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

• Low temperature Raman spectra of glycolic and lactic acid are presented.
• Spectra show smooth temperature variation.
• No phase transitions are indicated in the 10–300 K 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 295 K to 10 K 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 −0.337 eV/mol, −0.329 eV/mol, −0.262 eV/mol and −0.242 eV/mol, while in lactic acid two hydrogen bonds have energies of −0.283 eV/mol and −0.202 eV/mol.

Temperature dependent Raman spectra of glycolic and lactic acid reveal great stability of crystal lattices from 295 K to 10 K. Four different hydrogen bonds exist in glycolic acid, and two types in lactic acid. Their strengths were estimated to be between 0.2 eV/mol and 0.34 eV/mol.Figure optionsDownload as PowerPoint slide