22834-63-5Relevant academic research and scientific papers
Excited-state proton-coupled electron transfer within ion pairs
Hammarstr?m, Leif,Meyer, Gerald J.,Swords, Wesley B.
, p. 3460 - 3473 (2020)
The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.
Cationic Micellar Effect on the Kinetics of the Protolysis of Aromatic Carboxylic Acids studied by the Ultrasonic Absorption Method
Isoda, Teruyo,Yamasaki, Miyuki,Yano, Hiroshige,Sano, Takayuki,Harada, Shoji
, p. 869 - 874 (2007/10/02)
The protolysis of carboxylic acids has been kinetically studied by the ultrasonic absorption method in the presence of tetradecyltrimethylammonium bromide (TTAB) micelles in aqueous solution.The carboxylic acids studied were classified into two categories, one capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form the bond, namely the benzoic acid derivatives (BAD).The rate constant (γ2kf, kb), the apparent dissociation constant (Ka), and the volume change of the reaction (ΔV) were obtained.Different Ka dependences of the rate constants observed for SAD and BAD are discussed in relation to the effect of intramolecular hydrogen bond. pKa dependences were also observed for ΔV of SAD and BAD.These dependences are larger than those in aqueous solution.This result was attributed to the change of arrangement of water molecules around the solute in micellar solution and aqueous solution.
Cocatalysis by an internal carboxyl group and by α-cyclodextrin. The debromination of 4-bromo-4-methyl-2,5-cyclohexadienone-2-carboxylic acid
Takasaki, Bryan K.,Tee, Oswald S.
, p. 193 - 197 (2007/10/02)
Debromination of the title dienone (1), formed during the bromination of 5-methylsalicylic acid (2), is strongly catalyzed by α-cyclodextrin (CD).The bromination of 2 is also catalyzed by CD.The extent of the catalysis of the debromination of 1 is independent of pH in the range pH 3-5 and it tends to saturate at high and high .Analysis of the dependence data in the presence of 1 mM CD yields a value of 0.68 M for the dissociation constant of the CD*Br(-) complex.Also, from the dependence of the rate of debromination on , the anion of the dienone 1 forms a complex with CD with a Kd of 15 mM, which appears to react with Br(-) at a rate 170 times faster than does free 1(-).However, the catalysis is more likely due to the reaction of free dienone, reacting as 1, with the CD*Br(-) complex, which is 3400 times more reactive than uncomplexed Br(-).The combined effect of the internal catalysis by the carboxyl group of 1 and the external catalysis by CD is an overall rate enhancement of 12 million.Keywords: catalysis, cocatalysis, cyclodextrin, debromination, intramolecular
