Robert F. Höckendorf

Reactivity of Hydrated Monovalent First Row Transition Metal Ions M+(H2O)n, M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, toward Molecular Oxygen, Nitrous Oxide, and Carbon Dioxide

The reactions of hydrated monovalent transition metal ions M(+)(H(2)O)(n), M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, toward molecular oxygen, nitrous oxide, and carbon dioxide were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Clusters containing monovalent chromium, cobalt, nickel, or zinc were reactive toward O(2), while only hydrated cobalt was reactive toward N(2)O. A strongly size dependent reactivity was observed. Chromium and cobalt react very slowly with carbon dioxide. Nanocalorimetric analysis, (18)O(2) exchange, and collision induced dissociation (CID) experiments were done to learn more about the structure of the O(2) products. The thermochemistry for cobalt, nickel, and zinc is comparable to the formation of O(2)(-) from hydrated electrons. These results suggest that cobalt, nickel, and zinc are forming M(2+)/O(2)(-) ion pairs in the cluster, while chromium rather forms a covalently bound dioxygen complex in large clusters, followed by an exothermic dioxide formation in clusters with n ≤ 5. The results show that hydrated singly charged transition metal ions exhibit highly specific reactivities toward O(2), N(2)O, and CO(2).

Reactions of CH3SH and CH3SSCH3 with Gas-Phase Hydrated Radical Anions (H2O)n•–, CO2•–(H2O)n, and O2•–(H2O)n

The chemistry of (H(2)O)(n)(•-), CO(2)(•-)(H(2)O)(n), and O(2)(•-)(H(2)O)(n) with small sulfur-containing molecules was studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. With hydrated electrons and hydrated carbon dioxide radical anions, two reactions with relevance for biological radiation damage were observed, cleavage of the disulfide bond of CH(3)SSCH(3) and activation of the thiol group of CH(3)SH. No reactions were observed with CH(3)SCH(3). The hydrated superoxide radical anion, usually viewed as major source of oxidative stress, did not react with any of the compounds. Nanocalorimetry and quantum chemical calculations give a consistent picture of the reaction mechanism. The results indicate that the conversion of e(-) and CO(2)(•-) to O(2)(•-) deactivates highly reactive species and may actually reduce oxidative stress. For reactions of (H(2)O)(n)(•-) with CH(3)SH as well as CO(2)(•-)(H(2)O)(n) with CH(3)SSCH(3), the reaction products in the gas phase are different from those reported in the literature from pulse radiolysis studies. This observation is rationalized with the reduced cage effect in reactions of gas-phase clusters.

Thermal NH Bond Activation on Anionic and Cationic Platinum Clusters: Non‐Predetermined Reaction Pathways Indicate Transitions to a Bulk Surface Reactivity

Reactions of cationic and anionic platinum clusters Pt(n)(+/-), n=1-5, with NH(3) are studied by FT-ICR mass spectrometry and DFT calculations. With cationic clusters, radiative association of an intact NH(3) is the dominant reaction channel. On anionic clusters, NH(3) undergoes reductive elimination of molecular hydrogen, with nitride and hydride bound at remote sites of the cluster. Nitride assumes a bridge position, while hydride is bound atop a single platinum atom. On the [Pt(n),NH(3)](-) potential energy surface, for n=4 fifteen local minima and connecting transition states were identified with relevance to the hydrogen formation reaction, and 12 local minima were identified for n=5. These potential energy surfaces offer a rich variety of pathways, illustrating a key feature of a successful bulk catalyst: The variety of pathways helps the catalyst to work over a wide range of temperatures and pressures.

Thermochemistry of the Reaction of SF6 with Gas-Phase Hydrated Electrons: A Benchmark for Nanocalorimetry.

The reaction of sulfur hexafluoride with gas-phase hydrated electrons (H2O)n(-), n ≈ 60-130, is investigated at temperatures T = 140-300 K by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. SF6 reacts with a temperature-independent rate of 3.0 ± 1.0 × 10(-10) cm(3) s(-1) via exclusive formation of the hydrated F(-) anion and the SF5(•) radical, which evaporates from the cluster. Nanocalorimetry yields a reaction enthalpy of ΔHR,298K = 234 ± 24 kJ mol(-1). Combined with literature thermochemical data from bulk aqueous solution, these result in an F5S-F bond dissociation enthalpy of ΔH298K = 455 ± 24 kJ mol(-1), in excellent agreement with all high-level quantum chemical calculations in the literature. A combination with gas-phase literature thermochemistry also yields an experimental value for the electron affinity of SF5(•), EA(SF5(•)) = 4.27 ± 0.25 eV.

Competition between Birch reduction and fluorine abstraction in reactions of hydrated electrons (H2O)n― with the isomers of di- and trifluorobenzene

The reactions of the isomers of di- and trifluorobenzene with hydrated electrons (H(2)O)(n)(-), n = 19-70, have been studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. While Birch reduction, i.e. H atom transfer to the aromatic ring, was observed for all studied isomers, a strong dependence on the substitution pattern was observed for fluorine abstraction. Nanocalorimetry combined with G3 calculations are used to analyze the thermochemistry of the reactions. Fluorine abstraction is at least 100 kJ mol(-1) more exothermic than Birch reduction, yet the latter is the dominant reaction pathway for all three difluorobenzene isomers. Fluorine abstraction and Birch reduction face activation barriers of comparable magnitude. The relative barrier height is sensitive to the substitution pattern. Birch reduction occurs selectively with 1,3- and 1,4-difluorobenzene in a nanoscale aqueous environment.

