Environment-Imposed Selection Rules for Nuclear-Spin Conversion of H_{2} in Molecular Crystals

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 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 Δm=0 channels. Replacing CO_{2} with polar N_{2}O introduces rank-1 (dipole) components that partially open Δm≠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.