Energy and stability of protonated ketenes: inductive and resonance effects

The proton affinities (PA) of electronegatively-substituted ketenes RCH=C=O (R=H, CH3, NH2, OH and F) at various sites have been assessed by CBS-QB3 calculations. The most favorable protonation site was found to be the CH carbon atom to produce the acylium ions RCH2C+=O. The PA values, relative to that of CH2=C=O, can be interpreted in terms of destabilizing effects of the R group in RCH=C=O [J. Am. Chem. Soc. 113, 6021 (1991)] and by positive or negative inductive effects of R in RCH2C+=O. For HOCH=C=O the two destabilizing effects are of similar magnitude and this rationalizes that its PA (823 kJ mol-1) is virtually the same as that for CH2=C=O (820 kJ mol-1). For all ketenes (except for R=H), protonation leads to significant activation of the C=C bond. In the extreme case, protonation of H2NCH=C=O at CH (PA=917 kJ mol-1) leads to the weakly bonded complex H2NCH2+ .....C=O with a C-C length of 2.92 A and which only needs 15 kJ mol-1 to dissociate to H2NCH2+ + CO. In fact, the covalently-bonded species, H2NCH2C+=O, does not exist, the most stable configuration being CH2=N(H)-H+....C=O. When NH4+ (PA[NH3]=854 kJ mol-1) approaches the NH2 group of H2NCH=C=O (PA at N=845 kJ mol-1), the stable hydrogen-bridged cation H3N-H+....NH2-CH=C=O is produced. The NH4+ group can move over to the CH group or it can attack the CH group directly. After passing a transition state, dissociation follows to NH3+CH2NH2+ + CO. For neutral methylketene, CBS-QB3 calculates a heat of formation of -63 kJ mol-1, in good agreement with an experimental value (-67±5 kJ mol-1) but not with another experimental number(-95±5 kJ mol-1) and these matters are discussed. Suggestions for further experimental work are proposed to address this discrepancy.