Raman spectroscopic evidence of low temperature stability of d,l-glycolic and l-(+)-lactic acid crystals

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Molecular and biomolecular spectroscopy, 2015-04, Vol.140, p.35-43
• Main Authors: Mohaček-Grošev, Vlasta, Šoštarić, Vladimir, Maksimović, Aleksandar
• Format: Article
• Language: English
• Subjects: Biodegradable material; Glycolic acid; Lactic acid; Normal modes; Phonons; Temperature stability
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2014.12.048

Temperature dependent Raman spectra of glycolic and lactic acid reveal great stability of crystal lattices from 295K to 10K. Four different hydrogen bonds exist in glycolic acid, and two types in lactic acid. Their strengths were estimated to be between 0.2eV/mol and 0.34eV/mol. [Display omitted] •Low temperature Raman spectra of glycolic and lactic acid are presented.•Spectra show smooth temperature variation.•No phase transitions are indicated in the 10–300K interval.•Vibrational analysis for free molecules and crystals is performed. Raman and infrared spectra of polycrystalline d,l-glycolic and l-(+) lactic acid are presented and assigned both by an ab initio calculation of normal modes of free conformers and by self-consistent-charge density-functional-theory computational program DFTB+. Temperature dependent Raman spectra from 295K to 10K reveal great stability of crystal lattices, since no soft modes and no band splittings that could be attributed to changes of the number of molecules per unit cell were observed. A semiempirical calculation with GULP program was used to estimate the strength of hydrogen bonds in crystals: in glycolic acid they have energies of −0.337eV/mol, −0.329eV/mol, −0.262eV/mol and −0.242eV/mol, while in lactic acid two hydrogen bonds have energies of −0.283eV/mol and −0.202eV/mol.