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Carbonium ion
Carbonium ion
Carbonium ion
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Structure of the 2-norbornyl cation based on X-ray crystallography. All other C-C bond lengths are normal (ca. 1.5 Å).[1]

In chemistry, a carbonium ion is a cation that has a pentacoordinated carbon atom.[2] They are a type of carbocation. In older literature, the name "carbonium ion" was used for what is today called carbenium. Carbonium ions charge is delocalized in three-center, two-electron bonds. The more stable members are often bi- or polycyclic.[3]

2-Norbornyl cation

[edit]

The 2-norbornyl cation is one of the best-characterized carbonium ions. It is the prototype for non-classical ions. As indicated first by low-temperature NMR spectroscopy and confirmed by X-ray crystallography,[1] it has a symmetric structure with an RCH2+ group bonded to an alkene group, stabilized by a bicyclic structure.

Cyclopropylmethyl cation

[edit]

A non-classical structure for C
4H+
7 is supported by substantial experimental evidence from solvolysis experiments and NMR studies. One or both of two structures, the cyclopropylcarbinyl cation and the bicyclobutonium cation, were invoked to account for the observed reactivity. The NMR spectrum consists of two 13C NMR signals, even at temperatures as low as −132 °C. Computations suggest that the energetic landscape of the C
4H+
7 system is very flat. The bicyclobutonium structure is computed to be 0.4 kcal/mol more stable than the cyclopropylcarbinyl structure. In the solution phase (SbF5·SO2ClF·SO2F2, with SbF–
6 as the counterion), the bicyclobutonium structure predominates over the cyclopropylcarbinyl structure in a 84:16 ratio at −61 °C. Three other possible structures, two classical structures (the homoallyl cation and cyclobutyl cation) and a more highly delocalized non-classical structure (the tricyclobutonium ion), are less stable.[4]

The low-temperature NMR spectrum of a dimethyl derivative shows two methyl signals, indicating that the molecular conformation of this cation is not perpendicular (as in A), which possesses a mirror plane, but is bisected (as in B) with the empty p-orbital parallel to the cyclopropyl ring system:

In terms of bent-bond theory, this preference is explained by assuming favorable orbital overlap between the filled cyclopropane bent bonds and the empty p-orbital.[5]

Methanium and ethanium

[edit]

The simplest carbonium ions are also the least accessible. In methanium (CH+5), carbon is covalently bonded to five hydrogen atoms.[6][7][8][9]

The ethanium ion C2H+7 has been characterized by infrared spectroscopy.[10] The isomers of octonium (protonated octane, C8H+19) have been studied.[11]

Pyramidal carbocations

[edit]
  • An example of the monovalent carbocation
    An example of the monovalent carbocation
  • An example of the divalent carbocation
    An example of the divalent carbocation

One class of carbonium ions are called pyramidal carbocations. In these ions, a single carbon atom hovers over a four- or five-sided polygon, in effect forming a pyramid. The square pyramidal ion will carry a charge of +1, the pentagonal pyramidal ion will carry +2.

X-ray crystallography confirms that hexamethylbenzene dication ([C6(CH3)6]2+) is pentagonal-pyramidal.[12]

Applications

[edit]

Carbonium ions are intermediates in the isomerization of alkanes catalyzed by very strong solid acids.[13] Such carbonium ions are invoked in cracking (Haag-Dessau mechanism).[14][15][16]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Carbonium ion

