Kit H. Bowen

Zwitterion formation in hydrated amino acid, dipole bound anions: How many water molecules are required?

While the naturally occurring amino acids are not zwitterions in the vapor phase, they are in aqueous solutions, implying that water plays an important role in inducing zwitterion formation. Together, these observations inspire the question, “How many water molecules are required to induce zwitterion formation in a given amino acid molecule?” In this paper, we address this question in the context of mass spectrometric and size-selected photoelectron spectroscopic studies of hydrated amino acid anions. We utilize the facts that zwitterions possess very large dipole moments, and that excess electrons can bind to strong dipole fields to form dipole bound anions, which in turn display distinctive and recognizible photoelectron spectral signatures. The appearance of dipole-bound photoelectron spectra of hydrated amino acid anions, beginning at a given hydration number, thus signals the onset of greatly enhanced dipole moments there and, by implication, of zwitterion formation. We find that five water molecules are needed to transform glycine into its zwitterion, while four each are required for phenylalanine and tryptophan. Since the excess electron may also make a contribution to zwitterion stabilization, these numbers are lower limits for how many water molecules are needed to induce zwitterion formation in these amino acids when no extra (net) charges are involved.

Magic numbers in copper-doped aluminum cluster anions

Copper-doped aluminum cluster anions, CuAln− were generated in a laser vaporization source and examined via mass spectrometry (n=2–30) and anion photoelectron spectroscopy (n=2–15). The mass spectrum of the CuAln− series is dominated by CuAl13− with other magic numbers also appearing at n=6, 19, and 23. The electron affinity versus cluster size trend shows a peak at n=6 and a dip at n=13. These results are discussed in terms of the reordering of shell model energy levels and the enhanced stability of neutral CuAl13. Reordering, which is a consequence of the copper atom residing in the central region of these clusters, provides an anion-oriented electronic rationale for the observed magic numbers.

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.

Vibronic effects in the photon energy‐dependent photoelectron spectra of the CH3CN− dipole‐bound anion

Photoelectron spectra are reported for the ‘‘dipole‐bound’’ CH3CN− negative ion at three photodetachment energies (1.165, 2.331, and 3.496 eV), where the anion is prepared by photodissociation of the I−⋅CH3CN ion–molecule complex. While all three spectra are dominated by a single feature centered near zero electron binding energy, as expected for a dipole‐bound anion, vibrational structure is also observed and found to depend strongly on the photodetachment energy. This observation indicates that the vibrational excitation is not exclusively due to distortion between the ion and neutral, but also involves non‐‘‘Franck–Condon’’ effects. The origin of the energy dependence is traced to excitation of the πCN* shape resonance corresponding to the valence or ‘‘chemical’’ anion. The vibrational envelope of the nonresonant spectrum is surprisingly similar to the infrared spectrum of neutral acetonitrile, suggesting that even this excitation may not result from intramolecular distortions. We develop a simple mode...

Negative ion photoelectron spectroscopy of the ground state, dipole-bound dimeric anion, (HF)2−

We present the mass spectral and photoelectron spectroscopic results of our study of (HF)2−. Our main findings are as follows. The (HF)2− anion was observed experimentally for the first time, confirming the 20 year old prediction of Jordan and Wendoloski. The photoelectron spectrum of (HF)2− exhibits a distinctive spectral signature, which we have come to recognize as being characteristic of dipole bound anions. The vertical detachment energy (VDE) of (HF)2− has been determined to be 63±3 meV, and the adiabatic electron affinity (EAa) of (HF)2 was judged to be close to this value as well. Relatively weak spectral features, characteristic of intramolecular vibrations in the final (neutral dimer) state, were also observed. We have interpreted these results in terms of slight distortions of the dimer anion’s geometric structure which lead to an enhanced dipole moment. This interpretation is supported to a considerable extent by theoretical calculations reported in the companion paper by Gutowski and Skurski.

Barrier-free intermolecular proton transfer induced by excess electron attachment to the complex of alanine with uracil

The photoelectron spectrum of the uracil-alanine anionic complex (UA)(-) has been recorded with 2.540 eV photons. This spectrum reveals a broad feature with a maximum between 1.6 and 2.1 eV. The vertical electron detachment energy is too large to be attributed to an (UA)(-) anionic complex in which an intact uracil anion is solvated by alanine, or vice versa. The neutral and anionic complexes of uracil and alanine were studied at the B3LYP and second-order Møller-Plesset level of theory with 6-31++G(*) (*) basis sets. The neutral complexes form cyclic hydrogen bonds and the three most stable neutral complexes are bound by 0.72, 0.61, and 0.57 eV. The electron hole in complexes of uracil with alanine is localized on uracil, but the formation of a complex with alanine strongly modulates the vertical ionization energy of uracil. The theoretical results indicate that the excess electron in (UA)(-) occupies a pi(*) orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of alanine to the O8 atom of uracil. As a result, the four most stable structures of the uracil-alanine anionic complex can be characterized as a neutral radical of hydrogenated uracil solvated by a deprotonated alanine. Our current results for the anionic complex of uracil with alanine are similar to our previous results for the anion of uracil with glycine, and together they indicate that the BFPT process is not very sensitive to the nature of the amino acids hydrophobic residual group. The BFPT to the O8 atom of uracil may be relevant to the damage suffered by nucleic acid bases due to exposure to low energy electrons.

Solvent-induced stabilization of the naphthalene anion by water molecules: A negative cluster ion photoelectron spectroscopic study

We show that (a) only a single water molecule is needed to stabilize the naphthalene anion, (b) the EAa of naphthalene is −0.20 eV, in agreement with determinations by electron transmission spectroscopy, (c) the energetics are consistent with the number of waters required to stabilize the naphthalene anion, and (d) the excess electron is located on the naphthalene moiety of Nph1−(H2O)n.

Photoelectron spectroscopy of naphthalene cluster anions

Mass spectrometric and anion photoelectron spectroscopic studies of homogeneous naphthalene cluster anions, (Nph)n=2–7−, were conducted to characterize the nature of their anionic cores. The smallest stable species in this series was found to be the naphthalene dimer anion. The vertical detachment energies of naphthalene clusters, n=2–7, were determined and found to increase smoothly with cluster size. By extrapolation, the vertical detachment energy of the isolated naphthalene molecule was found to be −0.18 eV, in agreement with its adiabatic electron affinity value from literature. The strong similarity between the spectral profiles of (Nph)2− and (Nph)1−(H2O)1 implied that (Nph)2− possesses a solvated monomeric anion core. All of the naphthalene cluster anions studied here were interpreted as having monomer anion cores.

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.

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.