The Cyclobutadiene Problem

Maier, Günther

doi:10.1002/anie.197404251

Cited by 167 publications

(42 citation statements)

References 119 publications

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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: 59%

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: 59%

Concerning Jahn-Teller Effects

Pearson

1975

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

145

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“…This results in increased antiaromaticity of the system that destabilizes 1A. Hence, the system is relieved by pseudorotation to 1B, where the apical Y 2 -Pfour-membered rings such as cyclobutadiene (7), [1][2][3][4][5][6][7][8] fourmembered unsaturated lactones (8) and unsaturated lactams (9) [20] ( Figure 5). This phenomenon has, however, not been examined in the group of phosphorus compounds up until now.…”

Section: Electronic Structurementioning

confidence: 99%

“…[1] Antiaromaticity was formerly considered "homogenic" due to the involvement of similar p z orbitals perpendicular to the planar ring. Cyclobutadiene [2] and its derivatives [1,3,4] are prime examples of antiaromatic compounds. [5] In this paper, we present selected phosphorus-containing compounds as examples where antiaromaticity is "heterogenic".…”

Section: Introductionmentioning

confidence: 99%

Can Four‐Membered Heterophosphete Structures Exist? The Contribution of Phosphorus d Orbitals to Antiaromaticity

et al. 2007

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“…The challenge provided by anti-aromatic character and intrinsic instability of CBD inspired numerous experimental studies (4)(5)(6)(7)(8). The photochemical transformation of a-pyrone (9, 10) (Scheme 1) confined within a matrix emerged as a promising route to this elusive molecule without protection provided by bulky substituents or transition metals (11).…”

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confidence: 99%

Comment on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

Alabugin

Gold

Shatruk

et al. 2010

Science

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Legrand et al . (Reports, 16 July 2010, p. 299) reported the experimental observation of square-planar and rectangular-bent geometries of 1,3-dimethylcyclobutadiene (Me 2 CBD) confined within a crystalline matrix. However, we found no evidence for the Me 2 CBD formation. We argue that the experimental x-ray density data are better attributed to the bicyclic β-lactone intermediate where carbon dioxide is covalently bound to cyclobutadiene.

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The Cyclobutadiene Problem

Cited by 167 publications

References 119 publications

Concerning Jahn-Teller Effects

Concerning Jahn-Teller Effects

Can Four‐Membered Heterophosphete Structures Exist? The Contribution of Phosphorus d Orbitals to Antiaromaticity

Comment on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

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