Search Results for "Schreiner tunneling"

Perspective on Tunneling Control

Over the past nine years the Schreiner group, often in collaboration with the Allen group, have produced some remarkable studies demonstrating the role of tunneling control. (I have made quite a number of posts on this topics.) Tunneling control is a third mechanism for dictating product formation, in tandem with kinetic control (the favored product is the one that results from the lowest barrier) and thermodynamic control (the favored product is the one that has the lowest energy). Tunneling control has the favored product resulting from the narrowest mass-considered barrier.

Schreiner has written a very clear perspective on tunneling control. It is framed quite interestingly by some fascinating quotes:

It is probably fair to say that many organic chemists view the concept of tunneling, even of hydrogen atoms, with some skepticism. – Carpenter 19832

Reaction processes have been considered as taking place according to the laws of classical mechanics, quantum mechanical theory being only employed in calculating interatomic forces. – Bell 19333

Schreiner’s article makes it very clear how critical it is to really think about reactions from a truly quantum mechanical perspective. He notes the predominance of potential energy diagrams that focus exclusively on the relative energies and omits any serious consideration of the reaction coordinate metrics, like barrier width. When one also considers the rise in our understanding of the role of reaction dynamics in organic chemistry (see, for example, these many posts), just how long will it take for these critical notions to penetrate into standard organic chemical thinking? As Schreiner puts it:

It should begin by including quantum phenomena in introductory textbooks, where they are, at least in organic chemistry, blatantly absent. To put this oversight in words similar to those used much earlier by Frank Weinhold in a different context: “When will chemistry textbooks begin to serve as aids, rather than barriers, to this enriched quantum-mechanical perspective?”4

References

1) Schreiner, P. R., "Tunneling Control of Chemical Reactions: The Third Reactivity Paradigm." J. Am. Chem. Soc. 2017, 139, 15276-15283, DOI: 10.1021/jacs.7b06035.

2) Carpenter, B. K., "Heavy-atom tunneling as the dominant pathway in a solution-phase reaction? Bond shift in antiaromatic annulenes." J. Am. Chem. Soc. 1983, 105, 1700-1701, DOI: 10.1021/ja00344a073.

3) Bell, R. P., "The Application of Quantum Mechanics to Chemical Kinetics." Proc. R. Soc. London, Ser. A 1933, 139 (838), 466-474, DOI: 10.1098/rspa.1933.0031.

4) Weinhold, F., "Chemistry: A new twist on molecular shape." Nature 2001, 411, 539-541, DOI: 10.1038/35079225.

Schreiner &Tunneling Steven Bachrach 13 Nov 2017 No Comments

Conformationally selective tunneling

The Schreiner group has again reported an amazing experimental and computational study demonstrating a fascinating quantum mechanical tunneling effect, this time for the trifluoromethylhydroxycarbene (CF3COH) 2.1 (I have made on a number of posts discussing a series of important studies in this field by Schreiner.) Carbene 2 is formed, in analogy to many other hydroxycarbenes, by flash vapor pyrolysis of the appropriate oxoacid 1 and capturing the products on a noble gas matrix.

Carbene 2t is observed by IR spectroscopy, and its structure is identified by comparison with the computed CCSD(T)/cc-pVTZ frequencies. When 2t is subjected to 465 nm light, the signals for 2t disappear within 30s, and two new species are observed. The first species is the cis conformer 2c, confirmed by comparison with its computed CCSD(T)/cc-pVTZ frequencies. This cis conformer remains even with continued photolysis. The other product is determined to be trifluoroacetaldehyde 3. Perhaps most interesting is that 2t will convert to 3 in the absence of light at temperatures between 3 and 30 K, with a half-life of about 144 h. There is little rate difference at these temperatures. These results are quite indicative of quantum mechanical tunneling.

