One of the ubiquitous examples of the Hückel rule is cyclooctatetraene dianion (COT^2-). This annulene has, presumably, 10 π electrons and therefore should be aromatic, satisfying the 4n+2 rule. Therefore, the molecule should be planar, right? Well, an article by Dominikowska and Palusiak call into question these assumptions.¹

COT^2-

First off, with either B3LYP or MP2 and a variety of basis sets, optimization of COT^2- starting from the tub-shape of COT itself led to the planar or nearly planar structure most of the time. The exceptions include B3LYP/6-311++G(d,p) and MP2/aug-cc-pVDZ. More interesting is that a number of the MP2 planar structures have one or more imaginary frequency; for example, MP2/6-311G has four imaginary frequencies.

I reoptimized a number of these structures assuming D_8h symmetry, and looked for the number of imaginary frequencies. B3LYP/6-311G(d,p) had no imaginary frequencies, but B3LYP/6-31++G(d,p) and B3LYP/6-311++G(d,p) had 2 and 4 imaginary frequencies, respectively. Many of the MP2 optimizations had imaginary frequencies, with MP2/6-311G(d,p) having 3 imaginary frequencies. The optimized structures of COT^2- at ωB97X-D/6-311G(d,p) had no imaginary frequencies but with the 6-311++G(d,p) basis set, it had two imaginary frequencies. Interestingly, Truhlar’s M06-2x functional with both 6-311G(d,p) and 6-311++G(d,p) gives no imaginary.

This is reminiscent of the situation with benzene and other arenes, where certain combinations of method and basis set gave multiple imaginary frequencies.² The ultimate culprit was identified as intramolecular basis set superposition error. Dominikowska and Palusiak discount this explanation here for two reasons. First, multiple imaginary frequencies are seen with the Dunning correlation consistent basis sets – MP2/aug-cc-pVDZ has 7 imaginary frequencies (though my computation at D_8h gives only one imaginary frequency), something not observed for benzene. Secondly, they noticed that in the non-planar COT^2- optimized structure there are bond paths connecting the hydrogens to non-nuclear attractors situated way outside the molecule. They suggest that the COT^2- might really be a Rydberg state, with the extra electrons located outside the molecule. This implies that the π system has only 8 electrons, giving the tub shape. They note that COT^2- has a very short lifetime and suggest that it is not an aromatic compound, a larger annulene congener of benzene, at all.

It would be interesting to see what would happen with COT^2- correcting for intramolecular basis set superposition error via the method of Asturiol, Duran and Salvador,³ which I described in this post. This correction led to planar benzene having no imaginary frequencies. This type of computation would help assess just what is going on here – is COT^2- afflicted with basis set problems or is it a very unusual, non-aromatic system?

References

(1) Dominikowska, J.; Palusiak, M., "Cyclooctatetraene dianion—an artifact?," J. Comput. Chem., 2011, 32, 1441-1448, DOI: 10.1002/jcc.21730

(2) Moran, D.; Simmonett, A. C.; Leach, F. E.; Allen, W. D.; Schleyer, P. v. R.; Schaefer, H. F., III, "Popular Theoretical Methods Predict Benzene and Arenes To Be Nonplanar," J. Am. Chem. Soc., 2006, 128, 9342-9343, DOI: 10.1021/ja0630285

(3)  Asturiol, D.; Duran, M.; Salvador, P., "Intramolecular basis set superposition error effects on the planarity of benzene and other aromatic molecules: A solution to the problem," J. Chem. Phys., 2008, 128, 144108, DOI: 10.1063/1.2902974

While many pericyclic reactions proceed in a concerted fashion, the stepwise pathway is a distinct possibility. Fernandez, Sierra and Torres report on an interesting [8+2] cycloaddition that
is decidedly stepwise, confirmed through trapping of the intermediate zwitterion.¹

The reaction of 1 with 2 was examined at M06-2x/6-311+G(d) (optimized geometries of the critical points are shown in Figure 1). The first transition state (TS1) has nitrogen acting as a nucleophile, attacking the carbonyl carbon of ketene to give 3. The barrier is 11.6 kcal mol^-1, and 3 lies 0.7 kcal mol^-1 above reactants. While 3 might be described with a tropyllium cation resonance structure, the ring is in fact non-planar and both the NICS(0) and NICS(1)_zz values are positive. The ring is therefore antiaromatic, consistent with the endoergonicity of this step. Closure of the zwitterion through TS2 leads to the formal [8+2] product, with the barrier for this second step slightly lower than the barrier for the first step. Overall, the reaction is quite exothermic.

