Anion Potential Energy Surfaces Research Articles

Quasidegeneracy of (H2C = B = CH2)– borata-alkene synthon and (H3B–C≡CH)– boryl-substituted acetylene anion induced by an excess electron attachment

Using ab initio methods we examined neutral and anionic BC2H4 systems. The most stable neutral isomer was found to correspond to H2CBCH2 structure. Among other neutral isomers, H3B–CCH system was identified as the highest energy isomer. The energetic order of anionic isomers was found to be different as (H2CBCH2)– and (H3B–CCH)– species were identified as nearly isoenergetic and the most stable. The energetic effect caused by an excess electron attachment was found responsible for rendering (H3B–CCH)– isomer the most stable on the anionic potential energy surface. Vertical electron detachment energies (1.71–4.32 eV) characterizing (BC2H4)– anions were predicted and discussed.

Dissociative photodetachment of H3O2-: a full-dimensional quantum dynamics study.

The transition state is a central concept of chemistry. Photoelectron-photofragment coincidence (PPC) spectroscopy has been proven as an attractive method to study the transition state dynamics. Within a state-of-the-art full-dimensional quantum mechanical model, the dissociative photodetachment dynamics of H3O2- is investigated on accurate anion and neutral potential energy surfaces. The calculated PPC spectrum of H3O2- agrees well with the experimental measurement. The dissociative product OH is exclusively populated on the ground vibrational state, implying the character of the spectator bond. In contrast, the product H2O is predominantly populated in the ground and fundamental states of the symmetric and antisymmetric stretching modes, which is caused by the strong coupling between the antisymmetric motion of the transferred H atom in the transient intermediate [H3O2]* and both stretching modes of the product H2O.

Rich structural topology of the anion formed by the complex of acetonitrile with two water molecules

Ab-initio quantum-mechanical calculations are performed to determine structures and energetic of anions formed by the trimer of an acetonitrile molecule and two water molecules. In total, eight anion configurations corresponding to separate local minima of the anion potential energy surface are identified. All eight anions have positive vertical electron detachment energies. The identified anions belong to the following three categories: dipole-bound anions, anions where the excess electron is suspended between two fragments of the trimer, and solvated covalent anions.

Photoelectron-Photofragment Coincidence Studies on the Dissociation Dynamics of the OH-CH4 Complex.

Photoelectron-photofragment coincidence (PPC) spectroscopy was used to characterize the energetics and dynamics of the OH + CH4 → H2O + CH3 reaction initiated by photodetachment of the OH-(CH4) anion complex. PPC measurements at a photon energy of 3.20 eV yielded stable (OH-CH4 + e-) and dissociative (OH + CH4 (ν1 or ν3, v = 0, 1) + e-) channels. The main channel is dissociation to OH + CH4 + e- with a low kinetic energy release (KER), peaking at 0.04 eV. Interpretation of the experimental results was supported by quantum chemistry and quasiclassical trajectory calculations. The anion potential energy surface was constructed at the correlated coupled cluster singles, doubles, and perturbative triples level with augmented correlation consistent polarized valence triple-ζ basis set, and previously calculated neutral potential energy surfaces were used. Quasiclassical simulation of the dynamics of the OH-CH4 complex was carried out by selecting the momenta and coordinates from the Wigner distribution for the anion, providing the starting point for 4000 trajectories on the neutral potential energy surface. In agreement with the experimental results, most of the trajectories yield slowly recoiling OH + CH4 reactants while some are trapped in the entrance channel van der Waals well.

Theoretical calculations of photoelectron spectrum of (Au–CO2)− anion

Photoelectron spectra of the (Au–CO2)− anion complex have been calculated using both quantum path-integral and classical molecular dynamics methods to interpret the experimentally measured spectrum, showing a double-peak feature corresponding to two different anion configurations of Au·CO2− and Au−·CO2. The anionic potential energy surfaces have been developed on the basis of density-functional theory calculation results obtained with several different levels. We found that the calculated photoelectron spectra significantly depend on the relative energies of these two isomers and the isomerization barrier heights. Nuclear quantum effects are playing important roles for obtaining reliable spectra through vibrational quantization. The present calculations also suggest that the photoelectron spectrum intensity around the overlap region of the two anion peaks can provide a fingerprint of the Au·CO2− ↔ Au−·CO2 reaction.

Understanding Processes Following Resonant Electron Attachment: Minimum-Energy Crossing Points between Anionic and Neutral Potential Energy Surfaces.

The equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method with and without a complex absorbing potential (CAP) is applied for the study of the complex potential energy surfaces (CPES) of temporary anions and their parent neutral molecules. Crossings between the anionic state and the neutral state can be connected to the emission of nearly zero-energy electrons, which is demonstrated by the examples of acrylonitrile and methacrylonitrile. We show that the location of the minimum-energy crossing point (MECP) relative to the equilibrium structures of the neutral molecule and the anion can explain experimentally observed peaks on the threshold line of two-dimensional electron-energy loss spectra. The location and energy of the MECP is also crucial in dissociative electron attachment as we illustrate for chloro-substituted ethylenes. It is demonstrated that both the metastable region of the anionic CPES and the crossing with the neutral PES need to be considered to explain trends in the chloride ion formation cross sections of dichloroethylenes.

Ab initio study of reactive collisions between Rb((2)S) or Rb((2)P) and OH(-)((1)Σ(+)).

A theoretical rate constant for the associative detachment reaction Rb((2)S) + OH(-)((1)Σ(+)) → RbOH((1)Σ(+)) + e(-) of 4 × 10(-10) cm(3) s(-1) at 300 K has been calculated. This result agrees with the experimental rate constant of 2-1 (+2)×10(-10)cm(3)s(-1) obtained by Deiglmayr et al. [Phys. Rev. A 86, 043438 (2012)] for a temperature between 200 K and 600 K. A Langevin-based dynamics which depends on the crossing point between the anion (RbOH(-)) and neutral (RbOH) potential energy surfaces has been used. The calculations were performed using the ECP28MDF effective core potential to describe the rubidium atom at the CCSD(T) level of theory and extended basis sets. The effect of ECPs and basis set on the height of the crossing point, and hence the rate constant, has been investigated. The temperature dependence of the latter is also discussed. Preliminary work on the potential energy surface for the excited reaction channel Rb((2)P) + OH(-)((1)Σ(+)) calculated at the CASSCF-icMRCI level of theory is presented. We qualitatively discuss the charge transfer and associative detachment reactions arising from this excited entrance channel.

Photoelectron Spectroscopy of cis-Nitrous Acid Anion (cis-HONO(-)).

We report photoelectron spectra of cis-HONO(-) formed from an association reaction of OH(-) and NO in a pulsed, plasma-entrainment ion source. The experimental data are assigned to the cis-HONO(-) isomer, which is predicted to be the global minimum on the anion potential energy surface. We do not find evidence for a significant contribution from trans-HONO(-). Electron photodetachment of cis-HONO(-) with 1613, 1064, 532, 355, and 301 nm photons accesses the ground X̃ (1)A' (S0) and excited ã (3)A″ (T1) states of neutral HONO. The photoelectron spectrum resulting from detachment forming cis-HONO (S0) exhibits a long vibrational progression, dominated by overtones and combination bands involving the central O-N stretching and ONO bending vibrations. This indicates that there is a significant change in the central O-N bond length between cis-HONO(-) and cis-HONO (S0). The electron affinity (EA) of cis-HONO is determined to be 0.356(8) eV. We also report the dissociation energy (D0) of cis-HONO(-), forming OH(-) + NO, as 0.594(9) eV, which is a factor of 4 decrease in the central O-N bond strength compared to neutral cis-HONO. The T1 state of cis-HONO is shown to be ∼2.3 eV higher in energy than cis-HONO (S0). Electron photodetachment to form cis-HONO (T1) accesses a transition state along the HO-NO bond dissociation coordinate. The resulting photoelectron spectrum exhibits broad peaks spaced by the terminal N═O stretching frequency. Electronic structure calculations and photoelectron spectrum simulations reported here show very good agreement with the experimental data.

Energetics and transition-state dynamics of the F + HOCH3 → HF + OCH3 reaction.

