A PW91 density functional study of conformational choice in 2-phenylethanol, n-butylbenzene, and their cations: Problems for density functional theory?

Absolute energies, geometric structures, harmonic vibrational frequencies, and partial charge distributions (NPA) are obtained from ab initio (HF/MP2) and density functional theory (DFT) calculations of stable conformers of neutral and cationic 2-phenylethanol and n-butylbenzene, model aromatic molecules with flexible side chains. We focus on exploring the conformational preferences of the cations and find that cationic conformational preference is more pronounced in the system with the strongly interacting side chain. The DFT calculations presented use the Perdew−Wang exchange and correlation functional, PW91. For the neutral conformers, PW91 performs extremely well compared to MP2, indicating that this functional will be highly useful for future computational studies of neutral aromatic molecules with flexible side chains. For the cationic conformers, PW91 again performs well compared to MP2 for n-butylbenzene, a system in which the side chain interacts only weakly with the aromatic ring. However, considerable discrepancies occur between the MP2 and DFT calculations for 2-phenylethanol+. The results indicate that density functional theory does not provide a reliable description of the potential energy surface of 2-phenylethanol+, and that high levels of theory may be necessary to accurately treat similar cations. The present study represents the first systematic comparison of HF, MP2, and DFT calculations of cationic conformers of aromatic molecules with flexible side chains.