The effect of phenyl substitution on the thermochemistry of gas-phase ionsand their neutral counterparts

The available data for the heats of formation of phenyl-substituted organic s have been critically evaluated. Phenyl substitution is always thermochemi cally destabilizing in that Delta H-f(0)[R-C6H5] - Delta H-f(0)[R-H] is a p ositive quantity. The magnitude of the effect in neutrals depends on the su bstitution site, e.g. from + 79 kJ mol(-1) for (H) under bar COR or (H) und er bar CCR to + 155 kJ mol(-1) for RCO2(H) under bar. Multiple phenyl subst itution (where possible) at the same site is generally simply additive. For the corresponding cationic species, the first phenyl substitution is stabi lizing when the ionization energy of the substrate is lowered thereby and f or even-electron ions resonance-stabilization energies may also come into p lay. Additional phenyl substitution at the same site is, however, always de stabilizing. Phenyl substitutions were compared with vinyl substitution: fo r neutral species the effects are similar, whereas for even-electron ions, multiple vinyl substitution is only weakly stabilizing or destabilizing.