Ab Initio MCSCF Study on the Pseudo-Jahn−Teller Distortion from Planarity in Cycloheptatriene, Heptalene, and Heptafulvalene

Abstract

To elucidate the nature of a pseudo-Jahn-Teller (JT) distortion from planarity, ab initio MCSCF calculations with 6-31G(d) basis sets are carried out on the ground state of the titled molecules. Cycloheptatriene undergoes the pseudo-JT distortion from a planar C 2V to a boat C s conformation. In heptalene, the pseudo-JT distortion takes place in two stages, the initial step being from a symmetric planar D 2h to a skew planar C 2h structure and the subsequent step from the skew C 2h to a twisted C 2 one. In heptafulvalene, the symmetric planar D 2h structure exhibits a third-order saddle point and, hence, the pseudo-JT distortions take place from the D 2h to D 2 , C 2V , and C 2h ones through the respective out-of-plane nuclear deformations. Besides, the twisted D 2 structure exhibits a second-order saddle point with respect to the syn-and anti-folding motions of the sevenmembered rings and the structural changes take place from the D 2 to the C 2V and C 2h structures. A comparison of the total energy reveals that the ground state of heptafulvalene takes the anti-folded C 2h conformation. These structural characteristics are in good agreement with the available experimental facts. Energy component analyses reveal that in these molecules the pseudo-JT distortion from planarity should lead markedly to a lowering of the electron-nuclear attraction energy and concomitantly to raising of both the interelectronic and the internuclear repulsion energy. These energetic behaviors are accounted for in terms of the electrostatic interactions and the proximity between the nuclei and electron clouds involved in the CC and CH bonds, owing to the folding of the carbon skeleton.

Multiconfiguration Self-Consistent Field Study on Formonitrile Imine and N-Substituted Nitrile Imines HCN2–R: Energy Component Analysis of the Pseudo-Jahn–Teller Effect

“…In such a case, the structural change is regarded as a contraction or an expansion of the molecular skeleton, because the associated energy changes of the attractive and repulsive terms are out of phase with each other. On the basis of the present series of our investigations, ,− as the energy terms responsible for the pseudo-JT stabilization combine, no instances other than the above two groups have been, in fact, observed. Accordingly, we have been interested in the question of whether the energy variations of the two repulsive terms V nn and V ee should always be in the same direction.…”

“…On the contrary, Bertrand and co-workers showed by X-ray structural analyses that all stable nitrile imines prepared in the 1990s possess bent, helical backbones. − Many theoretical studies have been undertaken on equilibrium geometries and vibrational spectra for some substituted nitrile imines including formonitrile imine to explain available experimental data. − To the best of our knowledge, however, no theoretical research has been undertaken to elucidate why the C–N–N moiety in nitrile imines examined does not take a linear structure, but a bent structure. Our concern for nitrile imines is to explicate what occurs inside the molecule as it undergoes a pseudo-Jahn–Teller (JT) distortion from a symmetrical structure to a less symmetrical structure with a bent C–N–N moiety. − Our results obtained so far suggest that non-nearest neighbor interactions are important in such structural changes that the molecules bend their molecular skeletons. − This is because, with skeletal bending, the nuclei and electron clouds are brought spatially close to each other and, concomitantly, the electrostatic interactions between them are greatly affected. Thus, energy component analyses of the structural changes are expected to provide significant information regarding the chemical bonding in the nitrile imines.…”

The Fulvalenes

“…[188] Subsequent theoretical consideration of the stabilisation energy and Kekulé contribution (derived from the experimental bond lengths) were made, [189] but it is the recent high level calculations that replicate the structural data and confirm that the H···H interactions dictate the bend from planarity; the loss of overlap is partially compensated by pyramidalisation of the carbon centres of the seven-membered rings. [44,45,190] A unique though serendipitous synthesis of distinct syn-and anti-heptafulalvenes is provided by our own work on the thermal dimerisation of gem-dichlorocyclopropabenzene 144. [191] Heating 144 gives pale and deep yellow dimers that are isomeric heptafulvalenes as shown by crystallographic analysis.…”

Numerical Estimation of the Pseudo-Jahn–Teller Effect Using Nonadiabatic Coupling Integrals in Monocyclic and Bicyclic Conjugated Molecules

“…Even though the ground state is not degenerate, the geometrical structures of some kinds of molecules are known to be distorted by the pseudo-Jahn–Teller (pJT) effect (or the second-order Jahn–Teller effect). In the series of our research projects, − energy component analyses were performed to gain an insight into the nature of the pJT effect for various π-conjugated hydrocarbons. Cyclobutadiene (CBD) and cyclooctatetraene (COT) are known to be typical pJT molecules and have been investigated by many theoretical and experimental researchers in the past few decades. − Bicyclic nonalternant hydrocarbons, such as pentalene and heptalene, are also known to be good examples of pJT molecules.…”