The Molecular Origin of the “Continuous” Infrared Absorption in Aqueous Solutions of Acids: A Computational Approach

Iftimie, Radu; Tuckerman, Mark E.

doi:10.1002/anie.200502259

Cited by 52 publications

(47 citation statements)

References 33 publications

(20 reference statements)

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“…18 This distinctive featureless plateau spreads across nearly the entire IR frequency range and is not to the same as the broad so-called ABC features 61 seen in other H-bonded systems. 62 The cba is clearly a manifestation of unique, low-barrier H-bonding within the unique five -H-atom core of the H 13 O 6 + Stoyanov ion whose proton dynamics are faster than the IR timescale. We estimate that the cba accounts for ca.…”

Section: The Unique Structure Of H+ In Watermentioning

confidence: 99%

Myths about the Proton. The Nature of H⁺ in Condensed Media

2013

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CONSPECTUS Recent research has taught us that most protonated species are decidedly not well represented by a simple proton addition. What is the actual nature of the hydrogen ion (the “proton”) when H+, HA, H2A+ etc. are written in formulae, chemical equations and acid catalyzed reactions? In condensed media, H+ must be solvated and is nearly always di-coordinate – as illustrated by isolable bisdiethyletherate salts having H(OEt2)2+ cations and weakly coordinating anions. Even carbocations such as protonated alkenes have significant C-H---anion hydrogen bonding that gives the active protons two-coordinate character. Hydrogen bonding is everywhere, particularly when acids are involved. In contrast to the normal, asymmetric O-H---O hydrogen bonding found in water, ice and proteins, short, strong, low-barrier (SSLB) H-bonding commonly appears when strong acids are present. Unusually low frequency IR νOHO bands are a good indicator of SSLB H-bonds and curiously, bands associated with group vibrations near H+ in low-barrier H-bonding often disappear from the IR spectrum. Writing H3O+ (the Eigen ion), as often appears in textbooks, might seem more realistic than H+ for an ionized acid in water. However, this, too, is an unrealistic description of H(aq)+. The dihydrated H+ in the H5O2+ cation (the Zundel ion) gets somewhat closer but still fails to rationalize all the experimental and computational data on H(aq)+. Researchers do not understand the broad swath of IR absorption from H(aq)+, known as the “continuous broad absorption” (cba). Theory has not reproduced the cba, but it appears to be the signature of delocalized protons whose motion is faster than the IR timescale. What does this mean for reaction mechanisms involving H(aq)+? For the past decade, the carborane acid H(CHB11Cl11) has been the strongest known Brøsted acid. (It is now surpassed by the fluorinated analogue H(CHB11F11).) Carborane acids are strong enough to protonate alkanes at room temperature, giving H2 and carbocations. They protonate chloroalkanes to give dialkylchloronium ions, which decay to carbocations. By partially protonating an oxonium cation, they get as close to the fabled H4O2+ ion as can be achieved outside of a computer.

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Section: The Unique Structure Of H+ In Watermentioning

confidence: 99%

Myths about the Proton. The Nature of H⁺ in Condensed Media

2013

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“…[91][92][93][94][95][96][97][98] Likewise, acid-base neutralization reactions in water,a sw ell as proton transfer through membranes or ion channels,m ay have well-defined temporal beginning and end points in the protont ransmission chain of events,a lthough the elementary intermediates teps may be numerous and complex. Them ovement is as pontaneous one without aw ell-defined beginning and end.…”

Section: Time-resolved Ultrafast Proton Transfer Dynamics In Acidbasementioning

confidence: 99%

“…Laser-initiatedp rotont ransfer reactionsh ave aw ell-defined beginning, when the acid impulsively initiates the release of the proton to the nearby water molecule in the first solvation shell. [91][92][93][94][95][96][97][98] Likewise, acid-base neutralization reactions in water,a sw ell as proton transfer through membranes or ion channels,m ay have well-defined temporal beginning and end points in the protont ransmission chain of events,a lthough the elementary intermediates teps may be numerous and complex. To achieve temporal timing and to perform time-resolved studies,o ne usually utilizes the optically triggered enhanced photoacidity of suitable photoacids.A pplying photoacidity as am eans for timing proton transfer by short optical laser pulses has been routinely carried out for many decades.…”

