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 ΔE 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.