MoO-13


CATIONIC HEATS OF FORMATION FOR METHYLKETENE AND DIMETHYLKETENE

John C. Traeger

School of Chemistry, La Trobe University, Bundoora, Victoria 3083


A recent high level ab initio theoretical study [1] of methylketene and dimethylketene has produced 298 K heats of formation for both the neutral and cationic species that are significantly different from the evaluated experimental values of Lias et al. [2]. The calculated ionization energies are however in good agreement with experiment.

 
Theoretical [1]
Experimental [2]
methylketene
-68
-105
methylketene cation
797
759
dimethylketene
-92
-134
dimethylketene cation
726
681

Table 1. 298 K Heats of Formation (kJ/mol)

In this study, threshold photoionization mass spectrometry has been used to measure C3H4O+· appearance energies from cyclopentanone and 3-pentanone. Although these represent thermochemical upper limits, the respective experimental 298 K cationic heats of formation of 799.8 kJ/mol and 801.7 kJ/mol are in close agreement with the theoretical calculations for the methylketene cation. However, the corresponding appearance energy for 2-methyl-3-pentanone produces a significantly lower value of 783.5 kJ/mol.

The C4H6O+· appearance energy of 9.30 eV from 2,4-dimethyl-3-pentanone corresponds to a cationic heat of formation of 719.3 kJ/mol, in good agreement with theory. Photoionization of tetramethyl-1,3-cyclobutanedione produces the dimethylketene cation, together with neutral dimethylketene, as the major low energy fragmentation process. The appearance energy of 9.41 eV measured here corresponds to a combined product heat of formation of 632.9 kJ/mol. Again, this supports the theoretical calculation of 634 kJ/mol rather than the experimental value of 547 kJ/mol (Table 1).

  1. Scott, P. S.; Radom, L. Int. J. Mass Spectrom. Ion Processes, In Press.
  2. Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17 Suppl. 1.