Hyperfine interactions and internal rotation in methanol
In one sentence
A rigorous, first-principles treatment of methanol’s hyperfine structure, including the molecule’s troublesome internal rotation — the molecular-physics groundwork that subsequent astrophysical work would build on.
What’s the question?
Methanol — CH₃OH — is one of the most observed molecules in interstellar space, but it is also a small theoretical headache. Its CH₃ group rotates internally relative to the OH group, and that internal rotation couples to the rest of the molecule’s quantum mechanics in a way that ordinary symmetric-top theories don’t capture. To do anything quantitative with methanol’s hyperfine structure — the small splittings caused by interactions between nuclear spins and the rest of the molecule — you have to handle internal rotation properly.
What did we do?
We derived the nuclear spin-rotation and spin-torsion coupling terms in the hyperfine Hamiltonian for a molecule with internal rotation, computed the spin-rotation tensors with high-level ab initio electronic-structure methods, and used the resulting Hamiltonian to predict hyperfine transition frequencies and intensities for twelve torsion-rotation transitions of methanol.
Why does it matter?
The result is the first complete, theoretically clean description of methanol’s hyperfine structure. It is the foundation that the Nature Astronomy paper and many other observational works on methanol masers have stood on.
My role
First author. Did the theoretical derivation, ran the ab initio calculations, and wrote the paper.