Abstract-Terahertz Vibrations of Crystalline Acyclic and Cyclic Diglycine: Benchmarks for London Force Correction Models

Thomas Robert Juliano and Timothy Michael Korter

Terahertz spectroscopy provides direct information concerning weak intermolecular forces in crystalline molecular solids 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, acyclic diglycine and cyclic diglycine, as benchmark systems for gauging the performance of semi-empirical London force correction approaches. The diglycine samples were investigated using pulsed terahertz spectroscopy from 10 to 100 cm^-1 and then analyzed using solid-state density functional theory (DFT) augmented with existing London force corrections, as well as a new parameterization (DFT-DX) based on known experimental values. The two diglycine molecules provide a useful test for the applied models given their similarities, but more importantly the differences in the intermolecular forces displayed by each. It was found that all of the considered London force correction models were able to generate diglycine crystal structures of similar accuracy, but considerable variation occurred in their abilities to predict terahertz frequency vibrations. The DFT-DX parameterization was particularly successful in this investigation and shows promise for the improved analysis of low-frequency spectra.