) methodology. Magnetic exchange coupling constants are determined from the broken-symmetry approach. The calculated J values, -3.60 cm -1 for 3, 0.16 cm -1 for 4, and 0.67 cm -1 for 5, are in excellent agreement with the observed values. Because of the very large size of the diradicals 3-5, the CASSCF (10,10) calculations cannot yield realistic J values. Nevertheless, the CASSCF calculations support the antiferromagnetic nature of the magnetic coupling in 3 and the ferromagnetic nature of the coupling in 4 and 5. The existence of an intramolecular magnetic coupling in 3-5 is also confirmed through computations of the isotropic hyperfine coupling constants for monoradicals 1 and 2 as well as diradicals 3-5.
The Journal of Physical Chemistry A, 2013
Intramolecular magnetic exchange coupling constants are determined for seven isolated metaphenylene-based dinitroxide diradicals by unrestricted density functional methodology (UDFT) using a number of hybrid functionals such as B3LYP, B3LYP-D3, M06-2X, HSE, and LC-ωPBE. Geometry optimizations for both triplet and broken symmetry solutions are performed with the 6-311G(d,p) basis set for all the molecules. In all cases, B3LYP somewhat overestimates the coupling constant, and M06-2X produces a more realistic value. The range-separated HSE and LC-ωPBE functional yield large deviations from experiment. The nature of spin coupling agrees with the spin alternation rule and the calculated spin densities, in conjunction with the McConnell rule. It can also be explained in terms of the nondisjoint Single Occupied Molecular Orbital effect. Furthermore, it correlates with the calculated NICS(1) isotropic and zz and hyperfine coupling constants. We also put forward a method for the determination of the intramolecular (J) and intermolecular (J′) coupling constants from quantum chemical calculations on a one-dimensional chain of weakly bound diradicals. Two expressions are derived for the energies of different spin states in terms of J and J′. Exemplary UDFT computations are done on the N-mers (N = 2−6) of two diradicals for which the crystal coordinates are available. The intramolecular and intermolecular coupling constants are determined from the calculated UDFT energies. These are indeed in general agreement with the measured coupling constants.
Ferromagnetically Coupled S=1 Chains in Crystals of Verdazyl‐Nitronyl Nitroxide Diradicals
Angewandte Chemie International Edition, 2020
Thermally stable organic diradicals with a triplet ground state along with large singlet‐triplet energy gap have significant potential for advanced technological applications. A series of phenylene‐bridged diradicals with oxoverdazyl and nitronyl nitroxide units were synthesized via a palladium‐catalyzed cross‐coupling reaction of iodoverdazyls with a nitronyl nitroxide‐2‐ide gold(I) complex with high yields. The diradicals exhibit high stability and do not decompose in an inert atmosphere up to 180 °C. For the diradicals, both substantial AF (ΔEST≈−64 cm−1) and FM (ΔEST≥25 and 100 cm−1) intramolecular exchange interactions were observed. The sign of the exchange interaction is determined both by the bridging moiety (para‐ or meta‐phenylene) and by the type of oxoverdazyl block (C‐linked or N‐linked). Upon crystallization, diradicals with the triplet ground state form unique one‐dimensional exchange‐coupled chains with strong intra‐ and weak inter‐diradical ferromagnetic coupling.
The Journal of Organic Chemistry, 2003
A series of bis-iminonitroxide diradical derivatives of different lengths and geometry have been prepared that incorporate a conjugated phenylene-ethynylene bridge as a rigid spacer. This paper describes the synthesis of these new components and their main characterizations. An unexpected singlet ground state and substituent effects on the singlet-triplet gap have been found for substituted "m-phenylene"-based diradicals. The effects of the π-conjugation on the intramolecular through-bond spin coupling have been investigated by changing the length of the spacer within linear derivatives. The EPR studies demonstrate the intramolecular magnetic coupling between the radical spins within all compounds. This result is very attractive and unusual, given the large distance between the radicals from 15 Å in the dimer to 36 Å in the pentamer.
