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Speed Limits for Acidâœbase Chemistry in Aqueous Solutions 182 CHIMIA 2012, 66, No. 4 Laureates: awards and Honors, sCs FaLL Meeting 2011 doi:10.2533/chimia.2012.182 Chimia 66 (2012) 182–186 © Schweizerische Chemische Gesellschaft Speed Limits for Acid–Base Chemistry in Aqueous Solutions Mateusz L. Donten§, Joost VandeVondele, and Peter Hamm* §SCS-Metrohm Foundation Award for best oral presentation Abstract: Proton transfer reactions, including acid–base recombination, are commonly considered to occur ‘nearly instantaneously’. However, their actual time scales may stretch far into the microsecond range, as acid– base reactions are diffusion controlled and the concentrations are low near neutral pH. The interplay of competing bases in the pH relaxation is illustrated using a model acid–base system consisting of o-nitrobenzaldehyde (oNBA) as a proton cage and acetate ions and hydroxyl ions as bases. The kinetically controlled behavior leads to highly counterintuitive states, i.e. acetate ions are transiently protonated for hundreds of nanoseconds despite the presence of a much stronger base OH–. Keywords: Neutralization · o-Nitrobenzaldehyde · pH-jump · pH-relaxation · Proton release Introduction regarding proton transfer mechanisms[2,6–8] sets a speed limit for acid–base equilibra- and diffusion processes,[9–11] for solution tion in aqueous solutions already at the 100 Acid–base reactions are one of the phase systems are available. It is well es- µs time scale for near neutral conditions principal categories of chemical transfor- tablished that proton transfers are one of where the acid proton and hydroxyl con- mations, and are typically thought of in the fastest chemical processes and even in centrations fall below micromolar. These terms of pH determining the protonation diluted solution phase, when diffusion lim- rate limitations can become crucial for state of acids and bases in solution phase. ited, due to the Grotthus mechanism[9] their fundamental understanding of time evolu- Systems which undergo pH changes, due rates exceed other known reactions.[5,12,13] tion of acid base systems.[10,14–18] Naturally to ongoing reactions involving protons as On these grounds, chemists often take the when rapid pH changes are initiated the reactants or products, can be thought of advantage of treating dissociation process- ongoing relaxation must be taken into ac- in terms of two fundamental steps: proton es and pH equilibration as instantaneous count for a relatively long period. On a delivery and pH relaxation. When studied in everyday scientific practice. As a con- more general note the remark applies to in water solutions, and therefore in most sequence, most commonly acid–base sys- all systems in which protons are released cases, these two steps are described by two tems are discussed exclusively in terms of or taken in a reaction. In these cases if the very elementary chemical reactions: acid the equilibrium picture of pKa and pKw, reactive steps occur on time scales faster dissociation and autodissociation of water: which are the equilibrium constants for re- than the relatively slow neutralization time actions 1a and 1b, respectively. Describing scale, a stationary state for the reaction HA + H O ↔ H O+ + A– (1a) acid–base systems by their pKw, pKa and rather than the acid–base equilibrium will 2 3 H O + H O ↔ H O+ + OH– (1b) complementary their pKb values, bases be established. This difference might have 2 2 3 would be neutralized from strongest to tremendous implications regarding the ac- These two equations are an absolute weakest and pH sums with pOH to pKw tual reaction mechanisms and kinetics. corner stone of acid–base chemistry in by definition. In recent decades, chemical research Brønsted–Lowry definition which is one of Adding a kinetic dimension to the has been exploring new time regimes that the widest theories of chemical reactivity acid–base reactivity opens a whole new reach down to the femtosecond domain and explains all proton transfer reactions. space for consideration and experimental with modern laser spectroscopy tech- Acid–base reactions have been extensive- investigation. The first obvious but often niques. Pump-probe spectroscopy (con- ly researched,[1–5] are well understood in overlooked conclusion corresponds to the ceptually similar to flash photolysis) is a terms of their equilibria and many theories reaction orders for acid dissociation and powerful tool to see molecules act ‘live’ neutralization. Dissociation reaction 1a during reactions and structural reorienta- considered from left to right, follows a tions. In such experiments, two laser pulses pseudo first order kinetics due to the large are sent to the sample. The first one initi- excess of water molecules available to up- ates the chemical process, the second one take the released proton. The same is not is used to collect transient spectra of the true for neutralization which runs in the sample, and hence its chemical structure, opposite direction. Neutralization