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arXiv · 2510.16155

Environment-imposed selection rules for nuclear-spin conversion of H$_2$ in molecular crystals

Nathan Mclane·LeAnh Duckett·Leah G. Dodson
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Abstract

Nuclear-spin conversion in molecular hydrogen is governed by strict symmetry rules that typically require magnetic fields or catalytic surfaces to break. Here we demonstrate that the intrinsic tensor composition of a non-magnetic molecular crystal field can impose and relax these rules without external fields. High-resolution infrared spectra of H$_2$ in crystalline CO$_2$ reveal large rank-2 (quadrupolar) crystal-field splittings of the $m$ sublevels, while nuclear-spin conversion occurs only through $\Delta m = 0$ channels. Replacing CO$_2$ with polar N$_2$O introduces rank-1 (dipole) components that partially open $\Delta m \neq 0$ pathways, while incorporation of paramagnetic NO$_2$ fully lifts the restriction. These results establish a direct correspondence between crystal-field tensor rank and nuclear-spin dynamics, introducing a general symmetry-based framework for designing and controlling spin-isomer populations and quantum-state connectivity in molecular solids.

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Nathan Mclane, LeAnh Duckett, Leah G. Dodson. 2025-10-17. Environment-imposed selection rules for nuclear-spin conversion of H$_2$ in molecular crystals. https://doi.org/10.1103/2yw9-7h62

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