Search Results for "Schreiner tunneling"

Aminomethylene carbene does not rearrange by tunneling

Eckhardt and Schreiner have spectroscopically characterized the aminomethylene carbene 1.1 Their characterization rests on IR spectra, with comparison to the computed AE-CCSD(T)/cc-pCVQZ anharmonic vibrational frequencies, and the UV-Vis spectra, with comparison to the computed B3LYP/6–311++G(2d,2p) transitions.

1 can be converted to 2 by photolysis. Interestingly, 1 does not convert to 2 after 5 days on the matrix in the dark. This is in distinct contrast to hydroxycarbene and related other carbene which undergo quantum mechanical tunneling (see this post and this post). Examination of the potential energy surface for the reaction of 1 to 2 at AE-CCSD(T)/cc-pCVQZ (see Figure 1) identifies that the lowest barrier is 45.8 kcal mol-1, about 15 kcal mol-1 larger than the barrier for the hydroxycarbene rearrangement. Additionally, the barrier width for 12 is 25% larger than for the hydroxycarbenes. Both of these suggest substantially reduced tunneling, and WKB analysis predicts a tunneling half-life of more than a billion years. The stability of 1 is attributed to the strong π-donor ability of nitrogen to the electron-poor carbene. This is reflected in a very short C-N bond (1.27 Å).

Figure 1. Structures and energies of 1 and 2 and the transition states that connect them. The relative energies (kcal mol-1) are computed at AE-CCSD(T)/cc-pCVQZ.

References

1) Eckhardt, A. K.; Schreiner, P. R., "Spectroscopic Evidence for Aminomethylene (H−C̈−NH2)—The
Simplest Amino Carbene." Angew. Chem. Int. Ed. 2018, 57, 5248-5252, DOI: 10.1002/anie.201800679.

InChIs

1: InChI=1S/CH3N/c1-2/h1H,2H2
InChIKey=KASBEVXLSPWGFS-UHFFFAOYSA-N

2: InChI=1S/CH3N/c1-2/h2H,1H2
InChIKey=WDWDWGRYHDPSDS-UHFFFAOYSA-N

Schreiner &Tunneling Steven Bachrach 23 Jul 2018 1 Comment

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

Isotope Controlled Selectivity

I seem to be recently flooded with papers dealing with tunneling in organic systems. Well, here’s one more! Kozuch, Borden, Schreiner and co-workers seek out systems whereby isotopic substitution might lead to reaction selectivity.1 Their base system is cyclopropylmethylcarbene 1, which can undergo three different reactions: (a) the ring can expand to give 1-methylcyclobut-1-ene 2, (b) a hydrogen can shift from the terminal methyl group to give vinylcyclopropane 3, or (c) the methane hydrogen can shift to produce ethylidenecyclopropane 4. This last option can be neglected since its barrier (20.5 kcal mol-1) is so much higher than for the other two, 7.5 kcal mol-1 for the ring expansion and 12.1 kcal mol-1 for the [1,2]H-shift converting 13.

At high temperature, the ring expansion to 2 will dominate, but at low temperature the hydrogen shift to 3 might dominate by tunneling through the barrier due to the low mass and short distances involved. The reaction rates were computed using B3LYP/6-31G(d,p) and small-curvature tunneling. At low temperature, the rate for the hydrogen shift is 10 orders of magnitude faster than the ring expansion. Thinking that deuterium substitution of the terminal methyl group might slow down the rate of the [1,2]-shift, they computed the rates for the reactions of 1-d3, and in fact the rate of this shift does reduce by 104 but it is still much faster than the rate for ring expansion. What is needed is a system where the rate for ring expansion is slower than the rate for hydrogen migration but faster than the rate of deuterium migration.

They examine a number of different substituents that may help to lower the barrier for the ring expansion. The methoxy derivative 5 turns out to suit the bill perfectly. The methoxy group reduces the barrier for ring expansion from 7.5 kcal mol-1 with 1 to 2.5 kcal mol-1 with 5. With hydrogenated 5, the [1,2]H-shift is 103 times faster than ring expansion, but with deuterated 5, ring expansion is twice as fast as the deuterium migration.

The authors call this isotope controlled selectivity (ICS), and this is the first example of this type of control.

References

1. Nandi, A.; Gerbig, D.; Schreiner, P. R.; Borden, W. T.; Kozuch, S., Isotope-Controlled Selectivity by Quantum Tunneling: Hydrogen Migration versus Ring Expansion in Cyclopropylmethylcarbenes. J. Am. Chem. Soc. 2017, 139, 9097-9099, DOI: 10.1021/jacs.7b04593.

InChIs

1: InChI=1S/C5H8/c1-2-5-3-4-5/h5H,3-4H2,1H3
InChIKey=KJIJNBZLGHBOTI-UHFFFAOYSA-N

2: InChI<=1S/C5H8/c1-5-3-2-4-5/h3H,2,4H2,1H3
InChIKey=AVPHQXWAMGTQPF-UHFFFAOYSA-N

3: InChI=1S/C5H8/c1-2-5-3-4-5/h2,5H,1,3-4H2
InChIKey=YIWFBNMYFYINAD-UHFFFAOYSA-N

4: InChI=1S/C5H8/c1-2-5-3-4-5/h2H,3-4H2,1H3
InChIKey=ZIFNDRXSSPCNID-UHFFFAOYSA-N

5: InChI=1S/C6H10O/c1-3-6(7-2)4-5-6/h4-5H2,1-2H3
InChIKey=YMBSTCICUAORNN-UHFFFAOYSA-N

6: InChI<=1S/C6H10O/c1-5-3-4-6(5)7-2/h3-4H2,1-2H3
InChIKey=QBLNAZHAVPMLHB-UHFFFAOYSA-N

7: InChI<=1S/C6H10O/c1-3-6(7-2)4-5-6/h3H,1,4-5H2,2H3
InChIKey=FHYLDABSPVPDTJ-UHFFFAOYSA-N

Borden &Isotope Effects &Schreiner &Tunneling Steven Bachrach 22 Jan 2018 No 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

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