The F + HOCH3 → HF + OCH3 reaction is a system with 15 internal degrees of freedom that can provide a benchmark for the development of theory for increasingly complex chemical reactions. The dynamics of this reaction were studied by photoelectron-photofragment coincidence (PPC) spectroscopy carried out on the F-(HOCH3) anion, aided by a computational study of both the anion and neutral potential energy surfaces, with energies extrapolated to the CCSDT(Q)/CBS level of theory. Photodetachment at 4.80 eV accesses both the reactant and product channels for this reaction. In the product channel (HF + OCH3 + e-) of the neutral potential energy surface, vibrationally excited HF products in addition to the stable product-channel hydrogen-bonded complex (FH-OCH3) are observed in the PPC and photoelectron spectra. In addition, experimental evidence is observed for the reactant-channel van der Waals complex (F-HOCH3), in good agreement with the theoretical predictions. The relative stability of these long-lived complexes was probed by reducing the ion beam energy, increasing the product time-of-flight, indicating lifetimes on the microsecond timescale for the reactant- and product-channel complexes as well as providing evidence for long-lived vibrational Feshbach resonances associated with the HF(v > 0) + OCH3 product states. This system will provide a model for extending full-dimensionality quantum dynamics to larger numbers of degrees of freedom.

Quantum chemical study of dissociative electron attachment to d-ribose and d-fructose

We performed quantum chemistry calculations using long-range corrected density-functional theory to understand the interaction of an excess electron with d-ribose/d-fructose. Previous experimental studies have shown that these sugar molecules undergo dissociative electron attachment to form various fragments upon capture of a nearly zero-energy electron. First, we studied doorway states, including dipole-bound and valence-bound anions in pyranose, furanose, and open-chain structures. Subsequently, we explored the reaction pathways on the anionic potential energy surfaces for selected structures to understand the detailed reaction mechanisms of dissociative electron attachment to these sugar molecules.

Importance of Time Scale and Local Environment in Electron-Driven Proton Transfer. The Anion of Acetoacetic Acid

Anion photoelectron spectroscopy (PES) and electron energy-loss spectroscopy (EELS) probe different regions of the anionic potential energy surface. These complementary techniques provided information about anionic states of acetoacetic acid (AA). Electronic structure calculations facilitated the identification of the most stable tautomers and conformers for both neutral and anionic AA and determined their relative stabilities and excess electron binding energies. The most stable conformers of the neutral keto and enol tautomers differ by less than 1 kcal/mol in terms of electronic energies corrected for zero-point vibrations. Thermal effects favor these conformers of the keto tautomer, which do not support an intramolecular hydrogen bond between the keto and the carboxylic groups. The valence anion displays a distinct minimum which results from proton transfer from the carboxylic to the keto group; thus, we name it an ol structure. The minimum is characterized by a short intramolecular hydrogen bond, a significant electron vertical detachment energy of 2.38 eV, but a modest adiabatic electron affinity of 0.33 eV. The valence anion was identified in the anion PES experiments, and the measured electron vertical detachment energy of 2.30 eV is in good agreement with our computational prediction. We conclude that binding an excess electron in a π* valence orbital changes the localization of a proton in the fully relaxed structure of the AA(-) anion. The results of EELS experiments do not provide evidence for an ultrarapid proton transfer in the lowest π* resonance of AA(-), which would be capable of competing with electron autodetachment. This observation is consistent with our computational results, indicating that major gas-phase conformers and tautomers of neutral AA do not support the intramolecular hydrogen bond that would facilitate ultrarapid proton transfer and formation of the ol valence anion. This is confirmed by our vibrational EELS spectrum. Anions formed by vertical electron attachment to dominant neutrals undergo electron autodetachment with or without vibrational excitations but are unable to relax to the ol structure on a time scale fast enough to compete with autodetachment.

The benzene radical anion: A computationally demanding prototype for aromatic anions

The benzene radical anion is studied with ab initio coupled-cluster theory in large basis sets. Unlike the usual assumption, we find that, at the level of theory investigated, the minimum energy geometry is non-planar with tetrahedral distortion at two opposite carbon atoms. The anion is well known for its instability to auto-ionization which poses computational challenges to determine its properties. Despite the importance of the benzene radical anion, the considerable attention it has received in the literature so far has failed to address the details of its structure and shape-resonance character at a high level of theory. Here, we examine the dynamic Jahn-Teller effect and its impact on the anion potential energy surface. We find that a minimum energy geometry of C2 symmetry is located below one D2h stationary point on a C2h pseudo-rotation surface. The applicability of standard wave function methods to an unbound anion is assessed with the stabilization method. The isotropic hyperfine splitting constants (Aiso) are computed and compared to data obtained from experimental electron spin resonance experiments. Satisfactory agreement with experiment is obtained with coupled-cluster theory and large basis sets such as cc-pCVQZ.