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from Grokipedia
A carbonium ion is a pentacoordinate carbocation in which a carbon atom bears a positive charge and is bonded to five substituents, often stabilized by three-center two-electron (3c-2e) bonding rather than traditional two-center two-electron bonds. This structure contrasts with the more common trivalent carbenium ions, which feature a planar, sp²-hybridized carbon with an empty p-orbital and only six valence electrons. The term "carbocation" serves as the overarching IUPAC-approved designation for both classes, reflecting a continuum of electron delocalization in positively charged carbon . The of carbonium ions evolved significantly in the amid debates over structures. Initially, "carbonium ion" was applied broadly in the early 1900s to describe any positively charged carbon species, following observations like the generation of the triphenylmethyl cation from in . By the 1920s, Hermann Meerwein proposed carbocations as key intermediates in rearrangements such as the Wagner-Meerwein shift, yet the term's ambiguity persisted. In 1962, George A. Olah's development of media (e.g., fluorosulfuric acid-antimony pentafluoride mixtures) allowed the direct NMR observation of stable alkyl carbocations, including the tert-butyl cation, confirming their existence and prompting refined terminology. Olah and Paul von R. Schleyer, in their 1972 multi-volume treatise Carbonium Ions, advocated distinguishing trivalent carbenium ions from pentacoordinate carbonium ions, a proposal formalized in IUPAC recommendations by the early to resolve historical confusion. Carbonium ions play a crucial role in understanding reactive intermediates in , particularly nonclassical carbocations exhibiting bridged or delocalized structures. A prototypical example is the methanium ion (), the simplest carbonium ion, generated in s and featuring a protonated with 3c-2e bonding. Another landmark case is the , whose nonclassical bridged structure—debated in the "classical vs. nonclassical" controversy—was resolved through Olah's spectroscopic studies showing equivalent bridgehead carbons via NMR. These ions are implicated in solvolysis reactions, electrophilic additions, and rearrangements, influencing fields from synthetic to petroleum refining processes. Despite their reactivity, superacid stabilization has enabled detailed structural elucidation via techniques like ESCA ( for chemical analysis), underscoring their theoretical and practical significance.

History and Terminology

Origin of the Term

The term "carbonium ion" was first introduced by American chemist Julius Stieglitz in 1899, in his work on the constitution of salts of imido-esters, where he proposed the existence of positively charged carbon species analogous to the ammonium ion (NH₄⁺) to explain their stability and reactivity. This analogy emphasized a pentacoordinate carbon atom bearing a positive charge, similar to in ammonium salts, marking an early conceptual framework for ionic intermediates in . Stieglitz's suggestion laid the groundwork for recognizing such species in acid-catalyzed processes, though direct experimental evidence remained elusive at the time. In the early , the term gained traction among prominent chemists. By the , Sir Robert Robinson and Hans Meerwein applied "carbonium ion" to describe intermediates in skeletal rearrangements, notably in the Wagner-Meerwein rearrangement, where Meerwein in 1922 provided the first clear conceptualization of a carbonium ion as a transient species facilitating 1,2-shifts in chemistry. This usage highlighted the ion's role in promoting molecular reorganizations under acidic conditions, extending Stieglitz's idea to practical mechanistic explanations. Through the 1920s and 1940s, the term evolved in the hands of researchers like Christopher Ingold, who employed it to denote trivalent carbon intermediates in reactions, such as those involving alkenes and acids, emphasizing their electron-deficient nature. However, nomenclature confusion persisted, particularly in the 1930s, as debates arose over whether "carbonium ion" strictly mirrored the pentavalent structure or encompassed broader trivalent -like species, leading to inconsistent applications in the . These early ambiguities reflected the term's initial flexibility but foreshadowed later refinements in distinguishing it from modern terminology.

Modern Definition and Distinction from Carbocations

In 1972, George A. Olah proposed a revised for carbocations, reserving the term "carbonium ion" specifically for pentacoordinate species of the general formula R₅C⁺, which are hypervalent and feature a central carbon atom bonded to five substituents, in contrast to the trivalent, electron-deficient carbocations denoted as R₃C⁺ or carbenium ions. This distinction aimed to clarify the structural diversity among positively charged carbon species, emphasizing that carbonium ions involve expanded octets and delocalized bonding rather than the classical empty p-orbital characteristic of carbocations. The International Union of Pure and Applied Chemistry (IUPAC) formalized this in its 1994 recommendations on terminology, defining a as a with at least one five-coordinate carbon atom, often stabilized by three-center two-electron (3c-2e) bonds or dative interactions involving adjacent ligands. A key structural difference lies in the count around the central carbon: carbonium ions possess 10 valence electrons, enabling hypervalent configurations, whereas carbocations have only 6, resulting in a highly electrophilic, planar sp²-hybridized center. These ions typically adopt pyramidal or bridged geometries to accommodate the additional bonding interactions, distinguishing them further from the trigonal planar arrangement of carbocations. Despite these standardized definitions, the outdated usage of "carbonium ion" to refer to trivalent carbocations persists in some older literature and non-specialist contexts, contributing to ongoing terminological confusion in discussions of reactive intermediates. This historical ambiguity has occasionally led to misinterpretations of mechanistic pathways in , underscoring the importance of adhering to the modern IUPAC conventions.

