Cyclobutadiene and diphenylcyclobutadiene

Dewar, Michael J. S.; Kohn, Moritz; Trinajstić, Nenad

doi:10.1021/ja00743a023

Cited by 59 publications

(12 citation statements)

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“…The famous case of singlet cyclobutadiene, where a strong D2h deformation is calculated theoretically (22,23), is in doubt. The experimental evidence strongly supports a singlet with an undistorted D4h structure (24)(25)(26).…”

Section: T)mentioning

confidence: 99%

Concerning Jahn-Teller Effects

Pearson

1975

Proc. Natl. Acad. Sci. U.S.A.

145

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Jahn-Teller effects are grouped into two categories. The first arises from incomplete shells of degenerate orbitals. It includes the first-order Jahn-Teller effect, and the pseudo Jahn-Teller effect. The second arises from filled and empty molecular orbitals that are close in energy, and is the second-order Jahn-Teller effect. The two categories have quite different physical bases. As a result, geometric distortions produced by the first are quite small and normally lead to dynamic effects only. In favorable cases, the second-order Jahn-Teller effect produces very large distortions, including complete dissociation of a molecule. This can occur even when the relevant molecular orbitals are separated in energy by as much as 4 eV.An examination of the recent literature shows a rapidly increasing number of phenomena that are explained by invoking a Jahn-Teller effect. The examples come from many areas of physics and chemistry, including biochemistry.The effects invoked are those arising from orbitally degenerate states (Jahn-Teller effect proper), and from states which are nondegenerate but close in energy (pseudo, or secondorder Jahn-Teller effect).One attitude has been to lump these several effects together and to blur any distinction between them (1, 2). Another has been a reasonable belief that nondegenerate states must be very close in energy, less than 1 eV separation, before any distortion of molecular shape could be expected, and that second-order effects are very small (3-5). The purpose of this note is to point out that the nature of the first-order effect (FOJT) is such that distortions are inevitably very small, leading to the dynamic Jahn-Teller situation as a rule. Paradoxically, the second-order effect (SOJT) has a physical basis that can lead to very large distortions of molecular geometry, or even to complete dissociation.Following Jahn and Teller and Bader (6, 7), the potential energy, in terms of small displacements, Q, from an initial nuclear configuration, is given byEo -Ek The FOJT proper comes from the term linear in Q. For small displacements there is a complex mixing of electronic and vibrational states. This mixing is not under discussion here, but rather the possibility of a large distortion, which permanently affects the geometry of the molecule (static effect). Eq.[I] will not be valid for large distortions, but it still can be used to derive the symmetry constraints on the nuclear motions. The SOJT effect comes from the terms in [1] that are quadratic in Q. The Renner-Teller effect is a special second-order distortion which is important for linear molecules, but which will not be discussed further (8).Eq [1] is written in terms of state wave functions. It is necessary to interpret these in terms of molecular orbital (MO) theory to proceed further. The FOJT effect arises from incompletely filled shells. These shells are doubly and triply degenerate MO's which by their nature are not only of the same symmetry, but of the same kind. Examples are d orbitals in inorganic chemistry an...

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Section: T)mentioning

confidence: 99%

Concerning Jahn-Teller Effects

Pearson

1975

Proc. Natl. Acad. Sci. U.S.A.

145

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“…(A) The calculated resonance energy of thiophene (6.5 kcal/mol) is similar to that of pyrrole (5.3 kcal/ mol) 35 and greater than that of furan (4.3 kcal/mol). 35 Thiophene derivatives undergo Diels-Alder reactions only with difficulty, unlike corresponding derivatives of furan, but somewhat less easily than analogous derivatives of pyrrole.…”

Section: /V = Ksu (1)mentioning

confidence: 98%

Ground states of conjugated molecules. XX. SCF MO treatment of compounds containing bivalent sulfur

Dewar¹,

Trinajstić²

1970

J. Am. Chem. Soc.

Self Cite

108

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“…Herrn Professor Karl Hamann zum 65. Gehurtstag geividmet Nitriloxide addieren sich in einer 1,3-dipolaren Cycloaddition an zahlreiche ungesattigte Systeme unter Bildung von Derivaten des 1,2-O~azols~'~.…”

Section: Z U S C Hriften 13-dipolare Cycloaddition Von Nitriloxiden unclassified

Bifunktionelle Reagentien zur Quervernetzung von Proteinen

Fasold¹,

Klappenberger²,

Mayer³

et al. 1971

Angewandte Chemie

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Proteine konnen durch chemische Verbindungen quervernetzt werden, ohne daJ die komplizierte Raumstruktur sich dabei iindert. Man kennt inzwischen uiele bifunktionelle Reagentien, die mit funktionellen Gruppen in den Seitenketten der Aminosauren zu reagieren vermogen. Aus der bekannten Lange der eingefihrten Briicke kann nicht nur auf den Abstand der verkniipfen Aminosaurereste geschlossen werden, sondern es lassen sich u. a. auch Aussagen iiber Konformationsanderungen wahrend anderer Reaktionen und iiber die Anordnung der Bestandteile einer Quartarstruktur gewinnen.

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Cyclobutadiene and diphenylcyclobutadiene

Cited by 59 publications

References 0 publications

Concerning Jahn-Teller Effects

Concerning Jahn-Teller Effects

Ground states of conjugated molecules. XX. SCF MO treatment of compounds containing bivalent sulfur

Bifunktionelle Reagentien zur Quervernetzung von Proteinen

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