Resonant two-photon ionization, ultraviolet hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopy were used to assign and characterize the hydrogen-bonding topology of two conformers of the benzene-(water) 8 cluster. In both clusters, the eight water molecules form a hydrogen-bonded cube to which benzene is surface-attached. Comparison of the RIDIR spectra with density functional theory calculations is used to assign the two (water) 8 structures in benzene-(water) 8 as cubic octamers of D 2 d and S 4 symmetry, which differ in the configuration of the hydrogen bonds within the cube. OH stretch vibrational fundamentals near 3550 wave numbers provide unique spectral signatures for these “molecular ice cubes.”

Resonant ion-dip infrared (RIDIR) and UV-UV hole-burning spectroscopies are used to record the hydride stretch infrared spectra and S 1 rS 0 ultraviolet spectra of each of seven conformational isomers of tryptamine free from interference from one another. The different conformations of the ethylamine side chain produce unique S 1 rS 0 vibronic spectra, which can serve as the basis for RIDIR spectroscopy. The seven conformers possess unique spectral signatures in the alkyl CH stretch region of the infrared, which aid in the assignment of the observed transitions in the ultraviolet. Density functional theory (DFT) calculations of the structures, relative energies, and harmonic vibrational frequencies of nine low-energy minima are compared with the present and previous experimental data on tryptamine to assign the spectra of all seven conformers, all of which point the R carbon out of the plane of the indole ring. The nine conformers consist of all combinations of the three minimum-energy amino group positions (anti, gauche toward the phenyl side, and gauche toward the pyrrole side of indole) and three amino group orientations (180°, (60°) at each position. All three anti conformers are observed experimentally, whereas only the two lowest-energy of the three orientational conformers at each gauche position are observed. The dominant factor in determining the form of the CH stretch infrared spectrum is the orientation of the amino group, with the amino group position playing a secondary role. The frequencies of the S 1 rS 0 origin transitions, on the other hand, are most sensitive to the position of the amino group, whether it is anti, gauche phenyl, or gauche pyrrole. The prospects for using these methods more generally to characterize the conformations and energetics of flexible biomolecules are discussed.

A combination of resonant two-photon ionization (R2PI), resonant ion-dip infrared spectroscopy (RIDIRS), and infrared−ultraviolet (IR−UV) hole-burning spectroscopy is used to characterize the hydrogen-bonding topologies of indole−(water)1,2, 1-methylindole−(water)1 - 3, and 3-methylindole−(water)1 clusters formed and cooled in a supersonic expansion. The combination of methods provides a means of disentangling R2PI spectra that contain contributions from more than one species in the same mass channel due either to fragmentation or to the presence of conformational isomers. Density functional theory calculations (DFT Becke3LYP/6-31+G*) of the structures, harmonic vibrational frequencies, and infrared intensities provide a basis for distinguishing which structures are observed experimentally. The clusters studied exhibit a range of solvation structures around indole. In the indole−(water)1 and 3-methylindole−(water)1 complexes, the RIDIR spectra provide a benchmark frequency shift for the N−H···OH2 H-bonds in these structurally well-characterized complexes. In indole−(water)2, the two water molecules form a water dimer bridge between the N−H H-bond donor site and the indole π cloud acceptor site. For 1-methylindole−(water) n clusters, the N−H H-bonding site is blocked, favoring structures in which water acts as an H-bond donor to the indole π cloud. The RIDIR spectra show water to be π-bound either as a single molecule (n = 1), a water dimer (n = 2), or a water trimer cycle (n = 3). Two isomers of 1-methylindole−(water)3 with similar but highly entangled UV spectra are distinguished and assigned using IR−UV hole-burning spectroscopy. The isomers differ in the orientation of the H-bonds (clockwise or counterclockwise) in the water trimer cycle relative to 1-methylindole, effectively freezing out the two chiral structures of the cyclic water trimer. After the H-bonding topologies of these clusters are assigned, their electronic frequency shifts and Franck−Condon profiles are reevaluated in terms of the 1La−Lb character of the observed transitions. A coherent explanation of these data can be made without invoking 1La−1Lb energy reversal.

Resonant ion-dip infrared spectroscopy of benzene-(water) 9 : Expanding the cube The techniques of resonant two-photon ionization ͑R2PI͒, UV-UV ͑ultraviolet͒ hole-burning, and resonant ion-dip infrared ͑RIDIR͒ spectroscopies have been employed along with density functional theory ͑DFT͒ calculations to assign and characterize the hydrogen-bonding topologies of two isomers each of the benzene-͑water) 8 and (benzene͒ 2 ͑water) 8 gas-phase clusters. The BW 8 isomers ͑Bϭbenzene, Wϭwater͒ have R2PI spectra which are nearly identical to one another, but shifted by about 5 cm Ϫ1 from one another. This difference is sufficient to enable interference-free RIDIR spectra to be recorded. As with smaller BW n clusters, the BW 8 clusters fragment following photoionization by loss of either one or two water molecules. The OH stretch IR spectra of the two BW 8 isomers bear a close resemblance to one another, but differ most noticeably in the double-donor OH stretch transitions near 3550 cm Ϫ1 . Comparison to DFT calculated minimum energy structures, vibrational frequencies, and infrared intensities leads to an assignment of the H-bonding topology of the BW 8 isomers as nominally cubic water octamers of S 4 and D 2d symmetry surface attached to benzene through a H-bond. A series of arguments based on the R2PI and hole-burning spectra leads to an assignment of additional features in the R2PI spectra to two isomers of B 2 W 8 . The OH stretch RIDIR spectra of these isomers show them to be the corresponding S 4 and D 2d analogs of B 2 W 8 in which the benzene molecules each form a H-bond with a different dangling OH group on the W 8 sub-cluster.