To aid in confirming tunneling, they computed the potential energy surface at CCSD(T)/cc-pVTZ. The trans isomer is 0.8 kcal mol-1 lower in energy that the cis isomer, and this is much smaller than for other hydroxycarbenes they have examined. The rotational barrier TS1 between the two isomer is quite large, 26.4 kcal mol-1, precluding their interchange by classical means at matrix temperatures. The barrier for conversion of 2t to 3 (TS2) is also quite large, 30.7 kcal mol-1, and insurmountable at 10K by classical means. No transition state connecting 2c to 3 could be located. These geometries and energies are shown in Figure 1.

2c
0.8

TS1
26.4

2t
0.0

TS2
30.7

3
-49.7

Figure 1. Optimized geometries at CCSD(T)/cc-pVTZ. Relative energies (kcal mol-1) of each species are listed as well.

WKB computations at M06-2X/6-311++G(d,p) predict a half-life of 172 h, in nice agreement with experiment. The computed half-life for deuterated 2t is 106 years, and the experiment on the deuterated analogue revealed no formation of deuterated 3.

The novel component of this study is that tunneling is conformationally selective. The CF3 group stabilizes the cis form probably through some weak HF interaction, so that the cis isomer can be observed, but no tunneling is observed from this isomer. Only the trans isomer has the migrating hydrogen atom properly arranged for a short hop over to the carbon, allowing the tunneling process to take place.

References

1) Mardyukov, A.; Quanz, H.; Schreiner, P. R., "Conformer-specific hydrogen atom tunnelling in trifluoromethylhydroxycarbene." Nat. Chem. 2017, 9, 71–76, DOI: 10.1038/nchem.2609.

InChIs

1: =1S/C3HF3O3/c4-3(5,6)1(7)2(8)9/h(H,8,9)
InChIKey=GVDJEHMDNREMFA-UHFFFAOYSA-N

2: InChI=1S/C2HF3O/c3-2(4,5)1-6/h6H
InChIKey=FVJVNIREIXAWKU-UHFFFAOYSA-N

3: InChI=1S/C2HF3O/c3-2(4,5)1-6/h1H
InChIKey=JVTSHOJDBRTPHD-UHFFFAOYSA-N

carbenes &Schreiner &Tunneling Steven Bachrach 07 Feb 2017 2 Comments

Domino Tunneling

A 2013 study of oxalic acid 1 failed to uncover any tunneling between its conformations,1 despite observation of tunneling in other carboxylic acids (see this post). This was rationalized by computations which suggested rather high rearrangement barriers. Schreiner, Csaszar, and Allen have now re-examined oxalic acid using both experiments and computations and find what they call domino tunneling.2

First, they determined the structures of the three conformations of 1 along with the two transition states interconnecting them using the focal point method. These geometries and relative energies are shown in Figure 1. The barrier for the two rearrangement steps are smaller than previous computations suggest, which suggests that tunneling may be possible.

1tTt
(0.0)

TS1
(9.7)

1cTt
(-1.4)

TS2
(9.0)

1cTc
(-4.0)

Figure 1. Geometries of the conformers of 1 and the TS for rearrangement and relative energies (kcal mol-1)

Placing oxalic acid in a neon matrix at 3 K and then exposing it to IR radiation populates the excited 1tTt conformation. This state then decays to both 1cTt and 1cTc, which can only happen through a tunneling process at this very cold temperature. Kinetic analysis indicates that there are two different rates for decay from both 1tTt and 1cTc, with the two rates associated with different types of sites within the matrix.

The intrinsic reaction paths for the two rearrangements: 1tTt1cTt and → 1cTc were obtained at MP2/aug-cc-pVTZ. Numerical integration and the WKB method yield similar half-lives: about 42 h for the first reaction and 23 h for the second reaction. These match up very well with the experimental half-lives from the fast matrix sites of 43 ± 4 h and 30 ± 20 h, respectively. Thus, the two steps take place sequentially via tunneling, like dominos falling over.

