Solid‐state natural abundance ²⁵Mg NMR studies of Na₂MgEDTA·4H₂O—a possible new reference compound for ²⁵Mg NMR spectroscopy

Abstract

Natural abundance solid-state (25)Mg NMR measurements were made of the disodium salt of magnesium ethylenediaminetetraacetate tetrahydrate (Na(2)MgEDTA x 4 H(2)O). Both magic angle spinning (MAS) and static experiments were employed to determine the quadrupole coupling constant (C(q)) and the asymmetry parameter (eta(q)) of the electric field gradient (EFG) tensor associated with (25)Mg in this compound, giving the values C(q) = 1.675(5) MHz and eta(q) = 0.15(1). The isotropic chemical shift was determined to be delta(iso) = 0.25(10) ppm (relative to 11 M MgCl(2) aqueous solution) and a small chemical shift anisotropy (CSA) contribution (approximately -13 ppm) was detected, one of the first CSA reports in (25)Mg NMR. This compound exhibited remarkably good (25)Mg NMR sensitivity, due to its fast spin-lattice relaxation and modest quadrupole coupling, which allowed its use as a secondary shift reference and as a test sample for the implementation and optimisation of signal-enhancement methods in (25)Mg NMR spectroscopy, such as double frequency sweeps (DFS) and the use of adiabatic hyperbolic secant (HS) and WURST pulses.

Recent Advances in Solid-State 25Mg NMR Spectroscopy

“…97 Both HS and WURST pulses (among some other adiabatic pulses) have been extensively employed for the ST inversion and consequent CT enhancement in NMR experiments involving half-integer spin quadrupolar nuclei. 41,[83][84][85][86]91,100 Single HS pulse There are several parameters which need to be optimised for the achievement of the highest possible CT enhancement with minimal lineshape distortion. With optimal settings, HS and other adiabatic pulses usually give the highest enhancements (close to the theoretical limits of 2I for perfect ST inversion), when compared to other methods such as DFS and RAPT.…”

“…Due to these facts, this compound was extensively used as a ''test sample'' for the implementation of signal-enhancement methods, using the experimental set-up tuned at the 25 Mg NMR frequency. 41 In that work, the authors compared the performance of WURST and HS pulses with DFS, achieving enhancement factors of 2.2 for the adiabatic pulses and 2.6 for DFS (as compared to the CT intensity recorded in a standard Hahn-echo experiment). These somewhat low enhancement factors (compared with the ideal values of 3.0 and 5.0 in the cases of complete saturation and perfect inversion in the right order of all STs, respectively, as described before) were attributed to the fast spin-lattice relaxation found for 25 Mg nuclei in Na 2 MgEDTAÁ4H 2 O, which causes a rapid re-establishment of thermal equilibrium populations after the disturbing DFS or adiabatic pulses.…”

“…108 In 2008, Freitas et al 41 reported for the first time the use of adiabatic inversion pulses with the purpose of enhancing the CT signal in 25 Mg NMR spectroscopy. The model compound used in that work was magnesium disodium ethylenediaminetetraacetate tetrahydrate (Na 2 MgEDTAÁ4H 2 O).…”

“…Obviously, in this case, the shifts corresponding to the observed singularities will be dependent on the strength of the static magnetic field (B 0 ). Freitas et al 41 proposed the use of the compound magnesium disodium ethylenediaminetetraacetate tetrahydrate (Na 2 MgEDTAÁ4H 2 O), which gives a relatively narrow secondorder MAS CT lineshape with C q ¼1.675(5)MHz, q ¼015(1) and d iso ¼0.25 (10)ppm. This spectrum is easily observed in short experimental time due to a fast spin-lattice relaxation of 25 Mg in the compound.…”

“…The offset dependence of the enhancement factors obtained with use of HS pulses was also investigated, taking advantage of the ease to obtain 25 Mg NMR spectra with good S/N ratio in Na 2 MgEDTAÁ4H 2 O. 41 Some illustrative results are shown in Figure 12, where the enhancement factors obtained in 25 Mg MAS NMR spectra are plotted against the offsets of the HS pulses used to disturb the populations of the energy levels. The same plot also includes the theoretical firstorder quadrupole static powder pattern, calculated using the parameters experimentally determined from 25 Mg MAS NMR spectra in Na 2 MgED-TAÁ4H 2 O: C q ¼1.675(5)MHz, q ¼0.15(1), d iso ¼0.25 (10)ppm.…”