Svetlana A. Lyapustina

Dipole bound, nucleic acid base anions studied via negative ion photoelectron spectroscopy

The anions of the nucleic acid bases, uracil and thymine, were studied by negative ion photoelectron spectroscopy. Both monomer anions exhibit spectroscopic signatures that are indicative of dipole bound excess electrons. The adiabatic electron affinities of these molecules were found to be 93±7 meV for uracil and 69±7 meV for thymine. No conventional (valence) anions of these molecules were observed.

The dipole bound-to-covalent anion transformation in uracil

Nucleic acid base anions play an important role in radiation-induced mutagenesis. Recently, it has been shown that isolated (gas-phase) nucleobases form an exotic form of negative ions, namely, dipole bound anions. These are species in which the excess electrons are bound by the dipole fields of the neutral molecules. In the condensed phase, on the other hand, nucleobase anions are known to be conventional (covalent) anions, implying the transformation from one form into the other due to environmental (solvation) effects. Here, in a series of negative ion photoelectron spectroscopic experiments on gas-phase, solvated uracil cluster anions, we report the observation of this transformation.

On the binding of electrons to nitromethane: Dipole and valence bound anions

Conventional (valence) and dipole‐bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe(nf ) Rydberg atoms with CH3NO2 exhibit a pronounced maximum at an effective quantum number of n*≊13±1 which is characteristic of the formation of dipole‐bound anions [μ(CH3NO2)=3.46 D]. However, the breadth (Δn≊5, FWHM) of the n‐dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ‘‘doorway’’ dipole anion state. Studies of the electric field detachment of CH3NO−2 formed through the Xe(nf ) reactions at various n values provide further evidence for the formation of both a dipole‐bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 12±3 meV. Photodetachment of CH3NO−2 and CD3NO−2 formed via a supersonic...

In search of theoretically predicted magic clusters: Lithium-doped aluminum cluster anions

Lithium-doped aluminum cluster anions, LiAln− were generated in a laser vaporization source and examined via mass spectrometry and anion photoelectron spectroscopy (n=3–15). The mass spectrum of the LiAln− series exhibits a local minimum in intensity at n=13. The electron affinity vs cluster size trend also shows a dip at n=13. Agreement is quite good between our measured electron affinity values and those calculated by Rao, Khanna, and Jena, suggesting that their predictions about the structure and bonding of LiAl13 and other clusters in this series are also largely valid.

Electron binding to valence and multipole states of molecules: Nitrobenzene, para- and meta-dinitrobenzenes

Nitrobenzene anions (NB−) in both valence and dipole bound states are examined using laser (photodetachment) photoelectron and Rydberg electron transfer (RET) spectroscopies. Photoelectron spectroscopy of the valence NB− anion yields a valence (adiabatic) electron affinity of 1.00±0.01 eV. The reaction rates for charge transfer between atoms of cesium and xenon in high Rydberg states [Cs(ns,nd) and Xe(nf )] and NB exhibit a prominent peak in their n-dependencies consistent with the formation of a dipole bound anion having an electron affinity of 28 meV. Para-dinitrobenzene (pDNB) has a zero dipole moment and a large quadrupole moment. RET studies with pDNB show a complex n-dependence. The rate of formation of pDNB− ions exhibits a broad peak at low n-values and a second very broad feature extending to large n-values. The peak at low n is tentatively attributed to charge exchange into a quadrupole bound state (EAqb=25 meV). The absence of field-detachment for these ions suggests that if these are in a quad...

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.

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.

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.

Photoelectron spectroscopy of hydrated adenine anions.

We report the observation of hydrated adenine anions, A(-)(H(2)O)(n), n=1-7, and their study by anion photoelectron spectroscopy. Values for photoelectron threshold energies, E(T), and vertical detachment energies are tabulated for A(-)(H(2)O)(n) along with those for hydrated uracil anions, U(-)(H(2)O)(n), which are presented for comparison. Analysis of these and previously measured photoelectron spectra of hydrated nucleobase anions leads to the conclusion that threshold energies significantly overstate electron affinity values in these cases, and that extrapolation of hydrated nucleobase anion threshold values to n=0 leads to incorrect electron affinity values for the nucleobases themselves. Sequential shifts between spectra, however, lead to the conclusion that A(-)(H(2)O)(3) is likely to be the smallest adiabatically stable, hydrated adenine anion.