References

(1) Olbert-Majkut, A.; Ahokas, J.; Pettersson, M.; Lundell, J. "Visible Light-Driven Chemistry of Oxalic Acid in Solid Argon, Probed by Raman Spectroscopy," J. Phys. Chem. A 2013, 117, 1492-1502, DOI: 10.1021/jp311749z.

(2) Schreiner, P. R.; Wagner, J. P.; Reisenauer, H. P.; Gerbig, D.; Ley, D.; Sarka, J.; Császár, A. G.; Vaughn, A.; Allen, W. D. "Domino Tunneling," J. Am. Chem. Soc. 2015, 137, 7828-7834, DOI: 10.1021/jacs.5b03322.

InChIs

1: InChI=1S/C2H2O4/c3-1(4)2(5)6/h(H,3,4)(H,5,6)
InChIKey=MUBZPKHOEPUJKR-UHFFFAOYSA-N

focal point &Schreiner &Tunneling Steven Bachrach 11 Aug 2015 1 Comment

Another example of tunneling control

The notion of tunneling control has been a topic of interest within this blog a number of times. As developed by Schreiner and Allen,1,2 tunneling control is a third means for predicting (or directing) the outcome of a reaction, alongside the more traditionally recognized kinetic and thermodynamic control. Tunneling control occurs when tunneling through a higher barrier is preferred over tunneling through a lower barrier.

Kozuch and Borden propose another example of tunneling control, this time in the rearrangement of the noradamantyl carbene 1.3 This carbene can undergo a 1,2-carbon shift, driven by strain relief to form the alkene 2. The alternative as a 1,2-hydrogen shift that produces the alkene 3.

These two reaction pathways were explored using B3LYP/6-31G(d,p) computations coupled with canonical variational theory and small curvature tunneling corrections. Structures of the reactant 1 and the two transition states leading to the two products 2 and 3 are shown in Figure 1. The activation barrier at 300 K is 5.4 kcal mol-1 leading to 2 and 8.6 kcal mol-1 leading to 3. Tunneling is expected to be much more important for the hydrogen shift than for the carbon shift, but even including tunneling, the rate to form 2 is much faster than the rate to form 3 at 300 K.

1

TS 1→2

2

TS 1→3

3

Figure 1. B3LYP/6 optimized structures of 1-3 and the transition states leading to 2 and 3.

The situation is reversed however at cryogenic temperatures (< 20 K). Tunneling is now the only route for the reactions to occur, and the rate for formation of 3 is dramatically greater than the rate of formation of 2, which is inhibited by the movement of the much heavier carbon atom. Perdeuteration of the methyl group of 1, which drastically slows the rate of tunneling in the path to 3, nonetheless still favors this pathway (forming d33) over formation of d32. Thus, at low temperatures the formation of 3 is the preferred product, a manifestation of tunneling control.

Kozuch and Borden end their paper with a hope that an experimentalist will examine this interesting case. I concur!

References

(1) Schreiner, P. R.; Reisenauer, H. P.; Ley, D.; Gerbig, D.; Wu, C.-H.; Allen, W. D. "Methylhydroxycarbene: Tunneling Control of a Chemical Reaction," Science 2011, 332, 1300-1303, DOI: 10.1126/science.1203761.

(2) Ley, D.; Gerbig, D.; Schreiner, P. R. "Tunnelling control of chemical reactions – the organic chemist’s perspective," Org. Biomol. Chem. 2012, 10, 3781-3790, DOI: 10.1039/C2OB07170C.

(3) Kozuch, S.; Zhang, X.; Hrovat, D. A.; Borden, W. T. "Calculations on Tunneling in the Reactions of Noradamantyl Carbenes," J. Am. Chem. Soc. 2013, 135, 17274-17277, DOI: 10.1021/ja409176u.

