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

Abstract

The S₁← S₀ 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_α–C_β 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.

Acknowledgements

The authors gratefully acknowledge financial support from the Australian Research Council and a Monash University Logan Fellowship (ER). Quantum chemistry calculations were carried out using the Australian Partnership for Advanced Computing (APAC) National Facility and computers at Melbourne University. Special thanks to Dr. Chris Thompson for his timely assistance with calculations. Our team is part of the recently formed Australian Centre for Enabling Molecular Sciences. Finally, we are indebted to Professor John Simons for his enthusiastic, enduring and creative contributions to the field of gas phase biomolecules.