Minimum, antiaromatic

The potential surfaces of the ground and excited states in the vicinity of the conical intersection were calculated point by point, along the trajectory leading from the antiaromatic transition state to the benzene and H2 products. In this calculation, the HH distance was varied, and all other coordinates were optimized to obtain the minimum energy of the system in the excited electronic state ( Ai). The energy of the ground state was calculated at the geometry optimized for the excited state. In the calculation of the conical intersection... 

Whereas the benzene molecule possesses a structure of D6A symmetry with equal lengths of the CC bonds, for acyclic polyenes alternation of bond lengths is a characteristic [87JCS(P2)S1]. For antiaromatic molecules, alternation is even more pronounced and unlike the aromatic molecules, a high-symmetry structure of the lowest singlet state of the antiaromatic molecules does not correspond to a minimum on the PES. For... 

The aromaticity of pyridine and antiaromaticity of azete may be compared on the basis of the enthalpies of reactions (53) and (54). Both in structure (107) that corresponds to a minimum on the PES (unpublished results) and is an analog of the experimentally known X5-phosphorine (82ACR58) and in structure (108) a tetracoordinate nitrogen atom disturbs cyclic 7r-delocalization. In contrast to pyridine—Eq. (53)—the corresponding reaction for azete—Eq. (54)—is according to our AMI calculations exothermic. 

Non-planarity is the result of the dominance of the destabilizing interactions of the sulfur lone pair and tt- occupied MOs of the pentadienyl anion over the stabilizing interaction of that lone pair and the LUMO of the anion fragment. In fact thiabenzene is antiaromatic in a planar configuration. Pyramidalization reduces the antiaromaticity induced by the sulfur. Although no X-ray data are available on the parent system, kinetic data have been obtained supporting a minimum barrier to inversion at the pyramidal sulfur of a 2-thianaphthalene of 99.1 kJ mol-1 (75JA2718). The formulation of the system as a cyclic ylide is supported by the chemical reactivity of the compounds as related in the reactivity section below. 

The singlet and triplet states of the antiaromatic D5h SisH5+ (153) are not minima on the Si5H5+ PES. When the D5h symmetry of the singlet is released, it first collapses to the C2V isomers 154 and 155 which are also not minima on the PES, and upon release of the C2v constraints they further collapse to the Cs structure 156, which is the only minimum on the Si5H5+ PES. The pentasila-allylic-type cation 156, which has a structural... 

The antiaromatic oxirenes 11 are extremely elusive but their potential involvement in photochemical Wolff rearrangements by equilibration with the -oxocarbenes 12 has generated both theoretical and experimental interest and the subject has been reviewed <2004MR0291>. Experimental evidence for participation of oxirenes <2004MR0291> is supported by DFT calculations that suggest that oxirene corresponds to an energy minimum <2003JMT(629)263, CHEC-III(1.03.7)215>. 

Semiempirical calculations have been applied to antiaromatic three-membered rings and indicate that they correspond to energy minima <1973CC688>. For example, thiirene 13 corresponds to a local energy minimum relative to the isomeric carbene 14, zwitterion 15, heterocumulene 16, and heterocyclic carbene 17. 

Nitrogen pentafluoride represents an interesting contrast to oxirene. Oxirene is, on paper, a reasonable molecule there is no obvious reason why, however unstable it might be because of antiaromaticity [4] or strain, it should not be able to exist. On the other hand, NFs defies the hallowed octet rule why should it be more reasonable than, say, CHfi Yet a comprehensive computational study of this molecule left tittle doubt that it is a (relative) minimum on its potential energy surface [5]. The full armamentarium of post-HF methods, CASSDF, MRCl, CCSDT, CCSD(T), MP2 (section 5.4) and DFT (chapter 7) was employed here, and all agreed that Dan (section 2.6) NF5 is a minimum. 

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