InChIs

1: InChI=1S/C11H16/c1-2-11-6-8-3-9(7-11)5-10(11)4-8/h8-10H,3-7H2,1H3
InChIKey=CXFJINASYYTBBV-UHFFFAOYSA-N

2: InChI=1S/C11H16/c1-7-10-3-8-2-9(5-10)6-11(7)4-8/h8-10H,2-6H2,1H3
InChIKey=XDANPUSLLJWVEK-UHFFFAOYSA-N

3: InChI=1S/C11H16/c1-2-11-6-8-3-9(7-11)5-10(11)4-8/h2,8-10H,1,3-7H2
InChIKey=JHEPVTWREMDEMG-UHFFFAOYSA-N

Borden &Tunneling Steven Bachrach 27 Jan 2014 No Comments

Tunneling in t-butylhydroxycarbene

Sorry I missed this paper from much earlier this year – it’s from a journal that’s not on my normal reading list. Anyways, here is another fantastic work from the Schreiner lab demonstrating the concept of tunneling control (see this post).1 They prepare the t-butylhydroxycarbene 1 at low temperature to look for evidence of formation of possible products arising from a [1,2]-hydrogen shift (2), a [1,2]-methyl shift (3) or a [1,3]-CH insertion (4).

Schreiner performed CCSD(T)/cc-pVDZ optimizations of these compounds along with the transition states for the three migrations. The optimized geometries and relative energies are shown in Figure 1. The thermodynamic product is the aldehyde 2 while the kinetic product is the cyclopropane 4, with a barrier of 23.8 kcal mol-1 some 3.5 kcal mol-1 lower than the barrier leading to 2.

1
(0.0)

TS2
(27.3)

2
(-53.5)

TS3
(31.0)

3
(-41.0)

TS4
(23.8)

4
(-28.3)

Figure 1. CCSD(T)/cc-pVDZ optimized structures of 1-4 and the transition states for the three reaction. Relative energies in kcal mol-1.

At low temperature (11 K), 1 is found to slowly convert into 2 with a half-life of 1.7 h. No other product is observed. Rates for the three reactions were also computed using the Wentzel-Kramers-Brillouin (WKB) method (which Schreiner and Allen have used in all of their previous studies). The predicted rate for the conversion of 1 into 2, which takes place at 11 K solely through a tunneling process, is 0.4h, in quite reasonable agreement with experiment. The predicted rates for the other two potential reactions at 11 K are 1031 and 1040 years.

This is clearly an example of tunneling control. The reaction occurs not across the lowest barrier, but through the narrowest barrier.

References

(1) Ley, D.; Gerbig, D.; Schreiner, P. R. "Tunneling control of chemical reactions: C-H insertion versus H-tunneling in tert-butylhydroxycarbene," Chem. Sci. 2013, 4, 677-684, DOI: 10.1039/C2SC21555A.

InChI

1: InChI=1S/C5H10O/c1-5(2,3)4-6/h6H,1-3H3
InChIKey=ZGFKBRGJTPEEOC-UHFFFAOYSA-N

2: InChI=1S/C5H10O/c1-5(2,3)4-6/h4H,1-3H3

3: InChI=1S/C5H10O/c1-4(2)5(3)6/h6H,1-3H3
InChIKey=BZAZNULYLRVMSW-UHFFFAOYSA-N

4: InChI=1S/C5H10O/c1-5(2)3-4(5)6/h4,6H,3H2,1-2H3
InChIKey=MWWQKEGWQLBJBJ-UHFFFAOYSA-N

Schreiner &Tunneling Steven Bachrach 11 Nov 2013 No Comments

Review of tunneling in organic chemistry

Schreiner has written a very nice review of the role of tunneling in organic chemistry.1 This includes tunneling in the conformations of carboxylic acids and in hydrogen abstractions. But the major emphasis is on his own group’s contributions regarding tunneling on a variety of hydroxycarbenes (see these posts: cyclopropylhydroxycarbene, methylhydroxycarbene, phenylhydroxycarbene, dihydroxycarbene, and hydroxymethylene). This led to the development of a third means for controlling reactions: not just kinetic and thermodynamic control, but tunneling control as well.

Recommended reading for anyone interested in learning how quantum mechanical tunneling can have very real-world chemical consequences.

