Constructing organic superacids from superhalogens is a rational route as verified by DFT calculations.

The construction route of organic superacids from the combination of organic superhalogens and protons is verified to be a rational one based on a systematic theoretical study covering different planar conjugated backbones, e.g., [C5H5]- and [BC5H6]-, and electron-withdrawing substituents, e.g., -F, -CN and -NO2. In both the gas phase and the solution phase, the acidities of the composites here have a consistent strengthening with the increase of the vertical electron detachment energy of the superhalogen part. Decomposition of the acidity into different contributions further verifies the dominant role of the superhalogen part in the variation of the acidity. Thus, tuning of the acidity of systems of this type could be achieved via rational design of the constituent part of the superhalogen. That is to say, the design of a novel organic superacid with enhanced properties could be guided by the search for a new strong superhalogen of organic nature eventually. Having provided important contributions to the topic of superhalogens, theoretical calculation should be trusted to provide useful guidance for the research of organic superacids and could be expected to promote related experimental studies in the near future.

Why do higher VDEs of superhalogen not ensure improved stabilities of the noble gas hydrides promoted by them? A high-level ab initio case study.

A series of 20 composite structures, consisting of superhalogen and noble gas (Ng) hydrides, was explored via high-level coupled-cluster single, double and perturbative triple excitations calculations in this work. The existence of these composites, as local minima on the potential energy surface, arises from the charge transfer from the Ng hydride part to the superhalogen moiety. Clearly, this transfer could lead to stabilizing the interaction of the ionic type between the two components. The driving force of the charge transfer should be the high vertical electron detachment energy (VDE) of the superhalogen part leading to its enough capability of extracting the electron from the Ng hydride moiety. However, except triggering the ionic attractive interaction, there is nomonotonic correlation between the VDE value and the thermodynamic stability of the whole composite. This counter-intuitive result actually originates from the fact that, irrespective of various superhalogens, only two of their F ligands interact with the Ng atoms directly. Thus, although leading to higher VDE values, the increase in the number of electronegative ligands of the superhalogen moiety does not affect the stabilizing interaction of the composites here directly. In other words, with the necessary charge transfer generated, further increase of the VDE does not ensure the improvement of the thermodynamic stabilities of the whole composite. Moreover, in the transition state of the exothermic dissociation channel, more F atoms will give rise to higher probability of additional attractions between the F and H atoms which should lower the energy barrier. That is to say, increasing VDE, i.e., having more F atoms in many cases, will probably reduce the kinetic stability. Knowing the inevitable existence of the exothermic channel, kinetic stability is crucial to the ultimate goal of experimental observation of these Ng hydrides. Thus, in some cases, only the superhalogen itself may not provide enough information for the correct prediction on the properties of the whole composites. The understanding of the superhalogen-based composites will provide valuable information on the functional properties as well as the application potential of superhalogen clusters. Thus, the corresponding researches should focus on not only the superhalogen itself but also other related aspects, especially the details of the interaction between different parts.

The Combination of Superhalogens and Brønsted Acids HX (X = F, Cl, Br): An Effective Strategy for Designing Strong Superacids.

A series of 27 composite structures, consisting of superhalogen and Brønsted acid, is designed and systematically studied based on combined ab initio and DFT calculations focusing on their potentials as novel superacids. As indicated by high-level CCSD(T) results, all the composites here fulfill the theoretical criterion for superacid and the acidities of two of them are close to the strongest superacid ever reported. The influences of various factors on the superacid properties of these composites were analyzed in detail. Our results demonstrate that the acidity of these superacids is mainly determined by the superhalogen components while the effect of Brønsted acids, irrespective of their number or type, is relatively mild. Therefore, it is probable to design novel composite superacid with enhanced property through the regulation of the superhalogen component. It is encouraging that MP2 and DFT could also provide reliable results when compared with the high-level CCSD(T) method. The reliability of these low-cost methods implies the capability of theoretical calculations for future composite superacid of enlarged size, and thus it is highly probable that an effective guide to the related experimental research could be provided by the theory.

Could the increased structural versatility imposed by non-halogen ligands bring something new for polynuclear superhalogens? A case study on binuclear [Mg2L5]- (L = -OH, -OOH and -OF) anions.

