Energy and stability of protonated ketenes: Inductive and resonance effects
European Journal of Mass Spectrometry , Volume 10 - Issue 6 p. 791- 799
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 Å and which only needs 15 kJ mol-1 to dissociate to H2NCH 2+ + CO. In fact, the covalently-bonded species, H 2NCH2C+ = 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 H 3N-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.
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Ervasti, H.K, Burgers, P.C, & Ruttink, P.J.A. (2004). Energy and stability of protonated ketenes: Inductive and resonance effects. European Journal of Mass Spectrometry, 10(6), 791–799. doi:10.1255/ejms.690