The Reaction Electronic Flux Perspective on the Mechanism of the

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The Reaction Electronic Flux Perspective on the Mechanism of the Zimmerman Di-#-Methane Rearrangement Ricardo A. Matute, Patricia Pérez, Eduardo Chamorro, Nery Villegas-Escobar, Diego CortésArriagada, Barbara Herrera, Soledad Gutiérrez-Oliva, and Prof. Alejandro Toro-Labbé J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00499 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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The Journal of Organic Chemistry

The Reaction Electronic Flux Perspective on the Mechanism of the Zimmerman Di-π-Methane Rearrangement Ricardo A. Matute*,1,2,3 Patricia Pérez,4 Eduardo Chamorro,4 Nery Villegas-Escobar,1 Diego Cortés-Arriagada,5 Bárbara Herrera,1 Soledad Gutiérrez-Oliva1 and Alejandro Toro-Labbé*,1

1

Laboratorio de Química Teórica Computacional (QTC), Facultad de Química, Pontificia Universidad

Católica de Chile, Casilla 306, Correo 22, Santiago, Chile 2

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA

91125 3

Centro Integrativo de Biología y Química Aplicada (CIBQA), Universidad Bernardo O Higgins, Santiago

8370854, Chile 4

Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Ciencias Químicas,

Avenida República 275, 8370146 Santiago, Chile 5

Programa Institucional de Fomento a la Investigación, Desarrollo e Innovación, Universidad

Tecnológica Metropolitana, Ignacio Valdivieso 2409, P.O. Box 8940577, San Joaquín, Santiago

*E-mail (R.A.M.): [email protected] *E-mail (A.T.-L.): [email protected]

Abstract The reaction electronic flux (REF) offers a powerful tool in the analysis of reaction mechanisms. Noteworthy, the relation between aromaticity and REF can eventually reveal subtle electronic events associated to reactivity in aromatic systems. In this work, this relation was studied for the Triplet Zimmerman Di-πMethane rearrangement. The aromaticity loss and gain taking place during the reaction is well acquainted by the REF, thus shedding light on the electronic nature of reactions involving dibenzobarrelenes.

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Introduction The di-π-methane (DPM), also called Zimmerman rearrangement, 1 is a relevant photochemical reaction and formally a [π2 + σ2] rearrangement of 1,4-dienes. In bicyclic systems, it proceeds via triplet state upon sensitization, involving a complex two-steps transformation that involves, in the first step, the formation of a sigma bond together with loss of ring aromaticity.2 The second step comprises the breaking of a sigma bond together with the recovery of ring aromaticity.2 Despite of the extensive work on it, the nature of the electronic processes that drive the DPM rearrangement remains unclear. The most natural way to capture the electronic activity that explains the reaction mechanism is to track it along the intrinsic reaction coordinate (IRC) by means of the so-called Reaction Electronic Flux (REF): J(ξ)= -(dμ/dξ) with μ as the electronic chemical potential.3 From this approach, negative REF values indicate that bond weakening/breaking processes drive the reaction mechanism, whereas positive REF values indicate that the reaction is led by bond strengthening/formation processes.3 Nonetheless, although direct interpretation of REF profiles can be challenging for reactions involving both loss and recovery of aromaticity, such analysis is likely necessary to understand the electronic nature of some sigmatropic rearrangements. Hence, we study in this work the connection between REF and aromaticity for a disrupted Möbius system in the Zimmerman DPM rearrangement. The triplet sensitized rearrangement of dibenzobarrelene (DBB) (Scheme 1) is a

well-known

and

representative

Aryl-Aryl

DPM

reaction,

which

gives

dibenzosemibullvalene (SBV) as product.2 The central barrelene in DBB confers the molecule

the

Möbius

character,

since the

out-of-phase

overlap

between

perpendicular π-orbitals in its non-planar geometry (Scheme 1), and therefore its six π electrons make barrelene an antiaromatic molecule due to the 4n + 2 rule for Möbius antiaromaticity.