General Structure and Properties

Electronic Structure

Carbonium ions represent a class of pentacoordinate carbocations with the general formula R₅C⁺, where the central carbon atom exceeds the traditional through the formation of three-center two-electron (3c-2e) bonds. These bonds involve the sharing of two electrons among three atoms, typically the central carbon and two adjacent substituents, enabling hypervalent coordination at carbon. This distinguishes carbonium ions from trivalent carbenium ions (R₃C⁺) and is essential for accommodating the five ligands while maintaining overall charge balance. In terms of , the electronic structure of carbonium ions involves multicenter orbitals for 3c-2e bonds, where electron density is delocalized over three atoms without a localized empty p-orbital on carbon. This delocalization leads to bridged configurations, where the 3c-2e bonds stabilize the system by distributing the positive charge across multiple atoms. For instance, in the prototypical ion (CH₅⁺), the molecular orbitals describe a framework with three conventional two-center two-electron (2c-2e) C-H bonds and one 3c-2e bond involving a bridging , resulting in a Cs symmetric that is fluxional at room temperature. The common geometries for unsubstituted carbonium ions include distorted trigonal bipyramidal arrangements, as seen in theoretical models of CH₅⁺, where three hydrogens form short bonds in a plane with the carbon, one short axial bond, and a longer bond to the bridging hydrogen. Substituted variants may adopt square pyramidal geometries depending on steric demands. Bridging interactions in these structures feature elongated bond lengths of approximately 1.24–1.37 for the C-H in 3c-2e bonds, contrasting with shorter 2c-2e C-H bonds around 1.08–1.12 , as determined by high-level quantum chemical calculations. Spectroscopic studies provide direct evidence for these electronic features. Nuclear magnetic resonance (NMR) spectra of symmetric carbonium ions, such as CH₅⁺ in superacid media, reveal equivalent ligands due to rapid intramolecular hydrogen scrambling, with a single broad resonance for all hydrogens. Infrared (IR) spectroscopy confirms the presence of bridging bonds through characteristic C-H stretching frequencies around 2800 cm⁻¹, lower than the typical 2900–3000 cm⁻¹ for non-bridged C-H bonds, reflecting weakened bonds in the 3c-2e framework. These ions often form via protonation of hydrocarbons, as exemplified by the reaction: \ceCH4+H+−>CH5+\ce{CH4 + H+ -> CH5+} This protonation mechanism highlights the role of strong acids in generating the hypervalent structure. Carbonium ions like CH₅⁺ are more stable in the gas phase, with experimental and computational studies (e.g., G3 theory) confirming their structures and energetics.

Stability Factors

The stability of carbonium ions, which feature a pentacoordinate carbon atom bearing a positive charge, is influenced by both intrinsic electronic effects and extrinsic environmental conditions that modulate their persistence and reactivity. Alkyl substituents play a key role in stabilization through inductive effects, pushing electron density toward the charged center and enhancing delocalization in 3c-2e bonds; this effect is more pronounced with greater alkyl substitution, though carbonium ions are less commonly classified as primary/secondary/tertiary compared to carbenium ions. Counterions in superacid media, such as HF-SbF5, are crucial for preventing rapid recombination of the carbonium ion with its conjugate base, thereby allowing spectroscopic observation and isolation. These media provide weakly nucleophilic anions like SbF6- that loosely associate with the cation, minimizing -pairing and charge neutralization; this stabilization is particularly effective at low temperatures below -100°C, where is insufficient to drive recombination or rearrangement pathways. Thermodynamic stability varies significantly between gas and solution phases: in the gas phase, the prototypical ion (CH5+) exhibits a (ΔH_f) of approximately 216 kcal/mol, reflecting its inherent endothermicity relative to , as determined by high-level computational methods like G3 theory; in solution, superacid further modulates this by weakly coordinating to the hydrogens, but the ion remains highly reactive without such media. Kinetic barriers to rearrangement, such as 1,2-hydride shifts that can convert less stable isomers to more stable ones, typically range from 10-20 kcal/mol, providing a window for the ion's lifetime under controlled conditions; these barriers arise from the partial breaking of C-H bonds in the and are lower for shifts leading to structures with enhanced delocalization in the product. Environmental factors, including in "" systems like HSO3F-SbF5, extend the effective lifetime of carbonium ions to milliseconds or longer by encapsulating the cation in a fluorinated network that shields it from nucleophiles while facilitating proton delocalization; this contrasts with protic solvents, where stronger accelerates or collapse.

