Calculated geometries of dications of bis odd‐membered π‐ring systems containing a NCN fragment and related π‐systems. An opposite out‐of‐plane rotation of the 4nπ‐ring subsystems

Abstract

Quantum chemical calculations have been carried out on dications of bis odd-membered π-ring systems containing a NCN fragment and related π-systems. An opposite out-of-plane rotation of both subsystems was found if these systems contain 4n π-electrons (antiaromatic). A planar situation was found for 4n + 2 π-electrons (aromatic). The geometric representations could be compared with X-ray crystallographic three-dimensional structures of related compounds. Calculations at different levels clearly show that separation of the σ -and π-electron contribution is an effective way to elucidate the origin of the geometrical changes. We also give attention to some fundamental aspects of the subsystems related to the 1,3-azolium cations because of their biochemical relevance such as fast C 2 -H proton exchange. We postulate that at least two molecules of water are involved in this process. The significance of a trigonal pyramidal (TP) geometry has been emphasized.

Charge at the migrating hydrogen in the transition state of hydride transfer reactions from CH groups to hydride acceptors. Dynamics of the redox‐couple NADH‐NAD+

“…The axial C–H bond is 1.28–1.32 Å 4, which is in good correspondence with the bond lengths to the migrating hydrogen of 1.300 Å, as calculated for the degenerate reactions according to Scheme and given in Table I. An essential property of a TP is the presence of intramolecular charge transfer 9. The axial linkage accommodates the hydride transfer, whereas the equatorial bonds are electron‐donating.…”

Quantum chemical study of alternating N/B and N/C π bonds in (un)charged even‐membered 4n and 4n+2 cyclic systems and their related open systems

“… b Our data are obtained from the analytical expression: with B = 0.02 related to the calculated results for p ‐orb % = 100 corresponding to 1.75 Å (F), 2.13 Å (Cl), 2.28 Å (Br), and 2.48 Å (I) 13. …”

“…A number of four‐membered rings containing 6 π electrons have been studied with combinations as X=NH, Y=CH; X=O, Y=CH; the dication of X=O, Y=NH; and the dianion of X=NH, Y=BH 13, 28. In these rings the two highest occupied MOs, i.e., the HOMO and the POMO (penultimate occupied MO) are neighbor‐nonbonding and opposite‐antibonding resulting in opposite π repulsions.…”

“…For the calculations of geometries corresponding to a trigonal pyramidal (TP) configuration with CX in the axial location, we derived 1, 13: The values A and A ′ may be calculated by substitution of the experimental values R CX and E CX for, e.g., the tetrahedral configuration (% p ‐orb = 75). Support for this approach was given with ab initio values at the B3LYP/6‐31G** and B3LYP/6‐311 + G* level.…”

Calculated geometries and charge distributions of 4n and 4n + 2 π‐ring systems linked with −CH: An asymmetric versus symmetric π‐delocalization

“…For the isoelectronic ethylene imine we calculated that a disrotatory ring opening into azomethine ylide is possible between 1.51 and 1.81 Å. These calculations were also performed with the bond‐length–dependent resonance integral for the sp 2 hybridized CN linkages 25: in which However, for selected aziridines in which the crucial linkage is substituted with electron‐acceptor groups and NH is replaced by N‐aryl, a conrotatory ring opening is found 26. These factors “dilute” the rigid character of the three‐membered ring.…”