Dipole-bound Anions Research Articles

Vibrational Fano resonances in dipole-bound anions

This paper explores Fano resonances due to non-adiabatic coupling of vibrational modes and the electron continuum in dipole-bound anions. We adopt a simple one-electron model consisting of a point dipole and an auxiliary potential to represent the electron interaction with the neutral core. Nuclear motion is added by assuming that harmonic vibrations modulate the dipole moment. When the model is parameterized to simulate key features of the water tetramer anion, the resultant photodetachment lineshape closely resembles that observed experimentally and analyzed as a Fano resonance with a parameter q close to -1. Other parameterizations are explored for the model and it is found that large changes in the auxiliary potential are required to change the sign of q. This is consistent with the experimental finding that q is negative for all water cluster sizes studied.

Formation of negative ions in collisions between Rydberg atoms and neutral particles

We present a theoretical description of the formation of weakly bound (${E}_{b}\ensuremath{\sim}0.1\text{--}10$ meV) negative ions in collisions of Rydberg atoms with neutral atoms or molecules. Using the adiabatic approximation for the collision, we obtain an analytical expression for the formation cross section. It is shown that the cross section has a sharp peak in its dependence on the principal quantum number of the Rydberg electron. This result is in agreement with the experimental studies. We obtain a simple analytical expression that relates the peak position and the binding energy of the negative ion. This expression generalizes the empirical law previously established by Desfran\ifmmode \mbox{\c{c}}\else \c{c}\fi{}ois [Phys. Rev. A 51, 3667 (1995)]. Comparison of our calculations with the experimental data on dipole-bound anion formation shows that the obtained formula can be used for determination of binding energies of weakly bound negative ions from Rydberg electron transfer reactions with good accuracy.

Can a Dipole-Bound Electron Form a Pseudo-Atom? An Atoms-In-Molecules Study of the Hydrated Electron

Non-nuclear local maxima, or attractors, of electron density are a rare but very interesting feature of the electron density distribution in molecules and solids. Recently, non-nuclear attractors (NNAs) and the corresponding pseudoatoms of electron density have been identified with the quantum theory of atoms in molecules for some anionic clusters formed by several polar solvent molecules and an excess electron bound in either a solvated-electron or dipole-bound fashion. This contribution reports a detailed study of the topology of the electron density for a series of dipole-bound water cluster anions, as calculated with Hartree-Fock, Møller-Plesset perturbation theory, and coupled-cluster methods together with basis sets augmented with extra diffuse basis functions to accommodate the excess electron. For dipole-bound clusters, electron densities obtained with insufficient inclusion of electron correlation effects and tight basis sets feature a well-pronounced pseudoatom due to the excess electron, which ultimately disappears when a higher level of electronic structure theory and a more diffuse basis set are used. On the other hand, for solvated-electron clusters, where the excess electron is surrounded by solvent molecules, the existence of NNAs does not seem to be an artifact of the method employed, but rather a genuine feature of the electron density distribution. Pseudoatoms of electron density thus appear to be an exclusive feature of confined environments and are unlikely to be found on the tip of a cluster dipole or on solid surfaces.

Role of rotational states in the charge exchange of a dipole-bound anion with a polar molecule

The Landau-Herring method is applied to derive an analytical expression for the exchange interaction potential of a polar molecule with its own dipole-bound anion with account of the relative orientation of the dipole moments of the molecular cores. The potential obtained is used to calculate the cross-section of the resonant charge exchange of a polar molecule with a dipole-bound anion. The influence of the rotation of the molecular cores on the charge exchange cross section is analyzed. A new mechanism of transformation of the charge exchange reaction from quasi-resonant to resonant due to the compensation of the resonance detuning by changes in the rotational states of the colliding molecules is considered.

Photoelectron imaging spectroscopy of nitroethane anions

We present low-energy velocity map photoelectron imaging results for bare and Ar solvated nitroethane anions. We report an improved value for the adiabatic electron affinity of nitroethane of (191 ± 6) meV which is used to obtain a C-NO(2) bond dissociation energy of (0.589 ± 0.019) eV in nitroethane anion. We assign a weak feature at (27 ± 5) meV electron binding energy to the dipole-bound anion state of nitroethane. Photoelectron angular distributions exhibit increasing anisotropy with increasing kinetic energies. The main contributions to the photoelectron spectrum of nitroethane anion can be assigned to the vibrational modes of the nitro group. Transitions involving torsional motion around the CN bond axis lead to strong spectral congestion. Interpretation of the photoelectron spectrum is assisted by ab initio calculations and Franck-Condon simulations.

