Search Results for "Schreiner tunneling"

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

Topics for a new edition of Computational Organic Chemistry

I am very much contemplating a second edition of my book Computational Organic Chemistry, which is the basis of this blog. I have been in touch with Wiley and they are enthusiastic about a second edition.

Here is a list of some of the things I am contemplating as new topics for the second edition

  1. Discussion of the failures of many of the standard functionals (like B3LYP) to treat simple organics
  2. Predicting NMR, IR and ORD spectra
  3. Möbius compounds, especially aromatics
  4. π-π-stacking
  5. tunneling in carbenes (Schreiner and Allen’s great work)
  6. acidity of amino acids and remote protons
  7. bifurcating potential energy surfaces and the resultant need for dynamic considerations
  8. even more examples of dynamics – especially the roundabout SN2

So, I would like to ask my readers for suggestions of other ideas for new topics to add to the book. These can be extensions of the topics already covered, or brand new areas!

Additionally, I am planning on interviewing a few more people for the book, similar in spirit to the 6 interviews in the first addition. Again, I welcome any suggestions for computational chemists to interview!

Uncategorized Steven Bachrach 09 Aug 2011 6 Comments

Methylhydroxycarbene and tunelling control

Another remarkable piece of science from the Schreiner and Allen groups has appeared demonstrating the critical importance of combining experiment with computations.1 (This one will surely be in the running for computational chemistry paper of the year.) Once again they examine tunneling from a carbene intermediate, but this time with an amazing conclusion that will have impact on chemistry textbooks!

Schreiner and Allen have previously examined a number of hydroxycarbenes (see these posts: A, B, C) and have found tunneling to be the main exit channel from these carbenes. The tunneling passes through barriers that are as large as 30 kcal mol-1, and as expected, the deuterium labeled analogues have tunneling half lives that are exceptionally long, like 4000 years.

Now they examine methylhydroxycarbene 1,1 which is interesting because there are two possible exit channels, leading to acetaldehyde 2 or vinyl alcohol 3. Previous gas-phase pyrolysis of pyruvic acid suggested the intermediacy of 1, which rearranges to 2 much more rapidly than to 3. However, G1 computations predict the barrier to 3 is smaller than the barrier to 2,2 which should mean that 2 is the kinetic product!

Methylhydroxycarbene 1 was prepared by flash pyrolysis of pyruvic acid with capture of the products in an argon matrix. The carbene 1 was characterized by IR. The predicted frequencies (CCSD(T)/cc-pCVTZ – with corrections for anharmonicity) of 9 of the 11 bands of 1 are within 8 cm-1 of the experimental frequencies. The OH and OD stretches, the ones not in agreement, are likely to be perturbed by the matrix. The predicted (MRCC/aug-cc-pVTZ) and experimental UV spectrum are also in close agreement.

Holding the matrix at 11 K and following the spectra of 1-3 led to the following important kinetic results: the half-life for formation of 2 is 66 min with no 3 observed to form. In addition, the rate for the deuterium labeled carbene to form 2 was too long for measuring, but was 196 minutes in Kr and 251 minutes in Xe. CCSD(T)/cc-pCVCZ computations followed by focal point methods gives the barrier to form acetaldehyde from 1 as 28.0 kcal mol-1 while that to form vinyl alcohol 3 is much lower: 22.6 kcal mol-1. (The structures of these three molecules and the transition states connecting them are shown in Figure 1.) Apparently, the reaction passes through or over the higher barrier in large preference over passing through or over the lower barrier!

1

TS12

2

TS13

3

Figure 1. CCSD(T)/cc-pCVTZ optimizes structures of 1-3 and the transition states connecting 1 to 2 and 1 to 3.

Precise mapping of the intrinsic reaction path at CCSD(T)/cc-pCVTZ allows for computing the WKB tunneling probabilities. This leads to the prediction of the half-life for the reaction 12 as 71 minutes, in excellent agreement with experiment. The computed half-life for the deuterium labeled reaction of 400 years and the computed half-life for 13 of 190 days are both in fine agreement with experiment.

