Unsubstituted cyclopentadienyl cation, a ground-state triplet

Saunders, Martin; Berger, Reinhard; Jaffe, Annette B.; McBride, J. Michael; O’Neill, James A.; Breslow, Ronald; Hoffmȧnn, J.; Perchonock, Carl D.; Wasserman, E.; al., et

doi:10.1021/ja00790a049

Cited by 179 publications

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“…[33] Finally, we prepared cyclopentadienyl cation 22 itself, in collaboration with Martin Saunders and Mike McBride at Yale. [34] It was prepared from 5-bromocyclopentadiene and a very strong Lewis acid under special conditions. It also showed the ESR spectrum of a triplet, and the temperature effects of a groundstate triplet.…”

Section: Aromaticitymentioning

confidence: 99%

Novel Aromatic and Antiaromatic Systems

Breslow

2014

The Chemical Record

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Note from the Editor: August Kekulé's dream and his theory of the structure of benzene (1865). Julius Thomsen's logic that benzene contains equivalent electrons between its carbon atoms (ca. 1900). Richard Willstätter's synthesis and observation that 1,3,5,7‐cyclooctatetraene is not aromatic (early 1900s). Sir Robert Robinson's proposal of the ‘aromatic sextet’ (1925). Erich Hückel's molecular orbital treatment of benzene and other unsaturated compounds that separated sigma and pi electrons (early 1930s). Tetsuo Nozoe and Michael Dewar's independent proposal of a new aromatic structure for a cycloheptatrieneone (mid‐1940s). Ronald Breslow's demonstration that cyclopropenyl cation is aromatic (1958). Breslow's proposal of ‘antiaromaticity’ (1965). I offer several comments. First, in the field of aromaticity, Breslow is in mighty exceptional company. Second, his research has extended well beyond aromaticity. Yes, Breslow is applying the concepts of antiaromaticity toward the development of highly conductive organic materials. But his research has encompassed enzyme mimics, remote functionalization reactions, unnatural DNA analogues and cancer chemotherapy. Indeed, Breslow is the co‐discoverer of a highly successful drug, Vorinostat, which is FDA‐approved for the treatment of cutaneous T‐cell lymphoma. We thank Professor Breslow for joining our project honoring his and our friend, Tetsuo Nozoe. Tetsuo Nozoe would have would have been deeply touched. —Jeffrey I. Seeman Guest Editor University of Richmond Richmond, Virginia 23173, USA E‐mail: jseeman@richmond.edu Ronald Breslow with Koji Nakanishi (left) and Tetsuo Nozoe (center), Sendai, 1970. Abstract We contributed to the field of non‐benzenoid aromatic compounds by creating the cyclopropenyl cation and various of its derivatives, including cyclopropenone; it was the first aromatic system with other than six pi electrons in a single ring, and the simplest aromatic system. The pioneering work of Tetsuo Nozoe in tropolone chemistry was celebrated with the founding of ISNA, the International Symposium on Non‐Benzenoid Aromatic Compounds, where I described our work in the field. It fit the prediction that aromaticity would be found in systems with 4n + 2 pi electrons, where n is an integer. I was also concerned with the properties of monocyclic systems with 4n cyclically conjugated pi electrons. They were expected not to be aromatic, but the interesting question was whether they were actually antiaromatic, especially destabilized by the cyclic conjugation in such 4n species as the cyclopropenyl anion, cyclobutadiene, and cyclopentadienyl cation. The evidence supports antiaromaticity in these cases. We also examined compounds where 4n cyclic pi systems were fused with aromatic systems, and most interestingly systems in which two 4n pi systems were fused. In these cases the periphery of the molecules had 4n + 2 pi electrons, for aromaticity, but the components were antiaromatic. Recently we have studied electrical conductivities in aromatic molecules such as thiophe...