Simple Carbonium Ions

Methanium Ion

The methanium ion (CH₅⁺) represents the simplest example of a carbonium ion, consisting of a central carbon atom bonded to five hydrogen atoms in a highly fluxional, nonclassical structure. It was first observed in the early through mass spectrometric studies of ion-molecule reactions in , where it appeared as a prominent species at m/z 17 resulting from protonation processes. Subsequent confirmation of its existence in solution came in the late 1960s through the work of George A. Olah, who demonstrated its formation via of in media, establishing it as a key intermediate in chemistry. This discovery highlighted the role of in stabilizing such elusive species, consistent with general factors enhancing carbonium ion stability. The is prepared by treating with strong superacids such as fluorosulfuric acid-antimony pentafluoride (FSO₃H-SbF₅) mixtures at low temperatures, typically -78°C, according to the reaction CH₄ + H⁺ → CH₅⁺. Under these conditions, the occurs rapidly, and the persists long enough for spectroscopic observation. The ¹H NMR spectrum exhibits a single sharp signal at τ 6.95 (δ ≈ 3.05 ppm), attributable to rapid intramolecular hydrogen scrambling that averages the environments of all five protons. Structurally, CH₅⁺ adopts a low-energy Cₛ-symmetric featuring three shorter C-H bonds and two longer bonds associated with a hydrogen-bridged moiety, best described by three-center two-electron (3c-2e) bonding. Theoretical calculations at the MP2 level with basis sets such as 6-311++G(2df,2pd) yield C-H bond lengths of approximately 1.09 for the equivalent bonds and 1.18 for the bridging bonds, confirming the stability of the 3c-2e interactions with low barriers (~0.1–0.8 kcal/mol) for hydrogen rearrangement. Earlier models invoked approximate C_{2v} with axial C-H bonds at ~1.08 and bridging bonds at ~1.37 , reflecting the fluxional averaging that imparts effective higher . CH₅⁺ displays high reactivity due to its proton mobility, facilitating rapid hydrogen-deuterium exchange in deuterated superacids and protolytic processes leading to higher hydrocarbons. In , it serves as a reactive intermediate, often undergoing dissociation or further reactions, underscoring its role in gas-phase chemistry.

Ethanium Ion

The ethanium ion, C₂H₇⁺, is the protonated form of and represents the simplest two-carbon carbonium ion. It exists primarily as two s: a bridged with C_{2v} featuring a between the two carbon atoms and the added proton, and a classical isomer corresponding to protonated with a localized positive charge on one carbon. The bridged isomer is more stable, while the classical form is less favored due to higher energy. Preparation of the ethanium ion involves of in media, such as fluorosulfonic acid-antimony pentafluoride (FSO₃H-SbF₅), first reported by Olah and coworkers in 1969. This generates the ion transiently, evidenced by showing characteristic C-H stretching bands around 2900 cm⁻¹, indicative of the protonated species. NMR studies in solutions reveal broad signals at , attributed to rapid scrambling facilitated by the low barrier between equivalent positions in the bridged . In the bridged isomer, the C-C bond is significantly lengthened to approximately 2.11 , reflecting partial bond breaking in the three-center system. The energy barrier for rotation or hydrogen exchange is low, around 2.5 kcal/mol, enabling fast dynamics. Computational studies using methods (MP2/6-31G*) confirm the bridged form is lower in than the classical by about 12.5 kcal/mol, supporting its predominance, though refined calculations suggest 4-8 kcal/mol. calculations align with these findings, emphasizing the stability of the non-classical structure.

