Phosphorus-31 and vanadium-51 solid-state nuclear magnetic resonance spectroscopy of β-vanadyl phosphate — Effects of homo- and heteronuclear spin-spin, electrostatic, and paramagnetic interactions

Abstract

Field-dependent 31 P solid-state NMR studies demonstrate that the line shape in spectra of b-VOPO4 depends on 51 V-31 P direct and indirect spin-spin interactions (M 2 ( 51 V, 31 P) = 101(23) × 10 6 rad 2 s -2 , 2 J iso ( 51 V, 31 P) = 48(5) Hz) and, to a lesser extent, on 31 P chemical shift anisotropy (d iso = -10.4(2), U = d 11 -d 33 = 22(2) ppm) and 31 P-31 P interactions (M 2 ( 31 P, 31 P) = 6.7(1) × 10 6 rad 2 s -2 ). In contrast, homonuclear dipolar interactions play an important role for the field and spinning rate dependent 31 P spin-lattice relaxation via paramagnetic impurities (T 1 = 20-60 s). Vanadium-51 magic-angle spinning NMR spectra indicate a sizeable chemical shift anisotropy (d iso = -754(1), d 11 = -336(10), d 22 = -344(6), d 33 = -1581(8) ppm) and nuclear quadrupole interaction (c = 1.5(1) MHz, h = 0.35 (5)); the principal axis systems of both interactions are clearly not coincident, with an angle of 35(5)°between the greatest component of the electric field gradient tensor and d 33 .Résumé : Des études RMN du 31 P à l'état solide et dépendant du champ démontrent que la forme des raies dans les spectres du b-VOPO 4 dépend des interactions spin-spin directes et indirectes (M 2 ( 51 V, 31 P) = 101(23) × 10 6 rad 2 s -2 , 2 J iso ( 51 V, 31 P) = 48(5) Hz) et, à un degré moindre, sur l'anisotropie du déplacement chimique du 31 P (d iso = -10,4(2); U = d 11 -d 23 = 22(2) ppm) et aux interactions 31 P-31 P (M 2 ( 31 P, 31 P) = 6,7(1) × 10 6 rad 2 s -2 ). Par ailleurs, les interactions dipolaires homonucléaires jouent un rôle important pour la relaxation spin-réseau du 31 P qui varie avec le champ et la vitesse de rotation par le biais d'impuretés paramagnétiques (T 1 = 20-60 s). Les spectres RMN du vanadium à l'angle magique de rotation indiquent qu'il existe une anisotropie importante du déplacement chimique (d iso = -754(1), d 11 = -336(10), d 22 = -344(6), d 33 = -1581(8) ppm) et une interaction quadripolaire nucléaire (c = 1,5(1) MHz, h = 0,35(5)); il est clair que les axes principaux des systèmes des deux interactions ne coïncident pas, avec un angle de 35(5)°entre la composante principale du tenseur du gradient de champ électrique et d 33 .Mots-clés : spectroscopie RMN à l'état solide, RMN du 31 P, RMN du 51 V, relaxation spin-réseau, couplage spin-spin nucléaire indirect, anisotropie du déplacement chimique, tenseur du gradient de champ électrique, phosphate de vanadyle.[Traduit par la Rédaction]

Moderate MAS enhances local 1 H spin exchange and spin diffusion

“…A related earlier work is due to Eichele and Grimmer, who studied the longitudinal (T 1 ) relaxation of 31 P in a sample containing paramagnetic impurities, which provides an indirect measure of the homonuclear spin diffusion efficiency [16]. Similarly to our study, they observed a decrease of T 1 at slow (1-2 kHz) MAS, see Fig.…”

“…Based upon our previous work [15], we here studied local spin diffusion rates under MAS, and observed increased spin diffusion coefficients at low (<10 kHz) spinning rates as compared to static conditions, with a maximum spin diffusion efficiency around 2-3 kHz MAS. The effect is supported by previous T 1 relaxation data on a sample experiencing spin diffusion driven T 1 [16]. The fact that slow MAS enhances spin diffusion may be of particular interest for certain of the above-mentioned applications.…”

“…Turning to theoretical analyses, the indirect manifestation of maximum spin diffusivity at low MAS rates as observed by Eichele et al [16] in their study of T 1 times may be explained with a simple two-level picture. The 31 P spins experience an anisotropic chemical shielding in addition to the Zeeman splitting.…”

“…The dipolar interaction enters this picture as broadening of levels, which is a small effect compared to the anisotropy-caused splitting differences, diffusion is found to be most efficient. (b) T 1 minimum of 31 P in a sample containing paramagnetic centers [16]. In their vicinity, T 1 is very short, but for the whole sample it is limited by the transport of magnetization toward these sinks.…”

Electron Localization of Polyoxomolybdates with ε-Keggin Structure Studied by Solid-State 95Mo NMR and DFT Calculation

“…In paramagnetic compounds, the hyperfine interaction is included in the internal nuclear-spin interactions. , Fast paramagnetic relaxation can cause broadening of the NMR spectra, although in some cases it is advisedly used for speeding up NMR measurement. , The hyperfine coupling also affects the spectra as a change of line shape by the anisotropic component and as a whole shift by the isotropic component. Although numerous structural studies of paramagnetic materials have been conducted by solid-state NMR with 7 Li − and other HIQN such as 51 V, − 59 Co, 95 Mo, 137 Ba, and 139 La, the spectra have not been simulated by considering both isotropic and anisotropic effects from the second-order quadrupole, chemical shift and hyperfine interactions as far as we know. The present work performing numerical simulation of the 95 Mo spectra combined with DFT calculation − for the paramagnetic crystal would be a challenging attempt.…”