Photodetachment spectroscopy of the beryllium oxide anion, BeO. J Chem Phys 2017;146:054301. [PMID: 28178838 DOI: 10.1063/1.4974843]

Noble gas hydrides: theoretical prediction of the first group of anionic species

The first group of anionic noble-gas hydrides with the general formula HNgBeO- (Ng = Ar, Kr, Xe, Rn) is predicted through MP2, Coupled-Cluster, and Density Functional Theory computations employing correlation-consistent atomic basis sets. We derive that these species are stable with respect to the loss of H, H-, BeO, and BeO-, but unstable with respect to Ng + HBeO-. The energy barriers of the latter process are, however, high enough to suggest the conceivable existence of the heaviest HNgBeO- species as metastable in nature. Their stability arises from the interaction of the H- moiety with the positively-charged Ng atoms, particularly with the σ-hole ensuing from their ligation to BeO. This actually promotes relatively tight Ng-H bonds featuring a partially-covalent character, whose degree progressively increases when going from HArBeO- to HRnBeO-. The HNgBeO- compounds are also briefly compared with other noble-gas anions observed in the gas phase or isolated in crystal lattices.

Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Trimer and Tetramer

Computational studies of small beryllium clusters (BeN) predict dramatic, nonmonotonic changes in the bonding mechanisms and per-atom cohesion energies with increasing N. To date, experimental tests of these quantum chemistry models are lacking for all but the Be2 molecule. In the present study, we report spectroscopic data for Be3 and Be4 obtained via anion photodetachment spectroscopy. The trimer is predicted to have D3h symmetric equilibrium structures for both the neutral molecule and the anion. Photodetachment spectra reveal transitions that originate from the X2A2″ ground state and the 12A1' electronically excited state. The state symmetries were assigned on the basis of anisotropic photoelectron angular distributions. The neutral and anionic forms of Be4 are predicted to be tetrahedral. Franck-Condon diagonal photodetachment was observed with a photoelectron angular distribution consistent with the expected Be4-X2A1 → Be4X1A1 transition. The electron affinities of Be3 and Be4 were determined to be 11363 ± 60 and 13052 ± 50 cm-1, respectively.

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) is a high-resolution variant of anion photoelectron spectroscopy that has been applied with considerable success over the years to the study of radicals, size-selected clusters, and transition states for unimolecular and bimolecular reactions. Cryo-SEVI retains the versatility of conventional anion photoelectron spectroscopy while offering sub-meV resolution, thereby enabling the resolution of vibrational structure in the photoelectron spectra of complex anions. This Feature Article describes recent experiments in our laboratory using cryo-SEVI, including a new research direction in which anions are vibrationally pre-excited with an infrared laser pulse prior to photodetachment.

Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State

We report the observation of a symmetry-forbidden excited quadrupole-bound state (QBS) in the tetracyanobenzene anion (TCNB^-) using both photoelectron and photodetachment spectroscopies of cryogenically-cooled anions. The electron affinity of TCNB is accurately measured as 2.4695 eV. Photodetachment spectroscopy of TCNB^- reveals selected symmetry-allowed vibronic transitions to the QBS, but the ground vibrational state was not observed because the transition from the ground state of TCNB^- (A_u symmetry) to the QBS (A_g symmetry) is triply forbidden by the electric and magnetic dipoles and the electric quadrupole. The binding energy of the QBS is found to be 0.2206 eV, which is unusually large due to strong correlation and polarization effects. A centrifugal barrier is observed for near-threshold autodetachment, as well as relaxations from the QBS vibronic levels to the ground and a valence excited state of TCNB^-. The current study shows a rare example where symmetry selection rules, rather than the Franck-Condon principle, govern vibronic transitions to a nonvalence state in an anion.

Probing the Hydrogen Bonding in Microsolvated Clusters of Au1,2-(Solv)n (Solv = C2H5OH, n-C3H7OH; n = 1-3 for Au-; n =1 for Au2-)

The microsolvation of gold anions in different alcohol solvents is demonstrated by the combination of anion photoelectron spectroscopy and quantum chemical calculations on the Au_1,2^-(Solv)_n (Solv = C₂H₅OH, n-C₃H₇OH; n = 1-3 for Au^-; n = 1 for Au₂^-). The microsolvation structures of these clusters and their corresponding neutrals are assigned by comparing calculations with experiments. In terms of overall regularity, the increasing solvation number (n) and carbon chain extension both can increase the stability of the anion. When n ≥ 2, these clusters have low-energy isomers, where conventional hydrogen bonds (HBs) compete with nonconventional HBs (NHBs). NHBs are dominant when n ≤ 2 and when n is increased, vice versa. Interestingly, a variety of theoretical calculations show that after the hydroxy H atom of the ethanol molecule forms a weak ionic HB with Au^-, there are two lowest conformations of ethanol, trans and gauche, which could be coexisting in the molecular beams. Some theoretical methods also suggest that the gauche isomer is more stable than the trans one, which indicates that Au^- may exist as a gold gauche effect similar to fluorine.

