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Raman spectroscopy demonstrates that the rotational spectrum of solid hydrogen, and its isotope deuterium, undergo profound transformations upon compression while still remaining in phase I. We show that these changes are associated with a loss of quantum character in the rotational modes, ie. with increasing pressure, the angular momentum J gradually ceases to be a good quantum rotational number. Through isotopic comparisons of the rotational Raman contributions, we reveal that hydrogen and deuterium evolves from a quantum rotor to a harmonic oscillator. We find that the mechanics behind this transformation can be well described by a quantum mechanical single inhibited rotor, accurately reproducing the striking spectroscopic changes observed in phase I.