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 nonmagnetic molecular crystal field can impose and relax these rules without external fields. High-resolution infrared spectra of ${\mathrm{H}}_2$ in crystalline ${\mathrm{CO}}_2$ reveal large rank-2 (quadrupolar) crystal-field splittings of the $m$ sublevels, while nuclear-spin conversion occurs only through $\mathrm{\Delta }m=0$ channels. Replacing ${\mathrm{CO}}_2$ with polar ${\mathrm{N}}_2\mathrm{O}$ introduces rank-1 (dipole) components that partially open $\mathrm{\Delta }m\ne 0$ pathways, while incorporation of paramagnetic ${\mathrm{NO}}_2$ 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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• Chemical hydrogen storage
• Coherent control
• Nuclear spin & parity
• Potential energy surfaces
• Quantum state engineering
• Rotational states
• van der Waals interaction
• Density functional theory
• Optical spectroscopy