In this contribution, the solid-state low-temperature phase structure of [2,2]-paracyclophane is unambiguously characterised by single-crystal X-ray analysis. Additionally, a heat capacity measurement was undertaken, which proves the existence of a λ-type phase transition at 45 K, a transition that is connected with the formation of a secondary Cp/T feature at 60 K. The low-temperature phase (<45 K) crystallises in the lower symmetry space group P4n2, whereas the high-temperature phase (>60 K) crystallises in space group P4(2)/mnm. This proves what has been postulated both by experimental and theoretical chemists but has repeatedly been dismissed as speculation many times.
Keywords: paracyclophane; phase transitions; space group ambiguity; structure elucidation; thermal motion.
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