HEATS OF FORMATION OF GAS-PHASE IONS CALCULATED BY COMPOSITE AB-INITIO PROCEDURES

Heats of formation calculated with the G2(MP2), G2(MP2,SVP), CBS-Q, an d CBS-q methods for 46 aliphatic nitrogen- and oxygen-containing molec ules and ions with up to three carbon atoms are compared with experime ntal results and with each other. The values obtained theoretically fo r aliphatic amine radical cations and immonium ions are uniformly high er than the experimental values, in a few cases by more than 20 kJ mol (-1), possibly suggesting a systematic problem; the results for oxygen -containing ions do not show a similar trend. The heats of formation o btained with the G2(MP2) and G2(MP2,SVP) procedures differ only little , whereas the results based on the CBS-Q energies for closed-shell ion s are consistently higher than those based on the G2(MP2) energies. He ats of formation based on the CBS-q energies for ions are higher than those obtained with the other composite methods. (C) 1997 Elsevier Sci ence B.V.