Indeed,

barrelene

is

an

antiaromatic

Möbius

system like

4

cyclobutadiene is an antiaromatic Hückel system. Note that the benzo groups in DBB do not affect the Möbius character of barrelene.5 On the other hand, the reaction product (i.e, SBV) is a Hückel system (Scheme 1), since the extended delocalized πsystem is disrupted in T1 upon sensibilization. In the lowest-triplet-state (T1), the triplet 1,2-biradical (DBB*) rearranges first to a 1,4-biradical (BR-I) and then to a 1,3biradical (BR-II) that decays through an intersystem crossing (ISC) channel to the 2 ACS Paragon Plus Environment

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ground state product.2 The transition states in T1, i.e., TS-I and TS-II, lead the corresponding transitions. The rate-determining step is governed by TS-I in T1. According to Matute and Houk,2 the triplet surface of the DPM Rearrangement of DBB allows a shortcut on the potential energy surface (PES), bypassing the energetically shallow BR-I, thus leading to competing one-step and two-step mechanisms.

Scheme 1. Reaction mechanism for the DPM rearrangement of dibenzo-barrelene. Upon sensitization, competing one-step and two-step pathways take place on T1. The π-systems of DBB (Möbius reactant) and SBV (Hückel product) are shown below.

Computational Methods The electronic energies were calculated using unrestricted density functional theory (DFT) with the (U)M06-2X functional6 and 6-31G(d) basis set. The NICS(0) and NICS(1)7 for each ring of DBB*(T1) and benzene (S0 and T1) were calculated at M06-2X/6-31G(d) level of theory. All quantum chemistry calculations were performed using Gaussian09 package.8 In addition, the topological analysis of ELF was performed with the Topmod9 programs and the Multiwfn10 suite of tools for the HOMA, ELF, and AIM. 3 ACS Paragon Plus Environment


Aromaticity is related with changes in the electronic delocalization of the πsystem along the reaction coordinate. It was assessed along the minimum energy path (MEP) of the T1 state connecting DBB, BR-I, and BR-II. The MEP, defined by the reaction coordinate ξ in the PES, was explored by means of intrinsic reaction coordinate calculations. The Gonzalez-Schlegel method11 for IRC was used as implemented in Gaussian09. The reaction force analysis (RFA) of the MEP was performed to get insights regarding the chemical driving force along the reaction coordinate. The formalism of RFA has its base on the differentiation of the energy with respect to the intrinsic reaction coordinate: F(ξ)= -dE(ξ)/dξ.12 When deriving the reaction energy, leads to the Hellman-Feynman reaction force, and it is a global property that helps to get helpful information about the chemical events that are taking place in any given reaction.12 The partitioning of the energy ac-cording to the defined regions into W1 (reactant region), W2 (TS region, ξ < TS), W3 (TS region, ξ > TS), and W4 (product region), allows to characterize the chemical nature of the reaction and activation energies, since in most cases both W1 and W4 typically accounts for a structural work whereas W2 and W3 mostly accounts for electronic effects. 12 The Reaction Electronic Flux (REF) is defined as J(ξ)= -(dμ/dξ). It accounts for the escaping tendency of electrons from an equilibrium system and is associated to the negative of electronegativity defined by Mulliken. Owing to the discontinuity of the energy along the number of electrons, this index can be quantified using the finite difference approximation to values of the ionization energies (I) and the electron affinities (A) and with the Koopmans theorem to the energies of frontier orbitals HOMO and LUMO molecular orbitals εHOMO and εLUMO.12

Results and Discussion The MEP on T1 is defined by the IRC profile shown in Figure 1a. The reaction force analysis of the MEP (see Fig. 1a and Table 1) gives considerably higher values for W1 and W4 when compared to W2 and W3 respectively, for both TS-I and TS-II: the main contribution in the activation energy is coming from structural and not from electronic changes. 4 ACS Paragon Plus Environment

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Figure 1. Reaction plots on T1 for (a) IRC and RFA, (b) REF, (c) HOMA and BO, (d) electron density at the ring critical points (RCPs).

