Unexpected size distribution of Ba(H2O)n clusters: why is the intensity of the Ba(H2O)1 cluster anomalously low?

An experimental and theoretical study on the reactivity of neutral Ba atoms with water clusters has been conducted to unravel the origin of the irregular intensity pattern observed in one-photon ionization mass spectra of a Ba(H(2)O)(n)/BaOH(H(2)O)(n-1) (n = 1-4) cluster distribution, which was generated in a laser vaporization-supersonic expansion source. The most remarkable irregular feature is the finding for n = 1 of a lower intensity for the Ba(+)(H(2)O)(n) peak with respect to that of BaOH(+)(H(2)O)(n-1), which is opposite to the trend for n = 2-4. Rationalization of the data required consideration of a distinct behavior of ground-state and electronically excited state Ba atoms in inelastic and reactive Ba + (H(2)O)(n) encounters that can occur in the cluster source. Within this picture, the generation of Ba(H(2)O)(n) (n > 1) association products results from stabilizing collisions with atoms of the carrier gas, which are favored by intramolecular vibrational redistribution that operates on the corresponding collision intermediates prior to stabilization; the latter is unlikely to occur for Ba + (H(2)O) encounters. Overall, this interpretation is consistent with additional in-source laser excitation and quenching experiments, which aimed to explore qualitatively the effect of perturbing the Ba atom electronic state population distribution on the observed intensity pattern, as well as with the energetics of various possible reactions for the Ba + H(2)O system that derive from high level ab initio calculations.

Microsolvation and protonation effects on geometric and electronic structures of tryptophan and tryptophan-containing dipeptides

Photodissociation spectroscopy of solvated clusters of protonated tryptophan (TrpH(+)) and dipeptides containing tryptophan (Val-TrpH(+), Ala-TrpH(+), and Gly-TrpH(+)) has been carried out at low temperature to investigate the protonation and solvation effects on the electronic spectrum. For the protonated dipeptides, the S(1)-S(0) transition exhibits a substantial red shift due to the stronger interaction between the NH(3)(+) group and the indole pi ring. The S(1)-S(0) spectra of TrpH(+)(CH(3)OH)(n) clusters exhibit a drastic change with the number of methanol molecules. This behavior is interpreted in terms of the decrease in the interaction between the pi pi* and the repulsive pi sigma* states. Ala-TrpH(+) and Gly-TrpH(+) exhibit an extensive spectral change with addition of two methanol molecules. This change is ascribed to a conformational change, which is induced by the insertion of solvent molecule in between the NH(3)(+) group and the indole pi ring.

Fragmentation Pathways of [Mg(NH3)n]2+ Complexes: Electron Capture versus Charge Separation

New experimental results are presented from a detailed study of gas-phase [Mg(NH(3))(n)](2+) complexes and their fragmentation pathways. The reactions examined range from those observed as metastable (unimolecular) decompositions through to collision-induced processes, which have been accessed using a variety of collision gases. Measurements of ion intensity distributions coupled with unimolecular decay studies show that [Mg(NH(3))(4)](2+) not only is the most intense species detected but also sits at a critical boundary between complexes that are unstable with respect to charge separation and those that are sufficiently solvated to be deemed stable on the time scale of the experiment. Metastable fragmentation patterns have been used to provide information on the evolution of solvent structure around the central dication. In addition to highlighting the particular significance of [Mg(NH(3))(4)](2+), these measurements show some evidence to suggest the buildup of structures via a hydrogen-bonded network to give conformers of the form (4+1) and (4+2), respectively. Collision-induced dissociation studies show the ions to exhibit several fragmentation pathways, including the loss of NH(3) and NH(3) + H, which are promoted primarily through electron capture dissociation (ECD). This picture contrasts with the conclusion from a number of earlier studies that collisional activation mainly promotes charge separation. From the results presented it is suggested that electron capture may play a more dominant role in the charge reduction of multiply charged metal-ligand species than had previously been appreciated.