References

(1) Ley, D.; Gerbig, D.; Schreiner, P. R. "Tunnelling control of chemical reactions – the organic chemist’s perspective," Org. Biomol. Chem., 2012, 10, 3781-3790, DOI: 10.1039/C2OB07170C.

Schreiner &Tunneling Steven Bachrach 19 Jun 2012 No Comments

Tunneling in carboxylic acid conformations

The most favorable conformation of a carboxylic acid is the Z form. In fact, the E form is rarely found. Schreiner now offers an explanation for why this is so.1

Photolysis of matrix-deposited benzoic acid revealed only the Z form (1Z). However, photolysis of deuterated benzoic acid did reveal the E form 1E, however it disappeared with a half-life of 12 minutes on argon at 11 K and 20 K. The lack of temperature dependence, and the huge isotope effect suggested that the isomerization proceeds via tunneling.

The tunneling rate was computed by generating the reaction path at CCSD(T)/cc-pVTZ with
MP2/cc-pVDZ zero point energy. This gave a half-life of 2.8 h for the deuterium species and 10-5 min for the proton species. A Hammet-like relationship could be produced for the half-lives of para-substituted benzoic acids. Interestingly, a nice correlation is found between the computed width of the tunneling barrier and the half life with σ-donating ability.

References

(1) Amiri, S.; Reisenauer, H. P.; Schreiner, P. R., "Electronic Effects on Atom Tunneling: Conformational Isomerization of Monomeric Para-Substituted Benzoic Acid Derivatives," J. Am. Chem. Soc., 2010, 132 , 15902–15904, DOI: 10.1021/ja107531y

InChIs

Benzoic acid: InChI=1/C7H6O2/c8-7(9)6-4-2-1-3-5-6/h1-5H,(H,8,9)/f/h8H
InChIKey=WPYMKLBDIGXBTP-FZOZFQFYCI

Schreiner &Tunneling Steven Bachrach 01 Feb 2011 3 Comments

Structure of dihydroxycarbene

Dihdroxycarbene was the subject of a post a few years ago relating to how this carbene does not undergo tunneling,1 while related hydroxycarbene do undergo a tunneling rearrangement.

Now we have a gas-phase microwave determination of the trans,cis isomer of dihydroxycarbene.2 The computed CCSD(T)/cc-pCVQZ structure is shown in Figure 1. What is truly remarkable here is the amazing agreement between the experimental and computed structure – as seen in Table 1.The bond distance are in agreement within 0.001 Å and the bond angles agree within 0.3°! Just further evidence of the quality one can expect from high-level computations. And computing this structure was certainly far easier than the experiments!

Figure 1. CCSD(T)/cc-pCVQZ optimized geometry of dihydroxycarbene.

Table 1. Experimental and computed (CCSD(T)/cc-pCVQZ) geometric parameters of dihydroxycarbene.a


 

Expt.

Comp.

C-O

1.335

1.336

C-O

1.309

1.309

O-Htrans

0.961

0.960

O-Hcis

0.976

0.975

O-C-O

107.30

107.25

C-O-H­­trans

106.8

106.8

C-O-H­­cis

110.7

110.4


aDistances in Å and angles in deg.

References

(1) Schreiner, P. R.; Reisenauer, H. P. "Spectroscopic Identification of Dihydroxycarbene," Angew. Chem. Int. Ed. 2008, 47, 7071-7074, DOI: 10.1002/anie.200802105.

(2) Womack, C. C.; Crabtree, K. N.; McCaslin, L.; Martinez, O.; Field, R. W.; Stanton, J. F.; McCarthy, M. C. "Gas-Phase Structure Determination of Dihydroxycarbene, One of the Smallest Stable Singlet Carbenes," Angew. Chem. Int. Ed. 2014, 53, 4089-4092, DOI: 10.1002/anie.201311082.

