McConnell Model Research Articles

Dynamic and rapid deep synthesis of chemical exchange saturation transfer and semisolid magnetization transfer MRI signals

Model-driven analysis of biophysical phenomena is gaining increased attention and utility for medical imaging applications. In magnetic resonance imaging (MRI), the availability of well-established models for describing the relations between the nuclear magnetization, tissue properties, and the externally applied magnetic fields has enabled the prediction of image contrast and served as a powerful tool for designing the imaging protocols that are now routinely used in the clinic. Recently, various advanced imaging techniques have relied on these models for image reconstruction, quantitative tissue parameter extraction, and automatic optimization of acquisition protocols. In molecular MRI, however, the increased complexity of the imaging scenario, where the signals from various chemical compounds and multiple proton pools must be accounted for, results in exceedingly long model simulation times, severely hindering the progress of this approach and its dissemination for various clinical applications. Here, we show that a deep-learning-based system can capture the nonlinear relations embedded in the molecular MRI Bloch–McConnell model, enabling a rapid and accurate generation of biologically realistic synthetic data. The applicability of this simulated data for in-silico, in-vitro, and in-vivo imaging applications is then demonstrated for chemical exchange saturation transfer (CEST) and semisolid macromolecule magnetization transfer (MT) analysis and quantification. The proposed approach yielded 63–99% acceleration in data synthesis time while retaining excellent agreement with the ground truth (Pearson’s r > 0.99, p < 0.0001, normalized root mean square error < 3%).

Modified Superexchange Model for Electron Tunneling Across the Terminated Molecular Wire

The modified superexchange model shows wide possibilities to clarify the mechanism of formation of a tunneling current through terminated molecular wire. The model has no inherent rigid restrictions of the standard superexchange model. This allows to obtain the analytical expressions for the analysis of the current–voltage characteristics of the wire, in which the voltage bias does not destroy the delocalzation of molecular orbitals belonging to the interior region of the wire. The conditions under which the modified superexchange model leads to the results that follow from the Simmons model of tunneling through a rectangular barrier, or the McConnell model of superexchange tunneling, are presented. The mechanism by which the wire's terminal units with the biased energies that do or do not resonate with the Fermi‐levels of the electrodes control the current, is clarified. Using an example of a molecular wire with a regular bridging alkane chain, it is shown that the model predicts current–voltage characteristics that are in good agreement with the experimental data on the attenuation of the tunneling current with an increase of the chain length.

Distance Dependence of Triplet Energy Transfer in Water and Organic Solvents: A QM/MD Study

The possibility to optimize optoelectronic devices, such as organic light-emitting diodes or solar cells, by exploiting the special characteristics of triplet electronic states and their migration ability is attracting increased attention. In this study, we analyze how an intervening solvent modifies the distance dependence of triplet electronic energy transfer (TEET) processes by combining molecular dynamics simulations with quantum chemical calculations of the transfer matrix elements using the Fragment Excitation Difference (FED) method. We determine the β parameter characterizing the exponential distance decay of TEET rates in a stacked perylene dimer in water, chloroform, and benzene solutions. Our results indicate that the solvent dependence of β (βvacuum = 5.14 Å–1 > βwater = 3.77 Å–1 > βchloroform = 3.61 Å–1 > βbenzene = 3.44 Å–1) can be rationalized adopting the McConnell model of superexchange, where smaller triplet energy differences between the donor and the solvent lead to smaller β constants. We also estimate the decay of hole transfer (HT) and excess electron transfer (EET) processes in the system using the Fragment Charge Difference (FCD) method and find that βTEET can be reasonably well approximated by the sum of βEET and βHT constants.

The dependence of the electronic coupling on energy gap and bridge conformation – Towards prediction of the distance dependence of electron transfer reactions

