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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Magnetic Chemistry

Alternative title: Magnetisk kjemi

Awarded: NOK 10.0 mill.

On Earth, chemistry is determined by the electrical forces acting between the electrons and nuclei that make up molecules. However, the situation is different on other celestial bodies. On many white dwarfs, the final stage in the evolution of most stars, including the Sun, magnetic fields are so powerful that they alter chemistry. This is because the magnetic forces affecting the electrons and nuclei in the molecules there are as strong as the electrical forces. On these "magnetic white dwarfs," atoms shrink, and they bind to each other in new ways. This exotic chemistry can currently only be studied theoretically by simulating molecules in magnetic fields using quantum mechanics. Such methods have been developed in our research group in recent years, providing exciting insights into the chemistry on other celestial bodies. We have discovered a completely new mechanism for chemical bonding, called paramagnetic bonding, generated by electron currents in the magnetic field. However, all our previous studies of chemistry in strong magnetic fields have been limited to static properties such as molecular structure and chemical bonding. In the "Magnetic Chemistry" project, we have focused on the dynamics of molecules in strong magnetic fields and have developed the first general methods for calculating molecular motion in such fields. Such simulations of molecular dynamics are necessary to calculate molecular spectra in strong magnetic fields. Accurately calculated spectra are crucial for identifying molecules on magnetic white dwarfs, as the spectra serve as fingerprints for the molecules. Without knowing the fingerprints, we do not know what to look for. Molecular dynamics (and thus spectra) are strongly influenced by a strong magnetic field. Firstly, molecular structure and bonding change due to the field, which in turn affects the forces on the atoms and thus the molecule's dynamics. This effect is relatively easy to include in our calculations based on previous work. Secondly, a molecule consists of charged particles (positive nuclei and negative electrons), which are therefore influenced by the Lorentz force depending on the particles' motion in the magnetic field. Such forces have not been calculated quantum mechanically for molecules before. Within the Born-Oppenheimer approximation, there are two contributions to the Lorentz force on each atom in a molecule: one from the nucleus and one from the electrons. While the nuclear contribution is straightforward to calculate, the electron contribution is much more challenging and depends on the geometric vector potential in the magnetic field. We have developed advanced methods to calculate the vector potential and thus the electron contribution to the Lorentz force, for arbitrary geometries and orientations of the molecule in the magnetic field. We have then calculated the motions of molecules in a magnetic field by integrating the forces on each atom. From these motions, we have finally determined molecular spectra in magnetic fields corresponding to the conditions on magnetic white dwarfs. These spectra differ significantly from corresponding spectra on Earth. An important difference is related to the rotation of molecules. In the absence of a magnetic field, a molecule rotates freely. When a magnetic field is applied, free rotation is inhibited. In a sufficiently strong field, rotation will completely cease, and the molecule will instead rock back and forth about an axis perpendicular to the field direction. This gives rise to new lines in the molecule's spectrum, depending on the field strength. A typical feature is also that vibrational bands shift to higher energies in a magnetic field, as a result of the strengthening of bonds by the field.

Prosjektet har gitt en første innsikt i hvordan molekylers dynamikk påvirkes av sterke magnetfelt og for første gang gjort det mulig å beregne teoretisk spektra fra molekyler i slike magnetfelt. Slike spektra vil kunne benyttes som molekylære fingeravtrykk ved identifikasjon av molekyler på magnetisk hvite dverger. Det utviklingsarbeidet som ligger bak slike beregninger krever har gitt oss nye teoretiske verktøy og algoritmer til den kvantemekaniske beskrivelse av molekylære systemer, spesielt molekyler med komplisert elektronstruktur.

On Earth, chemistry is governed by the Coulomb interactions between electrons and nuclei, the effect of magnetism being weak and subtle. Elsewhere in the universe, the situation is different. On many white dwarf stars, the magnetic forces acting on the particles are as strong as the electric forces. Their complicated interplay sets up an exotic chemistry of egg-shaped atoms, dramatically affecting structure and reactivity of molecules. In the project Magnetic Chemistry, such chemistry will be studied theoretically, using the tools of quantum chemistry. Apart from shedding light on a fascinating chemistry that cannot be experienced by us directly, the study of chemistry in ultra-strong magnetic fields has experimental relevance in astrophysics. Atomic spectra have for a long time been observed on magnetic white dwarf stars and are used to map the field strength on their surface. Very recently, the first observation of molecules on nonmagnetic white dwarf stars has been made, indicating that there is every reason to believe that molecules exist also on magnetic white dwarf stars but they cannot be found without reliable quantum-chemical predictions. So far, only small molecules have been studied in strong magnetic fields and such studies have revealed many fascinating and unexpected phenomena such as a new chemical bonding mechanism. In the present project, we target larger and more complex systems, combining studies of energetics with molecular dynamics. The development of molecular dynamics will enable us to study for the first time the effect of strong magnetic fields on chemical reactions. We are, in particular, interested in the stability of larger molecules and their decay paths.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek