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, and CBS-q methods for 46 aliphatic nitrogen- and oxygen-containing molecules and ions with up to three carbon atoms are compared with experimental results and with each other. The values obtained theoretically for aliphatic amine radical cations and immonium ions are uniformly higher 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 obtained with the G2(MP2) and G2(MP2,SVP) procedures differ only little, whereas the results based on the CBS-Q energies for closed-shell ions are consistently higher than those based on the G2(MP2) energies. Heats of formation based on the CBS-q energies for ions are higher than those obtained with the other composite methods. (C) 1997 Elsevier Science B.V.