Chemical Physics
The physics of molecules and chemical reactions: how electrons and nuclei move when bonds form and break.
arXiv: physics.chem-ph
12 topics
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The outermost layer of water, where it meets air or oil, behaves differently from bulk water, and dissolved ions of equal charge behave very differently from one another. Reactions in tiny droplets have been reported to run up to millions of times faster, and in very concentrated salt water electric forces seem to reach farther than theory allows.
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Some of the most important reactions on Earth, such as splitting water in plants and making ammonia or methanol on an industrial scale, require moving several electrons and protons in a precise sequence. The key steps of how nature and industry do this are still not fully known.
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Chirality-induced spin selectivity
5 problemsElectrons passing through molecules with a handedness, like DNA or helical peptides, come out with their spins partly aligned, by tens of percent. This is far larger than standard theory predicts for molecules made of light atoms.
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Density functional theory computes molecules and materials from the electron density alone, but it relies on an approximate ingredient, the exchange-correlation functional, whose exact form is unknown. Its errors decide whether predictions of bond energies, band gaps and reaction barriers can be trusted.
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At the surface of a battery or fuel-cell electrode, a thin layer of water and ions called the electric double layer controls how fast electrons and protons cross. Its atomic structure, and why reaction rates depend on pH and on which ions are present, is poorly known.
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When a molecule absorbs light, an electron moves to a higher-energy arrangement called an excited state. Computing the energies and character of these states is harder than for the lowest state, and the cheapest standard method misses some kinds of excited states entirely.
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Water behaves unlike most liquids: it is densest at 4 C and its compressibility and heat capacity grow sharply as it is cooled below freezing without crystallizing. How an extra proton or an extra electron is arranged in water, and how protons move through it so fast, is also only partly understood.
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Nonadiabatic molecular quantum dynamics
6 problemsWhen light hits a molecule, electrons and atomic nuclei move together in a way that breaks the usual assumption that electrons instantly follow the nuclei. Describing this coupled motion decides what happens in vision, photosynthesis, sunscreen molecules and organic electronics.
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Placing molecules between two closely spaced mirrors can make their vibrations mix with trapped light, forming hybrid states called polaritons. Some experiments report that this changes chemical reaction speeds even in the dark, which current theory cannot explain.
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Light nuclei, especially hydrogen, behave partly like waves: they tunnel through energy barriers and are never fully at rest. Together with laser experiments that follow single collisions between molecules, this shows that reaction rates often depart from the simple picture of crossing one energy barrier.
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Chemists check cheaper methods against a few very accurate ones, such as coupled-cluster theory and quantum Monte Carlo (a method that samples electron positions at random). For large molecules and for metals these trusted methods start to disagree with each other or break down.
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In some molecules, especially those with several metal atoms, many electrons arrange themselves together so that no simple picture of one electron per orbital works. Computing their energies accurately is one of the hardest jobs in chemistry and a main target for quantum computers.