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

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 selfconsistent- 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.

glycolic acid; lactic acid; Raman; infrared; DFTB+; GULP; normal modes; phonons; temperature stability; biodegradable material

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