Nuclear Magnetic Resonance Spectroscopy. Thermodynamics and Spectral Analysis of 7-Phenyl-7-Carbomethoxynorcaradiene Valence Tautomerisms

The equilibria of 7-phenyl-, 7-(p-nitrophenyl)-, and 7-(p-methoxyphenyl)-7-carbomethoxynorcaradiene with their valence tautomers, the corresponding 1,3,5-cycloheptatrienes, have been studied by means of the 220-MHz pmr spectra at a series of temperatures. Although only time-averaged spectra of the rapidly equilibrating mixtures have been accessible, the enthalpy and entropy of the conversion of these three norcaradienes to the corresponding cycloheptatriene have been determined and found to be 5.4 ± 0.5 kcal mol^(-1) and 16.8 ± 2.0 cal mol^(-1) deg^(-1), 3.5 ± 0.2 kcal mol^(-1) and 11.0 ± 0.8 cal mol^(-1) deg^(-1), and 2.3 ± 0.3 kcal mol^(-1) and 7.4 ± 1.0 cal mol^(-1) deg^(-l), respectively. LAOCN3 analyses at two temperatures of the norcaradiene-cycloheptatriene portion of the pmr spectra of the three equilibrium mixtures have made possible the determination of approximate parameters for the pmr absorptions of each of the six individual compounds