Decorating (C60) n+, n = 1-3, with CO2 at low temperatures: Sterically enhanced physisorption.

Multiple attachment of CO2 to the monomer, dimer and trimer cations of C60 has been observed in the mass spectra of He nanodroplets sequentially doped with C60 and CO2 and exposed to electron ionization at 50 eV. Remarkable anomalies were seen in the ion yield for CO2 coverage for (C60)2+(CO2)8 and (C60)3+(CO2)1,2. These provide insight into the influence of steric properties on the nature of physisorption. The enhanced stabilities of (C60)2+(CO2)8 and (C60)3+(CO2)1,2 are attributed to physisorption inside the "groove" of the dimer and the two "dimples" in the trimer cations of C60. Molecular dynamics simulations provide a qualitative assessment of the observed physisorption and a useful visualization of structural aspects.

Ionization of doped helium nanodroplets: complexes of C60 with water clusters.

Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of (H(2)O)(n)H(3)O(+). Ejection of OH cannot be quenched by near-threshold ionization; it is only partly quenched when clusters are complexed with inert gas atoms. Mass spectra recorded by electron ionization of water-doped helium droplets show that the helium matrix also fails to quench OH loss. The situation changes drastically when helium droplets are codoped with C(60). Charged C(60)-water complexes are predominantly unprotonated; C(60)(H(2)O)(4)(+) and (C(60))(2)(H(2)O)(4)(+) appear with enhanced abundance. Another intense ion series is due to C(60)(H(2)O)(n)OH(+); dehydrogenation is proposed to be initiated by charge transfer between the primary He(+) ion and C(60). The resulting electronically excited C(60)(+*) leads to the formation of a doubly charged C(60)-water complex either via emission of an Auger electron from C(60)(+*), or internal Penning ionization of the attached water complex, followed by charge separation within {C(60)(H(2)O)(n)}(2+). This mechanism would also explain previous observations of dehydrogenation reactions in doped helium droplets. Mass-analyzed ion kinetic energy scans reveal spontaneous (unimolecular) dissociation of C(60)(H(2)O)(n)(+). In addition to the loss of single water molecules, a prominent reaction channel yields bare C(60)(+) for sizes n=3, 4, or 6. Ab initio Hartree-Fock calculations for C(60)-water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to C(60)(+). For n=3, 4, or 6, fissionlike desorption of the entire water complex from C(60)(H(2)O)(n)(+) energetically competes with the evaporation of a single water molecule.

Formation of even-numbered hydrogen cluster cations in ultracold helium droplets.

Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered H(n)(+). The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered H(n)(+). The use of high-resolution mass spectrometry allows the unambiguous identification of even-numbered H(n)(+) up to n approximately = 120 by their mass excess that distinguishes them from He(n)(+), mixed He(m)H(n)(+), and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n=6, pronounced drops in the abundance of even-numbered cluster ions are seen at n=30 and 114, which suggest icosahedral shell closures at H(6)(+)(H(2))(12) and H(6)(+)(H(2))(54). Possible isomers of H(6)(+) are identified at the quadratic configuration interaction with inclusion of single and double excitations (QCISD)/aug-cc-pVTZ level of theory.

Ozone reactions with alkaline-earth metal cations and dications in the gas phase: room-temperature kinetics and catalysis.

Room-temperature rate coefficients and product distributions are reported for the reactions of ozone with the cations and dications of the alkaline-earth metals Ca, Sr, and Ba. The measurements were performed with a selected-ion flow tube (SIFT) tandem mass spectrometer in conjunction with either an electrospray (ESI) or an inductively coupled plasma (ICP) ionization source. All the singly charged species react with ozone by O-atom transfer and form monoxide cations rapidly, k = 4.8, 6.7, and 8.7 x 10(-10) cm3 molecule(-1) s(-1) for the reactions of Ca+, Sr+, and Ba+, respectively. Further sequential O-atom transfer occurs to form dioxide and trioxide cations. The efficiencies for all O-atom transfer reactions are greater than 10%. The data also signify the catalytic conversion of ozone to oxygen with the alkaline-earth metal and metal oxide cations serving as catalysts. Ca2+ reacts rapidly with O3 by charge separation to form CaO+ and O2+ with a rate coefficient of k = 1.5 x 10(-9) cm3 molecule(-1) s(-1). In contrast, the reactions of Sr2+ and Ba2+ are found to be slow and add O3, (k >/= 1.1 x 10-11 cm3 molecule-1 s-1). The initial additions are followed by the rapid sequential addition of up to five O3 molecules with values of k between 1 and 5 x 10(-10) cm3 molecule(-1) s(-1). Metal/ozone cluster ions as large as Sr2+(O3)5 and Ba2+(O3)4 were observed for the first time.

Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene.

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.

Fullerometallic ion chemistry: reactions of C(60)Fe+ and C(20)H(10)Fe+ in the gas phase.

Fe+ has been attached to buckminsterfullerene, C(60), and corannulene, C(20)H(10), in the gas phase, and the reactivities of C(60)Fe+ and C(20)H(10)Fe+ have been measured with several small inorganic and organic molecules in helium bath gas at 0.35 Torr using a selected-ion flow tube (SIFT) mass spectrometer. Comparisons with measured reactivities of the bare Fe+ ion indicate that the presence of C(60) and C(20)H(10) leads to enhancements in reactivity at room temperature of up to 5 orders of magnitude. Ligation was the only chemistry observed with D(2), N(2), CO(2), CH(4), C(2)H(2), C(2)H(4), SO(2), C(6)D(6), NH(3), H(2)O, and CO, but other channels were observed to compete with adduct formation in the reactions with N(2)O and O(2). The number of molecules sequentially ligated to the ion was different: up to five molecules of ligand added sequentially to Fe+, up to four molecules of ligand were observed to attach to C(60)Fe+, while only up to three molecules added to C(20)H(10)Fe+. C(60)+ and C(20)H(10)+ were observed to be unreactive toward the same ligands. The kinetic results show the influence of carbonaceous surfaces on metal ion reactivity and are interpreted in terms of the nature of the coordination of Fe+ to the carbonaceous surface. Catalytic effects of the carbonaceous surfaces were identified for the reactions with N(2)O and O(2).

Corannulene as a Lewis base: computational modeling of protonation and lithium cation binding.

A computational modeling of the protonation of corannulene at B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) and of the binding of lithium cations to corannulene at B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p) has been performed. A proton attaches preferentially to one carbon atom, forming a sigma-complex. The isomer protonated at the innermost (hub) carbon has the best total energy. Protonation at the outermost (rim) carbon and at the intermediate (bridgehead rim) carbon is less favorable by ca. 2 and 14 kcal mol(-)(1), respectively. Hydrogen-bridged isomers are transition states between the sigma-complexes; the corresponding activation energies vary from 10 to 26 kcal mol(-)(1). With an empirical correction obtained from calculations on benzene, naphthalene, and azulene, the best estimate for the proton affinity of corannulene is 203 kcal mol(-)(1). The lithium cation positions itself preferentially over a ring. There is a small energetic preference for the 6-ring over the 5-ring binding (up to 2 kcal mol(-)(1)) and of the convex face over the concave face (3-5 kcal mol(-)(1)). The Li-bridged complexes are transition states between the pi-face complexes. Movement of the Li(+) cation over either face is facile, and the activation energy does not exceed 6 kcal mol(-)(1) on the convex face and 2.2 kcal mol(-)(1) on the concave face. In contrast, the transition of Li(+) around the corannulene edge involves a high activation barrier (24 kcal mol(-)(1) with respect to the lowest energy pi-face complex). An easier concave/convex transformation and vice versa is the bowl-to-bowl inversion with an activation energy of 7-12 kcal mol(-)(1). The computed binding energy of Li(+) to corannulene is 44 kcal mol(-)(1). Calculations of the (7)Li NMR chemical shifts and nuclear independent chemical shifts (NICS) have been performed to analyze the aromaticity of the corannulene rings and its changes upon protonation.

Collision-induced dissociation evidence for charge separation and "ball-and-chain" propagation in the addition of 1-butene to C 60 (2+).

The collision-induced dissociation of the adduct ions C60(C4H8) 2 (2+) and C60(C4H8) 3 (2+) formed by sequential reactions of C 60 (2+) with 1-butene has been investigated by using a selected-ion flow tube (SIFT) apparatus. Experiments at 295 ± 2 K in 0.35 ± 0.02 torr of helium indicated that C 60 (2+) adds at least five molecules of 1-butene in a sequential fashion with rates that decrease with the number of molecules added. Collision-induced dissociation experiments in which the downstream sampling nose cone of the SIFT was biased with respect to the flow tube indicated that the adduct ions C60(C4H8) 2 (2+) and C60(C4H8) 3 (2+) dissociate into C 60 (·+) and (C4H8) 2 (·+) and (C4H8) 3 (·+) , respectively. These observations provide evidence for the occurrence of charge separation in the derivatization of C60 dications and support the "ball-and-chain" mechanism first proposed by Wang et al. in 1992 for the sequential multiple addition of 1,3-butadiene to C 60 (2+) and C 70 (2+) .