Irina V. Stoyanova

The Structure of the Hydrogen Ion (Haq+) in Water

The hydrogen ion in water, H(aq)(+), is a unique H(13)O(6)(+) entity that defines the boundary of positive-charge delocalization. Its central unit is neither a C(3v) H(3)O(+) Eigen-type ion nor a typical H(5)O(2)(+) Zundel-type ion. IR spectroscopy indicates that the H(13)O(6)(+) ion has an unexpectedly long central O...O separation (>>2.43 A), showing that in comparison with the gas and solid phases, the environment of liquid water is uniquely proficient in delocalizing positive charge. These results will change the description of H(aq)(+) in textbooks of chemistry, and a more extensive delocalization of positive charge may need to be incorporated into descriptions of mechanisms of aqueous proton transport.

The nature of the hydrated proton H(aq)+ in organic solvents.

The nature of H(H2O)n(+) cations for n = 3-8 with weakly basic carborane counterions has been studied by IR spectroscopy in benzene and dichloroethane solution. Contrary to general expectation, neither Eigen-type H3O x 3 H2O(+) nor Zundel-type H5O2(+) x 4 H2O ions are present. Rather, the core species is the H7O3(+) ion.

The unique nature of H + in water

The H+(aq) ion in ionized strong aqueous acids is an unexpectedly unique H13O6+ entity, unlike those in gas phase H+(H2O)n clusters or typical crystalline acid hydrates. IR spectroscopy indicates that the core structure has neither C3vH3O+ Eigen-like nor typical H5O2+ Zundel-like character. Rather, extensive delocalization of the positive charge leads to a H13O6+ ion having an unexpectedly long central O⋯O separation of ∼2.57 A and four conjugated O⋯O separations of ∼2.7 A. These dimensions are in conflict with the shorter O⋯O separations found in structures calculated by theory. Ultrafast dynamic properties of the five H atoms involved in these H-bonds lead to a substantial collapse of normal IR vibrations and the appearance of a continuous broad absorption (cba) across the entire IR spectrum. This cba is distinguishable from the broad IR bands associated with typical low-barrier H-bonds. The solvation shell outside of the H13O6+ ion defines the boundary of positive charge delocalization. At low acid concentrations, the H13O6+ ion is a constituent part of an ion pair that has contact with the first hydration shell of the conjugate base anion. At higher concentrations, or with weaker acids, one or two H2O molecules of H13O6+ cation are shared with the hydration shell of the anion. Even the strongest acids show evidence of ion pairing.

The Strongest Brønsted Acid: Protonation of Alkanes by H(CHB11F11) at Room Temperature

What is the strongest acid? Can a simple Brønsted acid be prepared that can protonate an alkane at room temperature? Can that acid be free of the complicating effects of added Lewis acids that are typical of common, difficult-to-handle superacid mixtures? The carborane superacid H(CHB11 F11 ) is that acid. It is an extremely moisture-sensitive solid, prepared by treatment of anhydrous HCl with [Et3 SiHSiEt3 ][CHB11 F11 ]. It adds H2 O to form [H3 O][CHB11 F11 ] and benzene to form the benzenium ion salt [C6 H7 ][CHB11 F11 ]. It reacts with butane to form a crystalline tBu(+) salt and with n-hexane to form an isolable hexyl carbocation salt. Carbocations, which are thus no longer transient intermediates, react with NaH either by hydride addition to re-form an alkane or by deprotonation to form an alkene and H2 . By protonating alkanes at room temperature, the reactivity of H(CHB11 F11 ) opens up new opportunities for the easier study of acid-catalyzed hydrocarbon reforming.

Dialkyl chloronium ions

The carborane acid H(CHB(11)Cl(11)) reacts with chloroalkanes RCl to give isolable dialkylchloronium ion salts, [R(2)Cl][CHB(11)Cl(11)], that are stable at room temperature. X-ray crystal structures have been obtained for R = CH(3) and CH(2)CH(3), revealing bent cation structures with C-Cl-C angles of 101.5 and 105.8 degrees , respectively. The dimethylchloronium ion salt loses CH(3)Cl upon heating and forms sublimable CH(3)(CHB(11)Cl(11)), providing a clean synthetic route to an extremely potent electrophilic methylating agent. IR spectra of all species have been interpreted, including the C-Cl stretch in CH(3)-ClCHB(11)Cl(10).

