No abstract

No abstract

The conformational structures of tryptophan, isolated in the gas phase, have been assigned by combining the results of ultraviolet hole-burning and infrared ion dip spectroscopy with the predictions of ab initio calculations conducted at the MP2/6-311]G(d,p)//B3LYP/6-31]G(d) levels of theory. As in phenylalanine, the most strongly populated, and lowest energy conformer presents a folded alanyl side chain that is stabilised by a " daisy chain Ï of hydrogen-bonded interactions. These link the acidic proton, the amino group and the indole ring. There is a further interaction between the carbonyl oxygen and the neighbouring CH group on the pyrrole ring. A quantitative evaluation of the dipoleÈdipole interactions between the alanyl side chain and the indole ring in the and electronic states does not support the suggestion of electronic state mixing. In 1L a 1L b particular it casts doubt on the assignment of the Ñuorescence of the most stable, " special Ï conformer to emission from the state.

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.

The conformational landscapes of 2-amino-1-phenylethanol and its 1:1 water complexes have been investigated by UV band contour, UV−UV hole-burning, and IR−UV ion dip spectroscopy, coupled with ab initio computation. The two molecular conformers observed are both stabilized by an intramolecular hydrogen bond, located in the folded (gauche) OCCN side chain, which links the proton donor OH group to the terminal amino group and leads to a significant constriction of the side chain. In the dominant 1:1 water complex, the intramolecular hydrogen bond is disrupted by the first water molecule, which inserts between the OH group and the nitrogen atom, to form a cyclic H-bonded structure. The side chain expands significantly in order to accommodate the water molecule within the neighborhood of both the hydroxyl and amino groups.