Dipole-bound States Research Articles

Two thermodynamically stable states inSiO−andPN−

In contrast to the well-known CO and ${\mathrm{N}}_{2}$ molecules, isoelectronic SiO and PN form thermodynamically stable conventional (valence) and dipole-bound anion states upon attachment of an extra electron. According to the results of our high-quality ab initio calculations, binding energies of the electron in these states are 38 and 1.6 meV $({\mathrm{SiO}}^{\mathrm{\ensuremath{-}}})$ and 76 and 1.0 meV $({\mathrm{PN}}^{\mathrm{\ensuremath{-}}}),$ respectively. Dissociation trends of the ${\mathrm{SiO}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{PN}}^{\mathrm{\ensuremath{-}}}$ anions are discussed.

Electron and anion mobility in low density hydrogen cyanide gas. I. Dipole-bound electron ground states

We measured the mobility of excess electrons in the polar hydrogen cyanide gas (D=2.985 D) at low densities as a function of density and temperature by the so-called pulsed Townsend method. Experiments were performed at 294 and 333 K in the gas number density range 1.23×1017⩽n⩽3.61×1018 cm−3. We found a strong density dependence of the “zero-field” density-normalized mobility (μn). Only about 10% of the observed density variation can be qualitatively explained by coherent and incoherent multiple scattering effects. With increasing gas density an increasing number of linear HCN dimers is formed which due to the high dipole moment (D=6.552 D) represent much stronger electron scatterers than the HCN monomers. It was found that the dimers may be only in part responsible for the observed density effect. Therefore, we consider a transport process where short-lived dipole-bound electron ground states (lifetime ⩾12 ps) as quasilocalized states are involved. For comparison the electron mobility in saturated 2-aminoethanol vapor with a dipole moment of similar size (D=3.05 D) does not show any anomalous density behavior in the temperature range 298⩽T⩽435 K. In contrast to this the electron mobility in saturated but also in nonsaturated CH3CN gas (D=3.925 D) shows a density behavior similar to that in HCN.

Electron and anion mobility in low density hydrogen cyanide gas. I. Dipole-bound electron ground states

We measured the mobility of excess electrons in the polar hydrogen cyanide gas (D=2.985 D) at low densities as a function of density and temperature by the so-called pulsed Townsend method. Experiments were performed at 294 and 333 K in the gas number density range 1.23×1017⩽n⩽3.61×1018 cm−3. We found a strong density dependence of the “zero-field” density-normalized mobility (μn). Only about 10% of the observed density variation can be qualitatively explained by coherent and incoherent multiple scattering effects. With increasing gas density an increasing number of linear HCN dimers is formed which due to the high dipole moment (D=6.552 D) represent much stronger electron scatterers than the HCN monomers. It was found that the dimers may be only in part responsible for the observed density effect. Therefore, we consider a transport process where short-lived dipole-bound electron ground states (lifetime ⩾12 ps) as quasilocalized states are involved. For comparison the electron mobility in saturated 2-aminoethanol vapor with a dipole moment of similar size (D=3.05 D) does not show any anomalous density behavior in the temperature range 298⩽T⩽435 K. In contrast to this the electron mobility in saturated but also in nonsaturated CH3CN gas (D=3.925 D) shows a density behavior similar to that in HCN.

Electronic Structure of Dipole-Bound Anions

Dipole-bound anionic states of HCN, (HF)2, CH3CN, C3H2, C4H2, C5H2, and stretched CH3F are studied using extended one-electron basis sets at the coupled cluster level of theory with single, double, and noniterative triple excitations (CCSD(T)). Orbital relaxation and electron correlation corrections to the Koopmans' theorem prediction of electron binding energy are analyzed, and a physical interpretation of low-order corrections is proposed. It is demonstrated that the second-order dispersion interaction between the loosely bound electron and the electrons of the neutral host should be included into physical models of dipole-bound anions. Higher-order electron correlation corrections are also found to be important, and a slow convergence of the Moller−Plesset series for electron binding energies is documented. Modifications of the potential energy surfaces of the above polar molecules upon electron attachment are studied at the second-order Moller−Plesset level, and Franck−Condon factors for the anion/neu...

