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 S 1 r S 0 electronic transitions. Spectral features have been observed for clusters M(H 2 O) n , n ) 1-4, and their stoichiometry assigned on the basis of the ion fragmentation patterns. Ionization of hydrated PEA(H 2 O) n clusters leads to the observation of PEA(H 2 O) n-1 + and CH 2 NH 2 (H 2 O) 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., H water ‚‚‚π and O water ‚‚‚HC, lead to additional stabilization of certain complexes. The assignments are aided greatly by the extraordinary sensitivity of the S 1 r S 0 transition moment alignment to both side chain conformation and long-range intermolecular interactions.

Specific and nonspecific interactions in a molecule with flexible side chain: 2-phenylethanol and its 1:1 complex with argon studied by high-resolution UV spectroscopy

“…The binding situation may be compared with that of the fairly related system consisting of 1:1 complex of 2-PE with water. 14,15 Since there a water molecule is involved, the favorable binding sites are likely to be determined by suitability of specific hydrogen bond formations, while in the case of Ar binding to 2-POE the nonspecific dispersion interaction must be considered. The IR-UV double resonance spectrum of the 2-PE-water species indicates a formation of two isomeric hydrogen-bonded structures.…”

“…The IR-UV double resonance spectrum of the 2-PE-water species indicates a formation of two isomeric hydrogen-bonded structures. 14,15 In the first structure, the -HO hydrogen bond of the gauche form of 2-PE is preserved and the water binds to one of the lone pairs of alcoholic oxygen atom. In the second conformer, the -hydrogen bond between the aromatic ring and alcoholic OH is bridged by an intercalated water molecule.…”

“…Therefore, it is of paramount importance to know not only the most favorable conformations of the bare biomolecules 1-13 but also their conformational preferences in the presence of other species. 2,3,12,14,15 In the case of the bare 2-phenylethanol, recent spectroscopic measurements and theoretical calculations 3,5,7,11,15 indicate that its most favored conformation in the electronic ground state corresponds to a gauche geometry. The primary factor that stabilizes this conformeric structure is a nonclassical -hydrogen bonding of the terminal alcoholic group of the side chain with the aromatic ring.…”

“…Similarly, in the present case of 2-PE, for the preferred binding site we have to consider not only the nonspecific interaction of Ar with the aromatic ring but also the specific "hydrogen-bond-type" interaction with the OH group of the side chain. 14,15 There are three distinct possibilities that must be considered: ͑i͒ preserving the gauche conformation of the side chain, the Ar atom can bind to the aromatic ring at the opposite side of the OH group, ͑ii͒ the Ar is attached to the ring on the same side of the O-H group, and ͑iii͒ the Ar atom binding to the aromatic ring makes the side chain change its structure, assuming an extended geometry. To ascertain which scenario͑s͒ is realized in the conditions of the cold molecular beam high-resolution experiments have been carried out.…”

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate

“…[18,19] In addition, it can play an important role in stabilizing specific conformations of af lexible solute molecule, as observedf or some neutral aromatic amines such as tryptamine and PEA. [20][21][22][23][24][25][26] Rotational spectroscopy,i onization-loss stimulated Raman,l aser-induced fluorescence, and multiphoton ionization experimentso ft he PEA monohydrate (PEA-H 2 O) have revealed the hydration-induced locking of the PEA conformation, [15,23,24,27,28] which is also observed for the related tryptamine-H 2 Oc omplex by UV spectroscopy at the level of rotational resolution. [21] The four PEA conformers that were detectedi nasupersonic beam and which differ by the orientation of the ethylamino side chain collapse into one single structure in which the H 2 Om olecule, the Ha tom of which is directedt oward the lone pair of the N atom of PEA, locks the side chain conformation into afavorable NH···p configuration.…”