Why does the reaction preferentially tunnel through the higher barrier? Well, the tunneling rate is dependent on the square root of the barrier height and linearly on the barrier width. The width is much smaller for the rearrangement to 2. The hydrogen needs to move a shorter amount in proceeding from 1to 2 than to 3, and in the rearrangement to vinyl alcohol a second hydrogen must migrate downwards to form the planar vinyl group. Basically, width beats out the height.

The important conclusion from this paper is the following: in addition to reactions being under kinetic or thermodynamic control, we must now consider a third options – a reaction under tunneling control!

A nice perspective on this paper and its implications has been written by Carpenter, who points out how this adds to our general notion of significant limitations to transition state theory.3

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) Smith, B. J.; Nguyen Minh, T.; Bouma, W. J.; Radom, L., "Unimolecular rearrangements connecting hydroxyethylidene (CH3-C-OH), acetaldehyde (CH3-CH:O), and vinyl alcohol (CH2:CH-OH)," J. Am. Chem. Soc., 1991, 113, 6452-6458, DOI: 10.1021/ja00017a015

(3) Carpenter, B. K., “Taking the High Road and Getting There Before You,” Science, 2011, 332, 1269-1270, DOI: 10.1126/science.1206693.

InChIs

1: InChI=1/C2H4O/c1-2-3/h3H,1H3
InChIKey=JVKQHDUTAFISFX-UHFFFAOYAN

2: InChI=1/C2H4O/c1-2-3/h2H,1H3
InChIKey=IKHGUXGNUITLKF-UHFFFAOYAB

3: InChI=1/C2H4O/c1-2-3/h2-3H,1H2
InChIKey=IMROMDMJAWUWLK-UHFFFAOYAT

focal point &Schreiner &Tunneling Steven Bachrach 14 Jun 2011 3 Comments

Phenylhydroxycarbene

Somehow I missed this paper when it came out a few months ago, even though I was aware it was coming – as I mentioned it in one of my previous posts!

Anyways, Schreiner and Allen reported on their third study of hydroxyl carbenes (see these posts on dihydroxymethylene and hydroxymethylene), this time examining phenylhydroxycarbene.1 As I covered in my book, there is a lot of work on phenylcarbenes which typically ring expand to the cycloheptatetraene, see Reaction 1. One might expect phenylhydroxycarbene to do the same thing, i.e. 1 converting into 2 (Reaction 2). 1 is prepared by high-vacuum flash pyrolysis of phenylglyoxylic acid 3 and then capturing the product in an argon matrix at 11 K (Reaction 3).

Reaction 1

Reaction 2

Reaction 3

The carbene 1 is identified through comparison of its experimental and computed (anharmonic frequencies at CCSD(T)/cc-pVDZ) IR frequencies.

No ring expansion is observed at all – Reaction 2 does not occur. Instead, 1 rearranges to benzaldehyde 4 (Reaction 4) at 11 K with a half life of 2.46 h (and a half life of 2.55 h at 20 K). The deuterated analogue does not convert to benzaldehyde and 1-d appears to be completely stable.

Reaction 4

So, what is going on? The cis and trans forms of 1 interconvert through a barrier of 22.7 kcal mol-1. The trans isomer can convert to benzaldehyde (the reaction is very exothermic: -50.8 kcal mol-1) with a barrier of 28.8 kcal mol-1 through TS1, shown in Figure 1. The cis isomer can cleave into benzene and CO (not observed) with a huge barrier of 55 kcal mol-1. All of these barrier were computed at CCSD(T)/cc-pVQZ.

TS1

Figure 1. MP2/cc-pVDZ optimized transition state for the conversion of 1 into 4.

Benzaldehyde seems to be produced by passing through a huge barrier, something that is impossible from a thermal perspective (we’re at 11 K!). But this can be accomplished by tunneling. Tunneling probabilities were computed from the MP2/aug-cc-pVDZ intrinsic reaction path with barrier penetration integrals computed with the WKB approximation. The bottom line: the computed half-life is 3.3 h and the deuterated species is computed to have a half-life of 8700 years(!), both in excellent agreement with experimental observation. Quantum mechanical tunneling is clearly the explanation for this chemistry.

