Phillip M. Palmer

Simple water clusters of Coumarins 151 and 152A studied by IR–UV double resonance spectroscopy

Abstract The hydrogen-bonded clusters Coumarin 151/(H 2 O) 1 and Coumarin 152A/(H 2 O) 1 have been studied via IR–UV double-resonance spectroscopy. Each cluster exists in two structural forms having distinct electronic properties. Evidence is presented to show that the structures involve water hydrogen bonded to groups at opposite ends of the molecular dipole. These results were supported by semiempirical structure calculations and, for C151, by infrared measurements in the S 1 state.

Spectroscopy of jet-cooled polar complexes of aminophthalimides

Abstract The spectroscopy and excited-state lifetimes of 3- and 4-aminophthalimides (3AP, 4AP), N-methyl-4AP (NM4AP) and some complexes with polar molecules have been explored under supersonic jet conditions. Spectroscopic data, MM2-level structure calculations and rotational coherence spectroscopy show that a single water molecule attaches to 3AP by accepting a hydrogen bond from the imide > NH group. Whereas 3AP/H2O is present as a single species, the case of 4AP is complicated by the presence of two isomers, having significantly different fluorescence decay times and radiative lifetimes. This observation, which is repeated for methanol complexation, is evidence for the sensitivity of the electronic structure of 4AP to a site-specific interaction.

Electronic spectroscopy of jet-cooled anthracene/(H2O)n clusters (n=1–16): comparisons of inhomogeneous structure

Abstract Measurements of the S0→S1 electronic excitation spectra of anthracene/(H2O)n clusters (n=1–16) are reported. Preliminary data on perylene/(H2O)n, n=1–8, are included for comparison with the anthracene work and with literature data involving benzene. Near-threshold ionization and ion photodissociation measurements were employed to ensure correct identification of the species being studied. However, such measurements also reveal significant differences in the dissociation energies of the ionized clusters. Several cases are reported for anthracene/(H2O)n, including n=4, 5, 10, 14, and 15, where the spectra are much simpler than for other clusters of similar size. These results are discussed in connection with literature calculations for pure water clusters, which have suggested that water tetramers, and possibly pentamers, can act as building blocks for larger clusters.

Spectroscopic properties of jet-cooled methanol clusters, n=1–6, attached to perylene

Abstract Supersonically cooled clusters of perylene/(CH 3 OH) n ( n =1–6) have been studied using resonance-enhanced near-threshold two-photon ionization followed by time-of-flight mass-spectrometry. The perylene S 0 →S 1 transition is displaced to lower energy in all cases, but the amount of the displacement varies irregularly with cluster size over ≈150 cm −1 . Much greater variations are observed in the threshold ionization energy, over a range >3000 cm −1 .

Perylene complexes with p-dichlorobenzene and related species: comparison of rotational coherence data with structure calculations

Abstract Rotational coherence experiments have been carried out on complexes of perylene with p-dichlorobenzene (DCB), p-difluorobenzene (DFB) and hexafluorobenzene, largely to seek an explanation for the unusually small complexation shifts of the first two. All three complexes were found to have overlapped, parallel ring structures, similar to that of perylene/benzene. Moreover, in structures corresponding to the electronic ground state, the halogen-halogen axes of DFB and DCB were found to be aligned perpendicular to the perylene long axis.

Perylene/water clusters: some different trends in hydrogen-bonded structure induced by a large aromatic template

Abstract Measurements are reported of infrared-optical double-resonance spectra (3100–3800 cm −1 ) for jet-cooled perylene/(H 2 O) n clusters ranging up to n =8. Unusual patterns of OH-stretch resonances, which are particularly evident for the n =3–6 clusters, imply that some significant structural perturbations are taking place in the presence of the large aromatic molecule.

Structural measurements of hydrogen-bonded complexes of perylene with water and methanol

Abstract Rotational coherence experiments have been carried out for 1:1 van der Waals complexes of perylene with water and methanol. The observed inertial properties are consistent with structures corresponding to potential-energy minima calculated via MM2 techniques. The results show that both types of complex involve hydrogen-bonding to π-electrons on the perimeter of the aromatic molecule.