Archive for the 'Uncategorized' Category

Nitrogen substituted buckybowl fragment

Higashibayashi and co-workers prepared the hydrazine-substituted Buckyball fragment 1a and also its mono- and deoxidized analogues.1 To interpret their results, they also computed the parent structure 1b at ωB97Xd/6-311+G(d,p).


1a R = tBut
1b R = H

The optimized structure of 1b is a bowl, but a twisted geometry, where the lone pair on each
nitrogen is on the opposite face of the molecule, lies only 1.6 kcal mol-1 higher in energy. The barrier for moving from the bowl to the twist form is 2.0 kcal mol-1. The completely planar structure, which is also a transition state for inversion of the bowl, lies 5.1 kcal mol-1 above the lowest energy bowl structure. The geometries and energies of the conformations are shown in Figure 1.

1b bowl (0.0)

1b twist (1.6)

1b TS (2.0)

1b planar TS (5.11)

Figure 1. ωB97Xd/6-311+G(d,p) optimized
geometry and relative energy (kcal mol-1) of the conformations of 1b.

The mono oxidized 1b.+ structure is also a bowl, but there is no twist form and inversion takes place through a planar structure that is only 0.5 kcal mol-1 above the bowl ground state. The structures and energies of these conformations of 1b.+ are shown in Figure 2.

1b.+ bowl (0.0)

1b.+ planar TS (0.5)

Figure 2. ωB97Xd/6-311+G(d,p) optimized geometry and relative energy (kcal mol-1) of the conformations of 1b.+.

Lastly, the di-oxidized 1b2+ is planar, and its structure is shown in Figure 3.

1b2+ planar

Figure 2. ωB97Xd/6-311+G(d,p) optimized geometry of 1b2+.

These computations corroborate all of the experimental data observed with 1a. What is particularly of note is the fact that the potential energy surface is so dependent on charge state: a three-well potential for the neutral, and two-well potential for the monocation, and a single-well potential for the dication.

References

(1) Higashibayashi, S.; Pandit, P.; Haruki, R.; Adachi, S.-I.; Kumai, R. “Redox-Dependent
Transformation of a Hydrazinobuckybowl between Curved and Planar Geometries,” Angew. Chem. Int. Ed. 2016, 55, 10830-10834, DOI: 10.1002/anie.201605340.

InChIs

1a: InChI=1S/C40H44N2/c1-37(2,3)21-13-25-26-14-22(38(4,5)6)19-31-32-20-24(40(10,11)12)16-28-27-15-23(39(7,8)9)18-30-29(17-21)33(25)41(34(26)31)42(35(27)30)36(28)32/h13-20H,1-12H3
InChIKey=DKJNIDLSMMQIBX-UHFFFAOYSA-N

1b:InChI=1S/C24H12N2/c1-5-13-14-6-2-11-19-20-12-4-8-16-15-7-3-10-18-17(9-1)21(13)25(22(14)19)26(23(15)18)24(16)20/h1-12H
InChIKey=JQNPHLTXAOKXNQ-UHFFFAOYSA-N

Uncategorized Steven Bachrach 06 Sep 2016 No Comments

Identifying the n→π* interaction

The weak n→π* interaction has been proposed to explain some conformational structure. Singh, Mishra, Sharma, and Das have now provided the first spectroscopic evidence of this interactions.1 They examined the structure of phenylformate 1. This compound can exist as two conformational isomers, having the carbonyl oxygen pointing towards (cis) or away (trans) from the phenyl ring. They optimized the structures of these two conformers at M05-2X/aug-cc-pVDZ and find that the cis isomer is lower in energy by 1.32 kcal mol-1. Unfortunately, the authors do not provide the structures of these isomers, but since they are so small, I reoptimized them at ωB97XD/6-311g(d) and they are displayed in Figure 1. At this computational level, the cis isomer is lower in enthalpy than the trans isomer by 1.35 kcal mol-1.

1cis

1trans

Figure 1. ωB97XD/6-311g(d) optimized structures of the cis and trans conformations of 1.

One-color resonant 2-photon ionization (1C-R2PI) spectroscopy followed by UV-VIS hole burning spectroscopy identified two isomers of 1, one present in greater amount that the other. The IR spectra of the dominant isomer showed a carbonyl stretch at 1766 cm-1, in nice agreement with the predicted frequency of 1cis (1770 cm-1). The carbonyl stretch for the minor isomer is at 1797 cm-1, again in nice agreement with the computed frequency for 1trans (1800 cm-1). The cis isomer has the lower carbonyl frequency due to partial donation of the carbonyl oxygen electrons to the π* orbital of the phenyl ring.

References

(1) Singh, S. K.; Mishra, K. K.; Sharma, N.; Das, A. "Direct Spectroscopic Evidence for an n→π* Interaction," Angew. Chem. Int. Ed. 2016, 55, 7801-7805, DOI: 10.1002/anie.201511925.

InChIs

1: InChI=1S/C7H6O2/c8-6-9-7-4-2-1-3-5-7/h1-6H
InChIKey=GEOWCLRLLWTHDN-UHFFFAOYSA-N

Uncategorized Steven Bachrach 18 Jul 2016 1 Comment

Changes to the blog

I have been posting regularly on this blog for over nine years, beginning in July 2007. I have used this blog as a way to keep my book Computational Organic Chemistry current for its readers. I have also used it as a way for me to keep current with the literature.

It has been a terrific adventure for me, but an important change will be taking place in my life that will have an impact on the blog. Starting on August 1, 2016 I will become the Dean of the School of Science at Monmouth University in West Long Branch, NJ. (See the announcement.) I am extraordinarily excited to take on the challenges of leading the School. I suspect that my duties as Dean will keep me from finding the time to post as often as I have been for this past years. I will try to occasionally write a post as I intend to keep connected to the computational chemistry community. I have a few posts backlogged but expect a more infrequent posting schedule come August.

I fully intend to maintain the blog so that past posts remain accessible.

I want to thank all of the readers of this blog, those who read me through the Computational Chemistry Highlights blog, and especially those of you who have posted comments.

Uncategorized Steven Bachrach 11 Jul 2016 5 Comments

Interesting chemistry of biphenalenylidene

Uchida and co-workers reported on the preparation of biphenalenylidene 1 and its interesting electrocyclization to dihydroperopyrene 2.1 The experimental barrier they find by experiment for the conversion of 1-Z to 1-E is only 4.3 kcal mol-1. Secondly, the photochemical electrocyclization of 2-anti to 1-Z proceeds rapidly, through an (expected) allowed conrotatory pathway. However, the reverse reaction did not occur photochemically, but rather did occur thermally, even though this is formally forbidden by the Woodward-Hoffman rules.

To address these issues, they performed a number of computations, with geometries optimized at UB3LYP(BS)/6-31G**. First, CASSCF computations indicated considerable singlet diradical character for 1-Z. Both 1-Z and 1-E show significant twisting about the central double bond, consistent with the singlet diradical character. 1-Z is 1.8 kcal mol-1 lower in energy than 1-E, and the barrier for rotation interconverting these isomers is computed to be 7.0 kcal mol-1, in reasonable agreement with the experiment. These geometries are shown in Figure 1.

1-Z

1-E

TS (Z→E)

Figure 1. UB3LYP(BS)/6-31G** optimized geometries of 1-Z and 1- and the transition state to interconvert these two isomers.

The conrotatory electrocyclization that takes 1-Z into 2-anti has a barrier of 26.0 kcal mol-1 and is exothermic by 3.4 kcal mol-1. The disrotatory process has a higher barrier (34.2 kcal mol-1) and is endothermic by 8.4 kcal mol-1. These transition states and products are shown in Figure 2. So, despite being orbital symmetry forbidden, the conrotatory path is preferred, and this agrees with their experiments.

