J. M. Nilles

Cobalt–benzene cluster anions: Mass spectrometry and negative ion photoelectron spectroscopy

(Cobalt)n(benzene)m− cluster anions, (n,m) were generated by laser vaporization and studied by both mass spectrometry and anion photoelectron spectroscopy. Our assignment of the photoelectron spectrum of the (1,2) cluster anion suggests that it possesses a sandwich structure with the cobalt atom located between two parallel benzene rings, that the ground state of this anion is a singlet, and that the ground state of its corresponding neutral is a doublet. The photoelectron spectra of cobalt-rich cluster anions of the form (n,1) are interpreted as cobalt metal cluster anions which have been solvent-stabilized by their interaction with, in each case, a single benzene molecule. The photoelectron spectra of the benzene-rich cluster anions, (2,3), (2,2), and (3,3), are tentatively interpreted as suggesting extended sandwich structures for these anion complexes.

Photoelectron spectroscopy of As−,As2−,As3−,As4−, and As5−

The negative ion photoelectron spectra of As−, As2−, As3−, As4−, and As5− have been measured. From these, the electron affinities of As, As2, As3, As4, and As5 have been determined to be 0.814, 0.739, 1.45, <0.8, and ∼1.7 eV, respectively. In the case of As2−, the following molecular constants were also determined: re(As2−)=2.239 A, ωe(As2−)=293 cm−1, ωeχe(As2−)=4.9 cm−1, D0(As2−)=3.89 eV, and ΔE[2Πg(3/2)−2Πg(1/2)]=0.256 eV. In the case of As3−, vertical detachment energy (VDE) was measured to be 1.62 eV, and for As3, ΔE(2A2−2B1) was determined to be 0.36 eV. For As4−, VDE was found to be 1.52 eV. The relatively high stability of As5− suggests that it, like P5−, may be a candidate for forming cluster-assembled, ionic crystals of stoichiometry, MAs5, where M is an alkali metal atom. Similiarities with other small cluster anions of Group V elements are also discussed.

Microsolvation of small anions by aromatic molecules: An exploratory study

This work was motivated by the experimental finding that the O2−/benzene interaction energy is unexpectedly large. To further explore the interactions of small anions with aromatic molecules, anion photoelectron spectroscopy was utilized to measure interaction strengths of the seed anions, O2− and NO−, complexed with several aromatic molecules, including benzene, naphthalene, pyridine, and pyrimidine. As in the case of O2−(benzene), the anion(aromatic)1 binding energies for the other complexes studied were also higher than one might have anticipated. In addition, the interaction energy of O2− complexed with a given aromatic molecule was, in every case studied, higher (by a factor of ∼1.5) than that of NO− complexed with the same aromatic. While the dependence of interaction strengths on solvent dipole moments and/or polarizabilities implied a substantial electrostatic component to the binding in these complexes, differences in the binding of O2− and NO− with these aromatic molecules showed that there is a...

Photoelectron spectroscopy of nickel-benzene cluster anions

(Nickel)(n)(benzene)(m) (-) cluster anions were studied by both mass spectrometry and anion photoelectron spectroscopy. Only Ni(n)(Bz)(m) (-) species for which n > or =m were observed in the mass spectra. No single-nickel Ni(1)(Bz)(m) (-) species were seen. Adiabatic electron affinities, vertical detachment energies, and second transition energies were determined for (n,m)=(2,1), (2,2), (3,1), and (3,2). For the most part, calculations on Ni(n)(Bz)(m) (-) species by B. K. Rao and P. Jena [J. Chem. Phys. 117, 5234 (2002)] were found to be consistent with our results. The synergy between their calculations and our experiment provided enhanced confidence in the theoretically implied magnetic moments of several nickel-benzene complexes. The magnetic moments of small nickel clusters were seen to be extremely sensitive to immediate molecular environmental effects.

Double Rydberg anions: Photoelectron spectroscopy of NH4−, N2H7−, N3H10−, N4H13−, and N5H16−

We report the discovery and photoelectron spectroscopic study of the four double Rydberg anions: N2H7−, N3H10−, N4H13−, and N5H16−; of three solvated double Rydberg anions; and of a previously unseen feature in the spectrum of the double Rydberg anion, NH4−. In each case, vertical detachment energies were measured and vibrational features tentatively assigned. In the case of solvated double Rydberg anions, anion–neutral interaction energies were also determined.

Photoelectron spectroscopic studies of 5-halouracil anions

The parent negative ions of 5-chlorouracil, UCl(-) and 5-fluorouracil, UF(-) have been studied using anion photoelectron spectroscopy in order to investigate the electrophilic properties of their corresponding neutral halouracils. The vertical detachment energies (VDE) of these anions and the adiabatic electron affinities (EA) of their neutral molecular counterparts are reported. These results are in good agreement with the results of previously published theoretical calculations. The VDE values for both UCl(-) and UF(-) and the EA values for their neutral molecular counterparts are much greater than the corresponding values for both anionic and neutral forms of canonical uracil and thymine. These results are consistent with the observation that DNA is more sensitive to radiation damage when thymine is replaced by halouracil. While we also attempted to prepare the parent anion of 5-bromouracil, UBr(-), we did not observe it, the mass spectrum exhibiting only Br(-) fragments, i.e., 5-bromouracil apparently underwent dissociative electron attachment. This observation is consistent with a previous assessment, suggesting that 5-bromouracil is the best radio-sensitizer among these three halo-nucleobases.

Gas phase analogs of stable sodium-tin Zintl ions: Anion photoelectron spectroscopy and electronic structure

Mass spectrometry and photoelectron spectroscopy together with first principles theoretical calculations have been used to study the electronic and geometric properties of the following sodium-tin, cluster anion/neutral cluster combinations, (Na(n)Sn(4))(-)/(Na(n)Sn(4)), n = 0-4 and (NaSn(m))(-)/(NaSn(m)), m = 4-7. These synergistic studies found that specific Zintl anions, which are known to occur in condensed Zintl phases, also exist as stable moieties within free clusters. In particular, the cluster anion, (Na(3)Sn(4))(-) is very stable and is characterized as (Na(+))(3)(Sn(4))(-4); its moiety, (Sn(4))(-4) is a classic example of a Zintl anion. In addition, the cluster anion, (NaSn(5))(-) was the most abundant species to be observed in our mass spectrum, and it is characterized as Na(+)(Sn(5))(2-). Its moiety, (Sn(5))(2-) is also known to be present as a Zintl anion in condensed phases.

Computational and photoelectron spectroscopic study of the dipole-bound anions, indole(H2O)1,2−

We report our joint computational and anion photoelectron spectroscopic study of indole-water cluster anions, indole(H2O)1,2 (-). The photoelectron spectra of both cluster anions show the characteristics of dipole-bound anions, and this is confirmed by our theoretical computations. The experimentally determined vertical electron detachment (VDE) energies for indole(H2O)1 (-) and indole(H2O)2 (-) are 144 meV and 251 meV, respectively. The corresponding theoretically determined VDE values for indole(H2O)1 (-) and indole(H2O)2 (-) are 124 meV and 255 meV, respectively. The vibrational features in the photoelectron spectra of these cluster anions are assigned as the vibrations of the water molecule.