Conformational Choice, Hydrogen Bonding, and Rotation of the S1 ← S0 Electronic Transition Moment in 2-Phenylethyl Alcohol, 2-Phenylethylamine, and Their Water Clusters

• Published in: Journal of the American Chemical Society 1998-03, Vol.120 (11), p.2622-2632
• Main Authors: Dickinson, John A, Hockridge, Matthew R, Kroemer, Romano T, Robertson, Evan G, Simons, John P, McCombie, June, Walker, Melinda
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja972104o

Laser-induced fluorescence and one- and two-color, mass- selected R2PI excitation spectra of the S1 ← S0 electronic transitions in 2-phenylethyl alcohol and 2-phenylethylamine have been recorded in a jet-cooled environment. Five conformers of 2-phenylethyl alcohol and four of 2-phenylethylamine have been identified, together with a number of 1:1 hydrated water clusters. The fifth origin band in the excitation spectrum of 2-phenylethylamine has been reassigned to a water cluster, primarily on the basis of its ion fragmentation pattern. Analysis of their partially resolved rotational band contours has been aided by ab initio molecular orbital calculations, conducted at levels of theory ranging from MP2/3-21G* to MP2/6-311G** for the ground state and CIS/6-311G** for the first electronically excited singlet state. The reliability of the CIS method has also been tested through benchmark calculations, including computations on a related, experimentally known conformational system, methyl 3-hydroxybenzoate. 2-Phenylethylamine and 2-phenylethyl alcohol both display anti and gauche conformations (distinguished by their orientation about the Cα−Cβ bond) but the folded, gauche conformations, which allow the terminal hydroxyl or amino hydrogen atoms to be hydrogen bonded to the aromatic ring, are found to be the most stable. Their intramolecular binding energies are ∼5.5 kJ mol-1. The anti conformers display b-type rotational band contours, reflecting the 1Lb character of their first excited singlet states. In contrast, the band contours of the gauche conformers display a hybrid character, which reflects a strong rotation of the electronic transition moment in the molecular frame, attributed to electronic state mixing. The rotation of the transition moment is strongly modulated by the binding of a water molecule to the folded molecular conformer and, in the bare molecule, by changes in the orientation of the terminal hydroxyl or amino group. This effect allows a ready distinction to be made between the hydrogen-bonded and the non- hydrogen-bonded gauche conformers.