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The chemical reactions of size selected Si+n (n=3–24) clusters with C2H4 have been studied at thermal energies using selected ion drift tube techniques. Except for Si+3 (which dehydrogenates C2H4 to yield Si3C2H+2) the dominant products arise from adsorption of C2H4 onto the silicon clusters. The reaction rates show large variations with cluster size. Si+13 and Si+14 were found to be particularly unreactive. The reactions were studied over a temperature range of 143–298 K. The reactivity increases as the temperature is lowered indicating that variation in the rates observed for the different cluster sizes is not due to an activation barrier, but reflects the stability of the SinC2H+4 adducts. Statistical phase space theory was used to model the reactions of Si+4–Si+10 and provide an estimate of the binding energy of the first C2H4 to these clusters. The binding energies vary between 0.8 and 2.0 eV. Binding energies of this magnitude are too small to be accounted for by strong di-σ bonding, suggesting that the C2H4 molecule is bound to the silicon clusters by a weaker π-bonding interaction. Kinetic evidence for the presence of structural isomers was found for several clusters. With Si+9 the relative abundance of an unreactive isomer could be changed from ∼1% up to 17% by changing the source conditions. More than one C2H4 molecule will adsorb onto the silicon clusters (up to eight adsorb on Si+4). Rate constants for the adsorption of the first few (up to seven) C2H4 molecules on Si+n (n=4–10) have been determined. They show large variations with the number of adsorbed C2H4 molecules. [ABSTRACT FROM AUTHOR]

Mass resolved carbon cluster cations from C+3 to C+20 have been photofragmented using 248 and 351 nm light. For an initial cluster C+n , the dominant fragment observed is C+n-3 . Fluence dependence measurements of photofragmentation yield bracketed dissociation thresholds, photofragmentation cross sections, and product branching ratios. Photodissociation with 248 nm light is found to be primarily linear with laser fluence for n>5, but there are indications that C+3 and C+5 fragment only with the absorption of two or more photons. For 351 nm light, clusters with six and more atoms all show a linear dependence. The photofragmentation cross sections for both 351 and 248 nm light show a significant change as a function of cluster size. [ABSTRACT FROM AUTHOR]

Sequential clustering of atomic silicon-29 cations with SiD4 is observed at room temperature in the ion cell of a Fourier transform mass spectrometer. The clustering reactions proceed in a highly specific fashion. Si+ grows initially by sequential addition of three –SiD2 units. The measured reaction rates for these three steps are, respectively, 8.1±0.4, 0.36±0.04, and 2.0±0.3×10-10 cm3 molecule-1 s-1. A back reaction which results in loss of the silicon-29 isotopic label also occurs for these three reactions and represents ∼12%–15% of the reaction products, depending on the reaction step. This cluster growth mechanism then encounters a critical bottleneck and ceases. Further aggregation occurs only by slow bimolecular attachment of SiD4 at a rate of 1.0±0.3×10-13 cm3 molecule-1 s-1 at p(SiD4)=2.0×10-7 Torr. The fundamental mechanisms and energetics for the individual reaction pathways have been calculated by Raghavachari using ab initio electronic structure theory and are presented in a companion paper. The clustering mechanism involves insertion of a cluster ion into a SiD bond of SiD4 followed by elimination of D2. Addition of –SiD2 serves to increasingly saturate the bonds of all of the silicon centers which leads to the observed growth limitations at Si4D+6. The accuracy of these calculated potential surfaces is tested using statistical phase space theory. Since both the forward and reverse reaction rates are measured using isotopic labeling, the phase space theory calculations are used to determine both the insertion and the elimination transition state energies for each of the first three clustering steps as well as a lower limit for the well depth of the [Si5D+10] intermediate complex. Overall, good agreement is found between the transition state energies obtained from phase space theory and those determined by Raghavachari. These results indicate that Si+ clustering with SiD4 encounters an... [ABSTRACT FROM AUTHOR]

Data are reported on the effects of internal energy and angular momentum on the collision induced dissociation CID fragmentation pattern. For the ions studied changes in the relative intensities of the fragment ions as internal energy varied were found to be larger than suggested by McLafferty and coworkers. Possible effects of angular momentum on the CID fragmentation pattern are discussed. The charge stripping spectra of the ions studied were found to be strongly dependent on initial energy and/or angular momentum. Hence care must be taken if charge stripping spectra are used to distinguish ion structures. [ABSTRACT FROM AUTHOR]

