Hydrated Clusters of 2-Phenylethyl Alcohol and 2-Phenylethylamine: Structure, Bonding, and Rotation of the S1 ← S0 Electronic Transition Moment

Abstract

Hydrated clusters of 2-phenylethyl alcohol (PEAL) and 2-phenylethylamine (PEA) have been studied in a jet-cooled environment, using laser-induced fluorescence excitation and mass-selected resonant two-photon ionization (R2PI) spectroscopy of the S1 ← S0 electronic transitions. Spectral features have been observed for clusters M(H2O)n, n = 1−4, and their stoichiometry assigned on the basis of the ion fragmentation patterns. Ionization of hydrated PEA(H2O)n clusters leads to the observation of PEA(H2O)n-1+ and CH2NH2(H2O)n+ ions. Partially resolved rotational band contours of several n = 1, 2 clusters have been analyzed with the aid of ab initio molecular orbital calculations, conducted at the MP2/6-31G*//HF/6-31G* level for the ground state, and CIS/6-31G* for the first electronically excited singlet state. The analysis reveals the supramolecular structure: the host molecular conformation within these clusters and the binding sites of the water molecules. In n = 1 clusters of 2-phenylethylamine, the primary binding site involves hydrogen bonding to the nitrogen atom in the amine group. Cyclic hydrogen-bonded structures are observed for n = 2 clusters. In 2-phenylethyl alcohol, two different 1:1 clusters have been assigned in which the water molecule binds alternatively as a proton acceptor and proton donor. Further interactions between water molecules and the host, e.g., Hwater···π and Owater···HC, lead to additional stabilization of certain complexes. The assignments are aided greatly by the extraordinary sensitivity of the S1 ← S0 transition moment alignment to both side chain conformation and long-range intermolecular interactions.