Spin Alternation Rule in USCF for Through-Bond Magnetic Coupling─A New Look: Why and When Does It Arise and How To See It.

This work takes a new look at the spin alternation rule in unrestricted self-consistent-field (USCF) calculations in terms of structural characteristics such as periodicity, impurity location, and Coulomb exchange. For clarity, the systems considered are biradicals produced from linear conjugated hydrocarbons. Both site-parametrized Hamiltonian models for theoretical analysis and spin unrestricted density functional theory (DFT) calculations are used.

Quantum Chemical Investigation of Light-Activated Spin State Change in Pyrene Coupled to Oxoverdazyl Radical Center.

Low-spin ground states and low-lying excited states of higher spin were investigated for four pyrene oxoverdazyl monoradicals 1-4 and eight pyrene dioxoverdazyl diradicals 5-12. The ground states for quartet and quintet spin symmetries that are in reality excited states were found in the region of 565-775 nm above the respective electronic ground states. We calculated the "adiabatic" magnetic exchange coupling constant in the electronic ground state of each isolated biradical (5-12) by unrestricted density functional theory.

Quantum chemical investigation of meta-xylylene based one-dimensional polymer chain.

We have investigated unsubstituted and methyl substituted polyradical chains of meta-xylylene by using density functional theory-broken symmetry methodology (DFT-BS). Optimization of geometry in the high-spin and low-spin states have been done at B3LYP/6-31G(d,p) and M06-2X/6-31G(d,p) levels in unrestricted methodology. Single-point calculations on the high-spin optimized geometries have been done by using the 6-311G(d,p) basis set.

Solvation of CO2 in water: effect of RuBP on CO2 concentration in bundle sheath of C4 plants.

An understanding of the temperature-dependence of solubility of carbon dioxide (CO2) in water is important for many industrial processes. Voluminous work has been done by both quantum chemical methods and molecular dynamics (MD) simulations on the interaction between CO2 and water, but a quantitative evaluation of solubility remains elusive. In this work, we have approached the problem by considering quantum chemically calculated total energies and thermal energies, and incorporating the effects of mixing, hydrogen bonding, and phonon modes.

The Triplet-Singlet Gap in the m-Xylylene Radical: A Not So Simple One.

Meta-benzoquinodimethane (MBQDM) or m-xylylene provides a model for larger organic diradicals, the triplet-singlet gap being the key property. In the present work this energy difference has been the object of a systematic study by means of several density functional theory-based methods including B3LYP, M06, M06-2X, HSE and LC-ωPBE potentials and a variety of wave function-based methods such as complete active space self consistent field (CASSCF), Multireference second-order Møller-Plesset (MRMP), difference dedicated configuration interaction (DDCI), and Multireference configuration interaction (MRCI). In each case various basis sets of increasing quality have been explored, and the effect of the molecular geometry is also analyzed.

Metaphenylene-based nitroxide diradicals: a protocol to calculate intermolecular coupling constant in a one-dimensional chain.

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.

Theoretical investigation of photomagnetic properties of oxoverdazyl-substituted pyrenes.

We have investigated the ground state spin of 10 pairs of possible photochromic diradical isomers by quantum chemical methods. Dihydrogen pyrenes and dinitrile pyrenes have been chosen as spacers with radical centers attached at (1,7) and (1,8) locations. Oxoverdazyl has served as a radical center, and both C and N linkages have been investigated.

Theoretical investigation of stilbene as photochromic spin coupler.

Density functional theory (DFT) based calculations are used here to investigate the magnetic behavior, spectroscopic transitions, and possible photomagnetic properties of stilbene derivatives using photochromicity of cis- and trans-forms of the parent molecule. Nitronyl nitroxide (NN), iminonitroxide (IN), tetrathiafulvalene cation (TTF), and verdazyl (VER) are used as monoradical centers at the p, p' positions. The B3LYP functional with the usual broken symmetry approach and a sufficiently large basis set is chosen to obtain reliable estimates of the intramolecular exchange coupling constants (J).

High magnetic exchange coupling constants: a density functional theory based study of substituted Schlenk diradicals.

The Schlenk diradical has been known since 1915. After a detailed experimental work by Rajca, its magnetic nature has remained more or less unexplored. We have investigated by quantum chemical calculations the nature of magnetic coupling in 11 substituted Schlenk diradicals.

On the photomagnetism of nitronyl nitroxide, imino nitroxide, and verdazyl-substituted azobenzene.

The cis- and trans-azobenzenes are known as photochromic isomers with the trans- converting into the cis-form and vice versa upon irradiation with specific wavelengths. We have quantum chemically investigated the cis- and trans-forms of substituted azobenzene diradicals, with two nitronyl nitroxides, imino nitoxides, or verdazyls at para positions and serving as monoradical centers, to determine whether they can exhibit a photoassisted magnetic crossover. Geometries of both substituted and unsubstituted molecules have been optimized by density functional (DF) method UB3LYP using the 6-311G(d,p) basis set.

Photoswitching magnetic crossover in organic molecular systems.

We have theoretically designed efficient photomagnetic diradicals using both substituted and unsubstituted cyclophanediene (CPD) and dihydropyrenes (DHP) as spacers. Nitronyl nitroxide (NN), oxoverdazyl (o-VER), and tetrathiafulvalene (TTF) are chosen as monoradical centers. Molecular geometries have been optimized by the density functional method UB3LYP using the 6-311G(d,p) basis set.

Unusually large coupling constants in diradicals obtained from excitation of mixed radical centers: a theoretical study on potential photomagnets.

