Energy Component Analysis of the Pseudo-Jahn−Teller Effect in the Bicyclic Nonalternant Hydrocarbons:  The Pentalenoid and Heptalenoid Systems

Abstract

To gain insight into the nature of the pseudo-Jahn−Teller (JT) effect, an energy component analysis has been carried out for the bicyclic nonalternant hydrocarbons termed pentalenoid and heptalenoid systems by using the ab initio RHF method with 6-31G(d) basis set. Inspection of the energy component comprised in the total energy reveals that the stability of a less symmetrical nuclear configuration is largely responsible for the decrease in the internuclear repulsion energy and the interelectronic repulsion energy due to σ electrons. These observations are consistent with an expansion of the carbon skeleton brought about by the pseudo-JT distortion. Another energy component also plays an essential role in the pseudo-JT stabilization: For the examples, the preference for the C 2h structure rather than the D 2h one for the pentalene arises from a decrease in the interelectronic repulsion energy due to π electrons, while that for heptalene results from a lowering of the nuclear−electron attraction energy due to π electrons. This sharp distinction between the pentalenoid and heptalenoid systems is accounted for in terms of an electrostatic interaction combined with a charge relaxation attributed to π electrons.

Heavy atom tunneling in the automerization of pentalene and other antiaromatic systems

“…In the case of the rate constants for automerization of Me 3 PL a plateau is also reached, but only at a much lower temperature (approximately at 20 K) with a much longer half time (t 1/2 ¼ 1 s) in this temperature regime. 15…”

“…In contrast to Me 3 PL, the rate of automerization of 15 Me 3 PL at 10 K is computed to be essentially zero. To the extent that the hypothetical 15 H 3 C groups model the tert-butyl groups, calculations predict that tBu 3 PL will not automerize at cryogenic temperatures.…”

“…As shown by the results of our calculations on Br 3 PL and 57 H 3 PL, the masses of the substituents do not affect the tunneling rates for the bond-shiing. Since the automerization barrier heights are very similar for all the PL based systems, the disparity between the Arrhenius plots for automerization of Me 3 PL, 15 Me 3 PL, and PL must be attributed to the rotation of the Me groups. The een times greater mass of the hydrogens in the hypothetical 15 Me 3 PL, compared to the actual masses of hydrogens in Me 3 PL produces a computed tunneling rate at 10 K 10 24 times slower.…”

“…Of course, the effective masses of the hydrogens of a rotating methyl group in Me 3 PL are very different from those of a rotating tert-butyl group in tBu 3 PL, and this difference in effective mass could make the probability of tunneling very different in these molecules. 13 Therefore, we also performed QMT calculations on 15 Me 3 PL, an hypothetical molecule with an isotopic mass of 15 on the hydrogens of the methyl groups, since the mass of 15 H is equivalent to the mass of the methyl groups of each tert-butyl substituent. This model is far from perfect, because the C-H bonds in the methyl groups are ca.…”

“…RSC Advances PAPER symmetrical geometries. [15][16][17] They are highly reactive, with a tendency to dimerize even at low temperature. [18][19][20][21] HL was rst synthesized in 1961, 22 and the elusives APL and PL in 1995 and 1996, 17,18 respectively (although sterically shielded or electronically stabilized derivatives have been known for some decades [19][20][21][23][24][25] ).…”