Computational s-block thermochemistry with the correlation consistent composite approach

The correlation consistent composite approach (ccCA) is a model chemistry that has been shown to accurately compute gas-phase enthalpies of formation for alkali and alkaline earth metal oxides and hydroxides (Ho, D. S.; DeYonker, N. J.; Wilson, A. K.; Cundari, T. R. J. Phys. Chem. A 2006, 110, 9767).The ccCA results contrast to more widely used model chemistries where calculated enthalpies of formation for such species can be in error by up to 90 kcal mol(-1). In this study, we have applied ccCA to a more general set of 42 s-block molecules and compared the ccCA Delta H-f values to values obtained using the G3 and G3B model chemistries. Included in this training set are water complexes such as Na(H2O)(n)(+) where n = 1 - 4, dimers and trimers of ionic compounds such as (LiCl)(2) and (LiCl)(3), and the largest ccCA computation to date: Be(acac)2, BeC10H14O4. Problems with the G3 model chemistries seem to be isolated to metal-oxygen bonded systems and Be-containing systems, as G3 and G3B still perform quite well with a 2.7 and 2.6 kcal mol(-1) mean absolute deviation (MAD), respectively, for gas-phase enthalpies of formation. The MAD of the ccCA is only 2.2 kcal mol(-1) for enthalpies of formation (Delta H-f) for all compounds studied herein. While this MAD is roughly double that found for a ccCA study of >350 main group (i.e., p-block) compounds, it is commensurate with typical experimental uncertainties for s-block complexes. Some molecules where G3/G3B and ccCA computed Delta H-f values deviate significantly from experiment, such as (LiCl)(3), NaCN, and MgF, are inviting candidates for new experimental and high-level theoretical studies.