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Table 1. Energy (in kcal/mol) contributions from the reaction force analysis. Stage[a] W1

W2

W3

W4

TS-I

7.25

3.5

-3.67

-6.11

TS-II

1.75

1.04

-5.52

-11.33

[a]

TS-I: ΔE°= 0.97 and ΔE≠= 10.75; TS-II: ΔE°= -14.06 and ΔE≠= 2.79

The REF profile that involves TS-I and TS-II are shown in Figure 1b. The TS-I is associated with a negative REF curve, which reveals a process of bond weakening (aromaticity loss) before the formation of the C9a-C12 bond that is exposed by a positive peak leaving the TS-I region. The BO plot confirms the formation of the C9aC12 bond on the way from DBB* to the cyclopropane intermediate in BR-I. For the second step of the reaction, the REF plot shows a negative peak consistent with the C9-C9a bond breaking followed by a positive peak assigned to the recovery of aromaticity. In summary, the REF profile reveals that the DPM rearrangement is driven by four main chemical events sequentially appearing along the reaction coordinate: aromaticity loss complemented with C12-C9a bond formation in the first step and C9C9a bond breaking ensuing with aromaticity recovery in the second step. The projection of the REF profile onto the energy profile allows to quantify the energy involved in the main chemical events, in the first step of the reaction the aromaticity loss involves about 10 kcal/mol and the ensuing formation of the C12-C9a sigma bond engages roughly 8 kcal/mol. In the second step of the reaction the C9-C9a bond breaking and aromaticity recovery involves approximately 3 kcal/mol and 6 kcal/mol, respectively. Five different methods were used to characterize the change in aromaticity along the reaction coordinate: the Harmonic Oscillator Model of Aromaticity (HOMA),13 the Atoms-in-Molecules (AIM),14 the Electron Localization Function (ELF),15 the NucleusIndependent Chemical Shifts (NICS),7 and the Anisotropy Current Induced Density (ACID)16. HOMA is defined as the normalized sum of squared deviations of bond lengths from an optimal value (i.e., a fully aromatic system); ELF is the measure of the likelihood of finding an electron with the same spin in the neighborhood space of 6 ACS Paragon Plus Environment

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a reference electron; AIM provides the analysis of the electron density in the ring critical points (RCPs); the NICS (specifically NICS(0)) calculates the absolute magnetic shielding at the center of a ring, although it can be calculated at certain distance above or below the center of the ring (e.g., 1 Å above the molecular plane for NICS(1)); and the ACID scalar field defines the density of delocalized electrons. In addition, Bond Order (BO) calculations in the Mayer scheme 17 were performed to track bond formation and breaking from the cyclopropane intermediate, involving the C9aC12 and C9a-C9 atoms respectively. The slight shifting observed in the REF’s peaks with respect to the BOs and HOMAs descriptors indicates that previous electronic activity is needed to promote changes in these properties. Hence, the BO curves were contrasted in the same plot against the HOMA curves of both rings (Fig. 1c). The HOMA for ring B is suffering first a sharp decline in TS-I that is then recovered in TS-II, thus indicating a loss followed by a recovery of the aromaticity in that ring, this electronic π-delocalization in ring B is effectively reflected on the REF profile in the T 1 state. On the other hand, HOMA for ring A is steady along TS-I region and just slightly decreases for TS-II, reflecting the fact that the π-delocalization in ring A is slightly perturbed due to the formation of a benzyl radical at that step. Similar curves are obtained when the density at the RCP of ring A is computed (Fig. 1d). Further analysis by means of the calculation of the electron localization function (ELF) for DBB* in two molecular planes defined by the C8a-C5-C7 and C9a-C4-C2 centers (Fig. 2), with the map of a benzene (S0) as reference of the most delocalized system, shows that the ELF maps for both sixmember rings in DBB* are qualitatively similar to each other, revealing that both rings present the same delocalization topology pattern and high similitude to that showed by a ground state benzene. Consistency of the results from HOMA, AIM, and ELF confirms the REF findings concerning the role of π-delocalized electrons on the reaction mechanism.



