MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2006; 44: 553–565 Published online 14 March 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1791 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 † H. S. Vinay Deepak, 1 Anu Joy 2‡ and N. Suryaprakash 2* 1 Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2 NMR Research Centre, Indian Institute of Science, Bangalore 560 012, India Received 21 November 2005; Revised 27 December 2005; Accepted 15 January 2006 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. Identification of transitions belonging to each spin system is essential to determine heteronuclear dipolar couplings, which is a formidable task. In the present study, using 15 N– 1 H and 13 C– 1 H HSQC, and HMQC experiments we have selectively detected spectra of each rare spin coupled to protons. The 15 N– 1 H and 13 C– 1 H dipolar couplings have been determined in the natural abundance of 13 C and 15 N for the molecules pyrazine, pyrimidine and pyridazine oriented in a thermotropic liquid crystal. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: NMR spectroscopy; HSQC; HMQC; pyrazine; pyrimidine; pyridazine; dipolar couplings; natural abundance; liquid crystals; inverse experiment INTRODUCTION Analyses of NMR spectra of molecules aligned in the liquid crystalline media provide intramolecular dipolar couplings and thereby information on the molecular structure and orientation. 1–4 However, the major work in this area is restricted to proton NMR studies providing only the information on the proton– proton distances of the molecules. The information on the proton skeleton of the molecules is obtained from the 1 H– 1 H dipolar couplings (D HH ’s) derived by the analyses of proton NMR spectra. The geometric information involving proton and any other dilute spin such as 13 C or 15 N is obtained from the heteronuclear dipolar couplings (D ij ’s) determined by the analyses of satellite transitions in the proton NMR spectra or alternately by the analyses of fully coupled 13 C 5,6 or 15 N spectra. 7,8 The signal-to-noise ratio of the satellites is larger by a factor equal to the ratio of the sensitivity of protons to that of the coupled rare spin, i.e. the signal-to-noise ratio of 15 N satellites L Correspondence to: N. Suryaprakash, NMR Research Centre, Indian Institute of Science, Bangalore 560 012, India. E-mail: nsp@sif.iisc.ernet.in † Part of the work won the Best Poster Award at the Symposium on NMR, Drug Design and Bioinformatics, February 2004, Kolkata, India. ‡ Present Address: National Institute of Advanced Studies, Indian Institute of Science campus, Bangalore 560 012, India. Contract/grant sponsor: Department of Science and Technology, New Delhi; Contract/grant number: SR/S1/PC-13/2004. is 961 and that of 13 C satellites is 63. 7 Thus, the analysis of satellite spectra has a dramatic advantage compared to fully coupled rare spin spectra. However, the satellite spectral analysis has several limitations, i.e. (i) the addition of another interacting 13 C/ 15 N spin increases the number of allowed transitions by nearly fourfold, further increasing the complexity of the spectra; (ii) many of the satellite transitions are masked by the strong proton resonances arising out of 12 C/ 14 N-containing isotopomers constituting nearly 99% of the molecules in natural abundance; (iii) it is difficult to identify the resonance lines belonging to different spin systems arising from coupling of protons to different chemically inequivalent carbons/nitrogens in the molecule; and (iv) it is difficult to distinguish the satellite peaks from the impurity peaks that are of comparable intensities. Any technique that not only enhances the intensity of satellite transitions but also selectively detects the protons coupled to different 13 C or 15 N in their natural abundance will enable the easy analyses of the spectra and derivation of homo- and heteronuclear D ij ’s unambiguously. Two-dimensional HSQC 9 experiment has been employed earlier to determine the D CH ’s in proteins, nucleic acids and carbohydrates. 10 – 14 The orienting media in such studies are weak, and the experiment generally provides D CH between the directly bonded proton and carbon or the directly bonded proton and nitrogen. We have shown in our earlier study that when an organic molecule is aligned in a weakly orienting bicelle medium, both homo- and heteronuclear D ij ’s can be Copyright  2006 John Wiley & Sons, Ltd.