Methylhydroxycarbene: Tunneling Control of a Chemical Reaction

Schreiner, Peter R.; Ley, David; Gerbig, Dennis; Wu, Chia‐Hua; Allen, Wesley D.

doi:10.1126/science.1203761

Cited by 291 publications

(441 citation statements)

References 69 publications

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“…Tunnelling control of a chemical reaction through a pathway with a higher barrier was recently reported for isomerisation of a carbene intermediate [27]. These results further highlight the role that a pre-reaction hydrogen bonded OH complex plays in low temperature kinetics, in this case the adduct is sufficiently long lived to facilitate tunnelling, the majority proceeding via the higher activation barrier to form CH 3 O.…”

Section: Discussionsupporting

confidence: 51%

Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

et al. 2013

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Understanding the abundances of molecules observed in dense interstellar clouds requires knowledge of the rates of gas phase reactions between two neutral species. However, reactions possessing an activation barrier were considered too slow to play any important role at the low temperatures in these clouds. Here we show that despite the presence of a barrier the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol, one of the most abundant organic molecules in space, is almost two orders of magnitude larger at 63K than previously measured at ~200K. We also observe formation of the methoxy radical product that was recently detected in space. These results are interpreted through the formation of a hydrogen-bonded complex that is sufficiently long-lived to undergo quantum mechanical tunnelling to form products. We postulate that this tunnelling

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Section: Discussionsupporting

confidence: 51%

Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

et al. 2013

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“…For the second criterion, the ratio A H /A D was 0.28 (A H = 97.9, A D = 344.2), much less than 0.7, which implies tunneling. This suggestion is supported by ample precedent for tunneling in 1,2-H shifts, particularly in carbenes (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27), and also in cyclopentadienes (14). The barriers in these reactions vary from ca.…”

Section: Methodsmentioning

confidence: 66%

Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

Cheng

Doubleday

Breslow

2015

Proc. Natl. Acad. Sci. U.S.A.

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Hydrogen atom transfer reactions between the aldose and ketose are key mechanistic features in formose chemistry by which formaldehyde is converted to higher sugars under credible prebiotic conditions. For one of these transformations, we have investigated whether hydrogen tunneling makes a significant contribution to the mechanism by examining the deuterium kinetic isotope effect associated with the hydrogen transfer during the isomerization of glyceraldehyde to the corresponding dihydroxyacetone. To do this, we developed a quantitative HPLC assay that allowed us to measure the apparent large intrinsic kinetic isotope effect. From the Arrhenius plot of the kinetic isotope effect, the ratio of the preexponential factors A H /A D was 0.28 and the difference in activation energies E a(D) − E a(H) was 9.1 kJ·mol −1 . All these results imply a significant quantum-mechanical tunneling component in the isomerization mechanism. This is supported by multidimensional tunneling calculations using POLYRATE with small curvature tunneling.formose reaction | quantum tunneling | hydride shift | prebiotic reactions W e have described a new mechanism for the formose reaction ( Fig. 1) (1), essentially the same as we had proposed earlier (2) except that the isomerizations of aldose to ketose and the reversal involve a hydride shift rather than an enolization (3-7). Our evidence came from the finding that 2-deuteroglyceraldehyde was converted to 1-deuterodihydroxyacetone under conditions of the formose reaction, with catalysis by Ca 2+ at pH 12. In 2001 Nagorski and Richard had reported an extensive study of the interconversion of glyceraldehyde and dihydroxyacetone by either enolization or hydride shift and had seen that with Zn 2+ the hydride shift mechanism was the exclusive process, by a mechanism closely related to our more recent one (8). We also showed that the isomerization of the ketose erythrulose to the aldose aldotetrose in D 2 O did not lead to deuterium incorporation, as it would have in an enolization process, so it also uses the hydride shift mechanism (1). The first study of the formose reaction in D 2 O was performed by Benner, who saw that no deuterium was incorporated in the intermediates; for some reason he did not invoke hydride shift mechanisms (6).It should be mentioned that we saw that the presence of formaldehyde in the formose reaction in D 2 O led to trapping of any enols formed; without formaldehyde, as in our previous study, there is subsequent deuterium incorporation in dihydroxyacetone, but not significant in glyceraldehyde. We saw that the glyceraldehyde was present almost entirely as its hydrate, a gem diol, but dihydroxyacetone was present mainly as the ketone. This reverses the normally accepted enolization relative rates. Materials and MethodsIn a hydride shift the distance traveled by the proton is small, comparable to the range of its wave character, so we have investigated the possibility that there is a quantum-mechanical tunneling process involved (9). We find that there is indeed tunnelin...

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“…Assuming these calculations to be at least qualitatively correct, it seems likely that the UV photochemistry of PA with light of λ > 300 nm proceeds according to the sequence S 0 → S 1 → T 2 → T 1 . This intersystem crossing to the triplet surface is markedly different from the gas phase photochemistry, which occurs solely on the singlet surface to generate methylhydroxycarbene (28)(29)(30)54). However, in the aqueous study presented here, PA is seen to react from its T 1 state to produce acetoin, lactic and acetic acids, and oligomers, as described below and depicted in Scheme 1.…”

Section: Discussionmentioning

confidence: 88%

“…It is also interesting to note that gas phase photolysis does not require the presence of water for decarboxylation (37), whereas solution phase decarboxylation chemistry does. This is likely due to the fact that gas phase photochemistry of PA occurs predominantly on the singlet surface, with PA directly decarboxylating in a unimolecular reaction to form a methylhydroxycarbene, followed by H-atom transfer to form acetaldehyde and vinyl alcohol (28,54). In water, however, the chemistry occurs on the triplet surface, proceeding bimolecularly through reaction of one molecule of PA in the T1 state with one molecule of PA in its gem-diol form.…”

Section: Discussionmentioning

confidence: 93%

Photochemistry of aqueous pyruvic acid

Griffith

Carpenter

Shoemaker³

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

124

289

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The study of organic chemistry in atmospheric aerosols and cloud formation is of interest in predictions of air quality and climate change. It is now known that aqueous phase chemistry is important in the formation of secondary organic aerosols. Here, the photoreactivity of pyruvic acid (PA; CH 3 COCOOH) is investigated in aqueous environments characteristic of atmospheric aerosols. PA is currently used as a proxy for α-dicarbonyls in atmospheric models and is abundant in both the gas phase and the aqueous phase (atmospheric aerosols, fog, and clouds) in the atmosphere. The photoreactivity of PA in these phases, however, is very different, thus prompting the need for a mechanistic understanding of its reactivity in different environments. Although the decarboxylation of aqueous phase PA through UV excitation has been studied for many years, its mechanism and products remain controversial. In this work, photolysis of aqueous PA is shown to produce acetoin (CH 3 CHOHCOCH 3 ), lactic acid (CH 3 CHOHCOOH), acetic acid (CH 3 COOH), and oligomers, illustrating the progression from a three-carbon molecule to four-carbon and even six-carbon molecules through direct photolysis. These products are detected using vibrational and electronic spectroscopy, NMR, and MS, and a reaction mechanism is presented accounting for all products detected. The relevance of sunlight-initiated PA chemistry in aqueous environments is then discussed in the context of processes occurring on atmospheric aerosols.

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Methylhydroxycarbene: Tunneling Control of a Chemical Reaction

Cited by 291 publications

References 69 publications

Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

Photochemistry of aqueous pyruvic acid

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