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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.

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The mechanism for the unusual AlCl(3)-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane in pentane has been elucidated; the reaction, which also yields four isomeric dichloronorbornanes, occurs in three steps: (1). ionization to form the 2-chloro-2-norbornyl cation, which was fully characterized by two-dimensional (1)H and (13)C NMR in SbF(5)/SO(2)ClF; (2). Wagner-Meerwein shift to yield the 1-chloro-2-norbornyl cation, which was partially characterized by (1)H NMR; and (3). hydride abstraction.