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 similar to 103 and similar to 95 cm(-1) wavenumbers were detected, which corresponded to translational vibrations of O-H(D)center dot center dot center dot 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 Delta E of the ground vibrational state of potential minima for L- and R-tautomers is similar to 80 meV for COOH and similar to 70 meV for GOOD. At T >= 150 K, the vibrational frequency of the C=O bond in the COOH moiety exhibits an unusual temperature dependence.