Haq+ Structures in Proton Wires inside Nanotubes

The hydrated carborane acid H(CHB(11)I(11)).8H(2)O crystallizes in nanometer-diameter tubes of H(aq)(+) enclosed by walls of carborane anions. Three different types of H(aq)(+) clusters are found in these tubes: a symmetrical H(13)O(6)(+) ion with an unusually elongated Zundel-type H(5)O(2)(+) core, two hydrated H(7)O(3)(+) ions, and an unprecedented H(17)O(8)(+) ion having a nearly square core. All of the H(aq)(+) cations show unexpectedly longer O...O separations than in discrete H(aq)(+) ions, indicating greater delocalization of positive charge. The centrosymmetric H(aq)(+) ions are linked via short H bonds, forming a true one-dimensional proton wire.

Oligomeric Carbocation-like Species from Protonation of Chloroalkanes

The protonation of chloroethane by the strongest known solid superacid, the carborane acid H(CHB(11)Cl(11)), has been studied by quantitative IR spectroscopic methods to track mass balance and uncover previously unobserved chemistry. In the first step, an intermediate EtCl·H(CHB(11)Cl(11)) species without full proton transfer to EtCl can be observed when d(5)-deuterated chloroethane is used. It rapidly eliminates HCl (but not DCl) to form ethyl carborane, Et(CHB(11)Cl(11)), which binds a second molecule of chloroethane to form the Et(2)Cl(+) chloronium ion. This undergoes a slower, previously unrecognized HCl elimination reaction to form a butyl carborane, Bu(CHB(11)Cl(11)), beginning an oligomerization process whereby unsymmetrical dialkylchloronium ions decompose to alkyl carboranes of formula Bu(C(2)H(4))(n)(CHB(11)Cl(11)) up to n = 4. Over time, a parallel competing process of de-oligomerization take place in the presence of free carborane acid that finishes with the formation of hexyl or butyl carboranes. Upon heating to 150 C, the final products are all converted to the remarkably stable tert-butyl cation carborane salt.

The Basicity of Unsaturated Hydrocarbons as Probed by Hydrogen‐Bond‐Acceptor Ability: Bifurcated NH+⋅⋅⋅π Hydrogen Bonding

The competitive substitution of the anion (A(-)) in contact ion pairs of the type [Oct3NH+]B(C6F5)4 (-) by unsaturated hydrocarbons (L) in accordance with the equilibrium Oct3NH+...A(-) + nL right arrow over left arrow [Oct3NH+...Ln]A(-) has been studied in CCl4. On the basis of equilibrium constants, K, and shifts of nuNH to low frequency, it has been established that complexed Oct3NH...+Ln cations with n=1 and 2 are formed and have unidentate and bifurcated N--H+...pi hydrogen bonds, respectively. Bifurcated hydrogen bonds to unsaturated hydrocarbons have not been observed previously. The unsaturated hydrocarbons studied include benzene and methylbenzenes, fused-ring aromatics, alkenes, conjugated dienes, and alkynes. From the magnitude of the redshifts in the N--H stretching frequencies, Delta nuNH, a new scale for ranking the pi basicity of unsaturated hydrocarbons is proposed: fused-ring aromatics<or=benzene<toluene<xylene<mesitylene<durene<conjugated dienes approximately 1-alkynes<pentamethylbenzene<hexamethylbenzene<internal alkynes approximately cycloalkenes<1-methylcycloalkenes. This scale is relevant to the discussion of pi complexes for incipient protonation reactions and to understanding N--H+...pi hydrogen bonding in proteins and molecular crystals.

Evidence for C-H hydrogen bonding in salts of tert-butyl cation

Environmentally sensitive: A combination of C-H anion hydrogen bonding and hyperconjugative charge delocalization explains the sensitivity of the IR spectrum of the tert-butyl cation to its anion (see high-resolution X-ray structure with a CHB(11)Cl(11)(-) counterion). The νCH vibration of the cation scales linearly with the basicity of carborane anions on the νNH scale. The same also holds for the C(6)H(7)(+) benzenium ion.