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Section: Aromaticitymentioning

confidence: 99%

Novel Aromatic and Antiaromatic Systems

Breslow

2014

The Chemical Record

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“…Baird [14] argued in a classic paper, however, that aromaticity should also be associated with 4n p-electron monocycles in their triplet states. This argument, based on qualitative calculation of comparative energetics, is supported by the existence of planar triplet ground states of 4p species such as C 5 X Cl [15,16]). An extensive ab initio study by Gogonea et al [17] concluded that triplet aromaticity in 4n p systems does indeed exist and that is associated with planarisation, bond equalisation, stabilisation, negative NICS, downfield 1 H NMR shifts, and significant diamagnetic orbital susceptibility.…”

Section: Introductionmentioning

confidence: 98%

Ring-current aromaticity in triplet states of 4n π electron monocycles

Fowler

Steiner

Jenneskens

2003

Chemical Physics Letters

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The ipsocentric approach to orbital contributions to current density predicts that a triplet state of a 4n p-electron H€ u uckel monocycle should be aromatic, in the sense of supporting a diatropic ring current, and that this current should be carried by just four of the p electrons (three for n ¼ 1). The magnetic criterion of aromaticity is thus in agreement with BairdÕs classic energy-based prediction of aromatic stabilisation for these species.

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“…[50] X X-Bandensystems in beiden Molekülen als eine schwache (mit dem Symbol "j = 0" gekennzeichnete) Linie beobachtet, der sich unmittelbar eine we- …”

Section: Cyclopentadienylkationunclassified

Untersuchung des Jahn‐Teller‐Effekts in molekularen Kationen mithilfe von Photoelektronenspektroskopie und Gruppentheorie

Wörner

Merkt

2009

Angewandte Chemie

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Das klassische Kugel‐Stab‐Modell molekularer Strukturen verliert seine Gültigkeit, sobald die Bewegung der Elektronen an die Bewegung der Kerne gekoppelt ist. Diese Situation tritt beim Jahn‐Teller(JT)‐Effekt auf, der bei offenschaligen molekularen Systemen, insbesondere Radikalen und Ionen, häufig ist. Der JT‐Effekt ist jedem Chemiker als ein Mechanismus zur Verzerrung symmetrischer Systeme bekannt. Seine Auswirkung auf die Dynamik von Molekülen stellt jedoch in vielen Fällen ein ungelöstes Problem dar. Dieser Aufsatz gibt eine Übersicht über neue Erkenntnisse zur dynamischen Struktur molekularer Kationen von hoher Permutationssymmetrie, die durch rotationsauflösende Photoelektronenspektroskopie und Gruppentheorie gewonnen wurden. Unser besonderes Augenmerk gilt dem Cyclopentadienylkation, bei dem der Pseudo‐Jahn‐Teller‐Effekt eine Lokalisierung der elektronischen und eine Delokalisierung der Kernwellenfunktion hervorruft. Dabei werden auch die grundlegenden Mechanismen erörtert, die hinter dem vage definierten Konzept der “Antiaromatizität” stecken. Unsere Untersuchung des Methankations stellt die erste experimentelle Charakterisierung des JT‐Effekts in einem dreifach entarteten elektronischen Zustand dar. Eine besondere Form von JT‐induzierter Isomerie wurde entdeckt, deren Existenz in allen JT‐Systemen, in denen die Minima der Potentialfläche durch hohe Barrieren getrennt sind, vorausgesagt wird.

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Unsubstituted cyclopentadienyl cation, a ground-state triplet

Abstract: 13) Good Curie law behavior was observed for most samples down to liquid helium temperature. Although some showed deviations near 10°K, this was related to the prior history of the sample.

Cited by 179 publications

References 0 publications

Novel Aromatic and Antiaromatic Systems

Novel Aromatic and Antiaromatic Systems

Ring-current aromaticity in triplet states of 4n π electron monocycles

Untersuchung des Jahn‐Teller‐Effekts in molekularen Kationen mithilfe von Photoelektronenspektroskopie und Gruppentheorie

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