The attenuation factor, β, for the distance dependence of electron exchange reactions is a sensitive function of the donor–bridge energy gap and bridge conformation. In this work the electronic coupling for electron and triplet excitation energy transfer has been investigated for five commonly used repeating bridge structures. The investigated bridge structures are OF (oligo fluorene), OP (oligo phenylene), OPE (oligo p-phenyleneethynylene), OPV (oligo phenylenevinylene), and OTP (oligo thiophene). Firstly, the impact of the donor–bridge energy gap was investigated by performing calculations with a variety of donors appended onto bridges that were kept in a planar conformation. This resulted in, to our knowledge, the first presented sets of bridge specific parameters to be inserted into the commonly used McConnell model. Secondly, since at experimental conditions large conformational flexibility is expected, a previously developed model that takes conformational disorder of the bridge into account has been applied to the investigated systems [M.P. Eng, T. Ljungdahl, J. Mårtensson, B. Albinsson, J. Phys. Chem. B 110 (2006) 6483]. This model is based on Boltzmann averaging and has been shown to describe the temperature dependence of the attenuation factor through OPE-bridges. Together, the parameters describing the donor–bridge energy gap dependence, for planar bridge structures, and the Boltzmann averaging procedure, describing the impact of rotational disorder, have the potential to a priori predict attenuation factors for electron and excitation energy transfer reactions through bridged donor–acceptor systems.

Temperature Dependence of Electronic Coupling through Oligo‐p‐phenyleneethynylene Bridges

A series of donor-bridge-acceptor (D-B-A) systems with varying donor-acceptor distances has been studied with respect to the temperature dependence of the triplet excitation energy transfer (TEET) rates. The donor and acceptor, zinc(II) and free-base porphyrin, respectively, were separated by oligo-p-phenyleneethynylene (OPE) bridges, where the number of phenyleneethynylene groups was varied between two and five, giving rise to edge-to-edge separations ranging between 12.7 and 33.4 A. The study was performed in 2-MTHF between room temperature and 80 K. It was found that the distance dependence was exponential, in line with the McConnell model, and the attenuation factor, beta, was temperature dependent. The experimentally determined temperature dependence of beta was evaluated by using a previously derived model for the conformational dependence of the electronic coupling based on results from extensive quantum chemical, DFT and time-dependent DFT (TD-DFT), calculations. Two regimes in the temperature interval could be identified: one high-temperature, low-viscosity regime, and one low-temperature, high-viscosity regime. In the first regime, the temperature dependence of beta was, according to the model, well described by a Boltzmann conformational distribution. In the latter, the molecular motions that govern the electronic coupling are slowed down to the same order of magnitude as the TEET rates. This, in effect, leads to a distortion of the conformational distribution. In the high-temperature regime the model could reproduce the temperature dependence of beta, and the extracted rotational barrier between two neighboring phenyl units of the bridge structure, E(i)=1.1 kJ mol(-1), was in line with previous experimental and theoretical studies. After inclusion of parameters that take the viscosity of the medium into account, successful modeling of the experimentally observed temperature dependence of the distance dependence was achieved over the whole temperature interval.

A supramolecular network of 2‐(4,4,5,5‐tetramethyl‐3‐oxylimidazoline‐1‐oxide)‐5‐bromopyridine built through π–π stacking and hydrogen bonding interactions

AbstractThe X‐ray study of 2‐(4,4,5,5‐tetramethyl‐3‐oxylimidazoline‐1‐oxide)‐5‐bromopyridine (PyNN) shows an unusual 3‐D supramolecular assembly formed by the combined non‐covalent interactions of π–π stacking and H‐bonding forces. The mean π–π stacking distance between two pyridine units is ∼3.64 Å. The angle between the plane of the pyridine ring and the stack column axis (c‐axis) is 7.46°. The H‐bonding distance between the oxygen atom of the water and the nitronyl nitroxide (NN) oxygen [H···ON] is 1.891 Å. Two potential short NO···ON contacts are found (NO1···O1N = 3.673 Å and NO1···O2N = 3.695 Å) along the stacked column. Four water molecules through H‐bonding with four different radical oxygens form a perfect square in the a, b plane which extends as a tubular cage along the stack axis. Experimental spin densities of PyNN estimated from the 1H‐NMR spectroscopy and its bulk magnetic property have been correlated with the X‐ray structure in an attempt to understand the possible magnetic exchange interactions. The observed low temperature antiferromagnetic interaction is analyzed on the basis of McConnell's model I (spin polarization approach) and this magnetic behavior is probably due to the dominant NO···ON close contacts along the chain surpassing the stacking, H‐bonding and other interactions. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Distribution of GD3 in DPPC Monolayers: A Thermodynamic and Atomic Force Microscopy Combined Study