Figure 2. Colour-filled maps of the electron localization function (ELF), in two molecular planes of DBB* defined by the (a) C7-C5-C8a centers, (b) C2-C4-C9a centers, and (c) benzene (S0) at (U)M062x/6-31G(d) level. The maps use different colours to represent ELF value in different regions ranging from blue (no electron localization) to red (high electron localization). The topology of ELF in both sixmembered rings of DBB* are equivalent, depicting the same delocalization topology pattern as revealed for benzene (S0).

This is the very first example in which the REF accounts mostly for πreordering. An extension of the REF has been recently reported in which σ– and πelectronic activity can be tracked but restricted only for symmetric systems. 18 Nonetheless, we need to be cautious with the assignment of aromaticity in T1, since a reversal of the aromaticity could be operating along the MEP. The reversal of aromaticity for systems in the lowest triplet state (T 1) was predicted by N. C. Baird in 1972,19 hence known as Baird’s rule. Although the term “Baird’s rule” seems to have been used for the first time by Fowler in 2008 (before any experimental support). 20 Yet, Ottosson, Kilså and co-workers in a combined experimental and computational study in 2007 explored the scope and limitations of Baird’s theory on triplet state aromaticity,21 and earlier Peter Wan and co-workers pioneered the experimental studies on the excited state aromaticity concept (see e.g. Ref. 22), although Wan never cited Baird’s theory paper as he was unaware of its existence. Actually, there were experimental evidence on Baird-aromaticity shortly after Colin Baird published his seminal theory paper19 because Saunders, Breslow and collaborators discovered that the cyclopentadienyl cation is a ground state triplet with D5h symmetry.23 8 ACS Paragon Plus Environment

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Schleyer and co-workers later found it to be aromatic according to NICS and calculations of aromatic stabilization energies,24 and additional experimental studies on cyclopentadienyl cation were reported by Wörner and Merkt. 25 According to the Baird's rule summarized above, in case where reversal of aromaticity takes place, 4n π-electrons would lead aromaticity in detriment of 4n+2. In this work, then the question that arises is if the Baird’s rule19 is applicable to the Zimmerman DPM rearrangement of dibenzobarrelenes. In such a context, the REF analysis along the reaction coordinate, the spin density distributions as well as NICS and ACID methodologies were used to characterize the delocalization (aromaticity) pattern in these systems and to elucidate the either aromatic or antiaromatic nature of the six-member rings in DBB*. The spin density distribution shown in Figure 3a confirms that the DBB* is localized to the bridging ethylene bond; that in the 1,4-biradical intermediate, BR-I, one of the radical centers is delocalized in one of the rings; and that the delocalization is switched to the other ring in the 1,3-biradical intermediate, BR-II. It is quite remarkable, though, that the triplet excitation to DBB* is localized to the small ethylene moiety and not to the larger benzene rings. To get further insights on this regard, the Figure 3b compares the spin density distribution of DBB* with the compound having a saturated bridge, whereas the Figure 5 compares both (together with the other T1 structures) in terms of NICS(0) and NICS(1). The high positive NICS values for the compound having a saturated bridge are indicative of its antiaromatic nature. Hence, it seems the benzene rings of DBB* are able to completely avoid the unfavorable triplet excitation as the excitation instead localizes to the ethylene bridge, thus suggesting a T1 state antiaromaticity alleviation26,27 for the triplet excitation of dibenzobarrelenes. In addition, from Figure 4, the NICS(0) calculation for a ring of DBB* gives -9.1 ppm suggesting aromaticity, since ground state benzenes show negative values (NICS of -7.5 ppm in Fig. 5) whereas antiaromaticity is associated to high positive values (NICS of +31.6 ppm in Fig. 5).



