Rational Design of Cyclopentadiene-Based Super- and Hyperacids Based on Aromaticity

The design and synthesis of neutral organic superacids have been of interest recently due to their vast applications in chemistry and material sciences, for example, olefinic polymerization, isolation of highly reactive short-lived cations, etc. Cyclopentadiene behaves as a mild organic acid, producing a stable conjugate base by gaining aromaticity and conjugation after deprotonation. To stabilize conjugate bases of organic acids to show superacidities and hyperacidities, we considered aromatic phenyl substituents with cyclopentadiene (mono-, di-, and triphenyl-substituted cyclopentadiene and their cyano derivatives). The MP2, DFT (B3LYP, M06-2X), and CBS-QB3 methods were used to calculate the gas-phase proton affinities of the parent cyclopentadiene, and the DFT methods were chosen for the substituted cyclopentadiene as they yield an experimental proton affinity of cyclopentadiene of 253.66 kcal/mol (Expt. 253.6 ± 1.3 kcal/mol). The stable trisubstituted cyclopentadiene derivative shows gas-phase enthalpies of deprotonation (ΔHacid) of 245 and 239 kcal/mol with DFT B3LYP and M06-2X methods, respectively, with values in the range of hyperacidity. Some of the tautomers of cyclopentadiene derivatives show hyperacidity, with proton affinity values of 205-240 kcal/mol. Triphenyl-substituted cyclopentadiene behaves as a moderate acid but transforms into a superacid after replacing the phenyl group with nitrobenzene, which is a stronger acid than H2SO4 (ΔHacid = 298 kcal/mol). The nucleus-independent chemical shift (NICS) shows the extent of conjugation in the derivatives of cyclopentadienyl anions after deprotonation, which affects the stabilities of conjugate bases, i.e., the acidities of the protonated species. The calculated harmonic oscillator model of aromaticity and NICS indices reveal that the stability of conjugate bases as well as their acidities increase with increasing aromaticity of cyclopentadiene rings after deprotonation in all molecules.

Step-by-Step Replacement of Cyano Groups by Tricyanovinyls-The Influence on the Acidity

Acid-base properties are the simplest expression of compounds' coordinating ability. In the present work, we studied in silico how the gas-phase Brønsted acidity (GA) of several polycyano-substituted compounds change when cyano (CN) groups are replaced by 1,2,2-tricyanovinyl (TCNV) groups in (iso)cyanic acid, dicyanoamine, cyanoform, and hydrogen tetracyanoborate. Different tautomers and conformers/isomers are included in this study. Gas-phase acidity values are compared with the acidities of various acids, including percyanated protonated monocarba-closo-dodecaborate (carborane acid) and dodecaborate, as well as hydrogen cyanide and 1,2,2-tricyanoethene. An estimation of acetonitrile (MeCN), dimethylsufoxide (DMSO), and 1,2-dichloroethane (DCE) acidities is presented using the COSMO-RS method and correlation analysis. The strongest acid with four TCNV groups shows remarkable acidic properties.

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 ΔG_acid and solution-phase pK_a 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.

Superhalogen and Superacid

A superhalogen F@C₂₀ (CN)₂₀ and a corresponding Brønsted superacid were designed and investigated on DFT and DLPNO-CCSD(T) levels of theory. Calculated compounds have outstanding electron affinity and deprotonation energy, respectively. We consider superacid H[F@C₂₀ (CN)₂₀ ] to be able to protonate molecular nitrogen. The stability of these structures is discussed, while some of the previous predictions concerning neutral Brønsted superacids of record strength are doubted. © 2019 Wiley Periodicals, Inc.

A new strategy to generate super and hyper acids with simple organic molecules exploiting σ-hole interaction

Super and hyper acidity can be achieved using σ-hole non-covalent interactions with simple organic systems. The σ-hole interaction can stabilize the conjugate base of acids to such an extent that cyclopentadiene yields a ΔH_acid of 256 kcal mol^-1, which is ∼100 kcal mol^-1 lower than the reported results. The hyper acidic value of 209.0 kcal mol^-1 of organic acids has been achieved without electron-withdrawing substituents rather exploiting only σ-hole interaction. Remarkably, the 1st and 2nd acidity values (∼229 kcal mol^-1) of comparable strength in the hyperacid range have been observed for the first time using DFT computations. The simple organic acids were predicted to reach the pK_a values in the range of -7.3 to -7.8 in DMSO, which is remarkably lower than previous reports. These stable anions can be a potential candidate for application as an effective electrolyte in lithium-ion batteries.

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.

Superhalogen-based composite with strong acidity-a crossing point between two topics

Correlation between the acidity and the vertical electron detachment energy verifies the rationality of constructing superacid from superhalogen.

