Observation of Low-$γ$ Quadrupolar Nuclei by Surface-Enhanced NMR Spectroscopy

Abstract

We introduce a novel NMR approach that extends the capabilities of indirect dynamic nuclear polarization (DNP) under magic-angle spinning to probe the local environment of half-integer spin quadrupolar nuclei. Compared to cross-polarization, this novel method based on the refocused INEPT scheme with adiabatic dipolar recoupling is easier to optimize and does not distort the quadrupolar line shapes. Furthermore, the use of this technique, instead of the PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order) scheme or direct DNP, greatly improves the sensitivity of DNP-NMR for the detection of quadrupolar isotopes with small dipolar couplings to protons, including notably those located in the subsurface of inorganic materials or with low gyromagnetic ratio ($γ$). This technique has been applied to identify the atomic-level structure of Brønsted acid sites of hydrated titania-supported MoO${}_3$, MoO${}_3$/TiO${}_2$, a widely used heterogeneous catalyst. The spectra of protonated and unprotonated ${}^{17}$O sites, acquired in natural abundance, indicate the presence of various oxomolybdate species as well as HOMo${}_2$ and HOMo${}_3$ Brønsted acid sites. The enhanced sensitivity of this new method has also enabled the acquisition of the first DNP-enhanced spectra of ${}^{95}$Mo and ${}^{47,49}$Ti low-$γ$ quadrupolar isotopes. The proposed polarization transfer is also employed to acquire the first DNPenhanced spectrum of ${}^{67}$Zn, another low-$γ$ quadrupolar isotope. This possibility is demonstrated for Al-doped ZnO nanoparticles used in optoelectronic devices. The obtained ${}^{17}$O, ${}^{27}$Al, and ${}^{67}$Zn DNP-NMR data prove that the surface region of these nanoparticles contains ZnO phase as well as secondary phases, such as $α$-Al${}_2$O${}_3$ and partially inverse ZnAl${}_2$O${}_4$ spinel.