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^–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^–1) leads to the weakly bonded complex H₂NCH₂⁺····C=O with a C–C length of 2.92 Å and which needs only 15 kJ^–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 NH₂⁺ (PA[NH₃] = 854 kJ^–1) approaches the NH₂ group of H₂NCH=C=O (PA at N = 845 kJ^–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^–1, in good agreement with an experimental value (–67 ± 5 kJ^–1) but not with another experimental number (–95 ± 5) kJ^–1) and these matters are discussed. Suggestions for further experimental work are proposed to address this discrepancy.

Keywords: ketenes, proton affinity, bond activation, ab initio calculations, CBS-QB3