Broadband excitation in solid-state NMR of paramagnetic samples using Delays Alternating with Nutation for Tailored Excitation (‘Para-DANTE’)

Abstract

a b s t r a c tThis Letter shows that interleaved sequences of short pulses in the manner of 'Delays Alternating with Nutation for Tailored Excitation' (DANTE) with N = 1, 2, 3 . . . equidistant pulses per rotor period extending over K rotor periods can be used to excite, invert or refocus a large number of spinning sidebands of spin-1/2 nuclei in paramagnetic samples where hyperfine couplings lead to very broad spectra that extend over more than 1 MHz. The breadth of the response is maintained for rf-field amplitudes as low as 30 kHz since it results from cumulative effects of individual pulses with very short durations.

Broadband excitation in solid-state NMR using interleaved DANTE pulse trains with N pulses per rotor period

“…19 F spin-1/2 nuclei in paramagnetic CeF 3 sample [9], and 14 N spin-1 nuclei in c-glycine [20,21]. [9].…”

“…[9]. The 14 N nuclei in c-glycine have a quadrupolar coupling constant of C Q = 1.18 MHz, and an electric field gradient (efg) asymmetry parameter of g Q = 0.5 [13,22,35].…”

“…This technique has been demonstrated for the direct observation of spin-1/2 ( 1 H and 19 F) and spin-1 ( 6 Li) nuclei subject to paramagnetic interactions [9][10][11], as well as the direct observation and indirect detection of spin-1 nuclei ( 14 N) subject to large quadrupole interactions [20,21]. Furthermore, we have also recently introduced interleaved DANTE sequences comprising N > 1 pulses per rotor period [9,20,21]. These interleaved DANTE schemes excite transverse magnetization within a shorter time and are thus useful for wide MAS spectra with several different species and/or submitted to large losses.…”

“…In this context, trains of rotor-synchronized short rectangular pulses in the manner of Delays Alternating with Nutation for Tailored Excitation, DANTE [28,29], have been recently proposed as an efficient method for broadband excitation and inversion under MAS [9,10,20,21,30]. This technique has been demonstrated for the direct observation of spin-1/2 ( 1 H and 19 F) and spin-1 ( 6 Li) nuclei subject to paramagnetic interactions [9][10][11], as well as the direct observation and indirect detection of spin-1 nuclei ( 14 N) subject to large quadrupole interactions [20,21].…”

“…In that case, MAS powder spectra feature numerous spinning sidebands. Such wide spectra are encountered when: (i) the nuclei are close either to unpaired electrons in paramagnetic samples [2][3][4][5][6][7][8][9][10][11][12] or to conduction electrons in electrical conductors, (ii) the nuclei are subject to large quadrupole interactions (I > 1/2) [13][14][15][16][17][18][19][20][21][22][23][24][25], or (iii) their chemical shift anisotropy (CSA) is large, particularly when the static magnetic field is high [26,27].…”