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Molecules exist in various energy configurations called quantum states. The laws of physics that govern if and how molecules can switch from one state to another underpin the machinery of chemistry. Controlling those transitions with precision can affect the thermodynamic properties of materials and is a central challenge in quantum chemistry. A new study shows a surprisingly simple way to achieve this control<\/a> (Phys. Rev. Lett. 2026 DOI: 10.1103\/2yw9-7h62).<\/p>\n \u201cWe used dry ice and the simplest molecule in the universe: hydrogen,\u201d says coauthor Leah Dodson<\/a>, a chemical physicist at the University of Maryland, College Park.<\/p>\n Molecular hydrogen can exist in two forms, or quantum states, depending on the orientation of the spins of the two protons in the nucleus. In ortho-hydrogen, the two spins are aligned the same way, or parallel. In the para form, the nuclear spins are antiparallel. Most hydrogen is in the ortho form at room temperature, but when cooled to near 0 K, hydrogen molecules favor the para form.<\/p>\n Liquid hydrogen used as fuel and in medical technology needs to be in the para form, but converting between the two forms usually requires powerful magnetic fields or chemical catalysts. Dodson\u2019s team embedded hydrogen in dry ice (solid CO\u2082) and found that that simple step converted ortho-hydrogen to para-hydrogen.<\/p>\n What Dodson\u2019s team found interesting was that only some of the hydrogen was being converted. \u201cOrtho-hydrogen has three variants, and the dry ice environment converted only one of them to the para form,\u201d she explains. The other two stayed completely locked in ortho form even at temperatures close to absolute zero, where the para form is overwhelmingly favored. \u201cThis was a surprising result,\u201d Dodson says.<\/p>\n When the researchers replaced dry ice with solid nitrous oxide (N\u2082O), which has a different charge distribution, more variants began converting. And when they introduced a small amount of nitrogen dioxide (NO\u2082) into the hydrogen\u2019s environment, all the molecules began to convert. Each crystal environment opened a specific set of doors and controlled the hydrogen molecule\u2019s quantum state in a predictable way.<\/p>\n \u201cThe work shows a way to enable efficient nuclear spin conversion of hydrogen, which can be useful for liquid hydrogen storage and catalysis research,\u201d says Lee Liu<\/a>, a chemist at Purdue University, who was not involved in the study. \u201cI find the work very interesting,\u201d he adds. \u201cThey show one of the ways in which solids can open up new avenues for manipulating molecular states.\u201d<\/p>\n Dodson points out that the same science could also resolve a long-standing puzzle in astronomy: scientists currently estimate temperatures for comet formation by measuring the ratio of ortho to para forms of water released as comets pass the sun, a calculation built on the assumption that spin states have been frozen since the comet formed. Dodson believes that the spin states being fixed is unlikely, and her team now has the tools to test that assumption directly in the laboratory.<\/p>\n \u201cThis concept supersedes any one molecule,\u201d Dodson says. If the chemical character of molecules held in a crystal environment determines which quantum transitions the molecules can undergo, then crystal design becomes a new tool for quantum-state control across chemistry, Dodson explains. This concept is fundamental to everything from synthesizing commercially relevant chemicals to building stable quantum computers.<\/p>\n Next the researchers plan to extend the work to methane, an alternative fuel candidate with a more complex set of spin forms. They are also currently working out the detailed physical mechanism behind the observations.<\/p>\n