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- W2009796249 abstract "The proton affinities ( PA) of electronegatively-substituted ketenes RCH = C = O (R = H, CH 3 , NH 2 , 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 RCH 2 C + = O. The PA values, relative to that of CH 2 = 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 RCH 2 C + = 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 2 = 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 H 2 NCH = C = O at CH ( PA = 917 kJ mol −1 ) leads to the weakly bonded complex H 2 NCH 2 + •••• C = O with a C–C length of 2.92 Å and which only needs 15 kJ mol −1 to dissociate to H 2 NCH 2 + + CO. In fact, the covalently-bonded species, H 2 NCH 2 C + = O, does not exist, the most stable configuration being CH 2 = N(H)–H +•••• C = O. When NH 4 + ( PA[NH 3 ] = 854 kJ mol −1 ) approaches the NH 2 group of H 2 NCH = C = O ( PA at N = 845 kJ mol −1 ), the stable hydrogen-bridged cation H 3 N–H +•••• NH 2 –CH = C = O is produced. The NH 4 + group can move over to the CH group or it can attack the CH group directly. After passing a transition state, dissociation follows to NH 3 + CH 3 NH 3 + + 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." @default.
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- W2009796249 date "2004-12-01" @default.
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- W2009796249 title "Energy and Stability of Protonated Ketenes: Inductive and Resonance Effects" @default.
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- W2009796249 doi "https://doi.org/10.1255/ejms.690" @default.
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