Scheme 1 (relative energies in kcal mol^-1)

TS1	3
TS2	4

Figure 1. MO6-2x/6-311+G(d) optimized Structures of 3, 4, and the transition states leading to them (TS1 and TS2).

Experiments were performed with a variety of acyl chloride precursors to ketenes (Scheme 2), and along with the [8+2] product, a second product (5) incorporating 2 equivalents of ketene is found; in fact, if the R group is benzyloxy or t-butyl, 5 is the only observed product. This second product comes about via trapping of the intermediate 3. Mixing phenylketene with 4a (where the R group is phenyl) gives no reaction, thus precluding the intermediacy of 4 on the path to 5. MO6-2x computations of the trapping of 3 with phenylketenes indicates a barrier (TS3, see Figure 2) of 9.6 kcal mol^-1, very close to the barrier height of the second TS for ring closure of the [8+2] pathway, supporting the competition between trapping of the intermediate and progress on to the [8+2] product.

Scheme 2.

TS3

Figure 2. MO6-2x/6-311+G(d) optimized Structures of TS3.

References

(1) Lage, M. L.; Fernandez, I.; Sierra, M. A.; Torres, M. R., "Trapping Intermediates in an [8 + 2] Cycloaddition Reaction with the Help of DFT Calculations," Org. Lett., 2011, ASAP, DOI: 10.1021/ol200910z

InChIs

1: InChI=1/C8H6O/c9-7-6-8-4-2-1-3-5-8/h1-6H
InChIKey=RZGZTQYTDRQOEY-UHFFFAOYAC

2: InChI=1/C7H7N/c8-7-5-3-1-2-4-6-7/h1-6,8H
InChIKey=NHNIVEADUHCPRP-UHFFFAOYAI

3: InChI=1/C15H13NO/c17-15(12-13-8-4-3-5-9-13)16-14-10-6-1-2-7-11-14/h1-12,17H/b15-12-/f/h17h,16H
InChIKey=AFLGFPOCUCTUAS-UJUNEOEYDT

4: InChI=1/C15H13NO/c17-15-14(11-7-3-1-4-8-11)12-9-5-2-6-10-13(12)16-15/h1-10,12,14H,(H,16,17)/t12-,14+/m1/s1/f/h16H
InChIKey=BBJCGQGFRRQHHS-KEKMKNANDW

Changing the number of π-electrons in a molecule should alter its aromaticity and antiaromaticity – if the Hückel rule holds! Iron, Cohen and Rybtchinski¹ look at an interesting polycyclic aromatic perylene diimide 1 and its dianion and evaluate the aromaticity and reactivity of these species, using both experiment and computation.

The M06-2x/6-31++G** geometries of 1 and its dianion 1^2- are shown in Figure 1. 1 can be thought of as two naphthalenyl groups joined together by single bond (their distance is 1.479 Å). In the dianion, the bond distance between the two naphthyl groups (making the central 6-member ring) is shorter, 1.425 Å. In addition the C-C(O) bond of the imide ring shortens from 1.485 to 1.441 Å in going from the neutral to the dianion, with the C=O bond also lengthening, from 1.214 to 1.243 Å. These geometric changes suggest the formation of an aromatic central six-member ring, and perhaps some aromaticity in the diimide rings, too.

1

12-

Figure 1. M06-2x optimized structures of 1 and 1^2-.

The changes are reflected in the NICS(1) values. In 1, the NICS values for each ring of the naphthyl group is -9.31, as expected for an aromatic 6-member ring. The diimide has a NICS value of -0.01, indicating a non-aromatic ring, and the central ring has a NICS value of +1.78, suggesting some slight antiaromatic character. Significant changes come about with the reduction to the dianion. The central ring now has a NICS value of -9.10, indicating an aromatic ring. The naphthyl rings have NICS values of -3.94 and the diimide ring has a value of -4.53.

