“…KIEs of this magnitude are larger than the O-H/O-D classical limit of 7.9, pointing to the importance of quantum effects. 424 Large KIEs are known for other reactions involving H or D transfer, including (1) enzymatic C-H oxidations, 404,[425][426][427][428][429] which have been treated by a theoretical description of isotope effects originally developed by Bigeleisen and Mayer; 430,431 (2) values of up to 50 for proton transfer and up to 250 for H-atom abstraction by free radicals in the pre-1980s literature; 424 (3) a value >6 × 10 3 for alpha H-atom transfer from ethanol by methyl radicals in ethanol glasses at 77 K; 432 and (4) a value of >10 6 for intramolecular HAT in the triplet excited states of an aromatic ketone at 20 K. [433][434][435] In addition to k EPT , interpretation of KIEs for EPT must account separately for the equilibrium isotope effect in forming a H-bonded precursor complex, eq 105. Analogous to electron transfer in eq 57, the experimental rate constant, k obs , is related to K A and k EPT by k obs ) K A k EPT .…”
An initial review (PCET1) on proton-coupled electron transfer (PCET) by Huynh and Meyer appeared in Chemical Reviews in 2007. 1 This is a perennial review, a follow up on the original. It was intended for the special Chemical Reviews edition on Proton Coupled Electron Transfer that appeared in December, 2010 (Volume 110, Issue 12 Pages 6937-710). The reader is referred to it with articles on electrochemical approaches to studying PCET by Costentin and coworkers, 2 theory of electron proton transfer reactions by Hammes-Schiffer and coworkers, 3 proton-coupled electron flow in proteins and enzymes by Gray and coworkers, 4 and the thermochemistry of proton-coupled electron transfer by Mayer and coworkers. 5 Coverage for the current review is intended to be broad, covering all aspects of the topic comprehensively with literature coverage overlapping with the later references in PCET1 until late 2010. There is a growing understanding of the importance of PCET in chemistry and biology and its implications for catalysis and energy conversion. This has led to a series of informative reviews that have appeared since 2007. They include: "The possible role of Proton-coupled electron Transfer (PCET) in Water oxidation by Photosystem II" by Meyer and coworkers in 2007, 6 "Theoretical studies of proton-coupled electron transfer: Models and concepts relevant to bioenergetics" by Hammes-Schiffer and coworkers in 2008, 7 "Electrochemical Approach to the Mechanistic Study of Proton-Coupled Electron Transfer" by Costentin in 2008, 8 "Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems" by Nocera and Reece in 2009, 9 and "Integrating Proton-Coupled Electron Transfer and Excited States" by Meyer and coworkers in 2010. 10
“…KIEs of this magnitude are larger than the O-H/O-D classical limit of 7.9, pointing to the importance of quantum effects. 424 Large KIEs are known for other reactions involving H or D transfer, including (1) enzymatic C-H oxidations, 404,[425][426][427][428][429] which have been treated by a theoretical description of isotope effects originally developed by Bigeleisen and Mayer; 430,431 (2) values of up to 50 for proton transfer and up to 250 for H-atom abstraction by free radicals in the pre-1980s literature; 424 (3) a value >6 × 10 3 for alpha H-atom transfer from ethanol by methyl radicals in ethanol glasses at 77 K; 432 and (4) a value of >10 6 for intramolecular HAT in the triplet excited states of an aromatic ketone at 20 K. [433][434][435] In addition to k EPT , interpretation of KIEs for EPT must account separately for the equilibrium isotope effect in forming a H-bonded precursor complex, eq 105. Analogous to electron transfer in eq 57, the experimental rate constant, k obs , is related to K A and k EPT by k obs ) K A k EPT .…”
An initial review (PCET1) on proton-coupled electron transfer (PCET) by Huynh and Meyer appeared in Chemical Reviews in 2007. 1 This is a perennial review, a follow up on the original. It was intended for the special Chemical Reviews edition on Proton Coupled Electron Transfer that appeared in December, 2010 (Volume 110, Issue 12 Pages 6937-710). The reader is referred to it with articles on electrochemical approaches to studying PCET by Costentin and coworkers, 2 theory of electron proton transfer reactions by Hammes-Schiffer and coworkers, 3 proton-coupled electron flow in proteins and enzymes by Gray and coworkers, 4 and the thermochemistry of proton-coupled electron transfer by Mayer and coworkers. 5 Coverage for the current review is intended to be broad, covering all aspects of the topic comprehensively with literature coverage overlapping with the later references in PCET1 until late 2010. There is a growing understanding of the importance of PCET in chemistry and biology and its implications for catalysis and energy conversion. This has led to a series of informative reviews that have appeared since 2007. They include: "The possible role of Proton-coupled electron Transfer (PCET) in Water oxidation by Photosystem II" by Meyer and coworkers in 2007, 6 "Theoretical studies of proton-coupled electron transfer: Models and concepts relevant to bioenergetics" by Hammes-Schiffer and coworkers in 2008, 7 "Electrochemical Approach to the Mechanistic Study of Proton-Coupled Electron Transfer" by Costentin in 2008, 8 "Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems" by Nocera and Reece in 2009, 9 and "Integrating Proton-Coupled Electron Transfer and Excited States" by Meyer and coworkers in 2010. 10
