Gas phase conformation in the ibuprofen analogues isobutylbenzene and 2-phenylpropionic acid

The S-1 <- S-0 transitions of isobutylbenzene (IBB) and 2-phenylpropionic acid (2PPA) have been studied in a supersonic jet expansion by resonant 2-photon ionization (R2PI). Conformational assignments have been made with the aid of ab initio molecular orbital calculations on the ground and excited electronic states. Dominant features in the IBB spectrum are assigned to conformer AG, where A and G refer to anti and gauche arrangements of the two methyl groups with respect to the C-alpha-C-beta bond. Additional weak features are assigned to a higher energy GG conformer, present in low concentration in the jet. Four possible conformers of 2PPA are predicted, but the R2PI spectrum provides evidence of only one. A low barrier to interconversion between the two most stable conformers suggests that the higher energy form is depopulated by relaxation occurring in the jet expansion.