So, the dianion 1^2- species appears to be more aromatic than the neutral 1. This is reflected in its chemistry. The dianion of pegylated 1 is actually stable in water, though it is readily oxidized by air. Its stability in water is understood in terms of the computed reaction energy with water, which is quite positive due to that aromatic stabilization of 1^2-. This is compared with the computed negative free energies for the reaction of other dianions with water, such as the anthracenyl, and perylenyl dianions.

References

1) Iron, M. A.; Cohen, R.; Rybtchinski, B., "On the Unexpected Stability of the Dianion of Perylene Diimide in Water – A Computational Study," J. Phys. Chem. A, 2011, 115, 2047-2056, DOI: 10.1021/jp1107284

InChIs

1: InChI=1/C24H10N2O4/c27-21-13-5-1-9-10-2-6-15-20-16(24(30)26-23(15)29)8-4-12(18(10)20)11-3-7-14(22(28)25-21)19(13)17(9)11/h1-8H,(H,25,27,28)(H,26,29,30)/f/h25-26H
InChIKey=KJOLVZJFMDVPGB-SPEPDGBUCZ

Henry Rzepa brought this interesting paper to my attention via his blog post. I have a few comments too.

Isobe and co-workers prepared the interesting polycyclic aromatic compound 1, which they represent with the picture given below.¹ The molecule is non-planar due to the clash of the interior hydrogens. If you stare at this picture long enough you might decide that something looks amiss. If you start at the lower left phenyl ring of the top segment and move counterclockwise you would move upwards, out of the plane of the page. Then you cross into the lower half and continue the counter clockwise motion, again you climb upwards out of the page. And then you cross back over to the top half and your back where you started – but you’ve been climbing uphill all the time! This might remind you of the famous Escher drawing Ascending and Descending.

So, what’s the catch? Well, the molecule does exist and its crystal structure shows the key. The molecule is in fact of D₂ symmetry, so the upper and lower halves of the molecule are twisted in and out of the plane. B3LYP/6-31G(d,p) computations on the parent compound 2 indicate the D₂ symmetry of the ground state (see Figure 1). (Don’t forget to click on the structure to be able to manipulate the structure!) This is a chiral species. Racemization occurs through a barrier of 10.1 kcal mol^-1 (3) which connects to the meso structure 4, Passing then through the mirror image of 3 completes the process, joining to the mirror image of 2.

2

3

4

I think the title is a bit misleading here – the molecular expression is misleading as its drawn in 1, but the full 3D structure shows no such illusion; the molecule is in fact not a Penrose Stair. But 1 certainly possesses a structure of interesting shape and topology!

References

(1) Nakanishi, W.; Matsuno, T.; Ichikawa, J.; Isobe, H., "Illusory Molecular Expression of “Penrose Stairs” by an Aromatic Hydrocarbon," Angew. Chem. Int. Ed., 2011, ASAP, DOI: 10.1002/anie.201102210

InChIs

1: InChI=1/C52H52/c1-5-9-13-37-25-41-26-38(14-10-6-2)44-22-19-35-30-48(44)51(41)47-29-33(17-21-43(37)47)34-18-23-45-39(15-11-7-3)27-42-28-40(16-12-8-4)46-24-20-36(35)32-50(46)52(42)49(45)31-34/h17-32H,5-16H2,1-4H3
InChIKey=UNDWIIYPHJODRS-UHFFFAOYAO

2: InChI=1/C36H20/c1-9-25-10-2-22-6-15-29-18-32(22)35(25)31-17-27(13-5-21(1)31)28-14-7-23-3-11-26-12-4-24-8-16-30(29)20-34(24)36(26)33(23)19-28/h1-20H
InChIKey=KBDMNQAUFRTSJI-UHFFFAOYAA

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 1 → 2 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 1 → 3 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

Compounds like antipyrine 1 might be expected to have two pyramidal nitrogens with their substituents on opposite sides of the ring. Interestingly, a new polymorph of propyphenazone 2 has both N-methyl and N-phenyl groups on the same side of the ring. Just how unusual is this?