Characterization of the Ground States of BeC2 and BeC2- via Photoelectron Velocity Map Imaging Spectroscopy

Due to their potentially unique properties, beryllium carbide materials have been the subject of many theoretical studies. However, experimental validation has been lacking due to the difficulties of working with Be. Neutral beryllium dicarbide has been predicted to have a T-shaped equilibrium structure (C_2v), while previous quantum chemistry calculations for the structure of the anion had not yielded consistent results. In this study, we report photoelectron velocity map imaging spectra for the BeC₂^- X ²A₁ → BeC₂ X ¹A₁ transition. These data provide vibrational frequencies and the electron affinity of BeC₂. Ab initio electronic structure calculations, validated against the experimental data, show that both the anion and the neutral form have C_2v equilibrium geometries with polar covalent bonding between Be and the C₂ subunit. Computed vibrational frequencies and the electron affinity, obtained at the CCSD(T) level of theory, were found to be in good agreement with the measurements.

Dative Bonding between Closed-Shell Atoms: The BeF- Anion

Beryllium can exhibit unusually strong attractive interactions under conditions where it is nominally a closed-shell atom. Two prominent examples are the Be₂ dimer and the He-BeO complex. In the present study, we examine the bonding of the closed-shell Be-F^- anion. This molecule preserves the closed-shell character of the individual atoms as the electron affinity of F is high (328.16 kJ mol^-1) while that of Be is negative. Photodetachment spectroscopy was used to determine the vibrational frequency for BeF^- and the electron affinity of BeF (104.2 kJ mol^-1). The latter has been used to determine a lower bound of 343 kJ mol^-1 for the bond energy of BeF^-. Electronic structure calculations yielded predictions that were in good agreement with the observed data. A natural bond orbital analysis shows that BeF^- is primarily bound by a dative interaction.

Slow Photoelectron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled anions (cryo-SEVI) is a powerful technique for elucidating the vibrational and electronic structure of neutral radicals, clusters, and reaction transition states. SEVI is a high-resolution variant of anion photoelectron spectroscopy based on photoelectron imaging that yields spectra with energy resolution as high as 1-2 cm^-1. The preparation of cryogenically cold anions largely eliminates hot bands and dramatically narrows the rotational envelopes of spectral features, enabling the acquisition of well-resolved photoelectron spectra for complex and spectroscopically challenging species. We review the basis and history of the SEVI method, including recent experimental developments that have improved its resolution and versatility. We then survey recent SEVI studies to demonstrate the utility of this technique in the spectroscopy of aromatic radicals, metal and metal oxide clusters, nonadiabatic interactions between excited states of small molecules, and transition states of benchmark bimolecular reactions.

Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Sulfide Anion, BeS. J Phys Chem A 2017;121:5645-5650. [PMID: 28691819 DOI: 10.1021/acs.jpca.7b04894]

Slow electron velocity map imaging (SEVI) spectroscopy was used to examine the BeS^- anion to neutral ground-state transition, X ²Σ⁺ → X ¹Σ⁺. Rotational constants, vibrational intervals, and the electron binding energy of BeS^- were determined. Partially resolved rotational contours were seen due to the relatively small moment of inertia of beryllium sulfide. Upon analysis of the rotational contours, it was found that changes in the molecular rotational angular momentum, ΔN = -1, -2, -3, and -4, facilitated photodetachment at near-threshold photon energies. The electron affinity of BeS was found to be 2.3346(2) eV. SEVI spectra recorded using photon energies near the threshold for Δv = -1 processes exhibited features that were associated with a dipole-bound state (DBS) of BeS^-. Autodetachment spectroscopy was used to probe this state, and rotationally resolved data were obtained for the DBS ²Σ⁺, v' = 0 - X ²Σ⁺, v″ = 0 transition. Analysis of this structure provided the rotational constants for BeS^- X, v″ = 0, and the electron binding energy of the DBS. Electronic structure calculations, performed at the RCCSD(T) and MRCI levels of theory, gave predictions that were in good agreement with the experimental observations.

The nature of the chemical bond in BeO0,-, BeOBe+,0,-, and in their hydrogenated products HBeO0,-, BeOH, HBeOH, BeOBeH+,0,-, and HBeOBeH

The nature of the chemical bond in BeO^0,-, BeOBe^+,0,-, and in their hydrogenated products HBeO^0,-, BeOH, HBeOH, BeOBeH^+,0,-, and HBeOBeH has been studied through single and multi reference correlation methods. In all these species, excited and ionized atomic states participate in a resonant way making chemically possible molecules that have been termed hypervalent and explain also the "incompatible" geometrical structure of some species.