A combined ab initio and DFT study is performed in this work to explore the superhalogen properties of polynuclear structures based on the ligands of -OH, -OOH and -OF. According to high-level CCSD(T) results, all the structures here are superhalogens whose properties are superior to the corresponding mononuclear ones. Although inferior to similar structures based on F ligands, some of the superhalogens here are capable of transcending the traditional ones based on Cl atoms. Therefore the superhalogen properties of the anions here are still promising and they have an important advantage of high safety, which is crucial for practical applications. An increased degree of structural versatility is imposed by these non-halogen ligands because of the various ways in which they connect the central atoms and their multiple orientations. It is important that this increased versatility will bring new factors, e.g., the larger spatial extent of the whole cluster and the existence of intra-molecular hydrogen bonds, which should favour high VDE values. These factors are not available in traditional halogen-based systems and they may play an important role in the future search for novel superhalogens. (HF + MP2)/2, ωB97XD as well as M06-2X are capable of providing accurate VDE values, close to the CCSD(T) results, and their absolute errors are even lower than that of the OVGF. Due to the good balance between the accuracy and efficiency, these methods could provide reliable predictions on large systems which cannot be treated with CCSD(T) or even with the OVGF. Balanced distribution of the extra electron, between the terminal and bridging ligands, is also shown to be favourable to realize a high VDE value.

Probing the potential of halogen-free superhalogen anions as effective electrolytes of Li-ion batteries: a theoretical prospect from combined ab initio and DFT studies.

The potential of 23 superhalogen anions of halogen-free structures as high-performance electrolytes of Li-ion batteries is theoretically explored here. According to high-level ab initio results at the CCSD(T) level, eight candidates, obeying the Wade-Mingos rule, should be capable of forming electrolytes, which are better than the currently used commercial products. When comparing different methods, MP2 was found to be in good agreement with CCSD(T) in the calculation of ΔELi+ and ΔEH2O, which are parameters describing the performance of potential electrolytes. Thus, MP2 represents a good choice for such calculations, particularly for large potential electrolyte systems wherein CCSD(T) calculations are actually impractical. The five functionals selected here (ωB97XD, B2GP-PLYP, B2K-PLYP, B2T-PLYP and B3LYP) are also capable of reproducing the variational trends of the relative values of different structures at the CCSD(T) level. However, the actual DFT values of ΔELi+ are usually different from those of CCSD(T) by more than 1 eV. These significant deviations may be a problem when accurate ΔELi+ values are required.

Could the description on polynuclear superhalogens by DFT be comparable with high-level ab initio results? A comparison between DFT and CCSD(T).

A systematic density functional theory study including 17 exchange-correlation functionals was performed on different types of superhalogens with high level coupled-cluster single double including perturbative triple excitations (CCSD(T)) results as the reference. The superhalogens selected here cover the ranges from mononuclear to polynuclear structures and from structures with halogen-atom ligands to those with non-halogen ligands, e.g., [MgX3](-), [Mg2X5](-), and [Mg3X7](-) (X = F, Cl, CN). It is clearly indicated that three double-hybrid functionals B2T-PLYP, B2GP-PLYP, B2K-PLYP as well as the range-separated hybrid functional ωB97X are capable of providing results which approach the accuracy at the CCSD(T) level. The basis set effect is usually moderate and, in most cases, it is enough to utilize the basis set of triple-ξ quality, e.g., Def2-TZVP. In addition, the results of the HF and MP2 method are also acceptable here, especially for polynuclear superhalogens where CCSD(T) is probably unpractical.

Combining proton and silaborane-based superhalogen anions - an effective route to new superacids as verified via systematic DFT calculations.

Based on systematic DFT calculations, silaborane-based superhalogen anions, which obey the Wade-Mingos rule, are shown to be capable of giving rise to superacids via their combination with protons. Compared to previous carborane-based systems, the acidities of the composites here are stronger in both the gas phase and solution phase. Thus, the potential of candidates based on silaborane could be greater than those based on carborane in the search for ultra-strong acidic systems. Within a given group, a higher superhalogen anion vertical electron detachment energy (VDE) generally leads to stronger acidity. This consistency arises from the dominant role of the VDE, as established through the decomposition of the gas-phase acidity into different contributions. Thus, constructing superacids from superhalogens is a rational route whose future should be positive. Besides the VDE, other effects, i.e., the deformation energy (DE) and bond dissociation energy (BDE), could also be crucial, especially in terms of the differences between the acidities of composites belonging to different groups. A comparison between the results in the gas phase and solution phase indicates that complete calculations of both gas-phase ΔGacid and solution-phase pKa values are necessary to obtain an unbiased description of the acidity. The solvation free energies of the participants in the deprotonation process, especially the conjugate acid, are responsible for the discrepancies between gas phase and solution phase behavior.