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

Analysis of the Unusual Chemical Bonds and Dipole Moments of AeF- (Ae=Be-Ba): A Lesson in Covalent Bonding

Quantum chemical calculations of the anions AeF- (Ae=Be-Ba) have been carried out using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory employing BP86 with various basis sets. The detailed bonding analyses using different charge- and energy partitioning methods show that the molecules possess three distinctively different dative bonds in the lighter species with Ae=Be, Mg and four dative bonds when Ae=Ca, Sr, Ba. The occupied 2p atomic orbitals (AOs) and to a lesser degree the occupied 2s AO of F- donate electronic charge into the vacant spx(σ) and p(π) orbitals of Be and Mg which leads to a triple bond Ae F-. The heavier Ae atoms Ca, Sr, Ba use their vacant (n-1)d AOs as acceptor orbitals which enables them to form a second σ donor bond with F- that leads to quadruply bonded Ae F- (Ae=Ca-Ba). The presentation of molecular orbitals or charge distribution using only one isodensity value may give misleading information about the overall nature of the orbital or charge distribution. Better insights are given by contour line diagrams. The ELF calculations provide monosynaptic and disynaptic basins of AeF- which nicely agree with the analysis of the occupied molecular orbitals and with the charge density difference maps. A particular feature of the covalent bonds in AeF- concerns the inductive interaction of F- with the soft valence electrons in the (n)s valence orbitals of Ae. The polarization of the (n)s2 electrons induces a (n)spx hybridized lone-pair orbital at atom Ae, which yields a large dipole moment with the negative end at Ae. The concomitant formation of a vacant (n)spx AO of atom Ae, which overlaps with the occupied 2p(σ) AO of F-, leads to a strong covalent σ bond.

Laser Cooling and Electronic Structure of Be Halides Anions BeX- (X=Cl, Br, F, and I)

The adiabatic potential energy curves of the low lying electronic states of the Be halides anions BeX- (Cl, Br, F, and I) have been investigated in the representation 2s+1Ʌ(+/−) by using the complete active space self-consistent field (CASSCF) with multireference configuration interaction (MRCI+Q) method. The spectroscopic parameters Te,, Re, ωe, and Be, the static and transition dipole moment μe, and a rovibrational study of the investigated electronic states have been performed. New electronic states were investigated here for the first time. The highly diagonal FCF and the short radiative lifetime for the molecular anion BeF- prove its candidacy for direct Doppler laser cooling. The experimental proof of the stability and the calculated experimental parameters such as the vibrational branching ratio, the slowing distance, the recoil and Doppler temperatures with the experimental conditions of the buffer gas cell of this anion open the route for experimental work on the BeF- molecular ion.

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.

Superatomic nature of alkaline earth metal–water complexes: the cases of Be(H2O)0,+4 and Mg(H2O)0,+6

Beryllium– and magnesium–water complexes are shown to accommodate peripheral electrons around their Be²⁺(H₂O)₄ and Mg²⁺(H₂O)₆ cores in hydrogenic type orbitals.

MELEXIR: maximum entropy Legendre expanded image reconstruction. A fast and efficient method for the analysis of velocity map imaging or photoelectron imaging data

The MELEXIR program obtains a Legendre expansion of the 3D velocity distribution from 2D images of ions or photoelectrons. The maximum entropy algorithm avoids inverse Abel transforms, is fast and applicable to low-intensity images.

The Nature of the Chemical Bond in BeF- and Related Species

The beryllium atom represents a curiosity. Its ground state valency is zero, so it should not form chemical bonds. Nevertheless Be is a metal and can form stable chemical species largely due to the participation of its excited ³P (2s¹2p¹) state as shown in the cases of Be₂ and Be₃. BeF^- is a stable ( D₀ ≥ 81.4 kcal/mol) closed shell molecule that has been studied experimentally quite recently. Although it dissociates adiabatically to two closed shell atoms, Be (¹S) and F^-(¹S), the bonding is due to the excited ³P (2s¹2p¹) and ¹D (2p²) Be states.