13 C– 1 H HSQC experiment of probe molecules aligned in thermotropic liquid crystals: Sensitivity and resolution enhancement in the indirect dimension Bikash Baishya a,b , Raghav G. Mavinkurve c , N. Suryaprakash b, * a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India b NMR Research Centre, Indian Institute of Science, Bangalore 560 012, India c Department of Physics, Indian Institute of Science, Bangalore 560 012, India Received 10 October 2006; revised 24 December 2006 Available online 30 December 2006 Abstract The spectra of molecules oriented in liquid crystalline media are dominated by partially averaged dipolar couplings. In the 13 C– 1 H HSQC, due to the inefficient hetero-nuclear dipolar decoupling in the indirect dimension, normally carried out by using a p pulse, there is a considerable loss of resolution. Furthermore, in such strongly orienting media the 1 H– 1 H and 13 C– 1 H dipolar couplings leads to fast dephasing of transverse magnetization causing inefficient polarization transfer and hence the loss of sensitivity in the indirect dimension. In this study we have carried out 13 C– 1 H HSQC experiment with efficient polarization transfer from 1 H to 13 C for molecules aligned in liquid crystalline media. The homonuclear dipolar decoupling using FFLG during the INEPT transfer delays and also during evolution period combined with the p pulse heteronuclear decoupling in the t 1 period has been applied. The studies showed a significant reduction in partially averaged dipolar couplings and thereby enhancement in the resolution and sensitivity in the indirect dimension. This has been demonstrated on pyridazine and pyrimidine oriented in the liquid crystal. The two closely resonating carbons in pyrimidine are better resolved in the present study compared to the earlier work [H.S. Vinay Deepak, Anu Joy, N. Suryaprakash, Determination of natural abundance 15 N– 1 H and 13 C– 1 H dipolar couplings of molecules in a strongly orienting media using two-dimensional inverse experiments, Magn. Reson. Chem. 44 (2006) 553–565]. Ó 2007 Elsevier Inc. All rights reserved. Keywords: HSQC spectrum; FSLG; FFLG; INEPTRD 1. Introduction NMR spectra of molecules oriented in liquid crystals provide homo and heteronuclear dipolar couplings and thereby the geometry of the molecules. Several inequivalent dilute spins such as 13 C and 15 N coupled to protons form different coupled spin systems in their natural abundance and appear as satellites in the proton spectra. The identifi- cation of spectra corresponding to each coupled spin system is a prerequisite for the determination of dipolar couplings and it is a formidable task. We have shown earlier, using two-dimensional 15 N– 1 H and 13 C– 1 H Heteronuclear single quantum coherence (HSQC) and Heteronuclear multiple quantum coherence (HMQC) experiments, that selective detection of spectra of each rare spin coupled to protons is possible [1–3]. The dipolar decoupling has been carried out in the HSQC/HMQC experiments during the evolution period to obtain 13 C chemical shifts in the oriented phase. The normal spectrum dominated by proton–proton (D HH ’s) and proton–carbon (D CH ’s) dipolar couplings is obtained in the direct dimension. The spectra are obtained without any phase distortion. The cross section of the spectrum taken at the chemical shift positions of different carbons in the indi- rect dimension provides the spectrum of the particular car- bon coupled to protons, the analysis of which provided 1090-7807/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2006.12.014 * Corresponding author. Fax: +91 80 2360 1550. E-mail address: nsp@sif.iisc.ernet.in (N. Suryaprakash). www.elsevier.com/locate/jmr Journal of Magnetic Resonance 185 (2007) 221–229