Heteronuclear dipolar decoupling in liquid‐crystal NMR using supercycled SW_f‐TPPM sequences

Abstract

Heteronuclear dipolar decoupling is an essential requirement for extracting structural information from the (13)C NMR spectra of liquid crystals. Efficient schemes for heteronuclear dipolar decoupling in such systems are formulated here by supercycling SW(f)-TPPM, a sequence introduced recently for this purpose in rotating solids. These sequences are compared with two other commonly used decoupling schemes in liquid-crystal NMR, SPINAL-64 and SW(f)-TPPM, by analyzing the intensities of various resonances in the proton decoupled (13)C spectrum of the liquid-crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). The effectiveness of the decoupling programs with respect to experimental parameters such as RF field strength, decoupler offset frequency and phase angle is also presented.

Efficiency of heteronuclear dipolar decoupling schemes in solid-state NMR: Investigation of effective transverse relaxation times

“…Fig. 9 shows the transverse dephasing curves obtained using the decoupling schemes considered before along with supercycled SW f -TPPM (SW f -TPPM-sc) [16]. SW f -TPPM-sc consists of four SW f -TPPM blocks phase shifted with respect to each other in the following manner:…”

“…We here compare the performance of TPPM [2], SPINAL [3], SW f -TPPM [7], SW f -TPPM-sc (supercycled SW f -TPPM) [16], XiX [17], and PISSARRO [18] decoupling schemes for MAS frequencies 10, 22, and 60 kHz using high (m 1 = 196 kHz) and low (m 1 = 10 kHz) decoupling RF amplitudes with respect to the T 0 2 values. Decoupling efficiency using low RF amplitudes at high spinning frequencies has been explored in the past also [10,[19][20][21][22].…”

Sweep direction and efficiency of the swept-frequency two pulse phase modulated scheme for heteronuclear dipolar-decoupling in solid-state NMR

“…The direction of the sweep for a decoupling sequence is important because for more complex sequences like SPINAL-64 and recently introduced supercycled version of SW f -TPPM [23], changing the direction of the sweep produces different results. In the SW f -TPPM sequence, only the pulse durations are incrementally changed, whereas the phase angles remain constant.…”

Supercycled SWf-TPPM sequence for heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance

“…New sequences include small phase angle rapid cycling (SPARC) [6], small phase incremental alteration (SPINAL) [7], DROOPY sequences [8], XiX, a scheme with repeating units of two rotor-synchronized pulses with 180°phase shift [9], some symmetry based sequences [10,11], cosine modulated twopulse phase modulation [12], decoupling schemes based on Hahn-Echo trains [13], sequences based on Swept-frequency two-pulse phase modulation [14][15][16][17][18][19][20][21] and Phase-Wiggled TPPM [22].…”

“…Recently, we have demonstrated that the supercycled version of SW f -TPPM can be used effectively for removing heteronuclear dipolar interactions in liquid crystals [21]. The supercycled scheme was constructed in the form of R R R R, where R is the parent SW f -TPPM and R stands for the phase inverted counterpart of the parent sequence.…”