Photoelectron imaging of CH−

On the Formation of Interstellar CH- Anions: Exploring Mechanism and Rates for CH2 Reacting with H. J Phys Chem A 2020;124:5098-5108. [PMID: 32463233 PMCID: PMC7322726 DOI: 10.1021/acs.jpca.0c02412]

We present accurate ab initio calculations on the structural properties of a gas-phase reaction of possible interest for Saturn’s outer atmosphere chemistry, in which the CH₂ molecule has been detected. In the present study, that molecule is made to react with the H^– anion to form the CH^– species, one considered as a possible intermediate in ionic processes networks. The results indicate that this reaction is markedly exothermic and proceeds with the formation of an intermediate, which occurs via only a shallow barrier below the reagents and progresses directly to the product region. The corresponding rate coefficients of reactions are also computed by making use of the variational transition state theory modeling and found to efficiently lead to the formation of the final anion even at the lower temperatures of interstellar medium conditions.

The Electron Affinity as the Highest Occupied Anion Orbital Energy with a Sufficiently Accurate Approximation of the Exact Kohn-Sham Potential

Negative ions are not accurately represented in density functional approximations (DFAs) such as (semi)local density functionals (LDA or GGA or meta-GGA). This is caused by the much too high orbital energies (not negative enough) with these DFAs compared to the exact Kohn–Sham values. Negative ions very often have positive DFA HOMO energies, hence they are unstable. These problems do not occur with the exact Kohn–Sham potential, the anion HOMO energy then being equal to minus the electron affinity. It is therefore desirable to develop sufficiently accurate approximations to the exact Kohn–Sham potential. There are further beneficial effects on the orbital shapes and the density of using a good approximation to the exact KS potential. Notably the unoccupied orbitals are not unduly diffuse, as they are in the Hartree–Fock model, with hybrid functionals, and even with (semi)local density functional approximations (LDFAs). We show that the recently developed B-GLLB-VWN approximation [Gritsenko et al. J. Chem. Phys.2016, 144, 204114] to the exact KS potential affords stable negative ions with HOMO orbital energy close to minus the electron affinity.

Boron carbonyl complexes analogous to hydrocarbons

Recent studies on boron carbonyl complexes show their intriguing structural and bonding properties, enriching our knowledge on main group coordination chemistry. The isolobal relationships between BCO and CH and the more generally applicable CO/H- and B-/C analogies are employed to understand the structure and bonding of boron carbonyl complexes, bridging the boron carbonyl chemistry to the well-known hydrocarbon analogues.

Hardness maximization or equalization? New insights and quantitative relations between hardness increase and bond dissociation energy

It has been overlooked that the change of hardness, η, upon bonding is intimately connected to thermochemical cycles, which determine whether hardness is increased according to Pearson's "maximum hardness principle" (MHP) or equalized, as expected by Datta's "hardness equalization principle" (HEP). So far the performances of these likely incompatible "structural principles" have not been compared. Computational validations have been inconclusive because the hardness values and even their qualitative trends change drastically and unsystematically at different levels of theory. Here I elucidate the physical basis of both rules, and shed new light on them from an elementary experimental source. The difference, Δη = η _mol - <η _at>, of the molecular hardness, η _mol, and the averaged atomic hardness, <η _at>, is determined by thermochemical cycles involving the bond dissociation energies D of the molecule, D ⁺ of its cation, and D ^- of its anion. Whether the hardness is increased, equalized or even reduced is strongly influenced by ΔD = 2D - D ⁺  - D ^-. Quantitative expressions for Δη are obtained, and the principles are tested on 90 molecules and the association reactions forming them. The Wigner-Witmer symmetry constraints on bonding require the valence state (VS) hardness, η _VS, instead of the conventional ground state (GS) hardness, η _GS. Many intriguingly "unpredictable" failures and systematic shortcomings of said "principles" are understood and overcome for the first time, including failures involving exotic and/or challenging molecules, such as Be_2, B₂, O₃, and transition metal compounds. New linear relationships are discovered between the MHP hardness increase Δη _VS and the intrinsic bond dissociation energy D _i . For bond formations, MHP and HEP are not compatible, and HEP does not qualify as an ordering rule.

Photoelectron angular distributions for states of any mixed character: an experiment-friendly model for atomic, molecular, and cluster anions

We present a model for laboratory-frame photoelectron angular distributions in direct photodetachment from (in principle) any molecular orbital using linearly polarized light. A transparent mathematical approach is used to generalize the Cooper-Zare central-potential model to anionic states of any mixed character. In the limit of atomic-anion photodetachment, the model reproduces the Cooper-Zare formula. In the case of an initial orbital described as a superposition of s and p-type functions, the model yields the previously obtained s-p mixing formula. The formalism is further advanced using the Hanstorp approximation, whereas the relative scaling of the partial-wave cross-sections is assumed to follow the Wigner threshold law. The resulting model describes the energy dependence of photoelectron anisotropy for any atomic, molecular, or cluster anions, usually without requiring a direct calculation of the transition dipole matrix elements. As a benchmark case, we apply the p-d variant of the model to the experimental results for NO(-) photodetachment and show that the observed anisotropy trend is described well using physically meaningful values of the model parameters. Overall, the presented formalism delivers insight into the photodetachment process and affords a new quantitative strategy for analyzing the photoelectron angular distributions and characterizing mixed-character molecular orbitals using photoelectron imaging spectroscopy of negative ions.