Roumanos and Kertesz¹ have sampled the crystallographic database and found 334 structures with the antipyrine backbone. The vast majority of them have the nitrogen substituents on opposite sides, and a few structures have these groups essential co-planar with the ring. The new propyphenazone structure does seem to be unusual. However, they also performed a BLYP/DNP scan of the potential energy surface of 2. When this surface is overlayed on the distribution of the x-ray structures, one sees that most structures are within 3 kcal mol^-1 of the energy minimum (with the nitrogen groups on opposite sides). However, the structure with both groups on the same side is about 4 kcal mol^-1 higher in energy than the minimum energy structure, and the nearly planar structures are higher in energy still. Thus, the authors conclude that while this new structure is unusual, it is not an outlier.

References

(1) Roumanos, M.; Kertesz, M., "Conformations of Antipyrines," J. Phys. Chem.A, 2011, ASAP, DOI: 10.1021/jp201510w

InChIs

1: InChI=1/C11H12N2O/c1-9-8-11(14)13(12(9)2)10-6-4-3-5-7-10/h3-8H,1-2H3
InChIKey=VEQOALNAAJBPNY-UHFFFAOYAS

2: InChI=1/C14H18N2O/c1-10(2)13-11(3)15(4)16(14(13)17)12-8-6-5-7-9-12/h5-10H,1-4H3
InChIKey=PXWLVJLKJGVOKE-UHFFFAOYAH

Single-walled carbon nanotubes (SWNT) can be thought of as built from component macrocycles, often called nanohoops. So, for example, cycloparaphenylenes like 1 can be the thought of as the precursor (at least in principle) of armchair SWNTs. To create chiral SWNTs, Itami¹ has suggested that cycloparaphenylene-naphthalene (2) and other acene substituted macrocycles would serve as appropriate precursors.

Itami has synthesized 2 (having 13 phenyl groups and one naphthyl group) and also examined the ring strain energy and racemization energy of a series of these types of compounds at B3LYP/6-31G(d). As might be expected, based on studies of the cycloparaphenylenes themselves,^2,3 ring strain energy decreases with increasing size of the macrocycle. So, for example, the macrocycle with one naphthyl group and 5 phenyl rings has a strain energy of 90 kcal mol^-1, but the strain is reduced to 40 kcal mol^-1 with 13 phenyl rings.

The macrocycle 2 and related structures are chiral, existing in P and M forms. The racemization involves first rotation of the naphthyl group, as shown in Figure 1, with a barrier of about 8 kcal mol^-1. The direct product has the opposite stereochemistry but is not in the lowest energy conformation. Rotations of some phenyl groups remains to occur, but these rotations are expected to have a barrier less than that for the rotation of the naphthyl group, based on the previous study of cycloparaphenylenes. Again, the racemization barrier decreases with the size of the macrocycle.

(P)-2	2-TS
(M)-2’

Figure 1. B3LYP/6-31G(d) optimized structures along the racemization pathway of 2.

References

(1) Omachi, H.; Segawa, Y.; Itami, K., "Synthesis and Racemization Process of Chiral Carbon Nanorings: A Step toward the Chemical Synthesis of Chiral Carbon Nanotubes," Org. Lett., 2011, 13, 2480-2483, DOI: 10.1021/ol200730m

(2) Segawa, Y.; Omachi, H.; Itami, K., "Theoretical Studies on the Structures and Strain Energies of Cycloparaphenylenes," Org. Lett., 2010, 12, 2262-2265, DOI: 10.1021/ol1006168

(3) Bachrach, S. M.; Stuck, D., "DFT Study of Cycloparaphenylenes and Heteroatom-Substituted Nanohoops," J. Org. Chem., 2010, 75, 6595-6604, DOI: 10.1021/jo101371m

InChIs

2: InChI=1/C88H58/c1-2-60-4-3-59(1)61-5-9-63(10-6-61)65-13-17-67(18-14-65)69-21-25-71(26-22-69)73-29-33-75(34-30-73)77-37-41-79(42-38-77)81-45-49-83(50-46-81)85-53-55-88-58-86(54-56-87(88)57-85)84-51-47-82(48-52-84)80-43-39-78(40-44-80)76-35-31-74(32-36-76)72-27-23-70(24-28-72)68-19-15-66(16-20-68)64-11-7-62(60)8-12-64/h1-58H/b61-59-,62-60-,65-63-,66-64-,69-67-,70-68-,73-71-,74-72-,77-75-,78-76-,81-79-,82-80-,85-83-,86-84+
InChIKey=VXOGKWSXPGSUSO-ZMOMEJFTBU

