MX₃^- Superhalogens (M = Be, Mg, Ca; X = Cl, Br):  A Photoelectron Spectroscopic and ab Initio Theoretical Study

Abstract

Gas-phase alkaline earth halide anions, MgX3(-) and CaX3(-) (X = Cl, Br), were produced using electrospray and investigated using photoelectron spectroscopy at 157 nm. Extremely high electron binding energies were observed for all species and their first vertical detachment energies were measured as 6.60 +/- 0.04 eV for MgCl3(-), 6.00 +/- 0.04 eV for MgBr3(-), 6.62 +/- 0.04 eV for CaCl3(-), and 6.10 +/- 0.04 eV for CaBr3(-). The high electron binding energies indicate these are very stable anions and they belong to a class of anions, called superhalogens. Theoretical calculations at several levels of theory were carried out on these species, as well as the analogous BeX3(-). Vertical detachment energy spectra were predicted to compare with the experimental observations, and good agreement was obtained for all species. The first adiabatic detachment energies were found to be substantially lower (by about 1 eV) than the corresponding vertical detachment energies for all the MX3(-) species, indicating extremely large geometry changes between MX3(-) and MX3. We found that all the MX3(-) anions possess D3h ((1)A1') structures and are extremely stable against dissociation into MX2 and X-. The corresponding neutral species MX3, however, were found to be only weakly bound with respect to dissociation toward MX2 + X. The global minimum structures of all the MX3 neutrals were found to be C2v ((2)B2), which can be described as (X2(-))(MX+) charge-transfer complexes, whereas the MX2...X (C2v, (2)B1) van der Waals complexes were shown to be low-lying isomers.

Are superhalogens without halogen ligand capable of transcending traditional halogen-based superhalogens? Ab initio case study of binuclear anions based on pseudohalogen ligand

“…6 Owing to this character, superhalogens have wide application potentials in the fields of synthesis, 3,5,7,8 new dopants to improve the electrical conductivity of polymers 1,3 and production of new materials. [2][3][4][5]7,9 Gutsev and Boldyrev, who first introduced the term "superhalogen" in early 1980s, 1 have proposed a simple formula MX k+1 for traditional superhalogens, 1,2,10 where M is the central atom, k is the maximal formal valence of atom M, and X is halogen atom as ligand.…”

“…5,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Along with the deepening of the related researches, it has been a Author to whom correspondence should be addressed. Electronic mail: rayinyin@nwu.edu.…”

“…indicated that the VDEs of superhalogens could be improved both by strengthening the electronegativity of the ligand 12,16,20,23,[27][28][29][30][31] and by increasing the number of ligands. 10,11,29,30,[32][33][34] 8,42,43 have been utilized to design new superhalogens recently. However, most of the reported structures based on these new ligands belong to mononuclear superhalogens and the number of reported polynuclear superhalogens without halogen ligand is relatively infrequent.…”

Polynuclear Li12F13 − anion as a steric shielding agent with respect to selected metal ions

“…The very first experimental evidence of the superhalogen existence was provided in 1999 by the Wang's group who reported the photoelectron spectra of the selected triatomic MX 2 − superhalogen anions (M = Li, Na; X = Cl, Br, I) [8]. Since then, many other superhalogen anions [e.g., Na k Cl − k+1 (k = 1-4) and MX 3 − (where M = Be, Mg, Ca; X = Cl, Br)] have also been identified experimentally [9,10]. As indicated by both theoretical predictions and experimental measurements, the original superhalogen formula (MX k+1 ) can be extended to include the polynuclear M n X nk+1 neutral superhalogen compounds (containing n central atoms) whose corresponding polynuclear M n X − nk+1 anions exhibit even larger electron binding energies than their mononuclear counterparts [11][12][13][14][15][16][17].…”

Superhalogens beget superhalogens: a case study of (BO2)n oligomers

“…Subsequently, several experimental studies have confirmed the existence of superhalogens consisting of simple metal atoms, such as alkalis, Mg, and Al at the core and halogen atoms on the periphery. [5][6][7] Considerable research conducted in recent years has shown that superhalogens not only mimic the chemistry of halogens but also can be used to promote unusual reactions, 8 as a building block of energetic materials, 9 as dopants to increase the electrical conductivity of polymers, 10 for accessing high oxidation states of metal atoms, 11 as electrolytes in rechargeable batteries, 12 and in the production of organic superconductors and organic metals. 13 What has not been addressed in the literature is the electron affinity of species created by combing two or more superhalogens.…”