Valence and excited states ofLiH−

Valence and excited dipole-bound states of the ${\mathrm{LiH}}^{\mathrm{\ensuremath{-}}}$ anion are calculated with the recently developed electron-attachment equation-of-motion coupled-cluster technique. It is found that the first dipole-bound state of ${\mathrm{LiH}}^{\mathrm{\ensuremath{-}}}$ corresponds to the second dissociation channel ${\mathrm{LiH}}^{\mathrm{\ensuremath{-}}}{\ensuremath{\rightarrow}\mathrm{Li}}^{\mathrm{\ensuremath{-}}}{(}^{1}S)+\mathrm{H}{(}^{2}S).$ The second (excited) dipole-bound state of ${\mathrm{LiH}}^{\mathrm{\ensuremath{-}}}$ is below the neutral ground-state potential energy curve only for some range of the Li-H internuclear distance. This state appears at bond lengths larger than $\ensuremath{\approx}2.0\AA{}$ and decays at Li-H distances longer than \ensuremath{\approx}4.2 \AA{}, where the dipole moment of LiH becomes smaller than the critical value of 2.5 D. The adiabatic electron affinity of LiH calculated at the coupled-cluster level with the iterative inclusion of all single, double, and triple excitations and a large atomic natural orbital basis set is 0.327 eV, almost matching the recently obtained experimental value of $0.342\ifmmode\pm\else\textpm\fi{}0.012\mathrm{eV}.$

Dispersion Stabilization of Solvated Electrons and Dipole-Bound Anions

It has long been assumed that stability of “solvated electrons” and dipole-bound anions results primarily from the static Coulomb interaction of an excess electron with charge distribution of the neutral molecular host. Our results indicate, however, that the dispersion interaction between the loosely bound electron and the neutral molecular host is as important as the static Coulomb stabilization. A perturbation scheme is designed to analyze physically meaningful components of electron binding energy, and highly correlated electronic structure results are presented for dipole-bound anions and solvated electrons in the (HF) n clusters (n ) 2, 3). The vertical electron detachment energy for (HF)3 was found to be 0.63 and 0.21 eV for the solvated electron and dipole-bound anion, respectively. The equilibrium zigzag geometrical structure of the dipole-bound anion differs drastically from the cyclic C3h structure of the neutral trimer. The dispersion interaction is commonly associated with an attractive component of van der Waals interactions that displays asymptotic R -6 behavior, where R is the distance between interacting species. 1 As recognized by London, the dispersion effect results from correlated fluctuations of electron charge densities of interacting atomic or molecular species. 2 In this contribution we discuss the role of dispersion interaction in a different molecular context. We consider a loosely bound electron (lbe) as a chemical species, and we discuss its interaction with the neutral molecular host as if we were dealing with a weakly bound molecular dimer. Our theoretical framework applies to dipole-bound states (dbs) of molecular anions and to “solvated electrons” (se) because these anions share a feature that the excess electron is bound by physical rather than chemical interactions with its neutral molecular host. 3 The results presented here extend our earlier observations about the role of dynamic electron correlation in dipole-bound anions, 4-6 in particular in systems with hydrogen bonds such as (HF)2 - 7

Search for Stable Anions of Uracil−Water Clusters. Ab Initio Theoretical Studies

In this work we investigate the ability of the uracil·water complex to form stable anionic systems. As the experimental evidence and theoretical calculations have indicated, the isolated uracil molecule can only attach an excess electron into a diffuse dipole-bound state, while some recent experiments suggest that the uracil· water complex can form a more stable valence-type anion. In this work we demonstrate that it is possible to converge ab initio calculations of uracil·(H2O)3- to an equilibrium structure that is significantly different from the structure of the neutral cluster and that has a positive and remarkably significant vertical ionization potential. Apart from the valence anion, the uracil·(H2O)3 complex can form a stable dipolebound anion, but as the present calculations indicate the electron affinity, which corresponds to this attachment, is very small (13 meV). The structure of the dipole-bound anion is virtually identical with the structure of the neutral complex.

