Supercycled SW f -TPPM sequence for heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance Cyril Augustine ⇑ , Narayanan D. Kurur Department of Chemistry, Indian Institute of Technology, New Delhi 110 016, India article info Article history: Received 16 October 2010 Revised 29 December 2010 Available online 14 January 2011 Keywords: Solid-state NMR 13 C– 1 H dipolar decoupling Supercycled SW f -TPPM SW f -TPPM SPINAL-64 abstract The performance of a supercycled SW f -TPPM sequence for heteronuclear dipolar decoupling in solid-state NMR is analyzed here. The decoupling performance of this sequence with respect to experimental param- eters, such as, the phase angle, proton offset and MAS frequency is studied. A comparison is made with two other commonly used decoupling schemes in solid-state NMR namely, SPINAL-64 and SW f -TPPM, on a sample of U- 13 C-labeled tyrosine. Our results show that supercycled SW f -TPPM performs better than the former sequences. Also, numerical spin dynamics studies are presented which support the experi- mentally observed efficiency in the decoupling. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Spectral resonances in solid-state NMR are usually broad due to the overlapping of strong interactions that depend on the orienta- tion of nuclei in the magnetic field [1]. These interactions include homonuclear and heteronuclear dipole–dipole couplings, chemical shift anisotropy and electric quadrupolar couplings arising from the interaction of the non-spherical charge distribution of nuclei having spin I > ½ with the surrounding electric field gradient. How- ever, heteronuclear dipolar decoupling gives significant improve- ment in the resolution of NMR spectra of solid samples. For instance, high-resolution solid-state NMR spectra of rare nuclei such as 13 C can be recorded with appropriate radio-frequency (RF) irradiation on the abundant 1 H spins along with magic angle spinning (MAS) to remove the dipolar couplings between 13 C and 1 H nuclei [1,2]. A continuous burst of RF irradiation on the most abundant spins during the acquisition of the signals from the rare nuclei, called continuous-wave (CW) decoupling was once considered as an effective technique for removing heteronuclear dipolar interac- tions in solids [1]. Two-pulse phase modulation (TPPM) [3] led to a most significant development in heteronuclear dipolar decou- pling providing superior performance over the traditional CW decoupling. It was the first multi-pulse decoupling sequence in anisotropic systems. The TPPM sequence consists of repeating units of two RF pulses of equal length with alternating phases. It is of the form s +u s u , where s represents the pulse duration with a flip angle normally between 160° and 180° and u implies the phase angle around 15°. These two parameters s and u are care- fully optimized experimentally for superior performance. After the invention of TPPM, a continuous quest for more effec- tive decoupling sequences in anisotropic systems can be seen in the literature [6–21]. New sequences include small phase angle ra- pid 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 two- pulse phase modulation [12], decoupling schemes based on Hahn-Echo trains [13], sequences based on Swept-frequency two-pulse phase modulation [14–21] and Phase-Wiggled TPPM [22]. It has been proved in heteronuclear decoupling of isotropic liq- uids that the effectiveness of a decoupling sequence can be im- proved by combining different versions of the primitive cycle to form extended supercycles which results in the compensation of some of the residual pulse imperfections [4,5]. Later this idea was implemented in decoupling schemes for liquid crystals. For example, the SPARC-16 sequence was constructed by extending the basic TPPM scheme into a supercycle [6]. The SPARC-16, which stands for ‘small phase angle rapid cycling with 16 steps’, contains a step by step phase cycling of the parent TPPM decoupling se- quence using 16 pulses. The incorporation of phase increments in the basic elements of TPPM led to the group of sequences called SPINAL which stands for small phase incremental alteration [7]. Among them SPINAL-64 works more effectively than TPPM for het- eronuclear dipolar decoupling in anisotropic systems. In SPINAL-64 an extended supercycle is shaped from the basic element Q = s 1090-7807/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2011.01.007 ⇑ Corresponding author. Fax: +91 11 2658 1102. E-mail address: cyril.augustinev@gmail.com (C. Augustine). Journal of Magnetic Resonance 209 (2011) 156–160 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr