11B and 23Na solid-state NMR and density functional theory studies of electric field gradients at boron sites in ulexite

Abstract

Nuclear magnetic resonance (NMR) parameters of ¹¹B in borates and borosilicates, unlike those of many other nuclei such as ²⁹Si and ²⁷Al, vary only over limited ranges and have been thought to be insensitive to local structural environments. High-resolution NMR spectroscopy at high (14 T) and ultrahigh (21 T) fields yield precise ¹¹B and ²³Na NMR parameters for ulexite, which contains the pentaborate polyanion ([B₅O₆(OH)₆]³⁻) as the fundamental building block (FBB). These NMR parameters are compared with ab initio theoretical calculations as implemented in WIEN2K, including optimization of the ulexite structure, determination of the electric field gradients (EFG) and consequently the nuclear quadrupole interaction (QI) parameters at the five distinct B sites, and calculations of the density of states (DOS). These calculations show that the magnitudes and signs of the EFG for ^[3]B and ^[4]B are determined by multiple factors, including the electron distributions in the B 2p_z orbitals and their interactions with Ca-3p/O-2s orbitals. Most importantly, the calculated B 2p_z orbitals at all B sites in ulexite are predominantly affected by the atoms within the fundamental building block, resulting in the insensitivity of the ¹¹B QI parameters to the weak interunit interactions among FBB. Calculations with the water molecules removed from the ulexite structure provide further support for the strong intraunit interactions in FBB as a cause for the poor sensitivity of ¹¹B NMR parameters to local structural environments, including hydrogen bonding, in borates.