Computational study of the enthalpies of formation, DeltafH degrees, and mean bond enthalpies, mBEs, of H4-nEXn0/- and H3-nEXn+/0 (E=C, B; X=F-I)

To compensate for lacking experimental standard enthalpies of formation DeltafH degrees of haloboranes/-boranates as well as the isoelectronic halocarbenium ions and halomethanes, high-level quantum chemical calculations up to the ccsd(t)/(SDB-)aug-cc-pVQZ level have been performed to establish these values. Very reliable experimental data (e.g., DeltafH degrees of HCl, F, and CH4) or at the G3 level established values (e.g., DeltafH degrees of CF3+=410 kJ mol(-1)) were used as anchor points to obtain accurate absolute DeltafH degrees and mean bond enthalpy (mBE) values. To further minimize systematic errors of the protocol, all derived quantities were assessed in isodesmic reactions at the G3 and ccsd(t) level using the (SDB-)aug-cc-PVTZ basis set. The obtained DeltafH degrees values are in very good agreement to (scarcely available) accurate experimental and computational data. Almost all B-containing compounds have been assessed for the first time. We derived "best" DeltafH degrees values and used them to determine the mean E-X bond enthalpies in H4-nEXn-/0 and H3-nEXn0/+ (with n=1-3, E=B, C, and X=F-I). In each of the series, the DeltafH degrees values increase from fluorine to iodine, and except for the iodine-containing carbenium ions and the bromo- and iodomethanes, the DeltafH degrees values become lower with the more halogen atoms that are present in the particle. The boron containing species always have a lower DeltafH degrees than the isoelectronic carbenium ions and methanes, and the H4-nEXn-/0 are lower in energy than the parent H3-nEXn+/0. This reflects the greater average B-X bond strengths.