Photopotential decay delay on TiO 2 surface modified with p ‐benzaldehydes: consequences and applications

J of Physical Organic Chem

2017

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When N‐benzyl‐N′‐methylacetamidinium hydrochloride (pKa=11.8) is dissolved in D2O/DCl(1 M), an equilibrium of 2 54:46 stereoisomers in an ~2:1 =(R)Nδ+H(D) D/H ratio is formed. Therefore, 2 R =N‐benzyl (E and Z) and 2 R =N‐methyl (E and Z) groups attached to the corresponding H(D) (Z and E) for a total of 8 1H‐NMR signals are observed. Consequently, their rates of H and D transfer to D2O can be measured by means of the 1H‐NMR broadness (line shape) of the =(R)Nδ+H doublets and =(R)Nδ+D broad singlets. Acidity selectivity is observed for both processes. In fact, the relative proton and deuterium transfer rates follow the acidity order: =(PhCH2)Nδ+‐H(E) > =(PhCH2)Nδ+‐H(Z) > =(Me)Nδ+‐H(E) > =(Me)Nδ+‐H(Z). Proton transfer rates are in the range of 8 to 0.5 s‐1 with α = .92. This tendency is independently supported by the observed experimental chemical shift deuterium isotopic perturbation. The rate‐limiting step for proton exchange is the breaking of the hydrogen bond due to the fast amidine reprotonation (~1011 s). =(R)Nδ+D/=(R)Nδ+H equilibration is reached at ~80 s, and it can be measured by the relative =(R)Nδ+H versus =(R)Nδ+D signal integrations. The equilibrium of the 4 =(R)Nδ+H(D) centers is shifted toward deuterium, but they are further shifted in the more basic centers. Equilibrium is completely shifted toward D in the 4 centers when OD− contributes with the exchange process at pD > 3.

Section: Resultsmentioning

confidence: 99%

D₂O‐catalyzed proton transfer selectivity of 4 different NH protons in the same molecule

J of Physical Organic Chem

2017

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“…A linearization of Langmuir isotherm for 4SCA adsorption on TiO 2 was built: 1 / X = 1 / X max + 1 / ( X max K L [4SCA]). From the plot, the maximum adsorption of 4SCA, X max (34.84 mg 4SCA/g TiO 2) , and the adsorption equilibrium constant, K L (4.95 L/mg), were obtained.…”

Section: Resultsmentioning

confidence: 99%

“…The IR spectrum of the 4SCA‐TiO 2 has been previously reported by our group . The most remarkable observation in this spectrum is the disappearance of the aldehyde carbonyl group around 1700 cm − 1 .…”

Section: Resultsmentioning

confidence: 99%

“…The bathochromic movement of the E g value 3.21 eV (4SCA‐TiO 2 ) versus 3.32 eV (TiO 2 ), the 4SCA‐TiO 2 disappearance of the IR carbonyl group signal, the appearance of Ti‐O‐C signals, and the theoretical calculations of 4SCA interaction with a 10‐membered TiO 2 cluster performed in our group support the acetal formation in a geminal Ti as shown in Figure . Discussion on this matter has been previously reported …”

Section: Discussionmentioning

confidence: 99%

“…The trapping, as shown in Figure , is effective because of the stabilization via delocalization on the aromatic ring of the radical anion formed. In fact, we have shown that this radical anion has at pH 8, approximately 0.2 V stabilization as compared with the TiO 2 conduction band; therefore, electron transfer to the benzaldehyde trap occurs spontaneously and it is as fast as the electron‐hole recombination. The Naph oxidation rate‐limiting step becomes the 4SCA‐TiO 2 radical anion electron transfer to solution.…”

Section: Discussionmentioning

confidence: 99%

See 2 more Smart Citations

Minimizing electron-hole recombination in modified TiO₂photocatalysis: electron transfer to solution as rate-limiting step in organic compounds degradation

Rivas

Vargas

2016

J. Phys. Org. Chem.

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4‐Stilbenecarboxaldehyde (4SCA) at pH 3 was added to TiO2 anatase to form a new catalyst where the aldehyde carbonyl group reacts with the TiO2‐OH to form the corresponding acetal (4SCA‐TiO2). 4SCA‐TiO2 significantly retards the electron recombination when it is illuminated with ultraviolet B light because of the formation of a stable radical anion·−4SCA‐TiO2 that we have detected spectroelectrochemically. The light excited electron on the catalysis is transferred relatively slow to solution. Therefore, the electron transfer to solution is the rate‐limiting step for water‐dissolved organic compound degradation when 4SCA‐TiO2 is used as photocatalyst. For instance, degradation rate constants using naphthalene (Naph) and p‐nitrophenol (PNP) in an ample pH range support the proposal. Accordingly, rate constants are faster when the standard redox potential of the involved electron acceptor in the solution increases. In fact, this condition can be tuned to promote reactivity. The affinity between the organics being degraded and 4SCA‐TiO2 also influences on the degradation rate constants.

Unprecedented large solvent (H₂O vs D₂O) isotope effect in semiconductors photooxidation

J of Physical Organic Chem

Madriz

Carvajal

et al. 2019

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Glucose in water (L 2 O, L = H or D) at pH = 7 (phosphate buffer) is oxidized in presence of Bi 2 WO 6 and light. An unusually large solvent isotope effects, k H 2 O =k D 2 O ¼ 7:8 and 6.8, have been measured using solar light and solar simulator, respectively. These large values come from the contribution of the equilibrium L 2 O ⇄ L + + OL − previous to the rate-limiting step (rls) and the kinetic one (proton transfer) involved at the transition state. The reaction is faster when [L + ] increases; therefore, L + species instead of OL − one participates in the photooxidation. The rls is the 1 e − reduction-adsorption of H + on the Bi 2 WO 6 surface (Bi 2 WO 6 /Bi 2 WO 6 -H, −0.6 V vs NHE). Subsequently, O 2 reduction to L 2 O, as driving force, occurs at the catalyst conduction band (CB). Linear sweep voltammetry when Bi 2 WO 6 is used as cathode shows two reduction processes: H + /H 2 and O 2 /O 2 .− . The last one-electron reduction occurs at −0.2 V vs NHE and the first one at −0.4 V vs NHE. Both show solvent isotope effect, although the i 0H 2 O =i 0D 2 O are 2.4 and 1.9, respectively, quite smaller than the value obtained in the photooxidation due to the influence of the bias on the transition state symmetry in the first case and through water O 2 reduction in the second. Cyclic voltammetry indicates that the reduced and adsorbed H (Bi 2 WO 6 -H 2 ) is oxidized to Bi 2 WO 6 -H at approximately 0.2 V vs NHE. Positive conduction band potential of Bi 2 WO 6 (+0.5 V vs NHE) establishes the difference with other semiconductors where oxidation (L 2 O/L + ,OL . )instead of reduction has been proposed as rls in photooxidation.