The evolution of structural motifs of gold cluster anions, Au n − , in the size range n = 11-24 has been determined through a comparison of electron diffraction data with density functional calculations. The results provide clear evidence for a transformation from planar to three-dimensional structures in the range n = 12-14, the development of cage structures for n = 16 and 17, the appearance of a tetrahedral structure at n = 20, and the emergence of a highly symmetric tubular structure for n = 24.

We report the experimental structure determination of cold, mass selected Ag(55)(+/-) cluster ions using the recently developed technique of trapped ion electron diffraction. By comparison of experimental and theoretical molecular scattering functions and consideration of computed total energies, we show that Ag(55)(+) constitutes an ideal Mackay icosahedron, whereas Ag(55)(-) is a weakly Jahn-Teller distorted icosahedron. Isomers of other structural types, for example, decahedral or close-packed, could be ruled out. The candidate structures were obtained by density functional theory calculations.

No abstract

Bimolecular rate constants (kQ) were measured for the fluorescence quenching of a series of aromatic hydrocarbons by di-t-butylnitroxide (DTBN). In methylcyclohexane, measured values of kQ were close to the diffusion limit with only a 6.5-fold variation through the hydrocarbon series. A study of the viscosity dependence of the quenching reaction, in the Smoluchowski manner, suggests an interaction distance of 4–6 Å. A comparison is made of the efficiency of the DTBN and O2 fluorescence quenching reactions. Possible mechanisms are discussed.

We report the first direct measurements of the unfolding of a protein, Trp-cage, in the gas phase using laser-induced fluorescence of protein ions in a heated quadrupole ion trap. The changes in enthalpy and entropy associated with the observed conformational change are obtained by fitting a two-state model of protein unfolding to the fluorescence intensities plotted versus temperature. The enthalpy and entropy changes for the 2+ and 3+ charge states are greater than the values measured in solution and depend on charge state.