Structural effects in solvolytic reactions. 27. Solvolysis of the exo- and endo-1,2-diphenyl-2-norbornyl and -1,2-dimethyl-2-norbornyl p-nitrobenzoates and chlorides. Evidence for the classical nature of 1,2-disubstituted tertiary 2-norbonyl cations and structure of the parent 2-norbornyl cation