Hexachlorobenzene-negative ion formation in electron attachment experiments

Low-lying Negative Ion States Probed in Potassium - Ethanol Collisions

Dissociative electron transfer in collisions between neutral potassium atoms and neutral ethanol molecules yields mainly OH-, followed by C2H5O-, O-, CH3 - and CH2 -. The dynamics of negative ions have been investigated by recording time-of-flight mass spectra in a wide range of collision energies from 17.5 to 350 eV in the lab frame, where the branching ratios show a relevant energy dependence for low/intermediate collision energies. The dominant fragmentation channel in the whole energy range investigated has been assigned to the hydroxyl anion in contrast to oxygen anion from dissociative electron attachment (DEA) experiments. This result shows the relevant role of the electron donor in the vicinity of the temporary negative ion formed allowing access to reactions which are not thermodynamically attained in DEA experiments. The electronic state spectroscopy of such negative ions, was obtained from potassium cation energy loss spectra in the forward scattering direction at 205 eV impact energy, showing a prevalent Feshbach resonance at 9.36±0.10 eV withσ O H * / σ C H * ${{\sigma }_{OH}^{^{\ast}}/{\sigma }_{CH}^{^{\ast}}}$ character, while a less pronouncedσ O H * ${{\sigma }_{OH}^{^{\ast}}}$ contribution assigned to a shape resonance has been obtained at 3.16±0.10 eV. Quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom have been performed to support the experimental findings.

Analytic Evaluation of Nonadiabatic Couplings within the Complex Absorbing Potential Equation-of-Motion Coupled-Cluster Method

We present the theory for the evaluation of nonadiabatic couplings (NACs) involving resonance states within the complex absorbing potential equation-of-motion coupled-cluster (CAP-EOM-CC) framework implemented within the singles and doubles approximation. Resonance states are embedded in the continuum and undergo rapid decay through autodetachment. In addition, nuclear motion can facilitate transitions between different resonances and between resonances and bound states. These nonadiabatic transitions affect the chemical fate of resonances and have distinct spectroscopic signatures. The NAC vector is a central quantity needed to model such effects. In the CAP-EOM-CC framework, resonance states are treated on the same footing as bound states. Using the example of fumaronitrile, which supports a bound radical anion and several anionic resonances, we analyze the NAC between bound states and pseudocontinuum states, between bound states and resonances, and between two resonances. We find that the NAC between a bound state and a resonance is nearly independent of the CAP strength and thus straightforward to evaluate, whereas the NAC between two resonance states or between a bound state and a pseudocontinuum state is more difficult to evaluate.

Sensing the ortho Positions in C6Cl6 and C6H4Cl2 from Cl2− Formation upon Molecular Reduction

The geometrical effect of chlorine atom positions in polyatomic molecules after capturing a low-energy electron is shown to be a prevalent mechanism yielding Cl₂⁻. In this work, we investigated hexachlorobenzene reduction in electron transfer experiments to determine the role of chlorine atom positions around the aromatic ring, and compared our results with those using ortho-, meta- and para-dichlorobenzene molecules. This was achieved by combining gas-phase experiments to determine the reaction threshold by means of mass spectrometry together with quantum chemical calculations. We also observed that Cl₂⁻ formation can only occur in 1,2-C₆H₄Cl₂, where the two closest C–Cl bonds are cleaved while the chlorine atoms are brought together within the ring framework due to excess energy dissipation. These results show that a strong coupling between electronic and C–Cl bending motion is responsible for a positional isomeric effect, where molecular recognition is a determining factor in chlorine anion formation.