Gangliosides are the main component of lipid rafts. These microdomains, floating in the outer leaflet of cellular membrane, play a key role in fundamental cellular functions. Little is still known about ganglioside and phospholipid interaction. We studied mixtures of dipalmitoylphosphatidylcholine and GD3 (molar fraction of 0.2, 0.4, 0.6, 0.8) using complementary techniques: 1), thermodynamic properties of the Langmuir-Blodgett films were assessed at the air-water interface (surface tension, surface potential); and 2), three-dimensional morphology of deposited films on mica substrates were imaged by atomic force microscopy. Mixture thermodynamics were consistent with data in the literature. In particular, excess free energy was negative at each molar fraction, thus ruling out GD3 segregation. Atomic force microscopy showed that the height of liquid-condensed domains in deposited films varied with GD3 molar fraction, as compatible with a lipid aggregation model proposed by Maggio. No distinct GD3-rich domain was observed inside the films, suggesting that GD3 molecules gradually mix with dipalmitoylphosphatidylcholine molecules, confirming Δ G data. Morphological analysis revealed that the shape of liquid-condensed domains is strongly influenced by the amount of GD3, and an interesting stripe-formation phenomenon was observed. These data were combined with the thermodynamic results and interpreted in the light of McConnell's model.

Hybrid density functional theory studies on the magnetic interactions and the weak covalent bonding for the phenalenyl radical dimeric pair.

The phenalenyl radical (1) is a prototype of the hydrocarbon radical. Recently, the single crystal of 2,5,8-tri-tert-butylphenalenyl (2) was isolated and showed that the two phenalenyl radicals form a staggered dimeric pair, giving rise to strong antiferromagnetic interactions. The origin of the antiferromagnetic interactions and the nature of the chemical bond for the dimeric pair are challenging issues for chemists. First, spin-polarized hybrid DFT (Becke's half and half LYP (UB2LYP)) and CASSCF calculations were performed for 2 and its simplified model, the staggered-stacking phenalenyl radical dimeric pair (3a), to elucidate the origin of the strong antiferromagnetic coupling and the characteristics of the chemical bond. The calculated results showed that a SOMO-SOMO overlap effect was responsible for the strong antiferromagnetic interactions and weak or intermediate covalent bonding between phenalenyl radicals. The tert-butyl groups introduced at three beta-positions hardly affected the magnetic coupling, mainly causing steric hindrances in the crystalline state. Next, to obtain insight into ferromagnetic stacking, we investigated the stacking effect of staggered (3a)- and eclipsed (3b)-stacking phenalenyl radical dimeric pairs with a change of the SOMO-SOMO overlap on the basis of the extended McConnell model. We found that the stacking mode of the dimeric pair with both a small SOMO-SOMO overlap and a ferromagnetic spin polarization effect provided a ferromagnetic coupling.

Magnetic Properties of Nitronyl Nitroxide Derivatives Bearing N-H Site: An Approach to Ferromagnetic Self-Assemblies Using Hydrogen Bond

Nitronyl nitroxide derivatives bearing NH site, imidazolyl-2-nitronyl nitroxide (Im-NN), 4(5)-methyl-imidazolyl-2-nitronyl nitroxide (MeIm-NN), and benzimidazolyl-2-nitronyl nitroxide (BIm-NN) were synthesized, and were self-assembled by hydrogen bonding at NH site with nitrogen atom on imidazole ring or oxygen atom of nitroxide moiety. Ferromagnetic intermolecular interactions were observed in MeIm-NN and BIm-NN. The magnetic interaction can be rationally explained by McConnell model.

On the Validity of the McConnell-I Model of Ferromagnetic Interactions: The [2.2]Paracyclophane Example

A formal comparison between a rigorous implementation of a Heisenberg Hamiltonian model in a VB space with the McConnell-I model shows that the validity of the McConnell model rests upon a heuristic one-to-one correspondence between spin density products in the latter with the difference of the spin exchange density matrix elements in the former. Using a rigorous Heisenberg Hamiltonian, a numerical model computation of the singlet/quintet stability for pseudoortho, pseudometa, and pseudopara bis(phenylmethylenyl)[2.2]paracyclophanes (modeled with the correponding singlet/triplet bis(methyl)[2.2]paracyclophanes) shows that the McConnell model makes the correct prediction of low and high spin stability only because the contributions from “closest contact” carbon atoms that are not directly aligned is rather small. In systems where the alignment is not perfect, this cancelation may not hold. The association between spin density products in the McConnell model with the difference of the spin exchange density ...

