Organic Molecular Crystals

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Vibrational spectra of molecular crystals contain intermolecular modes that are not present in other phases. These vibrations typically occur at frequencies in the THz range and are very sensitive to the nature of the crystal lattice; these spectral features can be used to differentiate between materials that spectroscopy in the intramolecular frequency range cannot. For example, a particular molecule can often crystallize in different structures, a phenomenon known as polymorphism. Even though they are made of the same molecules, the different crystal structures can have distinct properties, such as solubility. This can make polymorph identification essential when designing optimal methods of pharmaceutical delivery. The intramolecular modes of two polymorphs can be extremely similar (the crystals are composed of exactly the same molecules, after all), but fortunately, the low-frequency intermolecular modes are directly affected by differences in crystal geometry and often allow one to immediately identify a particular polymorph.1

While the simplest picture of dynamics in molecular crystals assumes that the intermolecular modes are separate from the intramolecular modes, these low-frequency vibrations usually have more complicated, mixed character. Generally speaking, the terahertz region of vibrational spectra is the frequency range where these motions trend from predominantly intermolecular in character to predominantly intramolecular in character.2 The particular qualities of these vibrations result from the complex interplay of electrostatic, dipole-dipole, hydrogen bonding, and van der Waals interactions. Terahertz spectroscopy is a direct method for exploring this complicated energy landscape.

References

  1. True, A. B.; Schroeck, K.; French, T. A.; Schmuttenmaer, C. A., Terahertz Spectroscopy of Histidine Enantiomers and Polymorphs. J. Infrared Millim. Terahertz Waves 2011, 32 (5), 691-698.
  2. Williams, M. R. C.; True, A. B.; Izmaylov, A. F.; French, T. A.; Schroeck, K.; Schmuttenmaer, C. A., Terahertz Spectroscopy of Enantiopure and Racemic Polycrystalline Valine. Physical Chemistry Chemical Physics 2011, 13, 11719-11730.