QUANTUM CHEMICAL ROVIBRATIONAL DATA FOR THE INTERSTELLAR DETECTION OFc-C3H–

The anion chemistry of the interstellar medium (ISM) has almost exclusively been limited to linear hydrocarbons and cyanocarbons. Of the hydrocarbons, only the even n C n H– chains have been detected in the ISM, and lines hypothesized to originate with b-C3H– have been conclusively linked to the corresponding cation, as originally claimed. However, no reason has yet been provided as to why other anions cannot form, and the cyclic form of C3H– is actually the lowest-energy isomer on the anion's potential energy surface. As such, this work provides the necessary rovibrational reference data for the potential detection of this anion in the ISM or related laboratory experiments. Improvements over previously calculated rovibrational spectroscopic constants are contained herein along with graphical depictions of the pure rotational spectra at 100 K, 40 K, 20 K, and 2.7 K.

Single-Electron Induces Double-Reaction by Charge Delocalization

Injecting an electron by scanning tunneling microscope into a molecule physisorbed at a surface can induce dissociative reaction of one adsorbate bond. Here we show experimentally that a single low-energy electron incident on ortho-diiodobenzene physisorbed on Cu(110) preferentially induces reaction of both of the C-I bonds in the adsorbate, with an order-of-magnitude greater efficiency than for comparable cases of single bond breaking. A two-electronic-state model was used to follow the dynamics, first on an anionic potential-energy surface (pes*) and subsequently on the ground state pes. The model led to the conclusion that the two-bond reaction was due to the delocalization of added charge between adjacent halogen-atoms of ortho-diiodobenzene through overlapping antibonding orbitals, in contrast to the cases of para-dihalobenzenes, studied earlier, for which electron-induced reaction severed exclusively a single carbon-halogen bond. The finding that charge delocalization within a single molecule can readily cause concerted two-bond breaking suggests the more general possibility of intra- and also intermolecular charge delocalization resulting in multisite reaction. Intermolecular charge delocalization has recently been proposed by this laboratory to account for reaction in physisorbed molecular chains (Ning, Z.; Polanyi, J. C. Angew. Chem., Int. Ed. 2013, 52, 320-324).

Characterization of the t-Butyl Radical and Its Elusive Anion.

The t-butyl radical and its anion are studied theoretically using state-of-the-art quantum mechanical methods including coupled cluster theory with full single, double, and triple excitations (CCSDT) and CCSDT with perturbative quadruple excitations [CCSDT(Q)], in concert with large correlation-consistent cc-pVXZ and aug-cc-pVXZ (X = D, T, Q, 5) basis sets. The relative energies are extrapolated to the complete basis set limit (CBS). The lowest energy structure of the t-butyl radical has a nonplanar carbon backbone with overall C3v symmetry. Low-lying C3h and Cs symmetry transition states, for pyramidal inversion and methyl group rotation, respectively, between equivalent C3v minima are investigated. The barriers for these interconversions are both less than 1 kcal mol(-1), but the corresponding barriers on the anion potential energy surface are more pronounced. Using the focal point analysis technique, we obtain a value of -0.48 kcal mol(-1) for the t-butyl radical adiabatic electron affinity at the CCSDT(Q)/CBS level of theory, where the negative sign indicates that the formation of the t-butyl anion is adiabatically unfavorable. We show that the electron affinity, whose sign has been the subject of debate, is very sensitive to both the basis set and the correlation treatment, and previous experimental and theoretical estimates of its value bracket the value computed herein. Our results indicate that the t-butyl anion is classically metastable with a vertical detachment energy of over 10 kcal mol(-1) to reach the neutral potential energy surface. However, the inclusion of the zero-point vibrational effects seems to favor its nonexistence.

On the Temperature Dependence of the Thermal Electron Attachment to SF 6 , SF 5 Cl, and POCl 3

Abstract A simplified model for electron attachment to neutral molecules, separating electron--phonon coupling (or intramolecular vibrational redistribution, IVR) factors into an electronic and a vibrational contribution, is tested by comparison with experimental results for the nondissociative electron attachments to SF6, SF5Cl, and POCl3. It is shown that the experimental results can approximately be reproduced by the fit of two parameters, one of them characterizing the dependence of the IVR factor on the electron energy and one accounting for an effective threshold energy for overlap of vibrational wave functions on anionic and neutral potential energy surfaces. After fitting these parameters, temperature dependences over wide ranges of conditions can be represented. Likewise, kinetic modeling of dissociative processes of the highly excited anions produced by attachment is facilitated.

