Evaluation of Gas-to-Liquid ¹⁷O Chemical Shift of Water: A Test Case for Molecular and Periodic Approaches

Modeling liquid water features is a challenging and ongoing task that brings together a number of computational issues related to the description both of its electronic and geometrical structure. In order to go a step further in the understanding of this peculiar liquid, we present a thorough analysis of NMR gas-to-liquid ¹⁷O and ¹H shifts of water using density functional theory based molecular dynamics. In order to be as consistent as possible, we consider the influence of basis sets, exchange-correlation functionals, and structural models, in both molecular and periodic schemes, to evaluate ¹⁷O and ¹H nuclear shieldings. We show that strong error compensations between functional and basis-set expansion can be obtained in molecular approaches which artificially produces good ¹⁷O gas-to-liquid shifts with relatively small basis sets. We also demonstrate that, despite their ability to provide reliable liquid phase structures, generalized-gradient approximation based exchange-correlation functionals lead to strongly inconsistent values for ¹⁷O gas-to-liquid shift. This latter property is shown to be strongly influenced by intramolecular electronic delocalization, accentuated by the surrounded molecules. In contrast, ¹H is less sensitive to this effect. By including a Hartree–Fock exchange term, through the use of hybrid functionals which partially correct the self-interaction error, better agreement with experimental values is obtained. The present study provides a detailed guideline to properly evaluate gas-to-liquid shifts in hydrogen bonded systems and emphasizes that, for nuclear shieldings, an accurate electronic structure evaluation prevails over the description of the liquid structure