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In π systems with localized double bonds, the switch from one localization pattern to another symmetry-equivalent one, and the corresponding changes in electron distribution are often considered to proceed via a delocalized transition state of higher symmetry. We present a different mechanism, according to which the double-bond localization and the concomitant charge distributions can fluctuate without change in energy and without passing through a delocalized, higher-symmetry transition state. This result is obtained within a simple, independent-electron (HMO-type) treatment.

The He(Iα) PE spectra of the two pairs of 1(Ph)-3 and 2–4 have been recorded and assigned by comparison with spectra of related molecules, or by using simple MO arguments. This analysis shows that the PE spectra are in agreement with the assumption that the twist angles observed in the crystalline state are more or less conserved in the gas phase. The observed first adiabatic ionization energies I allow the calculation of the molar reaction enthalpy ΔrH which accompanies the enolization of the radical cations of the title compounds. It is found that ΔrH ≈ –−70 kJ mol−1 is roughly the same for both keto/enol pairs, in qualitative and quantitative agreement with expectation. These results complement and correct ionization energies determined previously by mass spectrometric methods and conclusions based on them.

The n ionization energies I and the gas-phase basicities GB of CH3-, Cl-, or CN-substituted quinuclidines have been measured by PE and ICR spectroscopy. The dependence of the shifts ΔI and ΔGB (relative to the values of the parent molecule) allow conclusions about the charge dispersal accompanying the n ionization or the protonation of quinuclidine in the gas phase. The agreement with the results of a minimal basis set abinitio calculation is excellent. Comparison of the solution pKa values with either I or GB reveals that 2-substituted quinuclidines exhibit sizeable solvent-induced proximity effects, i.e. that the corresponding quinuclidinium ions are more acidic in solution than expected on the basis of the gas-phase basicities. This agrees with earlier results concerning 2-substituted pyridines.

The propensity of the NN bond to undergo photo [2 + 2] cycloadditions has been further explored. In the specifically designed 1,5-azo/enes 1–3, no [2 + 2] cycloaddition has been observed upon either direct or sensitized excitation with light of various wave lengths at temperatures down to 77 K, in line with expectations based on X-ray (1: d = 2.71 A, ω = 129°) and PE measurements (1: I1 = 8.00, I2 = 9.05 eV; 2; I1 = 8.00, I2 = 9.25eV). The steric/stereoelectronic demands for the participation of the NN bond in pericyclic reactions are clearly more stringent than those for the CC bond.