Mind the correct basis set: A case study for predicting gas phase acidities of small compounds using calculations from first principles. Issue 1 (23rd September 2014)

• Record Type: Journal Article
• Authors: Tymińska, Nina; Włoch, Marta; Royappa, A. Timothy
• Is Part Of: International journal of quantum chemistry. Volume 115:Issue 1(2015:Jan. 05)
• Page Start: 42
• Page End: 49
• Publication Date: 2014-09-23
• Subjects: lithium monoxide anion -- acidity trend -- first principle calculations; Quantum chemistry -- Periodicals; 541.28
• DOI: 10.1002/qua.24792
• Languages: English
• ISSNs: 0020-7608

Abstract

Abstract : 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 space‐averaged quadratic coupled cluster and Brueckner Doubles with triple excitation contribution (BD(T)) within the aug‐cc‐pVQZ 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, Δ H acid 298 K ( CH 4 ) < Δ H acid 298 K ( 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 Δ H acid 298 K is achieved by augmenting relatively small cc‐pVXZ ( X = D, T ) basis sets. A similar effect is observed for the family of Pople's basis sets. Our estimate for Δ H acid 298 K ( LiOH ) with CCSD(T)/aug‐cc‐pVTZ was 423.8 kcal/mol, which agrees very well with the experimental value 425.7 ± 6.1 kcal/mol. An important finding is that the proper acidity trend may be reversed if the basis sets are not correctly selected. © 2014 Wiley Periodicals, Inc. Abstract : The proper acidity trend of the first‐row hydrides may be obtained with fairly inexpensive, "black‐box" type methods only if the basis set is properly chosen. The trend may be reversed if the basis set is not correctly selected. Theoretical studies confirm that the lithium monoxide anion is a stronger base than methyl anion, which was believed to be the strongest base for about three decades.