Solid-State ¹²⁷I NMR and GIPAW DFT Study of Metal Iodides and Their Hydrates: Structure, Symmetry, and Higher-Order Quadrupole-Induced Effects

Abstract

Central-transition (127)I solid-state nuclear magnetic resonance (SSNMR) spectra are presented for several anhydrous group 2 metal iodides (MgI(2), CaI(2), SrI(2), and BaI(2)), hydrates (BaI(2)·2H(2)O and SrI(2)·6H(2)O), and CdI(2) (4H polytype). Variable offset cumulative spectrum data acquisition coupled with echo pulse sequences and an 'ultrahigh' applied field of 21.1 T were usually suitable to acquire high-quality spectra. Spectral analysis revealed iodine-127 nuclear quadrupole coupling constants (C(Q)((127)I)) ranging in magnitude from 43.5 (CaI(2)) to 214 MHz (one site in SrI(2)). For very large C(Q), analytical second-order perturbation theory could not be used to reliably extract chemical shifts and a treatment which includes quadrupolar effects exactly was required (Bain, A. D. Mol. Phys. 2003, 101, 3163). Differences between second-order and exact modeling allowed us to observe 'higher-order' quadrupole-induced effects for the first time. This finding will have implications for the interpretation of SSNMR spectra of quadrupolar nuclei with large quadrupole moments. In favorable situations (i.e., C(Q)((127)I) < 120 MHz), measurements were also performed at 11.75 T which when combined with the 21.1 T data allowed us to measure iodine chemical shift (CS) tensor spans in the range from 60 (BaI(2)·2H(2)O) to 300 ppm (one site in BaI(2)). These measurements represent the first complete characterizations (i.e., electric field gradient and CS tensors as well as their relative orientation) of noncubic iodide sites using (127)I SSNMR. In select cases, the SSNMR data are supported with (127)I NQR measurements. We also summarize a variety of trends in the halogen SSNMR parameters for group 2 metal halides. Gauge-including projector-augmented wave DFT computations are employed to complement the experimental observations, to predict potential structures for the two hydrates, and to highlight the sensitivity of C(Q)((127)I) to minute structural changes, which has potential applications in NMR crystallography.

NMR parameters in alkali, alkaline earth and rare earth fluorides from first principle calculations

“…However, same kind of deviations have been noted previously for other halogens (Cl, Br and I) [105][106][107][108] and other nuclei such as 29 states. To overcome this PBE-DFT deficiency, the energy level of the 3d Ca orbitals was shifted to higher energy without changing the position of the s and p states in the Ca pseudopotential.…”

Assessment of DFT functionals with NMR chemical shifts

“…15,16 This intriguing and very systematic error has its origin in the deficiencies of the approximate density functional. For instance it has been shown before that for the 17 O chemical shift the error in CaO is significantly larger than for many other oxides and that the problem arises due to Ca 3d orbitals being too close to the valence band maximum, 13 which affects the Ca-3d O-2p hybridization.…”

A multinuclear solid-state magnetic resonance and GIPAW DFT study of anhydrous calcium chloride and its hydrates

“…[4][5][6] These tensors are important SSNMR observables, and a number of recent studies have illustrated that magnetic shielding and EFG tensor calculations using GIPAW DFT can provide additional insights in the study of a variety of chemical systems. [7][8][9][10][11][12][13][14][15][16][17] There is an increasing interest in "NMR crystallography", which pertains to obtaining molecular-level and crystal structure information for solid materials using SSNMR data. 18 Although there have been a number of exciting recent advances in this area of research, 6,[19][20][21][22][23][24][25] one cannot currently convert SSNMR data into a unique solid-state structure in a completely generalized manner.…”