There has been a great deal of work using THz time-domain spectroscopy (THz-TDS) to characterize the low frequency vibrational modes in organic molecular crystals (OMCs).  While the THz spectra of many OMCs have been reported, assigning these spectra has been exceedingly difficult.  Unlike high frequency modes which are localized to a single or several bonds in close proximity, low frequency modes involve motion of most or all of the atoms in the molecule.  Furthermore, there is strong coupling of intramolecular and intermolecular motions within the crystalline environment which further complicates matters.  Lastly, weak van der Waals interactions dramatically influence the THz spectra of OMCs, yet are difficult to treat computationally.  A new way to compare results from different calculations will be presented. In addition, recent progress toward the goal measuring the vibrational circular dichroism (VCD) spectrum of a molecular system in the THz spectral region will be reported.  VCD of chiral molecules in the mid-infrared has emerged over the last 3 decades as a powerful method to assign spectra, particularly with regard to their absolute stereochemical configuration.  The VCD spectrum in addition to the absorption spectrum will provide experimental data that will constrain possible peak assignments of THz spectra.

THz-TDS studies of low frequency modes in organic molecular crystals and progress toward THz vibrational circular dichroism

245th National Meeting (2013)

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