Neutral and cationic heats of formation for ketene, methylketene, and dimethylketene

Threshold photoionization mass spectrometry has been used to measure appear ance energies for the formation of C2H2O+., C3H4O+., and C4H6O+. from sever al different precursors, leading to derived 298 K cationic heats of formati on of 874.4 +/- 1.0, 783.5 +/- 0.3, and 719.3 +/- 3.6 kJ mol-L, respectivel y. Experimental appearance energies for the production of ionized phenol fr om phenyl acetate, phenyl propionate, and phenyl isobutyrate, and for ioniz ed dimethylketene from tetramethyl-1,3-cyclobutanedienone, provide 298 K he ats of formation for ketene (-53.6 +/- 0.5 kJ mol(-1)), methyl ketene (-66. 9 +/- 4.7 kJ mol(-1)), and dimethyl ketene (less than or equal to-86.4 kJ m ol(-1)). Apart from the value for the methylketene cation, these are all in excellent agreement with high level ab initio calculations. There is evide nce to suggest that the experimental ionization energy for methyl ketene ma y be up to 0.15 eV too high. (Int J Mass Spectrom 194 (2000) 261-267) (C) 2 000 Elsevier Science B.V.