Effects of two vibrational modes in the dissociative electron attachment toCF3Cl

We present a study of multimode effects in dissociative electron attachment to ${\text{CF}}_{3}\text{Cl}$ molecules using a time-independent version of the local complex potential theory. Symmetric stretch C-Cl vibrations ${\ensuremath{\nu}}_{3}$ and symmetric deformation (or so-called umbrella) vibrations ${\ensuremath{\nu}}_{2}$ are included. The neutral and anion potential energy surfaces are calculated using the second-order M\o{}ller-Plesset perturbation theory with an empirical adjustment of the vertical attachment energy. The final-state vibrational distribution in the ${\text{CF}}_{3}({\ensuremath{\nu}}_{2})$ fragment is dominated by the ${\ensuremath{\nu}}_{2}=2$ state. We also find an increase in the total cross section as compared with the one-dimensional calculations. This is explained by an increase in the anion survival probability.

Intermolecular Proton Transfer in Anionic Complexes of Uracil with Alcohols

A series of 18 alcohols (ROH) has been designed with an enthalpy of deprotonation in the gas phase (H(DP)) in the range 13.8-16.3 eV. The effects of excess electron attachment to the binary alcohol-uracil (ROH...U) complexes have been studied at the density functional level with a B3LYP exchange-correlation functional and at the second-order Møller-Plesset perturbation theory level. The photoelectron spectra of anionic complexes of uracil with 3 alcohols (ethanol, 2,2,3,3,3-pentafluoropropanol, and 1,1,1,3,3,3-hexafluoro-2-propanol) have been measured with 2.54 eV photons. For ROHs with deprotonation enthalpies larger than 14.8 eV, only the ROH...U- minimum exists on the potential energy surface of the anionic complex. For alcohols with deprotonation enthalpies in the range 14.3-14.8 eV, two minima might exist on the anionic potential energy surface, which correspond to the RO-...HU* and ROH...U- structures. For ROHs with deprotonation enthalpies smaller than 14.3 eV, the excess electron attachment to the ROH...U complex always induces a barrier-free proton transfer from the hydroxyl group of ROH to the O8 atom of U, with the product being RO-...HU*.

Femtosecond dynamics of Cu(H2O)2

The ultrafast relaxation dynamics of Cu(H(2)O)(2) is investigated using femtosecond photodetachment-photoionization spectroscopy. In addition, stationary points on the Cu(H(2)O)(2) anion, neutral, and cation potential energy surfaces are characterized by ab initio electronic structure calculations. Electron photodetachment from Cu(-)(H(2)O)(2) initiates the dynamics on the ground-state potential energy surface of neutral Cu(H(2)O)(2). The resulting Cu(H(2)O)(2) complexes experience large-amplitude H(2)O reorientation and dissociation. The time evolution of the Cu(H(2)O)(2) fragmentation products is monitored by time-resolved resonant multiphoton ionization. The parent ion, Cu(+)(H(2)O)(2), is not detected above background levels. The rise to a maximum of the Cu(+) signal from Cu(-)(H(2)O)(2), and the decay of the Cu(+)(H(2)O) signal from Cu(-)(H(2)O)(2) have similar tau approximately 10 ps time dependences to the corresponding signals from Cu(-)(H(2)O), but display clear differences at very short and long times. The experimental observations can be understood in terms of the following picture. Prompt dissociation of H(2)O from nascent Cu(H(2)O)(2) gives rise to a vibrationally excited Cu(H(2)O) complex, which dissociates to Cu+H(2)O due to coupling of H(2)O internal rotation to the dissociation coordinate. This prompt dissociation removes all intra-H(2)O vibrational excitation from the intermediate Cu(H(2)O) fragment, which quenches the long time vibrational predissociation to Cu+H(2)O previously observed in analogous experiments on Cu(-)(H(2)O).

DFT and MP2 investigation of the [formula omitted] anion potential energy surface

The B 2 H 3 - gas phase potential energy surface is studied at DFT(B3LYP/6-311+g(3df,2p)), MP2/6-311+g(3df,2p) and CCSD(T)//DFT and MP2 levels of theory. Many structures (isomers and transition states), which are not reported before, have been detected at these levels. Structural connectivities between these energetic extrema are brought to the fore and some H–H exchanges mechanisms are proposed. This leads to, on one hand H b–H t and on another hand B 1–B 2 possible spectral equivalences. Our results are briefly compared to Schaefer III’s ones on isoelectronic C 2H 3 radical and Si 2H 3.