Tautomeric Hydrogen Bond in Dimers of Ibuprofen

The Raman spectra of polycrystalline samples of ( RS)-2-(4-isobutylphenyl)-propionic acid of the common name ibuprofen have been measured in the temperature range 5-300 K. In the low-frequency spectrum of the normal C₁₂H₁₇(COOH) and deuterated C₁₂H₁₇(COOD) species, modes with ∼103 and ∼95 cm^-1 wavenumbers were detected, which corresponded to translational vibrations of O-H(D)···O hydrogen bonds of two different tautomers: left L and right R , respectively. At temperatures below 150 K, only the L-tautomer is found, and at T ≥ 150 K, both tautomers are observed. The energy difference Δ E of the ground vibrational state of potential minima for L- and R-tautomers is ∼80 meV for COOH and ∼70 meV for COOD. At T ≥ 150 K, the vibrational frequency of the C═O bond in the COOH moiety exhibits an unusual temperature dependence.