abstract
A new computationally-assisted methodology (PiMM), which accounts for the effects of intermolecular interactions in the crystal, is applied to the complete assignment of the Roman and infrared vibrational spectra of room temperature forms of crystalline caffeine, theobromine, and theophylline. The vibrational shifts due to crystal packing interactions are evaluated from ob initio calculations for a set of suitable molecular pairs, using the B3LYP/6-31G* approach. The proposed methodology provides on answer to the current demand for a reliable assignment of the vibrational spectra of these methyl-xanthines, and clarifies several misleading assignments. The most relevant intermolecular interactions in each system and their effect on the vibrational spectra are considered and discussed. Based on these results, significant insights are obtained for the structure of caffeine in the anhydrous form (stable at room temperature), for which no X-ray structure has been reported. A possible structure based on C-(8)-H center dot center dot center dot N-(9) and C-(1,C-3)-H center dot center dot center dot O intermolecular interactions is suggested.
keywords
ANHYDROUS THEOPHYLLINE; RAMAN-SPECTROSCOPY; CAFFEINE HYDRATE; SOLID-STATE; X-RAY; THEOBROMINE; METHYLXANTHINES; STABILITY; BEHAVIOR; DEHYDRATION
subject category
Chemistry; Physics
authors
Nolasco, MM; Amado, AM; Ribeiro-Claro, PJA
our authors
Mariela Martins Nolasco
Assistant Researcher