Theoretical and Computational Aspects of Magnetic Organic Molecules

The following sections are included:

• Contents
• Preface

The following sections are included:

• Historical Preamble and Significance of Magnetism
• Definitions
• Quantum Theory of Diamagnetism and Paramagnetism
• Ferromagnetism and Antiferromagnetism
• Organic Radicals
• References

Organic materials are of tremendous interest in a wide range of industrial applications including the production of essential everyday items such as paper, fuel, and adhesives, as well as the advancement of research and development in plastics, pharmaceuticals, and biotechnology…

In this chapter we will discuss the common quantum chemical methodologies for determining the ground state spin and intramolecular coupling constant. For a general molecule composed of atoms of low atomic number like C, H, N, O, etc., the spin–orbit interaction is exceedingly small and so we can separate the description of spins from the characterization of the motion of electrons and nuclei in space. The Born–Oppenheimer approximation, where to the zeroth order the effect of kinetic energy of the nuclei on the electronic movement is neglected, allows further simplification, separating nuclear motion from electronic motion. Electronic motion is governed by a Hamiltonian referring to the kinetic energy and coulombic interactions of electrons with nuclei and with one another. The electronic Hamiltonian for an organic molecule with N electrons, without any heavy atoms and in the absence of an external magnetic field is written from Eq. (1.7)…

Molecular modeling has become an essential part of chemical investigation, and is made easier by the wide availability of powerful and accessible software. For the largest molecules (thousands of atoms), semi-empirical molecular orbital methods augmented by molecular mechanics allows description of conformational change and docking; for systems of modest size (a few tens of atoms) powerful first-principles methods can estimate thermodynamic properties within a few kcal/mole and characterize a range of spectroscopic properties with high accuracy. Molecules of intermediate size may usefully be described by a variety of methods based on density functional theory (DFT)…

The following sections are included:

• Introduction
• A Few Remarks on Hund's Rule(s)
• Alternant Hydrocarbons and Nonbonded Levels
• Spin Multiplicity in Alternant Organic Molecules with Conjugated Bonds
• Spin Alternation: An Illustration from Unrestricted Calculations
• Spin Alternation in Unrestricted SCF Ground State
• Disjoint Orbitals and the Possibility of Hund's Rule Violations
• Spin Control: Dougherty's Non-Kekule Acenes
• Geometrical Distortion and Failure of Topological Rules
• Polyradicals and Topology
• Multiple Phenylene Couplers
• New Design Elements: Heterosubstitution and Net Charge
• Toward Materials: Larger Molecules, Chains, and Higher Dimensions
• Intermolecular Ferromagnetic Coupling: Evaluation of McConnell Mechanism I
• Evaluation of McConnell's Second Mechanism for Intermolecular Ferromagnetic Coupling
• Real Solid Organic Ferromagnets?
• Concluding Remarks
• References

The following sections are included:

• General Trends of Electronic Structure Calculations
• Two Case Studies: Diradicals TMM and TME
• Coupled Radicals
• Linear Couplers: Unsaturated Hydrocarbon Chains
• Aromatic Couplers
• Mixed Couplers: Molecular Tailoring
• Discussion: Aids to our Understanding
• Summary
• References

The practical application of molecular magnetism requires the design of molecules with two primary features, a high-spin ground state and a large (positive) intramolecular coupling constant, which give rise to prominent paramagnetic behavior. The latter may be observed in solution, in thin film of polymeric materials, or even in solid when the interaction between the nearest-neighbor substrate molecules is quite weak. For example, three derivatives of aminoxyl diradicals have been shown to have paramagnetic susceptibilities in solvent tetrahydrofuran (THF) as well as in polycrystalline form over a wide range of temperatures. Incidentally, paramagnetism in solution is ideally suited for biological applications…

The following sections are included:

• Introduction
• Spin Crossover — An Important Development from Inorganic Chemistry
• Photochromism — An Exotic Phenomenon in Organic Chemistry
• Photomagnetism — A Futuristic Phenomenon for Materials Science
• The CPD–DHP Coupler Pair
• Calculated and Observed Spectrum
• Cis- and Trans-photoconversion of Azobenzene Derivatives
• Photomagnetic Effect in Stilbene-Based Diradicals
• Photomagnetic Molecular Devices
• Conclusions
• References

The following sections are included:

• Introductory Remarks
• Spin Hamiltonians and Magnetic Coupling in Transition Metal Complexes
• The Heisenberg–Dirac–Van Vleck Hamiltonian
• Ising Model
• Beyond the HDVV Hamiltonian: The Hubbard Hamiltonian
• The Mapping Procedure: Linking Ab initio Calculations and Parameters of Low-Dimension Hamiltonian
• Beyond the Mapping Procedure: Effective Hamiltonian Theory
• What Needs to be Captured in the Effective Hamiltonian?
• On the Need for Highly Accurate Calculations
• UHF and DFT Methods: BS Open-Shell Calculations
• References

The following sections are included:

• Choice of Methods
• Inorganic Clusters
• Magnetic Solids with Localized Spin Moments
• Applications to Periodic Systems
• Selected Case Study Examples: Extended Systems
• Concluding Remarks
• References

The study of high-spin molecules and molecular magnetic materials has progressed over the past half century from the accidental discovery of a few exceptional molecules, describable in qualitative ways by approximate theory, to the preparation of an array of persistent high-spin molecules of considerable size, and even molecular solids exhibiting properties of ferromagnetism. With the advances in experiments have come improvements in computational modeling and a deeper understanding of the electronic states of the new materials. One can be confident that the advance will not relent, and what will come in the future will be as astonishing as the developments we have described already…

The following section is included:

• Index