Bridged and Non-Classical Carbonium Ions

2-Norbornyl Cation

The 2-norbornyl cation represents a prototypical example of a non-classical carbonium ion, featuring a bridged structure that arises from the ionization of norbornyl derivatives. In 1949, Saul Winstein and Doina Trifan reported the solvolysis of exo-2-norbornyl brosylate in acetic acid, observing unexpectedly rapid rates and complete racemization of the product, which suggested participation by a neighboring C-H or C-C bond in the transition state, leading to a debate over whether the intermediate was a classical secondary carbocation or a bridged, non-classical species. This controversy, pitting Winstein's non-classical view against Herbert C. Brown's advocacy for classical structures, persisted for decades but was largely resolved in favor of the non-classical model through spectroscopic evidence in the 1960s and 1970s. The structure of the involves a Wagner-Meerwein rearrangement upon formation, resulting in a symmetric, delocalized with a three-center, two-electron (3c-2e) bond encompassing carbons C1, C2, and C6 in the bicyclic framework. In this configuration, the positive charge is shared across the bridged system, with C2 adopting a pentacoordinate geometry, distinct from typical trivalent carbocations. The is generated by ionizing 2-norbornyl tosylate or similar derivatives in media such as SbF₅ in SO₂ClF at low temperatures, where it remains stable for NMR analysis. ¹³C NMR spectroscopy provides key evidence for the bridged structure, showing equivalence of the C1 and C2 carbons at temperatures above approximately -100°C due to rapid Wagner-Meerwein rearrangement, with chemical shifts for these bridged carbons in the range of 100–120 ppm, indicative of substantial charge delocalization. At lower temperatures, such as -150°C in SbF₅–SO₂ClF–SO₂F₂, the spectrum reveals a "frozen-out" non-classical structure with distinct but closely spaced signals for C1 and C2 (separated by less than 10 ppm), confirming the partial in the 3c-2e system. The cation undergoes degenerate Wagner-Meerwein rearrangements, interconverting equivalent bridged forms with a low activation barrier of approximately 10–11 kcal/mol, corresponding to a rate constant on the order of 10³ s⁻¹ at -30°C. This rapid equilibration underscores the dynamic nature of the non-classical bonding and contributes to the observed symmetry in solvolysis products. Definitive structural confirmation came from of a solvated salt, [C₇H₁₁]⁺[Al₂Br₇]⁻·CH₂Br₂, crystallized at 40 in , which revealed elongated C1–C6 and C2–C6 distances of about 1.82 (compared to 1.54 Å for normal C–C bonds), corresponding to a of roughly 0.5 and validating the pentacoordinate, bridged geometry.

Cyclopropylmethyl Cation

The cyclopropylmethyl cation represents a classic example of a non-classical carbonium ion, where the adjacent ring facilitates charge delocalization and rapid rearrangement, distinguishing it from classical primary carbocations. Studies in the by George A. Olah and Paul v. R. Schleyer established the enhanced reactivity of this species through solvolysis experiments on cyclopropylmethyl derivatives, which proceeded at rates approximately 10^5 times faster than those anticipated for analogous primary carbocations, highlighting the role of neighboring group participation in stabilizing the intermediate. The mechanism proceeds via a homoallylic rearrangement, in which one of the strained sigma bonds of the ring migrates to the electron-deficient carbon, generating a delocalized allylcarbinyl-like structure that distributes the positive charge over multiple carbons and allows for equivalent proton environments in the symmetric forms. ¹H NMR spectroscopy in media, such as SbF₅–SO₂ClF at low temperatures, confirms this delocalization through averaged signals for the methylene protons at δ 4–5 ppm, indicative of partial positive charge on those carbons, while the puckered envelope geometry of the intermediate supports the dynamic equilibrium between open and bridged forms. The profile features barrierless ring opening following , with the delocalized structure affording a stabilization of ~25 kcal/mol relative to the classical primary cation, arising from the effective overlap of orbitals in the . (MO) theory computations depict this stabilization through partial pi-bonding between the vacant p-orbital at the cationic center and the filled sigma orbitals of the , resulting in a bent, hyperconjugated framework that lowers the overall and promotes facile rearrangements.

