Energy and stability of protonated ketenes: inductive and resonance effects

摘要

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+ = 0. 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 CH, = C = 0 (820 U 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 angstrom 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.