Orbital Interaction Analysis of McConnell's Model for Through-Space Magnetic Coupling

McConnell's model for through-space magnetic interaction can be interpreted from the vantage point of molecular orbital interactions. Intermolecular magnetic interactions are analyzed in several stacking modes of diphenylmethyl radical. The nodal properties which lead to the overlap (or lack of it) of the singly occupied molecular orbitals in this system play an essential role in the ferromagnetic coupling of molecules in extended systems. These nodal characteristics, and the overlaps governing orbital interaction are as important as the spin-polarization effects.

The role of orbital interactions in determining ferromagnetic coupling in organic molecular assemblies

McConnell's model for through-space magnetic interaction can be interpreted from the vantage point of molecular orbital interactions. Intermolecular magnetic interactions are analyzed in several stacking modes of allyl, diphenylmethyl, and nitroxide radical systems. The nodal properties which lead to the overlap (or lack of it) of the singly occupied molecular orbitals in these systems play an essential role in the ferromagnetic coupling of molecules in extended systems. These nodal characteristics and the overlaps governing orbital interaction are as important, we argue, as the spin-polarization effects from electron correlation.

Buckminsterfullerene C 60 and organic ferromagnetism

Interest in the electronic properties of C 60 led to the discovery of metals, superconductors and a solid with a transition to a magnetic state. The approaches to organic ferromagnets are reviewed briefly, with emphasis on the McConnell model. The review is followed by the description of the magnetic properties of C 60 TDAE at ambient pressure and under hydrostatic pressure, showing that the 16 K transition is extremely pressure sensitive and that the effect of pressure is reversible. An explanation of the properties of C 60 TDAE in terms of a modification of the McConnell model is described for the first time.

Theoretical calculations of effective exchange integrals between nitronyl nitroxides with donor and acceptor groups

Semi-empirical calculations on the basis of an extended McConnell model have been carried out for dimers of phenyl nitronyl nitroxides with donor or acceptor groups. The intermolecular effective exchange integrals are positive (ferromagnetic) for appropriately stacked forms of these species because of large spin polarization effects. Implications of these results are discussed in relation to the organic ferromagnetism discovered recently for para-nitrophenyl nitronyl nitroxide.

Implications for the Design of Organic Ferromagnets Based Upon Hexaazaoctadecahydrocoronene

Abstract The molecular and electronic structures of [HOC]n, n = 0–4+, have been studied by a variety of experimental and theoretical techniques. The ground state structures for n = 0 and 4+ are closed shell systems. The structures for n = 1 + and 3+ are doublets and show Jahn-Teller distortions. The structure for n = 2+ in the solid is a Jahn-Teller distorted closed shell singlet. The Jahn-Teller distorted structures all resemble two cyanine dyes coupled by single C-C bonds. The 1:1 complex of [HOC]·+[TCNE]·− was synthesized following McConnell's model for ferromagnetism but the complex was found to be antiferromagnetic. A number of suggestions for designing triplet dications and dianions with degenerate orbitals based on benzene derivatives are presented following our experience with [HOC]n.

Mobile π Electrons From Donors and Acceptors

Abstract Concepts that evolved over the years for the design of organic metals and superconductors based on electron donor-acceptor charge transfer complexes are described. The preparation and complete characterization of three new acceptors; p-hexacyano divinylbenzene (HCDV), i-hexacyano divinylbenzene (i-HCDV) and 1,3,5-nonacyano trivinylbenzene (NCTV) are described. These were designed with different properties in mind. The latter for the preparation of organic ferromagnets based on the Breslow modification of the McConnell model.

Molecular/Organic Ferromagnets

Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.

Symmetry properties of exciton states in one-dimensional systems

We discuss a cyclic chain of N identical sites, each with two possible quantum states, whose Hamiltonian may be written in terms of the spin operators for N spins S = 1/2, and transformed into the Fermi site excitation representation. We show that in the Fermion wave representation the requirement that the states transform according to irreducible representations of the translational symmetry group demands that the quasiparticle momenta are drawn from the representations of the double group. They must be all integral or all half-integral when the number of quasiparticles is odd or even respectively. The requirement that a quantum-mechanical operator has a definite symmetry type leads to very complicated conditions unless the operator conserves the odd or even particle number. If it does the symmetry operator can be defined in terms of Fermi wave operators and projection operators. The time reversal transformation is developed in detail and applied to problems of degeneracy in McConnell's model of singlet excitons and in the anisotropic transverse Heisenberg antiferromagnet. In the Bogoliubov-Valatin transformation time reversal shifts the quasiparticle momenta by ± π, converting integral into half-integral momenta when N is odd. The energies of the excited states are doubly-periodic functions of the momentum.