The short conference paper The Acetylene-Butane Co-Crystal: A Potentially Abundant Molecular Mineral on Titan (mentioned in EurekaAlert's 'Bathtub rings' around Titan's lakes might be made of alien crystals) says:

Experimental: Acetylene (purified to remove acetone stabilizer) and butane (99% purity) were condensed into a liquid nitrogen-cooled cryostage (Linkam Scientific Instruments Ltd.) at 90 K or 130 K. Raman spectra within the cryostage were obtained using a high-resolution confocal dispersive micro- Raman spectrometer (Horiba Jobin Yvon LabRam HR) equipped with a 50 mW Nd:YAG laser (frequency- doubled 532 nm) as the excitation source. Thermal stability studies were performed by warming in 5 or 10 K increments and obtaining Raman spectra following a 5-minute equilibration time.

Results: Blue shifts (12–29 cm-1) of the C≡C and C-H stretching modes indicate the formation of a co-crystalline compound. We observed similar shifts for co-crystals comprised of benzene and ethane (2–12 cm- 1 red shifts; Vu et al. 2014) and of acetylene and ammonia (7–16 cm-1 red shifts; 42–66 cm-1 blue shifts; Cable et al. 2018). These shifts indicate a change in the chemical environment of the molecular species, typically modification of a host crystal lattice to accommodate a guest molecule.

Question: Is it common to conclude that such blue shifts in Raman spectra "...indicate the formation of a co-crystalline compound"? There is a solid with both molecules in close contact so they interact, but how does this point specifically to there being a o-crystalline compound?

  • $\begingroup$ No. Those are the results of a specific case. They could have said "Red shift (12-29 cm^-1....) and I would have kept reading without wondering. Indeed, this is the case for the benzene-ethane sample. Also, confocal microraman is a microscope, allowing to spatially resolve the spectroscopic features. That is what probably points to co-crystallisation, i.e. no or little segregation of phases. $\endgroup$ – Alchimista Jun 25 '19 at 8:08

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