Saelli, Nicolaou, and Bagno point out in a recent article how the determination of the structure of vannusal B might have been guided by DFT computed ¹³C NMR chemical shifts, had they been available.¹ The original structure was proposed in 1999 as 1,² but was ultimately settled as 2 in 2010.³

The ¹³C NMR chemical shifts of 1 and 2 and some other alternatives were computed at M06/pcS-2//B3LYP/6-31g(d,p), where the pcS-2 basis set⁴ is one proposed by Jensen for computing chemical shifts. The computed chemical shifts of 1 poorly correlate with the experimental chemical shifts of vannusal B, with a low correlation coefficient of 0.9580 and a maximum error of 16.2 ppm. On the other hand, the correlation between the computed chemical shifts of 2 with the experimental values is excellent (R²=0.9948) and a maximum error of 3.0 ppm. Comparison of computed and experimental H-H coupling constants of model compounds of the “northeast” section of the molecule verified the correct structure is 2.

References

(1) Saielli, G.; Nicolaou, K. C.; Ortiz, A.; Zhang, H.; Bagno, A., "Addressing the Stereochemistry of Complex Organic Molecules by Density Functional Theory-NMR: Vannusal B in Retrospective," J. Am. Chem. Soc., 2011, 133, 6072-6077, DOI: 10.1021/ja201108a

(2) Guella, G.; Dini, F.; Pietra, F., "Metabolites with a Novel C30 Backbone from Marine Ciliates," Angew. Chem. Int. Ed., 1999, 38, 1134-1136, DOI: 10.1002/(SICI)1521-3773(19990419)38:8<1134::AID-ANIE1134>3.0.CO;2-U

(3) Nicolaou, K. C.; Ortiz, A.; Zhang, H.; Dagneau, P.; Lanver, A.; Jennings, M. P.; Arseniyadis, S.; Faraoni, R.; Lizos, D. E., "Total Synthesis and Structural Revision of Vannusals A and B: Synthesis of the Originally Assigned Structure of Vannusal B," J. Am. Chem. Soc., 2010, 132, 7138-7152, DOI: 10.1021/ja100740t

(4) Jensen, F., "Basis Set Convergence of Nuclear Magnetic Shielding Constants Calculated by Density Functional Methods," J. Chem. Theory Comput., 2008, 4, 719-727, DOI: 10.1021/ct800013z

InChI

vannusal B (2):
InChI=1/C31H46O5/c1-16(2)18-6-7-19(17(18)3)20-8-9-21-22(20)14-29(15-32)24-11-13-31(29,25(21)33)27(35)30(24)12-10-23(26(30)34)28(4,5)36/h14-15,17-21,23-27,33-36H,1,6-13H2,2-5H3/t17-,18+,19+,20+,21-,23+,24+,25+,26?,27+,29+,30-,31-/m1/s1
InChIKey=KYOBJLKAZYUEHK-GYGUSHOLBX

A significant portion of Chapter 4 of my book is devoted to phenylnitrene 2 and phenylcarbene. Phenyloxenium cation 1 is isoelectronic with phenylnitrene and so one might expect similar behavior of the two. Winter has reported a nice computational study of the singlet and triplet phenyloxenium cation and finds some very striking differences between phenyloxenium cation and phenylnitrene.¹

Phenylnitrene has a triplet ground state, with the ¹A₁ state about 18 kcal mol^-1 higher in energy, and the ¹A₂ state higher still. CASPT2/pVTZ//CASSCF(8,8)/pVTZ computations of 1 find the singlet ¹A₁ to be the ground state. The lowest triplet is 22.1 kcal mol^-1 higher in energy, and the lowest ¹A₁ state lies 30.8 kcal mol^-1 above the ground state singlet. (The structures of the lowest singlet and triplet of 1 are shown in Figure 1.) Reanalysis of the ultraviolet photoelectron spectrum of the phenoxy radical² switches the assignments of the observed transitions and is in excellent agreement with these computed values. G3 and CCSD(T)/cc-pVTZ predicts a similar value for the singlet-triplet gap. B3LYP, MPW1PW91, and some other DFT methods predict the singlet to be lower in energy than the triplet, but with a gap half of the correct value of 22 kcal mol^-1.