Figure 3. (a) Spin densities of DBB*, BR-I and BR-II; (b) Spin density distribution of structures with double bond bridge and saturated bridge on T 1.

Figure 4. Calculated NICS(0) and NICS(1) on both rings of DBB* (a), its structure with saturated bridge (b), and the intermediates BR-I (c) and BR-II (d). 10 ACS Paragon Plus Environment

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From the ACID calculation (Fig. 5), the degree of electron delocalization can be roughly quantified by inspection of critical isosurface values (CIVs) in which the topology of the ACID boundary surface becomes separated into independent enveloping surfaces. The first critical isosurface value, i.e., for the two rings in DBB* (CIV = 0.076) is only slightly lower than the value of benzene (CIV=0.081). Moreover, the delocalization pattern in DBB* is analogous to the well-known observed for ground state benzene, i.e., portraying both strong diatropic ("aromatic") and weak paratropic ("antiaromatic") ring currents.16 In the case of triplet benzene, the ACID reveals only paratropic currents. Thus, in DBB* the two rings can be characterized as "aromatics" by contrasting against singlet and triplet benzene used as reference. In contrast, the paratropic currents of the triplet compound having a saturated bridge suggest antiaromaticity. These results are consistent with those found using the NICS methodology, thus indicating that the Baird’s rule is not applicable in this case due to that the unpaired parallel spins do not coexist in the same ring in DBB*, BR-I, or BR-II (see NICS and ACID for BR-I and BR-II in SI). Nevertheless, it is possible to argue that, at least in principle, the particular triplet state of DBB which has the excitation localized to the benzene rings could be destabilized because the Baird's rule to such an extent that another triplet state, one with the excitation localized to the ethylene bridge instead, becomes the T 1 state. If so, Baird’s rule on triplet state (anti)aromaticity could be still operational, but in an indirect way.



Figure 5. (U)M062x/6-31G(d) ACID isosurface (=0.05) of (a) Benzene (S0), (b) Benzene (T1), and (c) DBB*. The magnetic field vector is parallel to the z-axis pointing out of page (x-y plane), and the current density vectors plotted onto the ACID isosurface indicate the magnetically induced diatropic (clockwise) and paratropic (anticlockwise) flowing ring currents. The observed pattern in either direction will depend on the relative directions of the current flow and the applied magnetic field. Calculated NICS(0) values are given in units of ppm. Conclusion Overall, our results show that the electronic flux along the intrinsic reaction coordinate in the triplet DPM rearrangement of DBB is mostly affected by loss and recovery of aromaticity, thus leading to step-wise electronic events for both formation and breaking of sigma bonds. Hence, the non-statistical dynamics nature2 of the 12 ACS Paragon Plus Environment

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Zimmerman DPM rearrangement and the recently reported carbon tunneling of this reaction28 should be envisioned together with our REF perspective in order to get complete understanding of its intricated mechanism. Moreover, the coupling between REF and aromaticity certainly paves the way toward a better understanding of the electronic nature in reactions involving aromatic molecules. Acknowledgements The authors acknowledge the FONDECYT Grants No.1130072, No.1140341, No.1140343 and No.1170837. We are indebted to Prof Dr Rainer Herges for kindly providing us a copy of the ACID software package. Associated Content ACID and ELF backgrounds and the corresponding discussions, optimized geometries, energies of all computed species and additional computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