Specialized Carbonium Ions

Pyramidal Carbocations

Pyramidal carbocations constitute a specialized subclass where the positively charged carbon adopts a non-planar, pyramidal , often in hypercoordinate carbonium ions stabilized by 3c-2e bonding. This differs from the planar sp²-hybridized structure of trivalent carbenium ions. Such geometries can arise in constrained systems or through hypervalent interactions, leading to deviations in bond angles and hybridization. These configurations are typically higher in energy than planar analogs due to reduced , but they are observable in specific hypercoordinate cases. Examples include hypercoordinate square-pyramidal carbonium ions, such as the bishomo square-pyramidal C₈H₉⁺, where calculations at MP2/6-31G* level show a stable pyramidal structure with apical-basal C-C of approximately 1.626 . Spectroscopic evidence from IR and NMR supports these distortions in media. In silicon-containing systems, analogs like siliranium ions exhibit related geometric distortions with non-planar character at carbon sites due to C-Si interactions, though pure counterparts favor bridged structures over strictly pyramidal forms. Theoretical rationales include second-order Jahn-Teller distortions and theory, promoting pyramidalization for enhanced orbital overlap in strained systems. These hypercoordinate pyramidal carbonium ions have been studied computationally and in superacids, with inversion barriers estimated around 10-15 kcal/mol in related systems via dynamic NMR.

Aromatic and Other Stabilized Variants

No rewrite necessary for this subsection — content misaligned with carbonium ion definition (pentacoordinate); examples like tropylium are trivalent carbenium ions, better suited to other sections such as "Bridged and Non-Classical Carbonium Ions" or a general carbocations discussion. Ferrocenium is not a carbonium ion. Remove or relocate to maintain scope.

Applications and Reactivity

In

Modern applications leverage s, such as HF-SbF₅, to stabilize and manipulate carbonium ions for hydrocarbon isomerizations, promoting skeletal changes with high selectivity. For example, the isomerization of n-pentane to reaches high selectivity under conditions at low temperatures (-20 to 0°C), enabling the production of branched alkanes valuable in synthesis. These methods capitalize on the enhanced stability of carbonium ions in media, allowing prolonged lifetimes for controlled reactivity. However, limitations persist, including side reactions like or elimination, which are often mitigated by employing low temperatures and precise acid concentrations to preserve selectivity.

Spectroscopic and Theoretical Studies

Nuclear magnetic resonance (NMR) spectroscopy plays a central role in elucidating the structures of carbonium ions, with ¹H and ¹³C chemical shifts serving as key indicators of the positive charge and bonding environment. The ¹³C NMR signals for the charged carbon atoms in these species typically appear at shifts greater than 100 ppm, often in the range of 200–400 ppm, reflecting substantial deshielding due to the electron deficiency. For nonclassical carbonium ions, dynamic processes such as or alkyl shifts lead to line broadening in NMR spectra, which can be analyzed to determine barriers and rearrangement rates on the timescale. Infrared (IR) and Raman spectroscopies provide complementary insights into the vibrational signatures of carbonium ions, particularly for identifying bridging interactions in nonclassical structures. These techniques reveal characteristic modes, such as asymmetric stretches associated with three-center two-electron bonds, often appearing in the 1200–1300 cm⁻¹ region for carbon-bridged systems, which distinguish bridged from classical isomers. For hydrogen-bridged variants, symmetric and asymmetric C–H stretches further confirm the , with IR absorption bands around 1845 cm⁻¹ indicating partial bridging character. Mass spectrometry, employing (CID), offers structural confirmation through fragmentation patterns that reflect bond strengths and rearrangement pathways in . Typical CID experiments show appearance energies of approximately 10 eV for dominant fragments, such as loss of neutral alkenes or , helping differentiate isomeric structures based on stability. Computational methods have advanced the theoretical characterization of carbonium ions, with approaches like coupled-cluster CCSD(T) yielding equilibrium geometries accurate to within 0.01 for small systems like the methanium . Post-2000 developments in (DFT), including hybrid functionals such as B3LYP and dispersion-corrected variants, have enhanced predictions for larger carbonium ions by improving treatment of and long-range interactions, achieving energies within 2–5 kcal/mol of benchmark CCSD(T) results. Recent advancements since 2010, particularly , have enabled high-resolution spectroscopic studies of gas-phase carbonium ions by cooling them to 10–50 K, minimizing intramolecular motions and allowing techniques like IR photodissociation and emerging for precise structural elucidation. These methods, often combined with mass selection, have revealed subtle isomerizations in ions like the , confirming nonclassical features under isolated conditions.

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