(17)O NMR Investigation of Water Structure and Dynamics

Abstract

The structure and dynamics of the bound water in barium chlorate monohydrate were studied with (17)O nuclear magnetic resonance (NMR) spectroscopy in samples that are stationary and spinning at the magic-angle in magnetic fields ranging from 14.1 to 21.1 T. (17)O NMR parameters of the water were determined, and the effects of torsional oscillations of the water molecule on the (17)O quadrupolar coupling constant (CQ) were delineated with variable temperature MAS NMR. With decreasing temperature and reduction of the librational motion, we observe an increase in the experimentally measured CQ explaining the discrepancy between experiments and predictions from density functional theory. In addition, at low temperatures and in the absence of (1)H decoupling, we observe a well-resolved (1)H-(17)O dipole splitting in the spectra, which provides information on the structure of the H2O molecule. The splitting arises because of the homogeneous nature of the coupling between the two (1)H-(17)O dipoles and the (1)H-(1)H dipole.

Figure 1

Molecular and long-range crystal packing of Ba(ClO₃)₂·H₂O. (a) Molecular unit. (b) 2D long-range crystal structure shown with the b axis normal to the page. Colors represent different elements: barium (black), chlorine (green), chlorate oxygen (red), bound water oxygen (blue), and hydrogen (gray). (c) Interatomic distances and angles that are relevant to the structure of the bound water determined via neutron diffraction, where O_w is the water oxygen, and O(2)/O(3) are the nearest chlorate oxygen.

Figure 2

MAS and stationary ¹⁷O NMR spectra of Ba(ClO₃)₂·H₂¹⁷O. (a) MAS spectrum (solid, black) and simulation (dashed, black) at 21.1 T (ω_0H/2π = 900 MHz), (b) stationary ¹⁷O NMR spectrum (solid, red) and simulation (dashed, red) with continuous-wave ¹H decoupling (γB₁/2π = 100 kHz) at 17.6 T (ω_0H/2π = 748 MHz), and (c) stationary ¹⁷O NMR spectrum (solid, green) and simulation (dashed, green) without ¹H decoupling spectra at 17.6 T (ω_0H/2π = 748 MHz). NMR interactions from GIPAW calculations are included in each spectral simulation displayed with water molecule in the inset. NMR parameters used in the spectral simulations are given in Table 1.

Figure 3

EFG and NSA tensor components taken from GIPAW calculations in CASTEP on the structure of the bound water in Ba(ClO₃)₂·H₂O. (b,c) Visualization of the motional averaging of the V_zz component by the two librational modes of the bound water about the (b) V_yy (waving) and (c) V_xx (twisting) axes.

Figure 4

(a) ¹⁷O MAS NMR spectrum (solid, red) at 300 ± 5 K; spectral simulation (dashed, red): C_Q = 6.9 ±0.1 MHz, η_Q = 0.98 ± 0.05, δ_iso = 21 ± 1 ppm. (b) ¹⁷O MAS NMR spectrum (solid, green) at 170 ± 10 K, spectral simulation (dashed, green): C_Q= 7.4 ± 0.1 MHz, η_q = 0.95 ± 0.05, δ_iso = 21 ± 1 ppm. (c) ¹⁷O MAS NMR spectrum with 100 kHz continuous-wave ¹H decoupling (solid, blue) at 105 ± 5 K, spectral simulation (dashed, blue): C_Q = 7.55 ± 0.1₀ MHz, η_Q = 0.92 ± 0.05, δ_iso = 21 ± 1 ppm. Spectra were acquired with ω_R/2π = 14 kHz, spinning sidebands are noted by asterisks. The gray vertical dashed line indicates the left edge of all VT MAS spectra, while the colored vertical dashed lines indicate the right edge of each spectrum.

Figure 5

(a) ¹⁷O MAS NMR spectrum (solid, blue) at 105 ± 5 K without ¹H decoupling; spectral simulation (dashed, blue). (b) ¹⁷O MAS NMR spectrum (solid, black) at 105 ± 5 K with 100 kHz continuous-wave ¹H decoupling; spectral simulation (dashed, black). Spectra were acquired with a ω_R/2π = 14 kHz, spinning sidebands are noted by asterisks (*). NMR interactions that are included in each spectral simulation displayed with a water molecule in the inset. NMR parameters used in the spectral simulations are given in Table 1.