On the Electron Affinity of Nitromethane (CH3NO2)

A high-level systematic computational study is presented on an accurate value for the adiabatic valence electron affinity of nitromethane, CH(3)NO(2), to resolve literature disagreements in theoretical and experimental reported values. Density functional methods with triple-zeta quality basis sets gave good fortuitous agreement to early experimental determinations, while single-reference wave function based methods employing up to CCSD(T) gave poor or fortuitous agreement depending on the experimental reference value. DFT methods in general cannot accurately describe electron attachment from the result of unphysical self-interaction. It is found that multireference methods with aug-cc-pVTZ or similar basis sets are required to converge to an experimental value. Our highest level of theory 3S-MCQDPT2 and 7S-MCQDPT2 calculations with an aug-cc-pVTZ quality basis description yield values of 0.188 and 0.176 eV (0.170 eV with polynomial extrapolation), in excellent agreement with the most recent experimental value of 0.172 +/- 0.006 eV. CCSD(T)//aug-cc-pVTZ provides a fortuitously reasonable description. The isolated dipole-bound anion binding energy is tentatively calculated to be 7-8 meV.

The exchange interaction of an ion with a dipole-bound anion

An analytic study of the asymptotic properties of a quasimolecular system consisting of a dipole-bound anion A - p and a highly charged ion B Z b + was carried out. The influence of point dipole of the polar molecule on the asymptotic of the one-electron wave functions of the system (A p B) (Z b -1)+ for large distances between interacting particles was taken into account. The analytical expression for the leading term of the asymptotic of one-electron exchange interaction of a highly charged ion with a dipole-bound anion was obtained in the framework of a semiclassical approach.

Electron exchange between dipole-bound anion and polar molecule and dipole-bound anions dimer formation

We consider collision between a dipole-bound molecular anion and a neutral polar molecule and show that the excess electron can bind two neutral molecules into a dimer. Using a variational approach similar to the Heitler-London model of H+2 ion we obtain the energy terms of such a dimer. Their difference determines the cross-section of electron transfer from the anion to the neutral molecule in quasiclassical near-resonant Born-Oppenheimer approximation. We obtain for the cross-section an analytic expression containing the weak (logarythmic) factor depending on the molecular dipole moment, and collision velocity. Our analytic calculations are in a good accordance with the results of a recent experiment.

Electron exchange between a dipole-bound anion and a polar molecule and dimer formation in dipole-bound anions

We consider collision between a dipole-bound molecular anion and a neutral polar molecule and show that the excess electron can bind two neutral molecules into a dimer. Using a variational approach similar to the Heitler-London model of $\text{H}_{2}{}^{+}$ ion, we obtain the energy terms of such a dimer. Their difference determines the cross section of electron transfer from the anion to the neutral molecule in quasiclassical near-resonant Born-Oppenheimer approximation. We obtain for the cross section an analytical expression containing the weak (logarithmic) factor depending on the molecular dipole moment and the collision velocity. With the prelogarithmic factor proportional to inverse binding energy of the electron in the molecular anion, such weak dependence on the collision velocity is somewhat similar to the expressions appearing in the hard-sphere collision model. However the large logarithmic factor connected with long-range dipole-dipole interaction between the colliding molecules contradict to the hard-sphere approximation even for qualitative description of the charge transfer. Our analytical calculations are in a good accordance with the results of a recent experiment [Y. Liu, M. Cannon, L. Suess, F. B. Dunning, V. E. Chernov, and B. A. Zon, Chem. Phys. Lett. 433, 1 (2006)].