1 singlet (1A1)

1 triplet (3A2)

Figure 1. CASSCF(8,8) optimized geometries of the lowest singlet and triplet states of 1.

The origin of the difference between 1 and 2 lies in the description of the singlet state. The singlet state of 1 places the two lone pairs on oxygen into the sp-like orbital and into the in plane p orbital. However, in 2, the singlet is described by two determinants, one with the nitrogen lone pairs in the sp and in plane p orbital and the second determinant has them in the sp orbital and in the perpendicular p orbital. For 1, this single determinant allows for the positive charge to delocalize into the phenyl ring and off the very electronegative oxygen; this is manifest in a short C-O bond (1.211 Å). The greater electronegativity of oxygen then nitrogen brings the perpendicular p orbital lower in energy and better able to mix with the phenyl π-orbitals. In other words, the greater electronegativity of O over N results in a large symmetry break of the degenerate p orbitals.

References

(1) Hanway, P. J.; Winter, A. H., "Phenyloxenium Ions: More Like Phenylnitrenium Ions than Isoelectronic Phenylnitrenes?," J. Am. Chem. Soc., 2011, 133, 5086-5093, DOI: 10.1021/ja1114612

(2) Dewar, M. J. S.; David, D. E., "Ultraviolet photoelectron spectrum of the phenoxy radical," J. Am. Chem. Soc., 1980, 102, 7387-7389, DOI: 10.1021/ja00544a050

Lash has synthesized the simplified component of a porphyrin 1, which lacks two of the pyrrole rings.¹ This compound should act as a modified [18]annulene, and the NMR and x-ray structure support that notion. The NMR shows a multiplet at -2.52 ppm for the internal protons and the external protons show up at 9.88 and 9.96 ppm. The x-ray structure exhibits a nearly planar structure, with the C-C distances around the macrocycle varying from 1.379 to 1.418 Å. Interestingly, the UV-vis of 1shows a Soret-band at 401 nm, indicative of porphyrin-like behavior.

It is a simple thing to do some computations on a model of 1, and so I have computed (at B3LYP/6-311+G(d,p)) the structure and NMR of 2, shown in Figure 1. This compound is strictly planar. The C-C distances about the macrocycle vary from 1.386 to 1.415 Å, in excellent agreement with the experiment and indicating little bond alternation. The NICS values at the center of the macrocycle and 1 Å above this point are -12.6 and -11.9 ppm, supporting the aromatic [18]annulene structure. Further, the chemical shifts of the interior and exterior protons are computed to be -7.6 (interior) and 11.4 ppm (exterior) – in fair agreement with experiment. Nonetheless, simple computations provide support for the notion that this compound, and related porphyrins have a dominant [18]annulene character.

2

Figure 1. B3LYP/6-311+G(d,p) optimized structure of 2.

References

(1) Lash, T. D.; Jones, S. A.; Ferrence, G. M., "Synthesis and Characterization of Tetraphenyl-21,23-dideazaporphyrin: The Best Evidence Yet That Porphyrins Really Are the [18]Annulenes of Nature," J. Am. Chem. Soc., 2010, 132, 12786-12787, DOI: 10.1021/ja105146a

InChIs

1: InChI=1/C44H32N2/c1-2-18-30-38-42(34-23-11-6-12-24-34)44(36-27-15-8-16-28-36)40(46-38)32-20-4-3-19-3
1-39-43(35-25-13-7-14-26-35)41(37(45-39)29-17-1)33-21-9-5-10-22-33/h1-32H/b2-1-,4-3-,17-1+,18-2+,19-3+,20-4+,29-17+,30-18+,31-19+,32-20+,37-29-,38-30-,39-31-,40-32-
InChIKey=SOKXIWMGPPRDDE-VZLTUNCZBQ

2: InChI=1/C20H16N2/c1-2-6-10-18-15-16-20(22-18)12-8-4-3-7-11-19-14-13-17(21-19)9-5-1/h1-16H/b2-1-,4-3-,5-1+,6-2+,7-3+,8-4+,9-5+,10-6+,11-7+,12-8+,17-9-,18-10-,19-11-,20-12-
InChIKey=IWCJELFJBPNKQF-ICIIEBMOBH