Solid-state density functional theory is a powerful computational method used to provide insight into the low-frequency vibrations of crystalline solids. A known limitation of this method is its general underestimation of weak intermolecular forces. Semiempirical London force corrections have been developed to augment density functional theory calculations with the ultimate goal being corrections that are applicable to a range of compounds. In this study, two structurally similar amino acids, asparagine and glutamine, were chosen to gauge the proximity of the widely used DFT-D2 approach to this goal. Despite their chemical similarities, it was determined that the two molecular solids required considerably different semiempirical correction magnitudes, with asparagine requiring a 42% greater London force correction factor when compared to glutamine. To validate these findings, terahertz spectroscopy was used to investigate the intermolecular vibrations of both amino acids in the low-frequency, sub-100 cm-1 region. The excellent correlation between the experimental and the theoretical spectra demonstrates that the noncovalent interactions are well represented by the applied model despite the correction disparity. These results have highlighted a practical shortcoming of a common semiempirical method for the modeling of weak forces and emphasizes that care must be exercised for effective use of such corrections in crystalline solids. (Figure Presented).
ASJC Scopus subject areas
- Physical and Theoretical Chemistry