Protonation Energies and Basicities of Enamines

In calculations, the energy of the naked proton is zero, so that the corresponding protonation energy, PE, at 0 K in the gas phase for an isolated molecule is obtained by equation 10. 

Site of protonation and kind of molecules treated 3-21G 6-31G 6-31G Other types 

Molecular formulae and nomenclature of enamines and corresponding saturated amines for which experimental determination of basicities are reported in Table 16. 

PA Enaminesfl,b No. PAfl Amines Ref. P. N-proton- ation P a C-proton- ation Ref. 

0 PA values are given relative to ammonia, which has an experimental PA279 of 208.5 1.5 kcal mol - b Values taken from Reference 263. c Values taken from Reference 266. d Estimated values. e In 25% aqueous methanol at 28°C. f In acetonitrile. 

In the gas phase, the negative of the enthalpy of the reaction (equation 8) is defined experimentally as the proton affinity (PA) of B, indicated by equation 9. This quantity represents the intrinsic basicity of the base B in the absence of solvent . Experimental techniques for determination of gas-phase proton affinities are ion cyclotron resonance , high-pressure mass spectroscopy , the flowing afterglow technique and molecular beam experiments  

From the total energies presented in Table 9, theoretical gas-phase protonation energies, PE, are derived and presented in Table 15. In accordance with our calculation it has been shown experimentally that, in the gas phase, C protonation occurs with enamines and a large substituent effect due to substitution by an a-methyl group was observed . The intrinsic C-protonation basicity, PE, of vinylamine (115) is calculated to be larger by 18.7 kcal mol than its N-protonation basicity and by 6.8 kcal mol larger than that of ethylamine. The PE of N protonation of 115 is higher than that of 

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