This is another fine example of the power of joint experimental/computational studies. And be on the look-out for an even more exciting case from this group. I met with Wes Allen on my recent trip to the University of Georgia and was entertained with another hydroxycarbene that undergoes quite novel tunneling!

References

(1) 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

InChIs

1: InChI=1/C7H6O/c8-6-7-4-2-1-3-5-7/h1-5,8H
InChIKey=QVZIGMRPQWIGCV-UHFFFAOYAE

2: InChI=1/C7H6O/c8-7-5-3-1-2-4-6-7/h1-6H
InChIKey=QVWDCTQRORVHHT-UHFFFAOYAM

3: InChI=1/C8H6O3/c9-7(8(10)11)6-4-2-1-3-5-6/h1-5H,(H,10,11)/f/h10H
InChIKey=FAQJJMHZNSSFSM-KZFATGLACS

4: InChI=1/C7H6O/c8-6-7-4-2-1-3-5-7/h1-6H
InChIKey=HUMNYLRZRPPJDN-UHFFFAOYAE

carbenes &Schreiner &Tunneling Steven Bachrach 21 Sep 2010 3 Comments

From ACS San Francisco

Not particularly strong programming at the year’s spring ACS meeting – but one great session in the organic division yesterday. This was the awards session in honor of John Baldwin getting the James Flack Norris Award for physical organic chemistry.

First to speak was James Duncan, who discussed his recent CASSCF computations looking for pseudopericylic [3,3]-sigmatropic migrations. I will be commenting on his latest work in a post that will appear soon.

I had to skip the next talk, but came back to hear John Brauman discuss recent work on the solvation effect in the SN2 reaction. This is an interesting case of where the screening of larger substituents is counterbalanced by geometric changes that lead to greater charge distribution. The net effect is that they cancel each other out, and the methyl,ethyl, iso-propyl, butyl β-effect is negligible.

Next was Peter Schreiner who discussed his carbene work, specifically the enormous tunneling effect observed in hydroxymethylene (see this post). He discussed some new work, that is if anything even more fantastic on methylhydroxycarbene – look for this work perhaps later in 2011.

Last to speak was John Baldwin – and he described his truly tour de force efforts in examining the [1,3]-rearrangements of vinylcyclopropane and vinylcyclobutane. The former work is described in my book, while the later study is still ongoing.

John’s work is amazingly painstaking and careful. I am truly in awe of his dedication in taking on extremely difficult studies that require enormous care. John has really taught us a lot – not just about these rearrangements (they involve diradicals on a flat plateau demanding dynamic analysis – but how to think about a study and then carry it out to fruition so that all details are assessed. A truly deserving recipient!

pseudopericyclic &Schreiner &Tunneling Steven Bachrach 23 Mar 2010 1 Comment

Dihydroxycarbene

Following on the great study of hydroxycarbene1 (see my blog post), Schreiner now reports on the synthesis and characterization of dihydroxycarbene 1.2 It is prepared by high-vacuum flash pyrolysis of oxalic acid (Scheme 1).

Scheme 1

Dihydroxycarbene can exist in three different conformations characterized by the relationship about the C-O bond, either s-cis or s-trans. The three conformations are shown in Figure 1, and the s-trans,s-trans structure is the local energy minimum (computed at CCSD(T)/cc-pVTZ).

1tt (0.0)

1ct (0.1)

1cc (6.7)

Figure 1. CCSD(T)/cc-pVTZ optimized geometries and relative energies (kcal mol-1) of the conformers of 1.2

Identification of the 1 is made through comparison of the experimental and computed IR vibrational frequencies. As an example, the experimental and computed frequencies for the s-trans,s-trans conformer are listed in Table 1. The agreement is excellent.