InChIs

Dihydroxycarbene: InChI=1S/CH2O2/c2-1-3/h2-3H
InChIKey=VZOMUUKAVRPMBY-UHFFFAOYSA-N

carbenes Steven Bachrach 09 Jun 2014 No Comments

Computational Organic Chemistry, Second Edition – what’s new in Chapters 5-9

This post continues my presentation of what’s new in the second edition of my book Computational Organic Chemistry. I present here a brief summary of the new materials in chapters 5-9. (See this previous post for what’s new in chapters 1-4.)

Every chapter has been updated, meaning that the topics from the First Edition that remain in this Second Edition (and that’s most of them) have been updated with any new relevant work that have appeared since 2007, when the First Edition was published. In addition, the following new subjects have been included.

Chapter 5. Diradicals and Carbenes

One of the major additions to the entire book appears in Chapter 5: the discovery of tunneling in a variety of carbenes. This work, pioneered by Schreiner and Allen, led to the discovery of tunneling control, a third means, in conjunction with thermodynamic control and kinetic control, for controlling product formation. This work is an exemplar of the synergy provided by experiments done in partnership with computations. The chapter also includes an interview with Prof. Peter Schreiner.

Chapter 6. Organic Reactions of Anions

The discussion on proline-catalyzed aldols includes many new computations, especially dealing with the possible intermediacy of oxazolidinones. A section on thiurea-catalyzed Claisen rearrangements, from the Jacobsen group, concludes the chapter, showing how the computational approaches to organocatalyzed reactions can be extended beyond the aldol and aldol-like reactions.

Chapter 7. Solution-Phase Organic Chemistry

A discussion of solvent effects on amino acid structure has been added. This work focusses on the use of microsolvation to model local solvent effects, particularly in cases where proper accounting of strong hydrogen bonds can be critical in assessing behaviors.

Chapter 8. Organic Reaction Dynamics

A great deal of new materials appears in this chapter. Since the publication of the first edition of the book, many new studies have been published that greatly expand the types of organic reactions that are subject to dynamic effects. Of particular note are the many new examples of reactions on bifurcating surfaces. Some studies, principally by Singleton, now provide some guidance and hints towards predicting what types of reactions might exhibit non-statistical dynamics. Two new non-statistical dynamic types are presented: the roaming reactions and the roundabout mechanism in the SN2 reaction. The chapter ends with a detailed case study of the Wolff rearrangement.

Chapter 9. Computational Approaches to Understanding Enzymes

The last chapter is entirely new, and features how the techniques of computational organic chemistry, as discussed in the previous eight chapters, can be employed toward explicating enzymatic reactions. The chapter is not an in-depth survey of all of the activities in computational enzyme action – that would require its own full-length book – but rather it’s an overview to inspire you. The chapter begins with a brief discussion of enzymatic models, including the Pauling paradigm and Goodman’s model. Then computational strategies for addressing the large molecules involved in enzymatic studies are presented including QM/MM, adiabatic mapping, and the use of some very large-scale computations as benchmarks. Next, I present two case studies: of chorismate mutase and of catechol-O-methyltransferase (COMT). The chapter ends with a presentation of the progress in de novo design of enzymes capable of catalyzing specific reactions as developed by Baker and Houk.

Second Edition Steven Bachrach 16 Apr 2014 1 Comment

Cyclopropylhydroxycarbene

As we have noted in many previous posts, Schreiner has observed tunneling in hydroxycarbenes that is either very rapid (1a-c) or not at all (1d-f).1-4 In a recent paper his group investigates whether cyclopropylhydroxycarbene 2 might have an intermediate lifetime due to the π-donating effect of the three-member ring.5

Schreiner makes this carbene in his usual manner: flash pyrolysis of the cyclopropylglyoxylic acid. Let’s now consider three possible rearrangements of carbene 2. The hydrogen can migrate (Scheme 1, path a) to give cyclopropylcarboxyaldehyde 3 similar to what was observed with the related hydroxycarbenes. Carbon can migrate (Scheme 1, path b), opening up the three-member ring to give the cyclobutenol 4. This ring could open to the diene 5 and tautomerize to the ketone 6. Lastly, a hydrogen migration from carbon (Scheme 1, path c) would lead to 7. The relative energies of these species computed at CCSD(T)//cc-pVTZ//M06-2x//6-311++G(d,p) are shown in Scheme 1.

