Ion mobility mass spectrometry (IM-MS) can separate ions based on their size, shape, and charge as well as mass-to-charge ratios. Here, we report experimental IM-MS and IM-MS/MS data of the Au(25)(SCH(2)CH(2)Ph)(18)(-) nanocluster. The IM-MS of Au(25)(SCH(2)CH(2)Ph)(18)(-) exhibits a narrow, symmetric drift time distribution that indicates the presence of only one structure. The IM-MS/MS readily distinguishes between the fragmentation of the outer protecting layer, made from six [-SR-Au-SR-Au-SR-] "staples' where R = CH(2)CH(2)Ph, and the Au(13) core. The fragmentation of the staples is characterized by the predominant loss of Au(4)(SR)(4) from the cluster and the formation of eight distinct bands. The consecutive eight bands contain an increasing variety of Au(l)S(m)R(n)(-) product ions due to the incremental fragmentation of the outer layer of Au(21)X(14)(-), where X = S or SCH(2)CH(2)Ph. The mobility of species in each individual band shows that the lower mass species exhibit greater collision cross sections, facilitating the identification of the Au(l)S(m)R(n)(-) products. Below the bands, in the region 1200-2800 m/z, product ions relating to the fragmentation of the Au(13) core can be observed. In the low mass 50-1200 m/z region, fragment ions such as Au(SR)(2)(-), Au(2)(SR)(3)(-), Au(3)(SR)(4)(-), and Au(4)(SR)(5)(-) are also observed, corresponding to the large fragments Au(25-x)(SR)(18-(x+1)). The study shows that most of the dominant large fragments are of the general type Au(21)X(14)(-/+), and Au(17)X(10)(-/+) with electron counts of 8 and 6 in negative and positive mode, respectively. This suggests that geometric factors may outweigh electronic factors in the selection of Au(25)(SR)(18) structure.

Reaction cross sections and product velocity distributions are presented for the bimolecular gas-phase nucleophilic substitution (S(N)2) reaction Cl(-) + CH(3)Br --> CH(3)Cl + Br(-) as a function of collision energy, 0.06-24 eV. The exothermic S(N)2 reaction is inefficient compared with phase space theory (PST) and ion-dipole capture models. At the lowest energies, the S(N)2 reaction exhibits the largest cross sections and symmetrical forward/backward scattering of the CH(3)Cl + Br(-) products. The velocity distributions of the CH(3)Cl + Br(-) products are in agreement with an isotropic PST distribution, consistent with a complex-mediated reaction and a statistical internal energy distribution of the products. Above 0.2 eV, the velocity distributions become nonisotropic and nonstatistical, exhibiting CH(3)Cl forward scattering between 0.2 and 0.6 eV. A rebound mechanism with backward scattering above 0.6 eV is accompanied by a new rising feature in the CH(3)Cl + Br(-) cross sections. The competitive endothermic reaction Cl(-) + CH(3)Br --> CH(3) + ClBr(-) rises from its thermochemical threshold at 1.9 +/- 0.4 eV, showing nearly symmetrically scattered products just above threshold and strong backward scattering above 3 eV associated with a second feature in the cross section.

Energy-resolved, competitive threshold collision-induced dissociation (TCID) methods are used to measure the gas-phase acidities of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid relative to hydrogen cyanide, hydrogen sulfide, and the hydroperoxyl radical using guided ion beam tandem mass spectrometry. The gas-phase acidities of Delta(acid)H298(C6H5OH) = 1456 +/- 4 kJ/mol, Delta(acid)H298(3-CH3C6H4OH) = 1457 +/- 5 kJ/mol, Delta(acid)H298(2,4,6-(CH3)3C6H2OH) = 1456 +/- 4 kJ/mol, and Delta(acid)H298(CH3COOH) = 1457 +/- 6 kJ/mol are determined. The O-H bond dissociation enthalpy of D298(C6H5O-H) = 361 +/- 4 kJ/mol is derived using the previously published experimental electron affinity for C6H5O, and thermochemical values for the other species are reported. A comparison of the new TCID values with both experimental and theoretical values from the literature is presented.

Guided ion beam tandem mass spectrometry techniques are used to examine the competing product channels in the reaction of Cl(-) with CH(3)F in the center-of-mass collision energy range 0.05-27 eV. Four anionic reaction products are detected: F(-), CH(2)Cl(-), FCl(-), and CHCl(-). The endothermic S(N)2 reaction Cl(-) + CH(3)F --> CH(3)Cl + F(-) has an energy threshold of E(0) = 181 +/- 14 kJ/mol, exhibiting a 52 +/- 16 kJ/mol effective barrier in excess of the reaction endothermicity. The potential energy of the S(N)2 transition state is well below the energy of the products. Dynamical impedances to the activation of the S(N)2 reaction are discussed, including angular momentum constraints, orientational effects, and the inefficiency of translational energy in promoting the reaction. The fluorine abstraction reaction to form CH(3) + FCl(-) exhibits a 146 +/- 33 kJ/mol effective barrier above the reaction endothermicity. Direct proton transfer to form HCl is highly inefficient, but HF elimination is observed above 268 +/- 95 kJ/mol. Potential energy surfaces for the reactions are calculated using the CCSD(T)/aug-cc-pVDZ and HF/6-31+G(d) methods and used to interpret the dynamics.

Energy-resolved competitive collision-induced dissociation methods are used to measure the gas-phase acidity of phenol relative to hydrogen cyanide. The competitive dissociation of the [C6H5O·H·CN]- complex into C6H5OH + CN- and C6H5O- + HCN is studied using a guided ion beam tandem mass spectrometer. The reaction cross sections and product branching fractions are measured as a function of collision energy. The enthalpy difference between the two reaction channels is found by modeling the reaction cross sections near threshold using RRKM theory to account for the energy-dependent product branching ratio and kinetic shift. From the enthalpy difference, the phenol gas-phase acidity, Δacid H 0(C6H5OH) = 1448 ± 8 kJ/mol, is determined relative to the established literature value of hydrogen cyanide, Δacid H 0(HCN) = 1462.3 ± 0.9 kJ/mol. We then derive Δacid H 298(C6H5OH) = 1454 ± 8 kJ/mol and the bond dissociation energy of D 298(C6H5O−H) = 359 ± 8 kJ/mol.

The reactions of NO ϩ and NO 2 ϩ with water are considered to make an important contribution to the formation of proton hydrates in the upper atmosphere. There have been several recent studies of the relevant reactions, with the result that discrepancies have arisen in each case over the critical number of water molecules required to promote the appearance of a proton hydrate. Presented here are new results based on the collision-induced reactions of NO ϩ •͑H 2 O͒ n and NO 2 ϩ •͑H 2 O͒ n cluster ions, which we believe clarify the situation.

Guided ion beam tandem mass spectrometry techniques are used to measure the reaction cross sections of the collisionally activated process I -+ CH 3 Y f products, where Y ) Cl and Br. The Cland Brproducts are observed at the lowest collision energies. A back-side attack S N 2 reaction is responsible for the initial rise from the threshold. At higher collision energies, the ICland IBrions are observed and signify competition from a front-side-attack, halogen-abstraction reaction. All the reactions are endoergic and exhibit excess threshold energies, E 0 , when compared with established reaction endothermicities from the literature, ∆H 0 . The reaction mechanisms are explored with the aid of CCSD(T)/LanL2DZ and CCSD(T)/SDD molecular orbital calculations and phase space theory.

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