The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene - energetics, structure and interconversion of dihydrotropylium ions

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C7H9+ ions, [CHT + H](+). The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 +/- 4 kJ mol(-1), was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C7H9+ population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 +/- 4 kJ mol(-1), and the heat of formation of the cyclo-C7H9+ ion, DeltaH(r)(0)([CHT + H](+)) = 884 +/- 4 kJ mol(-1). Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H-(1)](+), generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 +/- 2 kJ mol(-1) and DeltaH(f 300)degrees([CHT + H-(1)](+)) = 892 +/- 2 kJ mol(-1). However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C-C bond formation, generating protonated norcaradiene [NCD + H](+), a valence tautomer being 19 kJ mol(-1) more stable than [CHT + H-(1)](+). The 1,4-dihydrotropylium ion, [CHT + H-(3)](+), generated by protonation of CHT at C-3, is 17 kJ mol(-1) less stable than [CHT + H-(2)](+). The bicyclic isomer [NCD + H](+) is separated by relatively high barriers, 70 and 66 kJ mol(-1) from the monocyclic isomers, [CHT + H-(1)](+) and [CHT + H-(3)](+), respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H-(1)](+) does not rearrange to the bicyclic isomer [NCD + H](+) under mild protonation conditions.