Section: Time-resolved Ultrafast Proton Transfer Dynamics In Acidbasementioning

confidence: 99%

Monitoring the Microscopic Molecular Mechanisms of Proton Transfer in Acid‐base Reactions in Aqueous Solutions

Pines

Nibbering

Pines

2015

Israel Journal of Chemistry

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This article provides a review of the authors’ collaborative work on acid‐base reactions under conditions allowing direct observation of mechanistic details of the proton transfer within reactive complexes of OH groups and the bases RCOO− (R=organic residues) in diffusion‐assisted neutralization reactions by using time‐resolved UV/Vis emission and IR absorption spectroscopy. We discuss the considerable insight gained by these experiments and outline the current status of the field.

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“…[46][47][48][49][50][51] This demonstrates the reliability of our theoretical approach. [a] Here, n refers to the water molecules number in the wire; o is the number of side-water molecules attached on the wire; p is the oxygen sequence number attached by water molecule.…”

mentioning

confidence: 63%

“…[44,45] Spectral signatures of hydrated proton vibrations in water clusters or in proteins are explored experimentally and theoretically. [46][47][48][49][50][51] Proton transfer (PT) along a Hbonded ammonia wire has been investigated [52][53][54] recently. The excitation of ammonia-wire vibrations induces H-atom transfer for 7-hydroxyquinoline (NH 3 ) 3 system with a reaction threshold % 200 cm À1 , while the substitution of H 2 O for NH 3 is disadvan-Proton transfer along a single-file hydrogen-bonded water chain is elucidated with a special emphasis on the investigation of chain length, side water, and solvent effects, as well as the temperature and pressure dependences.…”

Section: Introductionmentioning

confidence: 99%

Exploration on Regulating Factors for Proton Transfer along Hydrogen‐Bonded Water Chains

et al. 2007

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Proton transfer along a single-file hydrogen-bonded water chain is elucidated with a special emphasis on the investigation of chain length, side water, and solvent effects, as well as the temperature and pressure dependences. The number of water molecules in the chain varies from one to nine. The proton can be transported to the acceptor fragment through the single-file hydrogen-bonded water wire which contains at most five water molecules. If the number of water molecule is more than five, the proton is trapped by the chain in the hydroxyl-centered H(7)O(3) (+) state. The farthest water molecule involved in the formation of H(7)O(3) (+) is the fifth one away from the donor fragment. These phenomena reappear in the molecular dynamics simulations. The energy of the system is reduced along with the proton conduction. The proton transfer mechanism can be altered by excess proton. The augmentation of the solvent dielectric constant weakens the stability of the system, but favors the proton transfer. NMR spin-spin coupling constants can be used as a criterion in judging whether the proton is transferred or not. The enhancement of temperature increases the thermal motion of the molecule, augments the internal energy of the system, and favors the proton transfer. The lengthening of the water wire increases the entropy of the system, concomitantly, the temperature dependence of the Gibbs free energy increases. The most favorable condition for the proton transfer along the H-bonded water wire is the four-water contained chain with side water attached near to the acceptor fragment in polar solvent under higher temperature.

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The Molecular Origin of the “Continuous” Infrared Absorption in Aqueous Solutions of Acids: A Computational Approach

Cited by 52 publications

References 33 publications

Myths about the Proton. The Nature of H⁺ in Condensed Media

Myths about the Proton. The Nature of H⁺ in Condensed Media

Monitoring the Microscopic Molecular Mechanisms of Proton Transfer in Acid‐base Reactions in Aqueous Solutions

Exploration on Regulating Factors for Proton Transfer along Hydrogen‐Bonded Water Chains

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The Molecular Origin of the “Continuous” Infrared Absorption in Aqueous Solutions of Acids: A Computational Approach

Cited by 52 publications

References 33 publications

Myths about the Proton. The Nature of H+ in Condensed Media

Myths about the Proton. The Nature of H+ in Condensed Media

Monitoring the Microscopic Molecular Mechanisms of Proton Transfer in Acid‐base Reactions in Aqueous Solutions

Exploration on Regulating Factors for Proton Transfer along Hydrogen‐Bonded Water Chains

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Myths about the Proton. The Nature of H⁺ in Condensed Media

Myths about the Proton. The Nature of H⁺ in Condensed Media