  The laws of quantum mechanics are often tested against the behaviour of the
lightest element in the periodic table, hydrogen. One of the most striking
properties of molecular hydrogen is the coupling between molecular rotational
properties and nuclear spin orientations, giving rise to the spin isomers
ortho- and para-hydrogen. At high pressure, as intermolecular interactions
increase significantly, the free rotation of H2 molecules is increasingly
hindered, and consequently a modification of the coupling between molecular
rotational properties and the nuclear spin system can be anticipated. To date,
high-pressure experimental methods have not been able to observe nuclear spin
states at pressures approaching 100 GPa and consequently the effect of high
pressure on the nuclear spin statistics could not be directly measured. Here,
we present in-situ high-pressure nuclear magnetic resonance data on molecular
hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our
measurements confirm the presence of I=1 ortho-hydrogen at low pressures, above
70 GPa, where inter- and intramolecular distances become comparable, we observe
a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a
spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These
observations represent a unique case of a nuclear spin crossover phenomenon in
quantum solids.