Three sets of heterosubstituted, interconvertible, cyclophanediene (CPD), and dihydropyrenes (DDPs) and one such set involving dinitrilepyrenes were examined by UB3LYP broken-symmetry methodology with 6-311++g(d,p) bases. Nitronyl nitroxide and oxoverdazyl (with both N and C terminals) are monoradical centers, whereas CPD and DDP moieties serve as couplers. The photoexcited CPD converts to DDP.

Very strongly ferromagnetically coupled diradicals from mixed radical centers. II. Nitronyl nitroxide coupled to tetrathiafulvalene via spacers.

We predict large and positive intramolecular magnetic exchange coupling constants (J) for coupled diradicals constructed from nitronyl nitroxide and tetrathiafulvalene monoradical moieties. These diradicals have the general formula TTF-coupler-NN, where the couplers are mostly aromatic systems. Unrestricted density functional methodology (UB3LYP) has been used to optimize the molecular geometries of the triplet diradicals using the 6-311 g(d,p) basis set.

All-temperature magnon theory of ferromagnetism.

We present an all-temperature magnon formalism for ferromagnetic solids. To our knowledge, this is the first time that all-temperature spin statistics have been calculated. The general impression up to now is that the magnon formalism breaks down at the Curie point as it introduces a series expansion and unphysical states.

Influence of solute-solvent hydrogen bonding on intramolecular magnetic exchange interaction in aminoxyl diradicals: a QM/MM broken-symmetry DFT study.

We have investigated the effect of nitroxide radical-water hydrogen bonding (NO(*)...

Very strongly ferromagnetically coupled diradicals from mixed radical centers: nitronyl nitroxide coupled to oxoverdazyl via polyene spacers.

We predict extremely large and positive intramolecular magnetic exchange coupling constants (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 negative coupling constants.

Molecular tailoring and prediction of strongly ferromagnetically coupled trimethylenemethane-based nitroxide diradicals.

We have investigated the magnetic properties of four recently synthesized stable TMM-type nitroxide diradicals. Four new diradicals are proposed by tailoring one of the species in such a way that both conjugation and planarity increase. As a remarkable consequence, the intramolecular ferromagnetic exchange interaction was found to be quite high in the proposed radicals.

Theoretical determination of the standard reduction potentials of pheophytin-a in N,N-dimethyl formamide and membrane.

Quantum mechanical/molecular mechanics (QM/MM) calculations were performed on the neutral, anionic, and dianionic forms of Pheophytin-a (Pheo-a) in N,N-dimethyl formamide (DMF) in order to calculate the absolute free energy of reduction of Pheo-a in solution. The geometry of the solvated species was optimized by restricted open-shell density functional treatment (ROB3LYP) using the 6-31G(d) basis set for the molecular species while the primary solvent shell consisting of 45 DMF molecules was treated by the MM method using the universal force field (UFF). Electronic energies of the neutral, anionic, and dianionic species were obtained by carrying out single point density functional theory (DFT) calculations using the 6-311+G(2d,2p) basis set on the respective ONIOM optimized geometries.

Unique rate expression for glucose production in C(4) plants.

A physicochemical interpretation of a recently formulated temperature-dependent, steady-state rate expression for the production of glucose equivalent in C(4) plants is given here. We show that the rate equation is applicable to a wide range of C(4) plants.

Polyacene spacers in intramolecular magnetic coupling.

We predict the intramolecular magnetic exchange coupling constant (J) for eleven nitronyl nitroxide diradicals (NN) with different linear and angular polyacene couplers from broken-symmetry density functional treatment. For the linear acene couplers, J initially decreases with increase in the number of fused rings. But from anthracene coupler onward, the J value increases with the number of benzenoid rings due to an increasing diradical character of the coupler moiety.

Retarded boson-fermion interaction in atomic systems.

The retarded interaction between an electron and a spin-0 nucleus is derived from electrodynamical perturbation theory. The contribution of retardation at order v(2)c(2) mimics the Breit interaction [Phys. Rev.

Density functional theory prediction of enhanced photomagnetic properties of nitronyl nitroxide and imino nitroxide diradicals with substituded dihydropyrene couplers.

We predict the photoswitching magnetic properties of four substituted dihydropyrenes from density functional broken-symmetry calculations. The magnetic exchange coupling constants differ up to 9.44 cm(-1).

Theoretical determination of standard oxidation and reduction potentials of chlorophyll-a in acetonitrile.

QM/MM calculations were performed on ethyl chlorophyllide-a and its radical cation and anion, by using the density functional (DF) B3LYP method to determine the molecular characteristics, and a molecular mechanics (MM) method to simulate the solvating medium. The presence of the solvent was accounted for during the optimization of the geometry of the 85-atom chlorophyll-a system by using an ONIOM methodology. A total of 24 solvent molecules were explicitly considered during the optimization process, and these were treated by the universal force field (UFF) method.

Ab initio quantum chemical investigation of intramolecular magnetic interaction in some diradical derivatives of imino nitroxide and nitronyl nitroxide.

The magnetic properties of the monoradicals 2-(4-phenyl acetylene)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyl-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-imidozolyl-oxyl] (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.

Theoretical investigation of intramolecular magnetic interaction through an ethylenic coupler.

We show that an ethylenic coupler provides a very strong intramolecular magnetic interaction. A recently synthesized nitronyl nitroxide derivative, D-NIT2, is investigated by ab initio quantum chemical methods. The broken symmetry approach yields a coupling constant -541 K that is in good agreement with the observed value in solid state.