Ab initio theoretical study of dipole-bound anions of molecular complexes. Water trimer anion

We present results of ab initio calculations of our study of (H2O)3−. The main conclusions of this work are as follows: the most stable cyclic structure of (H2O)3 has a dipole moment too small to form a dipole-bound state with an excess electron; the dipole-bound anion of the water trimer observed experimentally appears to be a hydrated complex of the water dimer anion, (H2O)2−, by a single water molecule. The water trimer anion, (H2O)3−, has an open shape. The calculated vertical electron detachment energy of this anion is predicted to be equal to 141 meV, which is in good agreement with the experimental value of Bowen and co-workers, equal to 142±7 meV. Although the open optimal geometry of the (H2O)3− anion obtained in the present calculations is an equilibrium structure, its energy is higher than the energy of the cyclic equilibrium structure of the neutral complex, indicating that the anion is a metastable system. Based on calculations, we predict significant differences in the IR vibrational spectra of (H2O)3 and (H2O)3−, which may be used for identification of the two species.

Methylation Reduces Electron Affinity of Uracil. Ab Initio Theoretical Study

Following the experimental characterization of the N,N-dimethylated uracil anion by Bowen and co-workers, we have undertaken an investigation of the influence of the methylation on the electron affinity of the uracil molecule. Both experiment and theory agree that, as it is in the case of the isolated uracil molecule, the methylated uracils can only attach excess electrons into diffuse dipole-bound states. The corresponding electron affinities are very small (several MeV). The bonding effect in the dipole-bound state depends on the magnitude of the molecular dipole and on the size of the molecule. Selective methylation of the uracil molecule can be used to reduce or increase the dipole value and to change the electron affinity of the molecule. The present calculated results are consistent with the experimental determination that N,N-dimethylation of uracil results in reduction of the electron affinity.

Valence and excited dipole-bound states of polar diatomic anions: LiH−, LiF−, LiCl−, NaH−, NaF−, NaCl−, BeO−, and MgO−

Valence and excited states of the anions formed by the polar molecules LiH, LiF, LiCl, NaH, NaF, NaCl, BeO, and MgO are calculated by the recently developed electron-attachment equation-of-motion coupled-cluster (EA-EOMCC) method. The LiH−, NaH−, and LiF− anions are found to possess two excited dipole-bound states and the LiCl−, NaF−, NaCl−, BeO−, and MgO− anions have three excited dipole-bound states. The critical values of the dipole moment required to sustain the first, second, etc., excited state appears to be specific for each class of polar molecules.

Valence and excited dipole-bound states of polar diatomic anions: LiH −, LiF −, LiCl −, NaH −, NaF −, NaCl −, BeO −, and MgO −

Valence and excited states of the anions formed by the polar molecules LiH, LiF, LiCl, NaH, NaF, NaCl, BeO, and MgO are calculated by the recently developed electron-attachment equation-of-motion coupled-cluster (EA-EOMCC) method. The LiH −, NaH −, and LiF − anions are found to possess two excited dipole-bound states and the LiCl −, NaF −, NaCl −, BeO −, and MgO − anions have three excited dipole-bound states. The critical values of the dipole moment required to sustain the first, second, etc., excited state appears to be specific for each class of polar molecules.

Theoretical study of the dipole-bound anion (HF)2−

It has long been assumed that dipole-bound anionic states possess two properties; (i) the electron binding energy is dominated by the electrostatic electron–dipole interaction, and (ii) the geometrical distortion of the polar molecule upon attachment of a distant electron is negligible. Our results indicate, however, that the dispersion interaction between the loosely bound electron and the electrons of the neutral dimer is as important as the electrostatic electron–dipole stabilization. In addition, the hydrogen bond in (HF)2 is susceptible to a deformation upon attachment of a distant electron. This deformation enhances both the electrostatic and dispersion components of the electron binding energy. The calculated Franck–Condon factors indicate that neutral dimers formed in electron photodetachment experiments may be vibrationally excited in both stiff intramolecular HF stretching modes and soft intermolecular modes. The predicted value of the adiabatic electron detachment energy for (HF)2− is 396 cm−1 and the theoretical photoelectron spectrum is in remarkable agreement with the experimental data of Bowen et al. J. H. Hendricks, H. L. de Clercq, S. A. Lyapustina, and K. H. Bowen, Jr. [J. Chem. Phys. 107, 2962 (1997)].