A DFT study to design super- and hyperacids with 1-(cyclopenta-2,4-dien-1-yl)-4-nitrobenzene and 3-(cyclopenta-2,4-dien-1-ylmethylene)-6-methylenecyclohexa-1,4-diene molecules

DFT calculations reveal that poly-cyano-substituted 1-(cyclopenta-2,4-dien-1-yl)-4-nitrobenzene and 3-(cyclopenta-2,4-dien-1-ylmethylene)-6-methylenecyclohexa-1,4-diene can function as super- to hyper-acids.

Empirically Fitted Parameters for Calculating pKa Values with Small Deviations from Experiments Using a Simple Computational Strategy

Two empirically fitted parameters have been derived for 74 levels of theory. They allow fast and reliable pKa calculations using only the Gibbs energy difference between an acid and its conjugated base in aqueous solution (ΔGs(BA)). The parameters were obtained by least-squares fits of ΔGs(BA) vs experimental pKa values for phenols, carboxylic acids, and amines using training sets of 20 molecules for each chemical family. Test sets of 10 molecules per family-completely independent from the training set-were used to verify the reliability of the fitting parameters method. It was found that, except for MP2, deviations from experiments are lower than 0.5 pKa units. Moreover, mean unsigned errors lower than 0.35 pKa units were found for the 98.6%, 98.6%, and 94.6% of the tested levels of theory for phenols, carboxylic acids and amines, respectively. The parameters estimated here are expected to facilitate computationally based estimations of pKa values of species for which this magnitude is still unknown, with uncertainties similar to the experimental ones. However, the present study deals only with molecules of modest complexity, thus the reliability of the FP method for more complex systems remains to be tested.

Rational design of an efficient one-pot synthesis of 6H-pyrrolo[2,3,4-gh]perimidines in polyphosphoric acid

Several highly efficient one-pot synthetic protocols were developed, enabling polyphosphoric acid-activated nitroalkanes to act as electrophiles in reactions with aminonapthalenes.

Quantum mechanical based approaches for predicting pKa values of carboxylic acids: evaluating the performance of different strategies

The FP method is recommended for estimating pK_a values of carboxylic acids for which this information is still unknown, especially in combination with the PBE0 functional (MUE = 0.26) and the SMD solvent model.

Rational design of high-spin biradicaloids in the isobenzofulvene and isobenzoheptafulvene series

The relative energies of singlet biradicaloid and of triplet and singlet biradical electronic states for a series of benzannelated isobenzofulvenes and isobenzoheptafulvenes were calculated at the (u-)B3LYP/6-31G(d), full π-space CASSCF-CASPT2 (≤14 π-e(-)s), and full π-space RASSCF-RASPT2 (≤24 π-e(-)s) levels of theory. Both absolute and relative CASPT2 energies were reproduced quite well by the RASPT2 approach, which can be extended to much larger active spaces. RASPT2 (and DFT) calculations find that increasing benzannelation leads to triplet ground states in both hydrocarbon series, in violation of the classical principle of maximum bonding. This confirmed the expectations that the combined effects of resonance energy and aromaticity could compensate for the extra formal π-bond of the biradicaloid singlet, and that the strong exchange coupling inherent to the embedded trimethylenemethane (TMM) would manifest itself in the biradicaloids. The relative energy of the biradicaloid singlet rises rapidly upon benzannelation, as π-bonding between the high-energy delocalized GVB orbitals decreases. The underlying π-orbital topology is revealed when this weak π-bonding is artificially eliminated by a 1:1 mixing of the nondegenerate HOMO and LUMO to produce an overcorrelated valence bond (OCVB) orbital pair. For members of both biradicaloid series, the OCVB pairs are nondisjoint, revealing a limiting triplet preference with increasing benzannelation. Within the two-electron, two-orbital approximation, the effects of π-bonding in the singlet biradicaloids and orbital localization away from the acene π-system in the triplet biradicals can be analyzed as perturbations of the singlet OCVB biradicals. The application of a VB-based spin coupling scheme is discussed, in which the unpaired electrons of these species can be considered both ferromagnetically and antiferromagnetically coupled, with the strength of the latter strongly dependent on the acene subunit.

Physical origin of chemical phenomena: Interpretation of acidity, basicity, and hydride affinity by trichotomy paradigm

Some of the most important aspects of modeling in chemistry are discussed in detail. It is argued that the interpretive side of (quantum) chemistry is indispensable, since it gives sense to a myriad of experimental and computational results. The usefulness of some physical modeling is illustrated by the trichotomy approach in rationalizing acidity, basicity, and hydride affinities of neutral organic compounds. According to trichotomy paradigm, the simple chemical reaction of protonation and H- attachment can be decomposed into three separate sequential steps, which in turn mirror the initial-, intermediate-, and final-state effects. Ample evidence is given, which convincingly shows that the trichotomy approach has some distinct advantages in interpreting aforementioned properties that belong to the most important ones in chemistry and biochemistry.