Dunja Radisic

Photoelectron spectroscopy of chromium-doped silicon cluster anions

The photoelectron spectra of chromium-doped silicon cluster anions, CrSi-(n), were measured over the size range, n=8-12. Their vertical detachment energies were measured to be 2.71, 2.88, 2.87, 2.95, and 3.18 eV, respectively. Our results support theoretical calculations by Khanna, Rao, and Jena [Phys. Rev. Lett. 89, 016803 (2002)] which found CrSi12 to be an enhanced stability (magic) cluster with its chromium atom encapsulated inside a silicon cage and with its magnetic moment completely quenched by the effects of the surrounding cage.

Electron-Driven Acid-Base Chemistry : Proton Transfer from Hydrogen Chloride to Ammonia

In contrast to widely familiar acid-base behavior in solution, single molecules of NH3 and HCl do not react to form the ionic salt, NH+4Cl–, in isolation. We applied anion photoelectron spectroscopy and ab initio theory to investigate the interaction of an excess electron with the hydrogen-bonded complex NH3···HCl. Our results show that an excess electron induces this complex to form the ionic salt. We propose a mechanism that proceeds through a dipole-bound state to form the negative ion of ionic ammonium chloride, a species that can also be characterized as a deformed Rydberg radical, NH4, polarized by a chloride anion, Cl–.

The stabilization of arginine's zwitterion by dipole-binding of an excess electron

The arginine parent anion was generated by a newly developed, infrared desorption-electron photoemission hybrid anion source. The photoelectron spectrum of the arginine anion was recorded and interpreted as being due to dipole binding of the excess electron. The results are consistent with calculations by Rak, Skurski, Simons, and Gutowski, who predicted the near degeneracy of arginines canonical and zwitterionic dipole bound anions. Since neutral arginines zwitterion is slightly less stable than its canonical form, this work also demonstrates the ability of an excess electron to stabilize a zwitterion, just as ions and solvent molecules are already known to do.

Dipole-bound anions of highly polar molecules: Ethylene carbonate and vinylene carbonate

Results of experimental and theoretical studies of dipole-bound negative ions of the highly polar molecules ethylene carbonate (EC, C3H4O3, mu=5.35 D) and vinylene carbonate (VC, C3H2O3, mu=4.55 D) are presented. These negative ions are prepared in Rydberg electron transfer (RET) reactions in which rubidium (Rb) atoms, excited to ns or nd Rydberg states, collide with EC or VC molecules to produce EC- or VC- ions. In both cases ions are produced only when the Rb atoms are excited to states described by a relatively narrow range of effective principal quantum numbers, n*; the greatest yields of EC- and VC- are obtained for n*(max)=9.0+/-0.5 and 11.6+/-0.5, respectively. Charge transfer from low-lying Rydberg states of Rb is characteristic of a large excess electron binding energy (Eb) of the neutral parent; employing the previously derived empirical relationship Eb=23/n*(max)(2.8) eV, the electron binding energies are estimated to be 49+/-8 meV for EC and 24+/-3 meV for VC. Electron photodetachment studies of EC- show that the excess electron is bound by 49+/-5 meV, in excellent agreement with the RET results, lending credibility to the empirical relationship between Eb and n*(max). Vertical electron affinities for EC and VC are computed employing aug-cc-pVDZ atom-centered basis sets supplemented with a (5s5p) set of diffuse Gaussian primitives to support the dipole-bound electron; at the CCSD(T) level of theory the computed electron affinities are 40.9 and 20.1 meV for EC and VC, respectively.

Barrier-free proton transfer in anionic complex of thymine with glycine

We report the photoelectron spectrum of the thymine–glycine anionic complex (TG−) recorded with low energy photons (2.540 eV). The spectrum reveals a broad feature with a maximum between 1.6–1.9 eV. The measured electron vertical detachment energy is too large to be attributed to a complex in which an anion of intact thymine is solvated by glycine, or vice versa. The experimental data are paralleled by electronic structure calculations carried out at the density functional theory level with 6-31++G** basis sets and the B3LYP and MPW1K exchange–correlation functionals. The critical structures are further examined at the second order Moller–Plesset level of theory. The results of calculations indicate that the excess electron attachment to the complex induces an intermolecular barrier-free proton transfer from the carboxylic group of glycine to the O8 atom of thymine. As a result, the four most stable structures of the thymine–glycine anionic complex can be characterized as a neutral radical of hydrogenated thymine solvated by an anion of deprotonated glycine. The calculated vertical electron detachment energies for the four most stable anionic complexes lie in a range 1.6–1.9 eV, in excellent agreement with the maximum of the photoelectron peak.