reac- at a given time after the process has been tions involve protons and bases which nor- initiated. The most obvious application of mally exist at similar concentration levels pump probe spectroscopy involves studies *Correspondence: Prof. Dr. P. Hamm and the pseudo first order approximation of photoreactions that are directly initiated University of Zurich Institute of Physical Chemistry cannot be used. Therefore apparent time by light. However, other, more indirect Winterthurerstrasse 190 scales of neutralization will be concentra- chemical triggers broaden the range of CH-8057 Zurich tion dependent. For the text book example applications of pump probe spectroscopy Tel.: +41 44 635 44 50 of H O+ and OH– recombination this rate to very different classes of chemical reac- Fax: +41 44 635 68 13 3 11 –1 –1 E-mail: [email protected] is roughly 10 M s .[1,11–13] Effectively it tions. One example is that of temperature Laureates: awards and Honors, sCs FaLL Meeting 2011 CHIMIA 2012, 66, No. 4 183 jumps,[19,20] that may shift the chemical Scheme 1. equilibrium between several compounds, Mechanism of the so the kinetics leading to a new equilibri- photoreaction of um can be followed. Rapid proton release, oNBA in water. on the other hand, can trigger acid–base reactions[4,14,21,22] and protonation-depen- dent structural changes, such as protein folding.[23–25] Executing pH jump experi- ments,[14,15,17,26] and more generally speak- ing investigating dynamics of acid–base systems, there are always two critical as- pects that need to be understood before- hand: starting with reaction 1a, how fast are protons delivered to the system? And related to any acid–base equilibria 1b, how are these protons distributed after their re- lease? There are two principal mechanisms of combined time resolved UV pump–IR photo-excitation. It disappears with a 7 ps proton release in photoacids: excited state probe spectroscopy (TR-IR) with ab inito time constant as the reaction progresses to dissociation, when the electronic excitation molecular dynamics (AIMD) and obtained the second intermediate – nitrosobenzoic decreases the pKa value of an existing dis- a consistent picture of the reaction from acid – which can be identified by the car- sociable proton,[15,26–29] and caged protons, the reactant’s photoexcitation up to the fi- boxylic acid C=O band (Fig. 1B). The acid i.e. molecules in which an acidic proton is nal product formation.[30] The reaction is finally dissociates on a 21 ns time scale, created in a photochemical reaction.[30–32] summarized in Scheme 1. which is manifested by vanishing of the Understanding their functionality is es- The ketene intermediate can be clearly C=O band and the accompanying rise of sential to answer the first fundamental identified in the TR-IR data by a charac- the carboxylate peaks (Fig. 1C). Hence, question of how fast can they deliver free teristic band representing the =C=O group, the proton is delivered to the system with a protons? In the case of the excited state dis- presented in Fig. 1A. The band appears 21 ns time constant.[30] sociation, the actual proton release is the within a few hundred femtosconds after For the second reaction step from only step that needs to be discussed. For proton cages the problem gains complex- ity as the proton release depends on the reaction mechanism and kinetics in which the cage rearranges. Proton delivery in this case might happen from the final product or already during the reaction if an inter- mediate form is capable of acid dissocia- tion. In this paper, a model system for pH jumps will be discussed. The experiments were conducted with o-nitrobenzaldehyde (oNBA) as an optically triggered proton source[30] and acetate ions as a simple pro- ton acceptor.[14] oNBA Proton Cage Ortho-nitrobenzaldehyde is a caged proton molecule undergoing an internal redox reaction initiated by UV irradiation (Scheme 1). It produces o-nitrosobenzoic acid, a medium-strong carboxyl acid with pKa ~3. The reaction has been known for over a century[33] but its details remained controversial, in particular regarding the actual proton release time. The well-es- tablished first step of the reaction mecha- nism involves a ketene intermediate being formed as a result of a hydrogen transfer from the aldehyde group onto the nitro substitute.[34,35] Already this step results in the formation of a highly acidic -NOOH group, which was sometimes postulated to Fig. 1. Time-resolved difference IR spectra illustrating the oNBA photo reaction. A) ketene band [36] dissociate during the reaction. Further and kinetics of formation and disappearance of the first intermediate, B) the carbonyl stretch details of the reaction depend strongly on region with bands of the aldehyde and carboxyl group, kinetic trace shows the formation and dis- the solvent and are discussed controver- sociation of nitrosobenzoic acid, C) The nitro and carboxylate bands and carboxylate appearance sially in literature.[30,35,37–40] We recently kinetics.
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