Mass-resolved carbon cluster ions from C+3 to C+20 have been fragmented with 248 nm light. For any initial cluster C+n, the dominant fragment is C+n-3. The fragmentation is primarily linear in laser intensity for n>5, but there are indications that C+3 and C+5 fragment only with the absorption of two or more 248 nm photons. The photodissociation cross sections change dramatically with cluster size. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations have been performed to study the rehydration of compact and unfolded cytochrome c ions in the vapor phase. Experimental studies have shown that the compact conformations adsorb many more water molecules than unfolded ones when exposed to water vapor. MD simulations performed with up to 150 water molecules reproduce the key experimental observations, including a partial refolding caused by hydration. According to the calculations it is more energetically favorable to hydrate the compact conformation in the initial stages of hydration, because it is easier for a water molecule to interact simultaneously with several polar groups (due to their proximity). The protonated side chains are not favored hydration sites in the simulations because they have "self-solvation" shells which must be disrupted for the water to penetrate. For both conformations, the adsorbed water molecules are mainly located in surface crevices.

The conformations of unsolvated Ac-K(AGG)(5)+H(+) and Ac-(AGG)(5)K+H(+) peptides (Ac = acetyl, A = alanine, G = glycine, and K = lysine) have been examined by ion mobility measurements over a wide temperature range (150-410 K). The Ac-K(AGG)(5)+H(+) peptide remains a globule (a compact, roughly spherical structure) over the entire temperature range, while both an alpha-helix and a globule are found for Ac-(AGG)(5)K+H(+) at low temperature. As the temperature is raised the alpha-helix unfolds. Rate constants for loss of the helix (on a millisecond time scale) have been determined as a function of temperature and yield an Arrhenius activation energy and preexponential factor of 38.2 +/- 1.0 kJ mol(-1) and 6.5 +/- 3.7 x 10(9) s(-1), respectively. The alpha-helix apparently does not unfold directly into the globule, but first converts into a long-lived intermediate which survives to a significantly higher temperature before converting. According to molecular dynamics simulations, there is a partially untwisted helical conformation that has both a low energy and a well-defined geometry. This special structure lies between the helix and globule and may be the long-lived intermediate.

This article provides a review of recent studies of the properties of unsolvated (and partially solvated) peptides and proteins. The methods used to produce vapor-phase peptide and protein ions are described along with some of the techniques used to study them, such as H/D exchange, blackbody infrared radiative dissociation, and ion mobility measurements. Studies of unsolvated peptides and proteins provide information about their intrinsic intramolecular interactions. The topics covered include the role of zwitterions and salt bridges in the vapor phase, Coulomb interactions in multiply charged ions, the unfolding and refolding of vapor-phase proteins, and the stability of unsolvated helices and sheets. Finally, dehydration and rehydration studies of proteins in the vapor phase are described. These can provide exquisitely detailed information about hydration interactions, such as the enthalpy and entropy changes associated with adsorbing individual water molecules.

High-resolution ion mobility measurements and molecular dynamics simulations have been used to probe the conformations of protonated polyglycine and polyalanine (Gly(n)H and Ala(n)H+, n = 3-20) in the gas phase. The measured collision integrals for both the polyglycine and the polyalanine peptides are consistent with a self-solvated globule conformation, where the peptide chain wraps around and solvates the charge located on the terminal amine. The conformations of the small peptides are governed entirely by self-solvation, whereas the larger ones have additional backbone hydrogen bonds. Helical conformations, which are stable for neutral Alan peptides, were not observed in the experiments. Molecular dynamics simulations for Ala(n)H+ peptides suggest that the charge destabilizes the helix, although several of the low energy conformations found in the simulations for the larger Ala(n)H+ peptides have small helical regions.

Laser vaporization of graphite generates C(60)(+) cluster ions that are fullerenes and a mixture of roughly planar polycyclic polyyne ring isomers. Experimental studies of the annealing of the non-fullerene C(60)(+) ions indicate that they can be converted (in the gas phase) into the fullerene and an isomer that appears to be a large monocyclic ring. Some fragmentation is associated with conversion to the fullerene geometry, but the majority of the non-fullerene C(60)(+) isomers are cleanly converted into an intact fullerene. The emergence of the monocyclic ring (as the clusters are annealed) suggests that this is a relatively stable non-spheroidal form of these all carbon molecules. The estimated activation energies for the observed structural interconversions are relatively low, suggesting that these processes may play an important role in the synthesis of spheroidal fullerenes.