STABLE VALENCE ANIONS OF NUCLEIC ACID BASES AND DNA STRAND BREAKS INDUCED BY LOW ENERGY ELECTRONS

The last decade has witnessed immense advances in our understanding of the effects of ionizing radiation on biological systems. As the genetic information carrier in biological systems, DNA is the most important species which is prone to damage by high energy photons. Ionizing radiations destroy DNA indirectly by forming low energy electrons (LEEs) as secondary products of the interaction between ionizing radiation and water. An understanding of the mechanism that leads to the formation of single and double strand breaks may be important in guiding the further development of anticancer radiation therapy. In this article we demonstrate the likely involvement of stable nucleobases anions in the formation of DNA strand breaks – a concept which the radiation research community has not focused on so far. In Section refch21:sec21.1 we discuss the current status of studies related to the interaction between DNA and LEEs. The next section is devoted to the description of proton transfer induced by electron attachment to the complexes between nucleobases and various proton donors – a process leading to the strong stabilization of nucleobases anions. Then, we review our results concerning the anionic binary complexes of nucleobases with particular emphasize on the GC and AT systems. Next, the possible consequences of interactions between DNA and proteins in the context of electron attachment are briefly discussed. Further, we focus on existing proposal of single strand break formation in DNA. Ultimately, open questions as well perspectives of studies on electron induced DNA damage are discussed

On the interaction of electrons with betaine zwitterions

Betaine is a permanent zwitterion. The molecular betaine anion has been generated in a hybrid, infrared desorption-electron photoemission source and its photoelectron spectrum recorded. The photoelectron spectrum of the betaine anion is characteristic of a dipole bound anion, and its vertical detachment energy was measured to be 0.29+/-0.03 eV. Calculations by Rak, Skurski, and Gutowski [J. Chem. Phys. 114, 10673 (2001)] had found the betaine anion to be a dipole bound anion with a vertical detachment energy of 0.28 eV. We also measured the vertical detachment energy of deprotonated betaine to be approximately 1.9 eV.

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.

Photoelectron spectroscopy of the anions, CH3NH- and (CH3)2N- and the anion complexes, H-(CH3NH2) and (CH3)2N-[(CH3)2NH]

Abstract The photoelectron spectra of the negative ions, CH 3 NH − and (CH 3 ) 2 N − , as well as the anion complexes, H − (CH 3 NH 2 ) and (CH 3 ) 2 N − [(CH 3 ) 2 NH] have been measured. The adiabatic electron affinities of CH 3 NH and (CH 3 ) 2 N were determined to be 0.432±0.015 and 0.504±0.030 eV, respectively. The vertical detachment energies of H − (CH 3 NH 2 ) and (CH 3 ) 2 N − [(CH 3 ) 2 NH] were determined to be 1.157±0.025 and 1.1±0.1 eV, respectively, while the anion–neutral dissociation energies of H − (CH 3 NH 2 ) and (CH 3 ) 2 N − [(CH 3 ) 2 NH] were found to be 0.40 and 0.6 eV, respectively. These species are compared with their unmethylated counterparts, NH 2 − , H − (NH 3 ), and NH 2 − (NH 3 ).

Photoelectron Spectroscopic and Computational Study of Hydrated Pyrimidine Anions

The stabilization of the pyrimidine anion by the addition of water molecules is studied experimentally using photoelectron spectroscopy of mass-selected hydrated pyrimidine clusters and computationally using quantum-mechanical electronic structure theory. Although the pyrimidine molecular anion is not observed experimentally, the addition of a single water molecule is sufficient to impart a positive electron affinity. The sequential hydration data have been used to extrapolate to -0.22 eV for the electron affinity of neutral pyrimidine, which agrees very well with previous observations. These results for pyrimidine are consistent with previous studies of the hydrated cluster anions of uridine, cytidine, thymine, adenine, uracil, and naphthalene. This commonality suggests a universal effect of sequential hydration on the electron affinity of similar molecules.