Scheme 1. Relative energies in kcal mol-1.

The computed barriers for the initial step of each pathway is +30.4 kcal mol-1 for path a, +21.9 kcal mol-1 for path b and +35.8 kcal mol-1 for path c. Thus, one might expect to see only the reaction along path b at low temperature and mostly along b at high temperature with some small percent along path a. So what actually occurs?

After capturing the flash pyrolysis product in an Ar matrix, besides the unreacted cyclopropylglyoxylic acid, 6, 3, and 2 are observed in an approximate 8:5:1 ratio. 2 is identified on the basis of the nice agreement between the experimental and computed IR frequencies. Irradiation of 2 in the matrix leads to clean conversion to 4, also identified by comparison of the observed and computed IR frequencies. This is all consistent with the computed activation barriers. In the pyrolysis, at high T, 6 is the major product and 3 is the minor product. At very low T (11 K), irradiation of 2 produces 4 (crossing only the lowest barrier) and not continuing further along the rearrangement path to 6.

What is perhaps most exciting is that 2 disappears slowly in the dark at both 11 K and 20 K, converting at the same rate to 3. The half life is 17.7 h, much longer than for the alkyl and aryl substituted hydroxycarbenes 1a-c. This confirms the stabilization effect of the cyclopropyl group, as does its large singlet-triplet gap. The computed tunneling half-life using the WKB approach is 16.6 h, in excellent agreement with experiment. And as expected for a tunneling phenomenon, the dueterated analog has a much longer half-life, computed to be 105 years. Experimentally, 2-d persists with no conversion to 3-d observed.

As with methylhydroxycarbene, we see here an example of tunneling control vs kinetic control. At high T, the reaction crosses the lowest barrier (shown in Figure 1a), proceeding to 4 and subsequent rearrangement products. At low T, the reaction crosses a higher barrier (shown in Figure 1b), but this path involves tunneling of the very light hydrogen atom only, producing 3.

TS 2 → 3

TS 2 → 4

Figure 1. M06-2X/6-311++G(d,p) optimized geometry of the transition states connecting 2 to (a) 3 and (b) 4.

References

(1) Schreiner, P. R.; Reisenauer, H. P.; Pickard Iv, F. C.; Simmonett, A. C.; Allen, W. D.; Matyus, E.; Csaszar, A. G., "Capture of hydroxymethylene and its fast disappearance through tunnelling," Nature, 2008, 453, 906-909, DOI: 10.1038/nature07010.

(2) Schreiner, P. R.; Reisenauer, H. P., "Spectroscopic Identification of Dihydroxycarbene," Angew. Chem. Int. Ed., 2008, 47, 7071-7074, DOI: 10.1002/anie.200802105

(3) Gerbig, D.; Reisenauer, H. P.; Wu, C.-H.; Ley, D.; Allen, W. D.; Schreiner, P. R., "Phenylhydroxycarbene," J. Am. Chem. Soc., 2010, 132, 7273-7275, DOI: 10.1021/ja9107885

(4) Schreiner, P. R.; Reisenauer, H. P.; Ley, D.; Gerbig, D.; Wu, C.-H.; Allen, W. D., "Methylhydroxycarbene: Tunneling Control of a Chemical Reaction," Science, 2011, 332, 1300-1303, DOI: 10.1126/science.1203761.

(5) Ley, D.; Gerbig, D.; Wagner, J. P.; Reisenauer, H. P.; Schreiner, P. R., "Cyclopropylhydroxycarbene," J. Am. Chem. Soc., 2011, 133, 13614-13621, DOI: 10.1021/ja204507j

Schreiner &Tunneling Steven Bachrach 31 Aug 2011 4 Comments

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