Formation and photodestruction of dual dipole-bound anion (H2O)6{e−}CH3NO2

A new type of dipole-bound anion composed of water and nitromethane (CH(3)NO(2)) is formed via the incorporation of CH(3)NO(2) into argon-solvated water hexamer anions, (H(2)O)(6) (-)Ar(m). The reaction proceeds as an Ar-mediated process such that an effective energy dissipation through sequential Ar evaporation gives rise to the formation of [CH(3)NO(2)(H(2)O)(6)](-). Photoelectron spectroscopy is employed to probe the electronic properties of the [CH(3)NO(2)(H(2)O)(6)](-) anion, which reveals that the dipole-bound nature of (H(2)O)(6) (-) remains almost intact in the product anion; the vertical detachment energy of [CH(3)NO(2)(H(2)O)(6)](-) is determined to be 0.65+/-0.02 eV. This spectroscopic finding, together with other suggestive evidences, allows us to refer to [CH(3)NO(2)(H(2)O)(6)](-) as a dual dipole-bound anion described as (H(2)O)(6){e(-)}CH(3)NO(2), where the diffuse excess electron interacts with both the (H(2)O)(6) and CH(3)NO(2) moieties via the electron-dipole interactions. The photodestruction of (H(2)O)(6){e(-)}CH(3)NO(2) at 2134 nm (0.58 eV) occurs with a competition between electron detachment and fragmentation. The latter leads exclusively to the formation of CH(3)NO(2) (-)(H(2)O)(3), indicating that the dual dipole-bound anion serves as a precursor to the hydrated valence anion of CH(3)NO(2).

Important effects of neighbouring nucleotides on electron induced DNA single-strand breaks

In this Letter, we present Quantum Mechanics/Molecular Mechanics (QM/MM) calculations on molecules containing a 2-deoxycytidine-3′-monophosphate moiety (3′-dCMPH). In particular, we examine the effect that including neighbouring nucleotides at the Molecular Mechanic (MM) level has on the calculated electron affinities and on the energetic barriers of the C3′–O3′ bond cleavage. Our results demonstrate that the surrounding nucleotides relocate the excess electron from the π ∗ orbital of the base to a diffuse phosphate-centred orbital, leading to the formation of a dipole-bound anion state. Both the electron affinities and the activation energy of C3′–O3′ bond cleavage are strongly increased.

Theoretical study on the transformation mechanism between dipole-bound and valence-bound anion states of small uracil–water clusters and their photoelectron spectra

Microhydration effects on the transformation from the dipole-bound to valence-bound anions of uracil have been investigated at the density-functional BH&HLYP level of theory. It was found that the transformation barrier height decreases with an increase in the number of water molecules although the calculated barrier height somewhat depends on the water-binding site. Photoelectron spectra of the hydrated uracil anions, U −·(H 2O) n ( n = 1 and 2), have been calculated by a simplified Franck–Condon model and by an on-the-fly direct molecular dynamics method and then compared to experimental results. We found that the broad feature of the measured photoelectron spectra is mainly determined by vibrational motions including out-of-plane bending of the uracil anion core and that water solvation structures do not largely affect spectral broadening.

Rotational states of dipole-bound anions of hydrogen cyanide

Rotational states of dipole-bound anions of HCN are studied both experimentally and theoretically. The ground state experimental electron affinity (ea), 1.56 meV, is in good agreement with present and one earlier theoretical value. The predicted binding energy for the J = 1 state, 1.2 meV, is observed, whereas J = 2 is too fragile for observation and J = 3 is unbound.

Theoretical Study on the Mechanism of Low-Energy Dissociative Electron Attachment for Uracil

The lowest electronically adiabatic potential energy surface of the uracil anion has been theoretically investigated with density-functional theory methods in order to understand the mechanism of the N-H bond dissociation induced by low-energy electron attachment. We found that the BH&HLYP level can reasonably describe both the dipole-bound and valence anionic states in a balanced way. With this density-functional theory level, we have constructed two-dimensional potential energy surfaces as a function of appropriate internal coordinates and discuss the importance of electronic coupling between the dipole-bound and valence anion states in dissociative electron attachment of uracil. The transition state geometry for the electronic isomerization between the dipole-bound anion and the pi* valence anion was successfully optimized and the barrier height for this isomerization was found to be relatively low. It was found that the out-of-plane motion of H at the C6 position plays the most important role in this isomerization process. Reduced-dimensionality quantum wave packet calculations taking two active internal coordinates into account have also been performed to interpret the resonance structures observed in cross sections for the N-H dissociation channel at a qualitative level.

Excess Electrons Bound to Small Ammonia Clusters

Dipole-bound anions of small water clusters (H2O) N- (N >or= 2) are well-known from experiment and theory. In contrast, the smallest ammonia cluster anion detected so far is the 13-mer (NH3)13-. Here dipole-bound states of small ammonia clusters (NH3)N- (N = 2, 3, 4) are investigated using coupled-cluster ab initio methods. The trimer is found to be the smallest ammonia cluster able to form a dipole bound state, and its vertical detachment energy is predicted to be 27 meV, somewhat smaller than that of the water dimer. For the ammonia tetramer dipole-bound states with triple-acceptor monmers are identified akin to the well-studied double-acceptor binding motif of water cluster anions. Moreover, a (NH3)6-)hexamer that has been considered as a model for a cavity-bound state is examined. Ab initio results for this system challenge the notion that an electron localized in an ammonia cavity can be thought of as a delocalized radical anion.

