Exciplexes with Ionic Dopants: Stability, Structure, and Experimental Relevance of M⁺(²P)⁴He_<i>n</i> (M = Sr, Ba)

M⁺(²P)⁴He_<i>n</i> species, possibly involved in the post ²P ← ²S excitation dynamics of Sr⁺ and Ba⁺ in cold ⁴He gas or droplets, are studied employing both high level <i>ab initio</i> calculations to determine the potential energy curves (PEC) and diffusion Monte Carlo (DMC) to obtain information on their ground state structure and energetics. PEC for the excited M⁺(²P)He dimers were obtained using MRCI calculations with extended basis sets. Potential energy surfaces (PES) for M⁺(²P)­He_<i>n</i> were built with the DIM model including spin–orbit coupling via a perturbative procedure. DMC simulations indicated several exciplexes (<i>n</i> > 2) to be stable against He dissociation whatever the ion state, a finding that is at variance with what was previously suggested for Ba⁺(²P_1/2) due to the repulsive nature of the interaction potential obtained in [Phys. Rev. A 2004, 69, 042505]. Our results, instead, support the suggestion made in [J. Chem. Phys. 2012, 137, 051102] for the existence of Ba⁺(²P_1/2)­He_<i>n</i> exciplexes emitted following the excitation of the barium cation solvated into He droplets. In the ²P_1/2 state, the Ba ion also shows a peculiar behavior as a function of <i>n</i> with respect to the location and binding strength of the attached He atoms compared to Sr⁺. Although the latter forms the usual equatorial He ring, Ba⁺ deviates from this geometry for 1 ≤ <i>n</i> ≤ 4, with the radial distribution functions strongly depending on the number of solvent atoms. Finally, a putative species is proposed to explain the emission band at 523 nm that follows D1 or D2 excitation of Ba⁺ in superfluid bulk helium

Understanding the Reorientational Dynamics of Solid-State MBH₄ (M = Li–Cs)

The reorientational dynamics of crystalline MBH₄ (M = Li–Cs) have been characterized with the interacting quantum atom theory. This interpretive approach enables an atomistic deciphering of the energetic features involved in BH₄^– reorientation using easily graspable chemical terms. It reveals a complex construction of the activation energy that extends beyond interatomic distances and chemical interactions. BH₄^– reorientations are in LiBH₄ and NaBH₄ regulated by their interaction with the nearest metal cation; however, higher metal electronic polarizability and more covalent M···H interactions shift the source of destabilization to internal deformations in the heavier systems. Underlying electrostatic contributions cease abruptly at CsBH₄, triggering a departure in the otherwise monotonically increasing activation energy. Such knowledge concurs to the fundamental understanding and advancement of energy solutions in the field of hydrogen storage and solid-state batteries

A Theoretical Study on the Rotational Motion and Interactions in the Disordered Phase of MBH₄ (M = Li, Na, K, Rb, Cs)

The rotational motion in the high-temperature disordered phase of MBH₄ (M = Li, Na, K, Rb, Cs) is investigated utilizing two complementary theoretical approaches. The first one consists of high-level periodic DFT calculations which systematically consider several instantaneous representations of the structural disorder. The second approach is based on a series of in vacuo calculations on molecular complexes suitably extracted from the crystal and chosen as to possibly disentangle the energetic factors leading to the observed rotational barriers. The results of the first part demonstrate that the motion of the BH₄^– anion is dominated by 90° reorientations around the 4-fold symmetry axes of the cubic crystal, and depending on the instantaneous structural disorder activation energies are found to be between 0.00 and 0.31 eV for LiBH₄, 0.05 and 0.26 eV for NaBH₄, 0.16 and 0.27 eV for KBH₄, 0.22 and 0.31 eV for RbBH₄, and 0.21 and 0.32 eV for CsBH₄. The increasing rotational barriers as well as the movement of the transition state from 7° to 44° observed along the series of alkaline metals, M = Li–Rb, appear to be simply accounted for by an analysis of the energy profiles for the <i>C</i>₂ rotation of a BH₄^– group in M⁺–BH₄^– and BH₄^––BH₄^– in vacuo complexes. The energy gained from the introduction of disorder shows a trend opposite to that of the rotational barriers as it decreases along the Li–Rb series. Similar considerations apply to the <i>C</i>₃ rotational motion of the BH₄^– anion, which likewise has been studied in the crystal and in the in vacuo molecular complexes. CsBH₄ deviates from the systematic trends observed for LiBH₄–RbBH₄. Depending on the structural starting point of the rotation, its <i>C</i>₂ rotational barriers are found to be slightly higher or slightly lower than for RbBH₄, whereas its energy gain due to the introduction of disorder is found to be positioned between that of KBH₄ and RbBH₄. The <i>C</i>₃ rotational barriers of CsBH₄ are instead significantly smaller compared to those of RbBH₄ and even marginally below those of KBH₄