References 1. a) Zimmerman, H. E.; Grunewald, G. L. The Chemistry of Barrelene. III. A Unique Photoisomerization to Semibullvalene. J. Am. Chem. Soc. 1966, 88, 183. b) Zimmerman, H. E.; Binkley, H. E.; Givens, R. S.; Sherwin, M. A. Mechanistic organic photochemistry. XXIV. The mechanism of the conversion of barrelene to semibullvalene. A general photochemical process. J. Am. Chem. Soc. 1967, 89, 3932. c) Ciganek, E. The Photoisomerization of Dibenzobicyclo[2.2.2]octatrienes. J. Am. Chem. Soc. 1966, 88, 2882. 2. a) Matute, R. A.; Houk, K. N. The triplet surface of the Zimmerman di-π-methane rearrangement of dibenzobarrelene. Angew. Chem. Int. Ed. 2012, 51, 13097. b) Jimenez-Oses, G.; Liu, P.; Matute, R. A.; Houk, K. N. Competition between concerted and stepwise dynamics in the triplet di-π-methane rearrangement. Angew. Chem. Int. Ed. 2014, 53, 8664. c) Matute, R. A.; Garcia-Garibay, M. A.; Houk, K. N. Theory of substituent effects on the regioselectivity of di-π-methane rearrangements of dibenzobarrelenes. Org. Lett. 2014, 16, 5232.



3. a) Echegaray, E.; Toro-Labbe, A. Reaction Electronic Flux: A New Concept To Get Insights into Reaction Mechanisms. Study of Model Symmetric Nucleophilic Substitutions. J. Phys. Chem. A 2008, 112, 11801. b) Giri, S.; Echegaray, E.; Ayers, P. W.; Nunez, A. S.; Lund, F.; Toro-Labbe, A. Insights into the Mechanism of an SN2 Reaction from the Reaction Force and the Reaction Electronic Flux. J. Phys. Chem. A 2012, 116, 10015. 4. Yates, K. in Hückel Molecular Orbital Theory (Academic Press, Inc. Ltd.) 1978, pp. 301. 5. Haselbach, E.; Neuhaus, L.; Johnson, R. P.; Houk, K. N.; Paddon-Row, M. N. PI.-Orbital interactions in mobius-type molecules as studied by photoelectron spectroscopy. Helv. Chim. Acta 1982, 65, 1743. 6. Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215 7. Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. Nucleus-Independent Chemical Shifts (NICS) as an Aromaticity Criterion. Chem. Rev. 2005, 105, 3842. 8. Frisch, M. J. et. al. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT 2009. 9. Noury, S.; Krokidis, X.; Fuster, F.; Silvi, B. Computational tools for the electron localization function topological analysis. Comput. Chem. 1999, 23, 597. 10. Lu, T.; Chen, F. W. J. Multiwfn: a multifunctional wavefunction analyzer. Comput. Chem. 2012, 33, 580. 11. Gonzalez, C.; Schlegel, H. B. J. An improved algorithm for reaction path following. Chem. Phys. 1989, 90, 2154. 12. a) Toro-Labbé, A. Characterization of Chemical Reactions from the Profiles of Energy, Chemical Potential, and Hardness. J. Phys. Chem. A 1999, 103, 4398. b) Politzer, P.; Reimers, J. R.; Murray, J. S.; Toro-Labbé, A. Reaction Force and Its Link to Diabatic Analysis: A Unifying Approach to Analyzing Chemical Reactions. J. Phys. Chem. Lett. 2010, 1, 2858. 13. a) Krygowski, J. K. a. T. M. Definition of aromaticity basing on the harmonic oscillator model. Tetrahedron Lett. 1972, 13, 3839. b) Krygowski, T. M. Crystallographic studies of inter- and intramolecular interactions reflected in aromatic character of .pi.-electron systems. J. Chem. Inf. Comput. Sci. 1993, 33, 70. 14. Bader, R. F. in Atoms in molecules (John Wiley & Sons, Ltd.) 1990.