TS (con)

TS (dis)

2-anti

2-syn

Figure 2. UB3LYP(BS)/6-31G** optimized geometries of 2-anti and 2-syn and the transition states leading to them.

The authors argue that the large diradical character of 1 leads to both its low Z→E rotational barrier, and the low barrer for electrocyclization. The Woodward-Hoffmann allowed disrotatory barrier is inhibited by its highly strained geometry, making the conrotatory path the favored route.

References

(1) Uchida, K.; Ito, S.; Nakano, M.; Abe, M.; Kubo, T. "Biphenalenylidene: Isolation and Characterization of the Reactive Intermediate on the Decomposition Pathway of Phenalenyl Radical," J. Am. Chem. Soc. 2016, 138, 2399-2410, DOI: 10.1021/jacs.5b13033.

InChIs

1-E: InChI=1S/C26H16/c1-5-17-9-3-11-23-21(15-13-19(7-1)25(17)23)22-16-14-20-8-2-6-18-10-4-12-24(22)26(18)20/h1-16H/b22-21+
InChIKey=LOZZANITCNALJB-QURGRASLSA-N

1-Z: InChI=1S/C26H16/c1-5-17-9-3-11-23-21(15-13-19(7-1)25(17)23)22-16-14-20-8-2-6-18-10-4-12-24(22)26(18)20/h1-16H/b22-21-
InChIKey=LOZZANITCNALJB-DQRAZIAOSA-N

2-anti: InChI=1S/C26H18/c1-3-15-7-11-19-21-13-9-17-5-2-6-18-10-14-22(26(21)24(17)18)20-12-8-16(4-1)23(15)25(19)20/h1-14,19,21,25-26H/t19-,21-,25?,26?/m0/s1
InChIKey=BZIOOLOJBUBMSS-ATJINXRDSA-N

2-syn: InChI=1S/C26H18/c1-3-15-7-11-19-21-13-9-17-5-2-6-18-10-14-22(26(21)24(17)18)20-12-8-16(4-1)23(15)25(19)20/h1-14,19,21,25-26H/t19-,21+,25?,26?
InChIKey=BZIOOLOJBUBMSS-YXGNQKCYSA-N

Uncategorized Steven Bachrach 19 Apr 2016 No Comments

1,3,5-Trifluorenylcyclohexane

Reid, Rathore and colleagues report on the attempted preparation of the interesting molecule 1,3,5-trifluorenylcyclohexane (TFC) 1.1 They had hoped to prepare it by subjecting the precursor 2 to acid, which might then undergo a Friedel-Crafts reaction to prepare the last fluorenyl group, and subsequent loss of a proton would give 1. Unfortunately, they could not get this step to occur, even at high temperature and for long reaction times. What made it particularly frustrating was that they could get 3 to react under these conditions to give 1,4-difluorenylcyclohexane (14-DFC) 4, and convert 5 into 1,4-difluorenylcyclohexane (13-DFC) 6.

To get at why 1 could not be formed they utilized PCM(CH2Cl2)/M06-2X/6-31G(d) calculations. The lowest energy conformations of 1 and 4 are shown in Figure 1. While 4 is in a chair conformation, 1 is not in a chair conformation since this would bring the three fluorenyl groups into very close contact. Instead, the cyclohexyl ring of 1 adopts a twist-boat conformation, with a much flattened ring. They estimate that 1 is strained by about 17 kcal mol-1, with 10 kcal mol-1 coming from strain in the twist-boat conformation and another 7 kcal mol-1 of strain due to steric crowding of the fluorenyl groups.