Energies of dipole-bound anionic states

Dipole-bound anionic states of CH3CN, C3H2, and (HF)2 were studied using highly correlated electronic structure methods and extended one-electron basis sets. The electron detachment energies were calculated using the coupled cluster method with single, double, and noniterative triple excitations. Geometrical relaxation of the molecular framework upon electron attachment and the difference in the harmonic zero-point vibrational energies between the neutral and the dipole-bound anionic species were calculated at the MP2 level of theory. We demonstrate that the dispersion interaction between the loosely bound electron and the electrons of the neutral molecule is an important component of the electron binding energy, comparable in magnitude to the electrostatic electron–dipole stabilization. The geometrical relaxation upon electron attachment and the change in the zero-point vibrational energy is important for the weakly bound HF dimer. The predicted values of the vertical electron detachment energies for the dipole bound states of CH3CN and C3H2 of 112 and 188 cm−1, respectively, are in excellent agreement with the recent experimental results of 93 and 171±50 cm−1, respectively. For (HF)2−, the predicted value of adiabatic electron detachment energy is 396 cm−1, whereas the experimental vertical detachment energy is 508±24 cm−1. The possibility of formation of the neutral dimer in an excited vibrational state is considered. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 183–191, 1997

Excess electron mobility in low densityCH3CN gas:mShort-lived dipole-bound electron ground states as precursors of localized electron states

We measured the mobility of excess electrons in the strongly polar acetonitrile (${\mathrm{CH}}_{3}$ CN) gas at low densities as a function of density and temperature by the pulsed Townsend method. We found an unexpected strong density dependence of the 'zero-field' density-normalized mobility (\ensuremath{\mu}n) that cannot be explained only by different multiple-scattering effects. We propose a transport process where short-lived dipole-bound electron ground states as quasilocalized states are involved. Finally we discuss the experimentally observed anion mobilities as a function of density and temperature according to current theoretical approaches.

Autodetachment spectroscopy and dynamics of vibrationally excited dipole-bound states of H2CCC−

Direct observation of the rotational fine structure levels of a vibrationally excited negative ion dipole-bound state (DBS) is reported. Autodetachment resonances of H2CCC− were observed for the 2A1−2B1 transition in one quanta of ν6, ν4, and ν2 and two quanta of ν6 vibrational modes of the DBS. Rotational assignments for both the electronic ground state and the DBS were performed. Strong type (a) Coriolis coupling between ν6 and ν9 in both the electronic ground and excited states was observed, and coupling constants were determined. QCISD ab initio calculations were performed for the ground state, the negative ion, and the neutral state of H2CCC. The calculations on the neutral agree well with measured vibrational frequencies of the dipole-bound state. The autodetachment resonances contain information about the detachment dynamics via the observed linewidths, showing, e.g., that motions along the dipole moment axis significantly enhance autodetachment, indicating that the DBS is unstable with respect to neutral core motion which modulates the magnitude value of the dipole moment.

Autodetachment spectroscopy and dynamics of dipole bound states of negative ions: 2A1–2B1 transitions of H2CCC−

The H2CCC− ion is studied by autodetachment spectroscopy in a coaxial laser-ion beam spectrometer. Sharp resonances at photon energies near the photodetachment threshold energy are ascribed to a 2A1←2B1 transition followed by autodetachment of the dipole-bound state (DBS). Some 2500 rotational transitions are assigned and the band origin is determined to be 14 284.420(5) cm−1. The observation of individual rotational lines allowed determination of the rotational spectroscopic constants as A=9.651 53(4) cm−1 and (B+C)/2=0.346 461(3) cm−1 for the DBS as well as the spin-rotation coupling constant εaa=2.17(6)×10−3 cm−1. Based on an electron affinity of 14 469±64 cm−1, the binding energy of the electron in the DBS is 170±50 cm−1. Anomalous rotational line positions are found in the vicinity of Ka=7–10 in the DBS and have been attributed to the centrifugal distortion couplings caused by mixing with the CCC out-of-plane bending mode (ν6) and the CCC in-plane bending mode (ν9). The linewidths provide information about autodetachment rates that make it possible to obtain electron detachment dynamics for individual rotational states.