Theoretical Chemistry Accounts, 2014
We predict the magnetic exchange coupling constant (J) for 27 m-phenylene-based nitronyl nitroxide (NN) diradicals with nine different substituents in three unique (common ortho, ortho-meta and common meta) positions on the coupler unit by using the broken-symmetry density functional methodology. For all investigated diradicals, J values are computed using B3LYP, B3LYP-D3 and M06-2X functionals with 6-311?G(d,p) basis set. The J M06-2X value is larger than J B3LYP and closer to the observed value for the unsubstituted species. Substitutions at common ortho position always produce a greater angle of twist between the spin source and the coupler units. When the twist angle is very large, the nature of intramolecular magnetic interaction changes from ferromagnetic to antiferromagnetic. In these cases, the coupler-NN bond order becomes small. Substitution at the common meta position of m-phenylene in the diradical has little steric and hydrogen-bonding effects. Electron-withdrawing groups reveal a specific trend for single-atom substitution. An ortho substitution generally decreases J and a meta substitution always increases J with a decreasing-I effect. Variation of J with planarity as well as Hammett constant is investigated. The nucleus-independent chemical shift value is found to decrease from the corresponding monosubstituted phenyl derivatives. The dependence of J on these factors is explored. Keywords Diradical Á Magnetic coupling constant Á DFT Á Dihedral angle Á Inductive effect Á NICS(1)
C(sp 2 )-Coupled Nitronyl Nitroxide and Iminonitroxide Diradicals
Chemistry - A European Journal, 2014
Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences but the synthesis of stable high-spin organic molecules is still a challenge. We succeeded in synthesising heteroatom analogues of the 1,1,2,3,3-pentamethylenepropane (PMP) diradicals with two nitronyl nitroxide (DR 1 ) and with two iminonitroxide (DR 2 ) fragments linked through the C(sp 2 ) atom of the nitrone group. According to magnetic susceptibility measurements, EPR data and ab initio calculations at the (8,6)CASSCF and (8,6)NEVPT2 levels, DR 1 and DR 2 have singlet ground states. The singlet-triplet energy splitting (2J) is low (J/k = À7.4 for DR 1 and À6.0 K for DR 2 ), which comes from the disjoint nature of these diradicals. The reaction of [Cu(hfac) 2 ] with DR 1 gives rise to different heterospin complexes in which the diradical acts as a rigid ligand, retaining its initial conformation. For the [{Cu(hfac) 2 } 2 (DR 1 )(H 2 O)] complex, sufficiently strong ferromagnetic interactions (J 1 /k = 42.7 and J 2 /k = 14.1 K) between two coordinating Cu II ions and DR 1 were revealed. In [{Cu(hfac) 2 } 2 (DR 1 )(H 2 O)][Cu(hfac) 2 -(H 2 O)], the very strong and antiferromagnetic (J/k = À416.1 K) exchange interaction between one of the coordinating Cu II ions and DR 1 is caused by the very short equatorial CuÀO bond length (1.962 ).
Ferromagnetic Chain Based on Verdazyl-Nitroxide Diradical
Verdazyl-nitroxide diradicals were synthesized using the palladium-catalyzed cross-coupling reaction of the corresponding iodoverdazyls with a nitronyl nitroxide-2-ide gold(I) complex with high yields (up to 82%). The synthesized diradicals were found to be highly thermally stable and have a singlet (DEST » -64 cm–1) or triplet ground state (DEST ³ 25 and 100 cm–1), depending on which canonical hydrocarbon diradical type they belong to. Upon crystallization, triplet diradicals form unique one-dimensional (1D) spin S = 1 chains of organic diradicals with intrachain ferromagnetic coupling of J′/kB from 3 to 6 K.
Polyhedron, 2009
The effects of acceptor-donor interactions in thienyl substituted benzimidazole-nitronyl nitroxides (TBNN) on the absorption spectroscopy, spin density distribution, magnetic behavior, and crystallographic packing were explored through spectroscopy, computation, and characterization of structure and magnetic properties in the crystalline phase. The electronic spectra of the radicals exhibit a strong broad absorption in the NIR (k max $ 1000 nm) that exhibits solvatochromism consistent with charge transfer between the thienyl (donor) and benzonitronyl nitroxide (acceptor) dyads. Computational analysis allowed assignment of the transition as a HOMO-SOMO transition (TD-DFT UB3LYP/6-31G**). The TBNN radicals form highly disordered slipped p-stacks in the solid state that give rise to antiferromagnetic interactions consistent with 1D chain interactions. The magnetic behavior was well-fit to a Bonner-Fisher model to give exchange parameters of J = À2 to À10 cm À1 depending on substitution. The weak exchange parameters are attributed to the degree of solid-state disorder, and the observed properties can be rationalized by the effects of substitution on the electronic structure and topology of the radicals.