“…High yields of the triplet ketone 3 K are formed by a rapid ISC after the initial excitation of the aryl ketone chromophore. − An adiabatic hydrogen transfer from the o -methyl group to the carbonyl oxygen occurs on the triplet surface to give a triplet enol 3 E , followed by an ISC decay to the ground-state singlet enol 1 E , which finally reverts to the starting singlet ketone 1 K through a hydrogen-transfer reaction to complete an overall cyclic process. Several experimental studies with conformationally restricted compounds have concluded that both hydrogen-transfer reactions occur by quantum mechanical tunneling at ultralow reaction temperatures − (e.g., 4−100 K). 1 …”
Section: Introductionmentioning
confidence: 99%
“…[9][10][11][12] An adiabatic hydrogen transfer from the o-methyl group to the carbonyl oxygen occurs on the triplet surface to give a triplet enol 3 E, followed by an ISC decay to the ground-state singlet enol 1 E, which finally reverts to the starting singlet ketone 1 K through a hydrogen-transfer reaction to complete an overall cyclic process. Several experimental studies with conformationally restricted compounds have concluded that both hydrogentransfer reactions occur by quantum mechanical tunneling at ultralow reaction temperatures [13][14][15][16] 17 They have determined the hydrogen/ deuterium-transfer rates of the triplet ketone 3 K by emission spectroscopy at very low temperatures. These reaction rates were calculated from the total triplet decay rate by subtracting contributions from radiative (phosphorescence) and thermal processes, which were assumed from model compounds lacking the o-methyl group (for instance, anthrone or 2,3-dimethylanthrone).…”
The photoenolization of 1,4-dimethylanthrone (1,4-MAT) and 1,4-dimethylanthrone-d
8 (1,4-DMAT) in the
gas phase and in 2,2,2-trifluoroethanol (TFE) has been studied theoretically in this work. An electronic energy
profile with minima and saddle-point structures is determined by density functional theory methods in the
ground state (S0) and in the triplet state (T1). This study reveals that in the excited state an inversion of
stability of the tautomers and a lower energy barrier to overcome makes the proton transfer feasible. Our
molecular orbital analysis shows that upon proton transfer, T1 changes from n,π* to π,π* so that a diabatic
crossing takes place along the reaction coordinate. This crossing is not influenced by the presence of TFE.
Also, upon the photoexcitation to the first excited singlet state (S1) an intersystem crossing must occur to
access T1. Pure electronic calculations cannot tell us the exact point of the triplet potential energy surface
that will be accessed. For this reason we have performed a microcanonical dynamic calculation taking into
account the tunneling effect of the hydrogen and deuterium transfer rate constants (k
H and k
D, respectively)
on T1. After the rate-constant calculation we have also calculated the kinetic isotope effect (KIE) to compare
with the experimental value. Our results indicate that the predicted large KIE for this reaction can be explained
if the proton transfer takes place through tunneling at an energy slightly below the barrier.
“…Work from our group has focused on the excited-state H-atom transfer of o -methyl-substituted benzocycloalkanones such as anthrone and tetralone. In contrast to other studies where reaction rates as a function of temperature were determined by analysis of the spectra of overlapping laser-flash-photolysis transients, , we determined the rates of triplet reaction ( k H ) by emission spectroscopy at very low temperatures. − , We calculated reaction rates ( k H , eq 1) from the total triplet decay rate ( k dec ) by subtracting contributions from radiative ( k P ) and thermal processes ( k TS ), which were assumed from model compounds lacking the o -methyl group . It was shown that deuterium transfer ( k D ) in d 8 -1,4-dimethylanthrone (1,4-DMAT) and in d 8 -5,9-dimethyl-1-tetralone (DMT) at very low temperatures (ca.…”
The reactivity of 1,4-dimethylanthrone (1,4-MAT) and 1,4-dimethylanthrone-d
8 (1,4-DMAT) toward hydrogen
transfer was investigated in methylcyclohexane (MCH), ethanol (EtOH), and 2,2,2-trifluoroethanol (TFE) by
comparing their triplet-state emission spectra and lifetimes with those of compounds without the o-methyl
substituent. A predominantly n,π* lowest triplet state in nonpolar solvents such as MCH changes toward a
less reactive π,π* configuration in polar solvents such as EtOH and TFE. In EtOH, 1,4-DMAT exhibits a
temperature-dependent dual emission while the corresponding protio compound (1,4-MAT) decays by reaction
at all temperatures analyzed (ca. 15−90 K). Although compounds without the o-methyl group also presented
dual emission, their relative intensities and lifetimes were shown to be temperature-independent, indicating
the lack of thermal equilibration between the two triplets below 90 K. In the case of 1,4-DMAT in EtOH,
an Arrhenius plot based on the data from the short-lived component (mainly n,π*) and detection of the enol
between 19 and 80 K revealed that deuterium transfer occurs by quantum-mechanical tunneling (QMT). In
agreement with results in MCH where a short-lived n,π* state predominates, the D-tunneling rate constant
was estimated to be ca. (1.5−7.5) × 103 s-1. The phosphorescence of anthrone derivatives in TFE was
long-lived, with a strong π,π* character, and highly heterogeneous, but not dual. A large isotope effect on
phosphorescence is assigned to the isotope-dependent tunneling reaction from a predominant π,π* state.
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