Page 14 of 16

Page 15 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15. Becke, A. D.; Edgecombe, K. E. A simple measure of electron localization in atomic and molecular systems. J. Chem. Phys. 1990, 92, 5397. 16. a) Geuenich, D.; Hess, K.; Kohler, F.; Herges, R. Anisotropy of the Induced Current Density (ACID), a General Method To Quantify and Visualize Electronic Delocalization. Chem. Rev. 2005, 105, 3758. b) Herges, R.; Geuenich, D. Delocalization of Electrons in Molecules. J. Phys. Chem. A 2001, 105, 3214. 17. Mayer, I. Charge, bond order and valence in the AB initio SCF theory. Chem. Phys. Lett. 1983, 97, 270. 18. a) Villegas-Escobar, N.; Vogt-Geisse, S.; Gutiérrez-Oliva, S.; Toro-Labbé, A. Symmetryadapted reaction electronic flux in cycloaddition reactions. Theor. Chem. Acc. 2016, 135, 191. b) Vogt-Geisse, S.; Toro-Labbe, A. Chemical potential and reaction electronic flux in symmetry controlled reactions. J. Comput. Chem. 2016, 37, 1794. 19. Baird, N. C. Quantum organic photochemistry. II. Resonance and aromaticity in the lowest 3.pi..pi.* state of cyclic hydrocarbons. J. Am. Chem. Soc. 1972, 94, 4941. 20. Soncini, A.; Fowler, P. W. Ring-current aromaticity in open-shell systems. Chem. Phys. Lett. 2008, 450, 431. 21. Ottosson, H.; Kilså, K.; Chajara, K.; Piqueras, M.; Crespo, R.; Kato, H.; Muthas, D. Scope and limitations of Baird's theory on triplet state aromaticity: application to the tuning of singlet-triplet energy gaps in fulvenes. Chem. Eur. J. 2007, 13, 6998. 22. Wan, P.; Krogh, E. Evidence for the generation of aromatic cationic systems in the excited state. Photochemical solvolysis of fluoren-9-ol. Chem. Commun. 1985, 0, 1207 23. Saunders, M.; Berger, R.; Jaffe, A.; McBride, J. M.; O'Neill, J.; Breslow, R.; Hoffmann, J. M.; Perchonock, C.; Wasserman, E.; Hutton, R. S.; Kuck, V. J. Unsubstituted cyclopentadienyl cation, a ground-state triplet. J. Am. Chem. Soc. 1973, 95, 3017. 24. Gogonea, V.; Schleyer, P. v. R.; Schreiner, P. R. Consequences of Triplet Aromaticity in 4nπ-Electron Annulenes: Calculation of Magnetic Shieldings for Open-Shell Species. Angew. Chem. Int. Ed. 1998, 37, 1945. 25. Wörner, H. J.; Merkt, F. J. Diradicals, antiaromaticity, and the pseudo-Jahn-Teller effect: electronic and rovibronic structures of the cyclopentadienyl cation. Chem. Phys. 2007, 127, 034303. 26. Mohamed, R. K.; Mondal, S.; Jorner, K.; Delgado, T. F.; Lobodin, V. V.; Ottosson, H.; Alabugin, I. V. The Missing C1-C5 Cycloaromatization Reaction: Triplet State



Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes. J. Am. Chem. Soc. 2015, 137, 15441. 27. Papadakis, R.; Li, H.; Bergman, J.; Lundstedt, A.; Jorner, K.; Ayub, R.; Haldar, S.; Jahn, B. O.; Denisova, A.; Zietz, B.; Lindh, R.; Sanyal, B.; Grennberg, H.; Leifer, K.; Ottosson, H. Metal-free photochemical silylations and transfer hydrogenations of benzenoid hydrocarbons and graphene. Nat. Commun. 2016, 7 , 12962. 28. Li, X.; Liao, T.; Chung, L. W. Computational Prediction of Excited-State Carbon Tunneling in the Two Steps of Triplet Zimmerman Di-π-Methane Rearrangement. J. Am. Chem. Soc. 2017, 139, 16438.

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The Reaction Electronic Flux Perspective on the Mechanism of the

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