They next optimized the structures of the intermediates and transition states on the path taking 2 into 1 and 3 into 4. The transition states of the Friedel-Crafts reaction are the highest points on these paths, and their geometries are shown in Figure 1. The barrier through the TS for the Friedel-Crafts step forming 1 is about 17 kcal mol-1 higher than for the barrier to form 4. This very large increase in activation barrier, due to the strains imposed by that third fluorenyl group, explains the lack of reaction. Furthermore, since the reaction 21 is 2.0 kcal mol-1 endothermic, at high temperature the reaction is likely to be reversible and favors 2.

1

4

TS to 1

TS to 4

Figure 1. PCM(CH2Cl2)/M06-2X/6-31G(d) optimized geometries.

References

(1) Talipov, M. R.; Abdelwahed, S. H.; Thakur, K.; Reid, S. A.; Rathore, R. "From Wires to Cables: Attempted Synthesis of 1,3,5-Trifluorenylcyclohexane as a Platform for Molecular Cables," J. Org. Chem. 2016, DOI: 10.1021/acs.joc.5b02792.

InChIs

1 (TFC): InChI=1S/C42H30/c1-7-19-34-28(13-1)29-14-2-8-20-35(29)40(34)25-41(36-21-9-3-15-30(36)31-16-4-10-22-37(31)41)27-42(26-40)38-23-11-5-17-32(38)33-18-6-12-24-39(33)42/h1-24H,25-27H2
InChIKey=CXXRVQFQMRJLAI-UHFFFAOYSA-N

4 (14-DFC): InChI=1S/C30H24/c1-5-13-25-21(9-1)22-10-2-6-14-26(22)29(25)17-19-30(20-18-29)27-15-7-3-11-23(27)24-12-4-8-16-28(24)30/h1-16H,17-20H2
InChIkey=ZZTDGVHNROVFMK-UHFFFAOYSA-N

6 (13-DFC):InChI=1S/C30H24/c1-2-11-22(12-3-1)24-14-4-5-15-25(24)23-13-10-20-30(21-23)28-18-8-6-16-26(28)27-17-7-9-19-29(27)30/h1-9,11-19H,10,20-21H2
InChIKey=TTZIUDUAWUTKAI-UHFFFAOYSA-N

Uncategorized Steven Bachrach 28 Mar 2016 No Comments

Really short non-bonded HH distances

Setting the record for the shortest non-bonded HH contact has become an active contest. Following on the report of a contact distance of only 1.47 Å that I blogged about here, Firouzi and Shahbazian propose a series of related cage molecules with C-H bonds pointed into their interior.1 The compounds were optimized with a variety of computational methods, and many of them have HH distances well below that of the previous record. The shortest distance is found in 1, shown in Figure 1. The HH distance in 1 is predicted to be less than 1.2 Å with a variety of density functionals and moderate basis sets.

1

Figure 1. Optimized geometry of 1 at ωB97X-D/cc-pVDZ.

References

(1) Firouzi, R.; Shahbazian, S. "Seeking Extremes in Molecular Design: To What Extent May Two “Non-Bonded” Hydrogen Atoms be Squeezed in a Hydrocarbon?," ChemPhysChem 2016, 17, 51-54, DOI: 10.1002/cphc.201501002.

InChIs

1: InChI=1S/C41H44/c1-7-13-25-17-9-3-40-5-11-19(35(17)40)27-15-8-2-39(1,33(13)15)34-14(7)26-18-10-4-41-6-12-20(36(18)41)28(16(8)34)32(27)30-23(11)37(40)21(9)29(31(25)26)22(10)38(41)24(12)30/h7-38H,1-6H2
InChIKey=DMWSEJFKSWCSLI-UHFFFAOYSA-N

Uncategorized Steven Bachrach 26 Jan 2016 6 Comments

An amazing barrel structure

I don’t really have anything to say about this recent paper by Anderson, et al.1 They have simply prepared a very beautiful structure, an aryllated analogue of 1. They even optimized the structure of 1 at BLYP/6-31G(d) and it’s shown in Figure 1. That must have taken some time!

