Cryogenic terahertz spectrum of (+)-methamphetamine hydrochloride and assignment using solid-state density functional...

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Terahertz (THz) spectroscopy is a valuable tool for analyzing molecular vibrations.
• Previous THz studies of (+)-methamphetamine hydrochloride were conducted at room temperature.
• Cryogenic conditions can enhance spectral resolution and reveal subtle features.

Purpose of the Study:

• To present the cryogenic terahertz spectrum of (+)-methamphetamine hydrochloride.
• To perform a complete structural analysis and vibrational assignment using computational methods.
• To investigate the necessity of solid-state simulations for accurate THz spectral assignment.

Main Methods:

• Cryogenic terahertz spectroscopy (10.0–100.0 cm⁻¹).
• Solid-state density functional theory (DFT) calculations.
• Comparison of eight density functionals using solid-state and isolated-molecule methods.
• Vibrational assignment based on the BP density functional.

Main Results:

• The cryogenic THz spectrum revealed multiple spectral features not observed in room-temperature studies.
• Solid-state DFT simulations were essential for accurate assignment of the experimental THz spectrum.
• The BP density functional provided the best agreement between simulated and experimental spectra.
• Thirteen infrared-active vibrational modes were predicted, with most intensity attributed to external crystal vibrations.

Conclusions:

• Cryogenic THz spectroscopy provides enhanced insights into the vibrational dynamics of (+)-methamphetamine hydrochloride.
• Accurate assignment of solid-state THz spectra necessitates the use of solid-state computational simulations.
• The BP/DNP level of theory effectively models the vibrational modes responsible for the observed spectral features.