Ab initio study of the stability of [n]paracyclophanes and their Dewar benzene-type isomers

Abstract

Summary. Ab initio calculations are carried out for [n]paracyclophanes andtheir Dewar benzene isomers for n = 5, 6, and 7 as well as for the benzene and Dewar benzene itself. The benzene isomers are studied b y employing various AO basis sets ranging from 6-31G type to such of triple-zeta type including two d and one f function on the carbons and two p and one d on the hydrogens. The correlation energy contribution is computed by employing MP2, CAS-SCF, MRD-CI and MCPF procedures. Potential curves for the low-energy states in the isomerization from benzene to its Dewar form are also computed under certain geometrical assumptions. The energy difference between the two potential minima is calculated to be 3.35 eV; neglect of electron correlation increases the value by about 0.3 eV, deficiencies in the polarization description (6-31G basis) overestimates it by another 0.65 eV. The calculations suggest that the experimental Dewar benzene geometry determination needs refinement and that the isomerization energy of hexamethylbenzene is considerably smaller than that of benzene itself.The geometries of the [n]paracyclophanes in both isomer forms are optimized in the 6-31G basis; the energy difference is determined by employing an AO basis of double-zeta quality plus polarization functions. Correlation effects are considered. The electronic isomerization energies are 2.58eV (n = 7), 1.63 eV (n = 6) and 0.7 eV (n = 5) for the unsubstituted compounds. The relative energetics are discussed in terms of potential surface diagram having the benzene ring deformation as the "reaction coordinate". Comparison with the anthracene isomerization is made and the effect of substituents is pointed out.

[1.1]Paracyclophane. Photochemical Generation from the Corresponding Bis(Dewar benzene) Derivative and Theoretical Study of Its Structure and Strain Energy

“…The total deformation angles (α + β), however, agree within 0.4° (50.1 ± 0.2°). The angle (α + β) in 1a is much smaller than that calculated for [4]paracyclophane, even slightly smaller than that for [5]paracyclophane . In accord with the calculation results, 1a is much superior to the former and is comparable to the latter in kinetic stability.…”

“…Computational analysis on 1 − 3 , in fact, indicates that the first step will be slightly endothermic whereas the second step will be highly exothermic, suggesting that the latter step should be much easier than the former. Calculations also reveal that the degree of deformation of aromatic ring in 1 will be comparable to that in [5]paracyclophane, much less than that in [4]paracyclophane, implying that 1 may possibly be isolable unless electronic interactions between the aromatic rings held in close proximity significantly destabilize the system . In a preliminary account of this work 8 we reported spectroscopic evidence for the generation of the first [1.1]paracyclophane, the bis( methoxycarbonyl) derivative.…”

Aromaticity of bent benzene rings: A VBSCF study

“…The p orbitals cannot point to the carbon atoms of the bridge and must to some degree be perpendicular to the plane of the atoms connected to the carbon that is bent out of plane. This will lead to an easier transition to the Dewar benzene when decreasing the bridge length as shown by w x various authors 14,15,17,33 . A commonly used criterion for judging aromaticity of distorted molecules is the strain energy for the molecule.…”

Theoretical Characterization of Photoisomerization Channels of Dimethylpyridines on the Singlet and Triplet Potential Energy Surfaces

“…Isomerization to the Dewar form can involve the Möbius form 14, or it can proceed directly from excited dimethylpyridine 2 to the Dewar forms 7 and 9 by 3 ± 6 or 1 ± 4 bonding as has been studied for the isomerization of benzene and [n]paracyclophanes (n 5, 6, 7) to their Dewar-analogues. [31] Formation of 7 from 14 proceeds via a transition state 16 with a low barrier of 8.2 kcal mol À1 at the B3LYP level, while 9 is formed through a transition state 17 with a barrier of 14.4 kcal mol À1 . Dewar dimethylpyridine 7 is more stable than 9 by 7.4 kcal mol À1 .…”