Mind the correct basis set: A case study for predicting gas phase acidities of small compounds using calculations from first principles

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© 2014 Wiley Periodicals, Inc. Some of the most popular computational methods have been utilized to determine a dependency of the acidity trend of the first-row hydrides on a choice of basis set. For about three decades, methyl anion (CH-3) was known as the strongest base but after Tian et al. were able to produce the gas phase lithium monoxide anion (LiO-) they discovered it was a stronger base than CH-3 (Tian et al., Proc Natl Acad Soc USA 2008, 105, 7647). Furthermore, the authors confirmed their experimental results using high-level ab initio methods, namely W1 and W2C composite methods, as well as complete active spaceaveraged quadratic coupled cluster and Brueckner Doubles with triple excitation contribution (BD(T)) within the aug-ccpVQZ basis set. These methods are highly demanding in terms of the computational effort as well as a level of expertise needed from the user to correctly conduct such calculations. We have shown that the proper acidity trend, that is, δH298Kacid < δH4 < δH298K acid {LiOH}, can be obtained with less expensive, "black-box" type methods if only the basis set is properly chosen. Our results prove that the diffuse augmented basis sets are absolutely necessary for appropriate predictions of acidities. Our calculations show that the correct order of D H298K acid is achieved by augmenting relatively small cc-pVXZ (X5D,T) basis sets. A similar effect is observed for the family of Pople's basis sets. Our estimate for δH298K acid {LiOH} with CCSD(T)/aug-cc-pVTZ was 423.8 kcal/mol, which agrees very well with the experimental value 425.±66.1 kcal/mol. An important finding is that the proper acidity trend may be reversed if the basis sets are not correctly selected.

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International Journal of Quantum Chemistry