Reactions of hydrated singly charged first-row transition-metal ions M+(H2O)n (M=V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) toward nitric oxide in the gas phase.

Reactions of M(+) (H2 O)n (M=V, Cr, Mn, Fe, Co, Ni, Cu, Zn; n≤40) with NO were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Uptake of NO was observed for M=Cr, Fe, Co, Ni, Zn. The number of NO molecules taken up depends on the metal ion. For iron and zinc, NO uptake is followed by elimination of HNO and formation of the hydrated metal hydroxide, with strong size dependence. For manganese, only small HMnOH(+) (H2 O)n-1 species, which are formed under the influence of room-temperature black-body radiation, react with NO. Here NO uptake competes with HNO formation, both being primary reactions. The results illustrate that, in the presence of water, transition-metal ions are able to undergo quite particular and diverse reactions with NO. HNO is presumably formed through recombination of a proton and (3) NO(-) for M=Fe, Zn, preferentially for n=15-20. For manganese, the hydride in HMnOH(+) (H2 O)n-1 is involved in HNO formation, preferentially for n≤4. The strong size dependence of the HNO formation efficiency illustrates that each molecule counts in the reactions of small ionic water clusters.

Gas Phase Ion Chemistry of Gold–Silicon Clusters

The chemistry of gold differs from other coinage metals, because the d electrons of gold participate in bonding due to strong relativistic effects. Interest in the structural and electronic properties of small gold clusters as a function of their size has been sparked by the discovery of the unusual catalytic activity of supported gold clusters. It comes as a surprise that gold is very reactive on silicon surfaces even at room temperature, given that gold is a noble metal. Studies on the structure and reactivity of pure and oxidized gold anions shed new light on the mechanisms of fundamental gas-phase reactions. The negatively charged free gold dimers Au2 are able to catalyze the gas-phase oxidation reaction of CO to CO2 in the presence of molecular oxygen. Spectroscopically, the nature of absorbed oxygen species on gold clusters has been resolved by vibrationally resolved UV photoelectron spectroscopy of AunO2 . Infrared spectra of AunNO + exhibit a pronounced odd–even alternation with the number of gold atoms in the cluster, whereas for AunCO + no oscillation is found for the CO stretching mode. In contrast to the pure gold clusters, considerably less information is available about structural and electronic properties of gold–silicon clusters. Several metastable Si–Au alloys, including a [SiAu4] phase, have been observed to form at the Si–Au interface. Recent photoelectron spectroscopy has shown that gold acts as hydrogen analogue in AunSim [n=1–4, m=1–2]. AunSim are structurally and electronically similar to HnSim . In addition, the comparison of structures and chemical bonding between Au3Si3 /0/+ and the corresponding silicon hydrides further extends the isolobal analogy between Au and H, which extends the LAu/H analogy introduced by Schmidbaur. However, nothing is known so far about the reactivity of AunSi in the gas phase. Testing several small molecules, we found AunSi to be particularly unreactive. In search for a suitable reactant, we calculated the proton affinity of AunSi quantum chemically. The results showed that trifluoroacetic acid (TFA), which is routinely used as a protonation promoter agent, has a high enough gas-phase acidity, and was found to react with AunSi ions in an intriguing way. AunSi (n=2–4) are produced by laser vaporization, mass selected and reacted with different molecules in the ion trap of an FT–ICR mass spectrometer. The products are analyzed by high-resolution mass spectrometry for different reaction delays. Efficient reactions are observed with the strong Brønsted acid CF3COOH. The mechanistically straightforward pathways between AunSi and CF3COOH are proton transfer to AunSi (n=2–4), shown in reaction R1, resulting in the formation of neutral AunSiH and the base CF3COO , and for n=3 radiative association of CF3COOH to form Au3SiCF3COOH , (R2). A more complicated process, illustrated in R3 is the etching of Si, leading to pure Aun (n=2–4) clusters. Au4Si in R5 preferentially undergoes loss of a gold atom, while for n=2, 3 an interesting CO2 elimination process is observed in R4 leading to the ionic product AunSiCF3H. Among the secondary reactions are radiative association of CF3COOH to CF3COO (R6), and radiative association of a second CF3COOH molecule to Au3SiCF3COOH (R7), which may be accompanied by loss of H2, (R8). Absolute rate constants for primary (R1–R5) and selected secondary (R6–R8) reaction channels are listed in Tables 1 and 2, respectively.

Ion–molecule reactions of CoAr6+ with nitrogen oxides N2O, NO, and NO2: measuring absolute pressure by shock-freezing of the collision complex

A new method to determine the absolute pressure in an ultra-high vacuum apparatus is tested using ion molecule reactions with CoAr6+. In a collision with a neutral reactant the complex between Co+ and the collision partner is stabilized by evaporation of argon atoms. If CoAr6+ reacts at the collision rate, the absolute pressure can be determined by comparing the experimental collision rate with the collision rate calculated from average dipole orientation theory. The experimental results with N2O, NO and NO2 do indeed show that the collision complex is frozen out. Comparing the rates of primary, secondary and tertiary reaction products suggests that not all collisions of CoAr6+ are reactive.