On the vibrational fine structure in the near-threshold photofragmentation spectrum of the I−⋅CH3I complex: Spectroscopic observation of nonadiabatic effects in electron-molecule scattering

Photofragmentation of the I−⋅CH3I ion-molecule complex is observed to accompany photoexcitation in the vicinity of its electron detachment thresholds. The I− photofragment action spectrum displays a vibrational progression in the ν3 (largely C–I stretching) mode of neutral CH3I, the same mode which is excited upon photodetachment of the complex. The extent of this vibrational activity in the I−⋅CH3I photoelectron spectrum is found to strongly depend on the photodetachment energy, becoming very pronounced as the photon energy approaches the detachment threshold. This indicates that the vibrational features in the photoelectron spectrum arise from non-Franck–Condon effects. These observations of selective excitation of ν3 in both the photoelectron and photofragmentation spectra are correlated to nonadiabatic effects arising from the repulsive state of the CH3I− anion, which is thought to evolve into a resonance near the equilibrium separation of neutral CH3I. The I−⋅CH3I photochemistry is discussed in the context of the mechanisms postulated to govern electron-molecule scattering (i.e., vibrational inelastic and dissociative electron attachment) in bare CH3I. Finally, we cast the scattering mechanism in a spectroscopic picture and suggest that the threshold fragmentation in I−⋅CH3I can be viewed as the predissociation of a transient or virtual dipole-bound electronic state.

A theoretical study of the valence- and dipole-bound states of the nitromethane anion

The valence- and dipole-bound states of CH3NO−2 are studied at the CCSD(T), HFDFT (B3LYP), and EA-EOMCC levels of theory. At both CCSD(T) and HFDFT levels, we have found a positive valence EA in nice agreement with the experimental data. The binding energy of the dipole-bound electron is about 13 meV according to the EA-EOMCC calculations. Interaction of the valence- and dipole-bound states (DBS) of CH3NO−2 is complicated, since the dipole-bound state exists at the equilibrium geometry of the anion and corresponds to an excited state of the valence-bound anion. Hence, excitations of the valence anionic state could lead to both the detachment of an electron or formation of a DBS, whose geometry is similar to the geometry of the neutral parent. At the equilibrium geometry of the anion, the energies of the dipole-bound and valence states are close to each other. Since typical lifetimes of rovibrational excited states of a DBS are two orders of magnitude higher than the lifetimes of ordinary vibrationally excited states, it might be possible to transform the DBS into the valence ground state under certain experimental conditions.

Contribution of electron correlation to the stability of dipole-bound anionic states.

It has long been assumed that electron correlation effects are relatively unimportant for describing dipolebound anionic states. It is shown here that this assumption is incorrect: high-level electronic structure calculations on the dipole-bound anion states of CH 3CN, C 3H 2, and C 5H 2 reveal that for these species a large fraction of the electron binding energy derives from the dispersion-type interaction between the loosely bound electron and the neutral molecule. The predicted values of the electron affinities of the dipole bound states of CH 3CN and C3H 2, 108 and 173 cm 21 , respectively, are in excellent agreement with the recent experimental results, 93 cm 21 and 171650 cm 21 , respectively. The predicted value for C 5H 2 is 614 cm 21 . @S1050-2947~96!09208-6#

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 model to illustrate that vibrational excitation can occur upon photodetachment of a dipole-bound electron due to the perturbation of the weakly bound electron by the fluctuating dipole moment of the vibrating neutral molecule. We treat this effect in a Herzberg–Teller interaction picture where the dipole-bound state is mixed with the low lying electron continuum through a dipolar interaction with the neutral molecule.