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 amide, N-phenyl formamide (formanilide), and its water clusters have been studied in a jet expansion using laser-induced fluorescence excitation and mass-selected, resonant two-photon ionization (R2PI) techniques. The isomer with a trans configuration of the amide group (defined in Figure ) is identified through analysis of the partially resolved contour of its S1 ← S0 band origin. Ion-dip “hole-burn” spectra of the nonplanar cis isomer contain either symmetric or antisymmetric components of low-frequency progressions, providing evidence of a double-minimum ground state potential. Excited-state vibrations at 76 and 152 cm-1, which are strongly Franck−Condon active, show evidence of Duschinski mixing of the ground-state modes including Cring−N torsion. Water clusters have been observed for trans-formanilide only:  two distinct 1:1 hydrates, two 1:2 hydrates, and a complex with at least four bound water molecules. (The cis isomer is also expected to form extremely stable complexes with water, but none have been detected experimentally in the present study.) The observed clusters are assigned using spectroscopic data, including band contours, to structural alternatives computed ab initio at the HF/6-31G* level. The 1:1 hydrates are assigned to a cluster in which water binds at the NH site and one in which water binds at the HCO site. In the 1:2 clusters, the addition of a further water molecule to each of the 1:1 clusters results in cyclic hydrogen-bonded structures, with the water dimer bridging between proton donor and proton acceptor sites of the host. The interactions are HCO···HOH and OCH···OH2 in one case and NH···OH2 and πring···HOH in the other. At the MP2/6-31G*//HF/6-31G* level, these structures are ca. 10 kJ mol-1 more stable than the nearest competitor, in part because of cooperative effects. The R2PI spectrum of the NH bound 1:2 cluster “C” is very similar to that of the indole(H2O)2 complex assigned by Zwier and co-workers. Its origin is red-shifted 482 cm-1 from trans-formanilide, and the electronic transition excites long intermingled vibrational progressions with frequencies of 29, 38, and 51 cm-1.

Fluorescence excitation, resonant two-photon ionization (R2PI) and IR-UV ion dip spectroscopy have been used to study conformers of N-benzylformamide (NBFA) and associated clusters including hydrates with up to n ) 3 water molecules. The most stable conformer has a trans arrangement of the HNCO atoms. It is distinguished from the cis conformer by a higher frequency for the NH stretch (3478 cm -1 , compared to 3443 cm -1 ) and lower frequency for the amide I overtone (3435 cm -1 , compared to 3465 cm -1 ). The cis conformer forms cyclic H-bonded structures with one or two water molecules, binding via strong H-bonds to the neighboring CdO and NH groups. With the addition of a third water molecule, the cyclic water trimer binds to both these groups in preference to a linear chain of three waters. For trans-NBFA, a single water binds to the carbonyl group and is further stabilized by dispersive CH‚‚‚O water interactions. Two water molecules bind to the NH group instead and form a bridge to the π-system of the aromatic ring. A heterodimer species is also observed, composed of cis-and trans-NBFA. It is stabilized by NH trans ‚‚‚OdC cis and NH cis ‚‚‚π trans H-bonds, which give rise to shifts in the NH stretch frequency of -102 and -28 cm -1 , respectively. Flexibility of the amide side chain plays a key role in promoting additional CH‚‚‚O water interactions in these clusters. When compared to the unsolvated conformers, some of the clusters exhibit considerable distortion in the dihedral angle τ 1 (C 2 C 1 C R N) and in τ 2 (C 1 C R NC), equivalent to the Ramachandran angle φ in proteins. Solvation also affects the photophysics of NBFA, as the clusters show normal fluorescence behavior while the S 1 states of the isolated molecules are affected by a competing, nonfluorescent decay process.

Fluorescence excitation, resonant two-photon ionisation (R2PI) and IR-UV ion-depletion spectroscopy were used to study the structures of 2-phenylacetamide (2PA) and its dimer and water clusters in their and S 0, S 1 ionic states. The n \ 1È3 hydrates have " daisy chain Ï structures in which the water molecules link the amide NH and CO sites. The dimer is symmetric, connected via two strong NHÉ É ÉOC hydrogen bonds. Hydrogen-bonded NH and OH modes of the 1 : 1 hydrates are strongly coupled, to the extent that the symmetric combination is almost IR forbidden. Examination and reinterpretation of previously published IR data on the 1 : 1 hydrates of formamide and 2-pyridone revealed similar coupling. Solvation a †ects the photophysics of 2PA as it does N-benzylformamide ; the isolated 2PA molecule does not appear to Ñuoresce but its clusters do. Hydrogen bonding to the amide carbonyl group could account for this behaviour by raising the energy of the lowest amide (n,p*) states above that of the Ñuorescent (p,p*) level. S 1