Terahertz spectroscopy is a useful technique for examining and characterizing the low-frequency vibrations of crystalline molecular solids.  Terahertz data provides direct information concerning weak intermolecular forces in such materials and therefore acts as an excellent method for calibrating and evaluating computational models for non-covalent interactions.  In this study, the low-frequency vibrations of two dipeptides were compared, cyclic diglycine and acyclic diglycine.  They were investigated using pulsed terahertz spectroscopy (10 to 100 cm-1) and solid-state density functional theory (DFT).  The terahertz spectra were recorded at both room temperature and 78 K, showing the temperature dependence of the spectra and aiding in the spectral feature resolution.  Each of the spectra were unique, with the linear and cyclic diglycines showing four and three distinct absorptions, respectively, at 78 K.  Solid-state DFT calculations utilizing PBE/cc-pVTZ with the addition of semi-empirical London force dispersion corrections were used to assign the modes seen in the cold spectra.  To analyze the accuracy of these dispersion corrections, several parameterizations were tested for their ability to reproduce the experimental crystal structures and terahertz vibrational spectra.  It was found that a new set of experiment-based parameters (dispersion coefficients and van der Waals radii) yielded results that were superior to traditional DFT-D2 methods.  These modifications will be discussed in detail and the broad applicability of this approach considered.

Utilizing terahertz spectroscopy in the evaluation of London dispersion forces in cyclic and acyclic dipeptides

246th National Meeting (2013)

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