Comment on “Electron structure of a dipole-bound anion confined in a spherical box” and “Addendum to ‘Electron structure of a dipole-bound anion confined in a spherical Box’: The case of a finite dipole”

This Comment is motivated by some conceptual inconsistencies in the paper by Ronen [Phys. Rev. A 68, 012106 (2003)] and its Addendum [S. Ronen, Phys. Rev. A 68, 064101 (2003)]. While some of the inconsistencies are trivial---such as not distinguishing between anions and cations, and stating that polar molecules are made up of nuclei with positive and negative electric charges---the crucial one concerns the binding energy for an electron in a polar molecule, especially in the context of a confined molecular ion.

Molecular Anions

The experimental and theoretical study of molecular anions has undergone explosive growth over the past 40 years. Advances in techniques used to generate anions in appreciable numbers as well as new ion-storage, ion-optics, and laser spectroscopic tools have been key on the experimental front. Theoretical developments on the electronic structure and molecular dynamics fronts now allow one to achieve higher accuracy and to study electronically metastable states, thus bringing theory in close collaboration with experiment in this field. In this article, many of the experimental and theoretical challenges specific to studying molecular anions are discussed. Results from many research groups on several classes of molecular anions are overviewed, and both literature citations and active (in online html and pdf versions) links to numerous contributing scientists' Web sites are provided. Specific focus is made on the following families of anions: dipole-bound, zwitterion-bound, double-Rydberg, multiply charged, metastable, cluster-based, and biological anions. In discussing each kind of anion, emphasis is placed on the structural, energetic, spectroscopic, and chemical-reactivity characteristics that make these anions novel, interesting, and important.

Dipole-bound CH3CN− ions: Temperature dependence of ion production rates and lifetimes

The formation of long-lived (tau less, similar10 mus) dipole-bound CH(3)CN(-) ions through electron transfer in K(14p)CH(3)CN collisions is investigated as a function of target temperature. The rate for their formation is observed to decrease steadily with increasing target temperature. The results are consistent with earlier suggestions that only target molecules in the ground vibrational state and low-lying rotational states can form long-lived dipole-bound anions. For CH(3)CN, the data indicate that creation of long-lived ions requires that the target molecules be in states with rotational quantum numbers j less, similar20. The measurements further demonstrate that the lifetime of the longest-lived (tau greater, similar50 mus) ions is limited by blackbody-radiation-induced photodetachment.

ANIONS AND ANOMALIES

We analyze the recent claim that experimental measurements of binding energies of dipole-bound anions can be understood in terms of a quantum mechanical anomaly. The discrepancy between the experimental critical dipole moments and that predicted by the anisotropic inverse square potential of a static dipole precludes such an explanation. As has long been known, in the physical problem one must include rotational structure so that the long distance behavior changes from 1/r2 to 1/r4. In a simple model this can be shown to lead to a modification of the critical dipole moment of 20% or so, bringing it into agreement with experiment. This, together with the fact that inclusion of finite size effects does not change the critical dipole moment of the static point dipole, strongly suggests that the quantum mechanical anomaly interpretation of the formation of dipole-bound anions cannot be correct.

Low-Energy Electron Capture by 6-Aza-2-thiothymine: Investigations by Electron Attachment and Electron Transmission Spectroscopies

The interaction of low-energy (0-10 eV) electrons with 6-aza-2-thiothymine is investigated in the gas phase by studies of sharp structure in the total electron scattering cross section and by mass analysis of the stable or long-lived negative ions produced by electron attachment. The most efficient fragmentation process, occurring at 0.15 eV, involves the ejection of a closed-shell neutral molecule (CH3CN). Ab initio calculations support our proposal that this process leads to ring closure to form a stable four-member heterocyclic anion. A long-lived parent anion with an approximate lifetime of 75 microseconds is observed near zero electron energy, and evidence is also seen for the slow decay of this anion by ejection of CH3CN. Near 3.3 eV, an anion of m/e 41 is produced that is likely to be a metastable valence anion of bent CH3CN, but the dipole-bound anion cannot be ruled out.