Evaluating London Dispersion Force Corrections in Crystalline Nitroguanidine by Terahertz Spectroscopy

The low-frequency vibrations of the crystalline explosive nitroguanidine have been investigated using pulsed terahertz (THz) spectroscopy (10 to 100cm^-1 ) and analyzed with solid-state density functional theory simulations. The THz spectrum recorded at 78 K exhibited five distinct absorptions that have been successfully assigned using PBE/cc-pVTZ augmented with semi-empirical London force dispersion corrections. This computational investigation of the nitroguanidine structure and dynamics in the solid-state also included a study of the utilized dispersion force correction parameters, resulting in the development of new values for use in the analysis and prediction of the THz spectra of molecular solids. The applied models enabled the observed spectral absorptions to be assigned to external rigid rotations and translations of the molecules in the crystal. The assignments were further validated by a complementary study of perdeuterated nitroguanidine.