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

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• 作者：Xingyu Lu ; Julien Tr茅bosc ; Olivier Lafon ; Diego Carnevale ; Simone Ulzega ; Geoffrey Bodenhausen ; Jean-Paul Amoureux
• 关键词：1D solid-state NMR ; 14N ; Broadband excitation ; DANTE ; MAS ; Paramagnetic samples
• 刊名：Journal of Magnetic Resonance
• 出版年：November, 2013
• 年：2013
• 卷：236
• 期：Complete
• 页码：105-116
• 全文大小：3629 K

文摘

We analyze the direct excitation of wide one-dimensional spectra of nuclei with spin I = 1/2 or 1 in rotating solids submitted to pulse trains in the manner of Delays Alternating with Nutations for Tailored Excitation (DANTE), either with one short rotor-synchronized pulse of duration 蟿_p in each of K rotor periods or with N interleaved equally spaced pulses 蟿_p in each rotor period, globally also extending over K rotor periods . The excitation profile of scheme is a comb of rf-spikelets with N谓_R = N/T_R spacing from the carrier frequency, and a width of each spikelet inversely proportional to the length, KT_R, of scheme. Since the individual pulse lengths, 蟿_p, are typically of a few hundreds of ns, scheme can readily excite spinning sidebands families covering several MHz, provided the rf carrier frequency is close enough to the resonance frequency of one the spinning sidebands. If the difference of isotropic chemical shifts between distinct chemical sites is less than about 1.35/(KT_R), scheme can excite the spinning sidebands families of several sites. For nuclei with I = 1/2, if the homogeneous and inhomogeneous decays of coherences during the DANTE sequence are neglected, the K pulses of a train have a linearly cumulative effect, so that the total nutation angle is 胃_tot = K2蟺谓₁蟿_p, where 谓₁ is the rf-field amplitude. This allows obtaining nearly ideal 90掳 pulses for excitation or 180掳 rotations for inversion and refocusing across wide MAS spectra comprising many spinning sidebands. If one uses interleaved DANTE trains with N > 1, only spinning sidebands separated by intervals of N谓_R with respect to the carrier frequency are observed as if the effective spinning speed was N谓_R. The other sidebands have vanishing intensities because of the cancellation of the N contributions with opposite signs. However, the intensities of the remaining sidebands obey the same rules as in spectra obtained with 谓_R. With increasing N, the intensities of the non-vanishing sidebands increase, but the total intensity integrated over all sidebands decreases. Furthermore, the NK pulses in a train do not have a simple cumulative effect and the optimal cumulated flip angle for optimal excitation, , exceeds 90掳. Such pulse trains allow achieving efficient broadband excitation, but they are not recommended for broadband inversion or refocusing as they cannot provide proper 180掳 rotations. Since pulse trains with N > 1 are shorter than basic sequences, they are useful for broadband excitation in samples with rapid homogeneous or inhomogeneous decay.

For nuclei with I = 1 (e.g., for ¹⁴N), the response to basic pulse train is moreover affected by inhomogeneous decay due to 2nd-order quadrupole interactions, since these are not of rank 2 and therefore cannot be eliminated by spinning about the magic angle. For large quadrupole interactions, the signal decay produced by second-order quadrupole interaction can be minimized by (i) reducing the length of pulse trains using N > 1, (ii) fast spinning, (iii) large rf-field, and (iv) using high magnetic fields to reduce the 2nd-order quadrupole interaction.