Table 1. Computed and experimental vibrational frequencies (cm-1) and intensities (in parentheses) of the s-trans,s-trans conformation of 1.2


vibration

computed

experiment

1

3876.4 (23.5)

3633.2 / 3628.6 (w)

2

3871.4 (234.1)

3625.1 (s)

3

1443.1 (124.4)

1386.2 (m)

4

1370.5 (58.3)

1289.0 / 1287.4 (w)

5

1157.8 (470.6)

1110.3 / 1109.3 (vs)

6

1156.6 (1.4)

 

7

742.4 (178.8)

706.6 (s)

8

672.4 (0.0)

 

9

641.6 (11.2)

 


Unlike hydroxycarbene, dihydroxycarbene is stable. The amazing instability of hydroxycarbene is due to tunneling through a large barrier: nearly 30 kcal mol-1.1 The tunneling route for the decomposition of 1 is more difficult for two reasons. First, its C-O bond is quite strong; the C-O distance is quite short, 1.325 Å. This makes a long distance that must be traversed in the tunneling mode. (The strong bond is due to π-donation from the oxygen lone pair into the empty carbon p orbital; this is noted by the large rotational barrier about the C-O bonds of 17 kcal mol-1!) Second, the activation barrier for decomposition is very high, at least 34 kcal mol-1.

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 Dihydroxycarbene13," Angew. Chem. Int. Ed., 2008, 47, 7071-7074, DOI: 10.1002/anie.200802105

InChIs

1: InChI=1/CH2O2/c2-1-3/h2-3H

carbenes &Schreiner &Tunneling Steven Bachrach 06 Oct 2008 2 Comments

An update on Hydroxymethylene

A nice summary of the tunneling behavior of hydroxymethylene1 was just published by Bucher in Angewandte Chemie.2 Bucher strongly points out that the really novel part of this work is the very large barrier through which the proton tunnels. My blog post on this topic is here.

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) Bucher, G.; “Hydroxycarbene: Watching a Molecular Mole at Work,” Angew. Chem. Int. Ed., 2008, 47, 6957 – 6958, DOI: 10.1002/anie.200803195

carbenes &Schreiner &Tunneling Steven Bachrach 28 Aug 2008 No Comments

Hydroxymethylene tunnels through a large barrier

The very simple carbene hydroxymethylene, HOCH, has finally been prepared and characterized.1 Glyoxylic acid CHOCO2H is subjected to high-vacuum laser photolysis. It fragments into HOCH, which is then trapped into an argon matrix. The experimental IR frequencies match up very well with the CCSD(T)/cc-pVQZ harmonic frequencies of the trans isomer 1t that are also adjusted for anharmonic effects. The computed vertical excitation energy of 415 nm matches well with the experimental value of the maximum absorption in the UV/vis spectra of 427 nm.

The other very interesting experimental result is that HOCH has a lifetime of about 2 hours in the matrix, while the deuterated species DOCH is stable. To explain these results, Schreiner, Allen and co-workers optimized a number of structures on the PES at CCSD(T)/cc-pVQZ and computed their energies using the focal point technique. The optimized structures and their relative energies are given in Figure 1.

1t (0.0)

TS2 (29.7)

2 (-52.1)

TS1(26.8)

 

 

1c (4.4)

 

 

Figure 1. Optimized CCSD(T)/cc-pVQZ structures of HOCH isomers and their Focal Point relative energies (kcal mol-1).1

The barriers for rearrangement from 1t are both very high. Rearrangement to formaldehyde 2 requires crossing a barrier of 29.7 kcal mol-1, while the barrier to convert to the cis isomer 1c is 26.8 kcal mol-1. (Note that from 1c a cleavage into CO and H2 can occur, but this barrier is another 47.0 kcal mol-1.) These barriers are too large to be crossed at the very low temperatures of the matrices. However, using the intrinsic reaction potential at CCSD(T)/cc-pVQZ and WKB theory, the tunneling lifetime of HOCH is computed to be 122 minutes, in excellent accord with the experiment. The lifetime for DOCH is computed to be over 1200 years. Thus, the degradation of hydroxymethylene is entirely due to tunneling through a very large classical barrier! This rapid tunneling casts serious doubt on the ability to ever identify any hydroxymethylene in interstellar space.

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.

InChI

1: InChI=1/CH2O/c1-2/h1-2H
2: InChI=1/CH2O/c1-2/h1H2

carbenes &focal point &Schreiner &Tunneling Steven Bachrach 19 Aug 2008 4 Comments

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