The 2-Norbornyl Cation is not a Single Minimum Energy System

Simple and dual variable linear regression equations are presented which can estimate the sensitivity constants ρ for the solvolysis reactions of hundreds of bicyclic and tricyclic compounds. These are the first QSARs which utilize dihedral angles to estimate/predict ρ constants. These QSARs and other analyses herein support the new theory that the bridgehead hydrogen bond orbital assists the displacement of the exo leaving group in the solvolyses of 2-norbornyl derivatives. Alternative interpretions of the 1H NMR and 13C NMR spectra indicate that C1 is hypercoordinated rather than C6. Consequently, in normal hydroxylic solvents the 2-norbornyl cation is not symmetrical, does not require C—C σ-bond bridging and is not a single minimum energy system, i.e. it is a pair of rapidly equilibrating cations, eq 1, structure 2b. At ≤ −158°C, the 2-norbornyl cation is proposed to be an H—C1—H σ-bond delocalized resonance hybrid structure 16/16’. Hypotheses are presented which suggest that the norbornane system can be viewed as a saturated counterpart of a conjugated π system, e.g. benzene. This new delocalization and bond concept, sigma aromaticity, can also help to explain the preferential “exo'’ reactions of norbornane and norbornene systems, the unusual stability of the 2-norbornyl cation, and perhaps provide new insight concerning the ubiquitousness of six-membered rings. Sigma aromaticity may also help account for the greater efficiency of singlet energy transfer between chromophores when the molecular spacer group is rigid rather than flexible, and electron transfer in DNA.

ELECTRONIC DELOCALIZATION: A QUANTITATIVE STUDY FROM MODERN AB INITIO VALENCE BOND THEORY

An ab initio spin-free valence bond code called Xiamen-99 has been developed based on an efficient algorithm called paired-permanent-determinant approach, where Hamiltonian and overlap matrix elements are expressed in terms of paired-permanent-determinants. With this tool, we probed the electronic delocalization phenomenon in a few typical examples including benzene, formamide and ethane. Our computations revealed that ab initio valence bond methods are able to estimate the energetic contribution from the delocalization effect to the stabilization of molecules, thus pave the way to illuminate the resonance theory at the quantitative level. In particular, we analyzed the cyclic electronic delocalization in benzene and showed that different understandings on the resonance may originate from the different usage of one-electron orbitals in the valence bond theory. Our investigation into the hyperconjugative interaction in ethane demonstrated that the hyperconjugation effect is not the dominating factor in the preference of the staggered conformer of ethane.

Planar transition structures in the epoxidation of alkenes. A DFT study on the reaction of peroxyformic acid with norbornene derivatives

We studied, with the RB3LYP/6-311+G(d,p) method, the mechanism of peroxyformic acid epoxidation of norbornene, norbornadiene, tetramethylethene, and anti- and syn-sesquinorbornenes. The transition structures (TSs) for the reaction of tetramethylethene and norbornene show a perfect spiro geometry (the peroxy acid plane is perpendicular to the C=C bond axis) with synchronous bond formation. Also three out of the four TSs of the norbornadiene reaction are spiro-like, but the highly asynchronous syn,endo-TS has a planar-like geometry. anti- and syn-sesquinorbornenes are substrates that, because of steric constraints, cannot easily accommodate spiro-like TSs. In fact, we managed to locate only a planar-like TS and a planar TS (the peroxy acid plane contains the C=C bond axis), respectively, for these substrates. These planar TSs are "nonconcerted" since they are strongly unsymmetrical and only one of the C-O bonds of the oxirane ring is significantly formed. IRC analysis, while confirming that formation of one C-O bond fully precedes that of the other, also suggests that all this can take place without formation of intermediates, that is, within a "nonconcerted one-step process". Our theoretical data correctly reproduce the experimental facial syn selectivity of norbornene and norbornadiene epoxidations and compare well with the experimental activation free energies of the peroxy acid epoxidation of all the olefins reported here. This accord validates the method used as adequate to deal with the reactivity of these systems.

Chemistry of the fullerenes: the manifestation of strain in a class of continuous aromatic molecules

Within the wr-orbital axis vector theory, the total rehybridization required for closure of the fullerenes is approximately conserved. This result allows the development of a structure-based index of strain in the fullerenes, and it is estimated that about 80 percent of the heat of formation of the carbon atoms in C60 may be attributed to a combination of v strain and steric inhibition of resonance. Application of this analysis to the geometries of structurally characterized organometallic derivatives of C60 and C70 shows that the reactivity exhibited by the fullerenes may be attributed to the relief of a combination of local and global strain energy. C60 is of ambiguous aromatic character with anomalous magnetic properties but with the reactivity of a continuous aromatic molecule, moderated only by the tremendous strain inherent in the spheroidal structure.