Figure 1. BLYP/6-31G(d) optimized structure of 1.
(Remember that you can manipulate this structure by simply clicking on in, which will launch the JMol app.)

References

(1) Neuhaus, P.; Cnossen, A.; Gong, J. Q.; Herz, L. M.; Anderson, H. L. "A Molecular Nanotube with Three-Dimensional π-Conjugation," Angew. Chem. Int. Ed. 2015, 54, 7344-7348, DOI: 10.1002/anie.201502735.

Uncategorized Steven Bachrach 04 Aug 2015 2 Comments

Hypercubane

Three-dimensional objects can be projected into four-dimensional objects. So for example a cube can be projected into a hypercube, as in Scheme 1.

Scheme 1.

Pichierri proposes a hydrocarbon analogue of the hypercube. The critical decision is the connecting bridge between the outer (exploded) carbons. This distance is too long to be a single carbon-carbon bond. Pichierri opts to use ethynyl bridges, to give the hypercube 1.1

Now, unfortunately he does not supply any supporting materials. So I have reoptimized this Oh geometry at B3LYP/6-31G(d), and show this structure in Figure 1. Pichierri does not report much beyond the geometry of 1 and the perfluoronated analogue. One interesting property that might be of interest is the ring strain energy of 1, which I will not take up here.


1

2

But a question I will take up is just what bridges might serve to create the hydrocarbon hypercube. A more fundamental choice might be ethanyl bridges, to create 2. However, the Oh conformer of 2 has 13 imaginary frequencies at B3LYP/6-31G(d). Lowering the symmetry to D3 give a structure that has only real frequencies, and it’s shown in Figure 1. An interesting exercise is to ponder other choices of bridges, which I will leave for the reader.

1

2

Figure 1. B3LYP/6-31G(d) optimized structures of 1 and 2.
As always, be sure to click on the image to enable Jmol for interactive viewing of these interesting structures!

References

(1) Pichierri, F. "Hypercubane: DFT-based prediction of an Oh-symmetric double-shell hydrocarbon," Chem. Phys. Lett. 2014, 612, 198-202, DOI: j.cplett.2014.08.032.

InChIs

1: InChI=1S/C40H24/c1-2-26-7-9-29-15-11-27-5-3-25(1)4-6-28-12-16-30(10-8-26)20-23-32(22-19-29)24-21-31(17-13-27,18-14-28)39-35(27)33(25)34(26)37(29,35)40(32,39)38(30,34)36(28,33)39/h1-24H
InChIKey=FFMFUIDOGFAUOP-UHFFFAOYSA-N

2: InChI=1S/C40H48/c1-2-26-7-9-29-15-11-27-5-3-25(1)4-6-28-12-16-30(10-8-26)20-23-32(22-19-29)24-21-31(17-13-27,18-14-28)39-35(27)33(25)34(26)37(29,35)40(32,39)38(30,34)36(28,33)39/h1-24H2
InChIKey=MCSZKKKJCDSRIV-UHFFFAOYSA-N

Uncategorized Steven Bachrach 15 Dec 2014 2 Comments

History of the development of ChemDraw

I suspect that the majority of my readers have no experience in drawing chemical structures by hand for publication purposes. That’s because of one software product: ChemDraw. I remember using the Fieser triangle (unfortunately no longer sold by Aldrich – click to see a pic and product description!), a plastic template that had standard-sized rings, like nice pentagons and hexagons, and chair and boat conformations of cyclohaxane, and you’d take your fancy ink pen and careful follow the template. Then you moved the template to draw say a bond off of the ring and hoped to god that the ink didn’t smudge. There were other templates for drawing letters and numbers – or you used scratch-off transfer decals. (By this point all of you under 40 are thinking “what the hell is he talking about?”)

Well all of that changed with three seminal events for organic chemists: the introduction of the original Macintosh computer, the introduction of the Apple LaserWriter and the introduction of ChemDraw. The Mac allowed one to sketch in a much more intuitive way – again for those less than 50, computers use to come without a mouse! Imagine trying to draw a chemical structure using a keyboard. That’s why there were no structure drawing tools prior to the Mac. The LaserWriter meant that you could print an output that looked as good as what you had on the screen, and could thus be submitted for publication. And ChemDraw – well this was just astonishing! I still remember the day during my post-doc when the Mac and LaserWriter arrived and we launched ChemDraw and were able to quickly draw molecules – steroid, and conformations, and stereoisomers and they all looked beautiful and we could get them done in a flash!

When I started my first academic position at Northern Illinois University in August 1987 I purchased a Mac and a LaserWriter and ChemDraw as part of my start-up – and I was the first in the department to have a Mac – but that changed rapidly!

So, why all of the teary reminiscences? Well David Evans has just published1 a nice romp through the mid-1980s recalling how Stewart Rubinstein, aided by Evans and his wife, developed ChemDraw and started CambridgeSoft, and as Stuart Schreiber says “ChemDraw changed the field in a way that has not been replicated since.”

Today, there are other chemical structure drawing tools available, and in fact I no longer use ChemDraw, but it is still a wonder to be able to create drawings so easily and so nicely. Maybe one day I’ll reminisce about the day I got EndNote and my life changed again!

References

1) Evans, D. A. “History of the Harvard ChemDraw Project,” Angew. Chem. Int. Ed. 2014, 53, 1521-3773, DOI 10.1002/anie.201405820.

Uncategorized Steven Bachrach 08 Oct 2014 3 Comments

Splitting CO2 with a two-coordinate boron cation

This paper is a bit afield from the usual material I cover but this is an interesting reaction. Shoji and coworkers have prepared the two-coordinate boron species 1,1 and confirmed its geometry by an x-ray crystal structure. What I find interesting is its reaction with CO2, which gives 2 and organoboranes that are not identified, though presumably derived from 3.

M06-2x/6-311+G(d,p) computations support a hypothetical mechanism whereby first a complex between 1 and CO2 is formed (CP1), that is 4.4 kcal mol-1 above isolated reactants. Then passing through TS1, which is 4.2 kcal mol-1 above CP1, an intermediate is formed (INT), which is almost 6 kcal mol-1 below starting materials. A second transition state is then traversed (about 1 kcal mol-1 below starting materials), to form an exit complex between 2 and 3, which can then separate to the final products with an overall exothermicity of 10.6 kcal mol-1. The structures of these critical points are shown in Figure 1.

1
(0.0)

CP1
(4.4)

TS1
(8.6)

INT
(-5.7)

TS2
(-1.1)

CP2
(-9.0)

Figure 1. M06-2x/6-311+G(d,p) optimized structures. Relative energy in kcal mol-1.

References

(1) Shoji, Y.; Tanaka, N.; Mikami, K.; Uchiyama, M.; Fukushima, T. "A two-coordinate boron cation featuring C–B+–C bonding," Nat. Chem. 2014, 6, 498-503, DOI: 10.1038/nchem.1948.

InChIs

1: InChI=1S/C18H22B/c1-11-7-13(3)17(14(4)8-11)19-18-15(5)9-12(2)10-16(18)6/h7-10H,1-6H3/q+1
InChIKey=WLUJABFTLHAEMI-UHFFFAOYSA-N

2: InChI=1S/C10H11O/c1-7-4-8(2)10(6-11)9(3)5-7/h4-5H,1-3H3/q+1
InChIKey=CUJVTHUIQVMVHD-UHFFFAOYSA-N

3: InChI=1S/C9H11BO/c1-6-4-7(2)9(10-11)8(3)5-6/h4-5H,1-3H3
InChIKey=ZJKBFARYTPYYGV-UHFFFAOYSA-N

Uncategorized Steven Bachrach 04 Aug 2014 1 Comment

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