Discerning the degenerate transitions of scalar coupled 1 H NMR spectra: Correlation and resolved techniques at higher quantum G.N. Manjunatha Reddy a , T.N. Guru Row b , N. Suryaprakash a, * a NMR Research Centre, Indian Institute of Science, SIF, Bangalore, Karnataka 560 012, India b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, Karnataka 560 012, India article info Article history: Received 16 August 2008 Revised 14 October 2008 Available online 31 October 2008 Keywords: Multiple quantum Multiple quantum J-resolved Scalar couplings Spectral analyses Degenerate transitions Fluorinated benzamides abstract The blend of spin topological filtering and the spin state selective detection of single quantum transitions by the two dimensional multiple quantum-single quantum correlation and higher quantum resolved techniques have been employed for simplifying the complexity of scalar coupled 1 H NMR spectra. The conventional two dimensional COSY and TOCSY experiments, though identify the coupled spin networks, fail to differentiate them due to severe overlap of transitions. Non-selective excitation of homonuclear higher quantum of protons results in filtering of spin systems irrespective of their spin topologies. The spin state selection by passive 19 F spins provides fewer transitions in each cross section of the single quantum dimension simplifying the analyses of the complex spectra. The degenerate single quantum transitions are further discerned by spin selective double and/or triple quantum resolved experiments that mimic simultaneous heteronuclear and selective homonuclear decoupling in the higher quantum dimension. The techniques aided the determination of precise values of spectral parameters and relative signs of the couplings. Ó 2008 Elsevier Inc. All rights reserved. 1. Introduction The development of several two dimensional NMR techniques over the decades have aided the analyses of very complex 1 H NMR spectra of molecules [1–3]. The identification of the group of resonances pertaining to different inequivalent protons and their coupling network is generally obtained from the conventional two dimensional COSY and TOCSY experiments [1]. This paves the way for the analyses of the first order spectra of weakly scalar cou- pled spin systems, where the chemical shift separation between the coupled spins is several orders of magnitude larger than the spin–spin couplings. For extracting the precise magnitudes of the scalar couplings in such weakly coupled spin systems several two dimensional experiments such as, J-resolved [4–6] and quan- titative J correlation experiments [7–9] have been reported. An- other methodology which has drawn considerable attention is the spin state selective detection, where the high field and low field components of both resolved and unresolved multiplets are either separated or individually detected, for the measurement of scalar couplings in biological macromolecules and also in high-res- olution NMR studies in the solid state [10–34]. It is well know that the single quantum NMR spectra of weakly coupled spins are invariant to the change of relative signs of the couplings. Neverthe- less both the magnitudes and signs of the couplings play a domi- nant role in assessing the stereochemistry and the conformation of biomolecules. There are several experimental schemes available in the literature to obtain information on the relative signs of cou- plings, viz. Z-COSY [35], Soft-COSY [36] and E-COSY [37]. Though tedious, the second order effects on the one dimensional spectra of double and triple resonance experiments also provide the rela- tive signs of the couplings [38–40]. Our recent study employing the spin selective and non-selective multiple quantum single quantum correlation techniques have provided the magnitudes and relative signs of couplings [41] in addition to spectral simplification by the spin state selection of passive spins. The application of non-selective multiple quantum excitation with spin system filtering to unravel the overlap of tran- sitions and to simplify the analyses of the scalar coupled complex 1 H spectra of difluorinated benzamides have been demonstrated by us recently [42]. The fluorinated benzamides are the class of compounds that have been used actively as drugs and finds poten- tial applications as anti-inflammatory agents and antiviral agents [43] whose 1 H NMR spectra have been reported in the literature [44–46]. However, all the NMR spectral parameters are not avail- able presumably due to many degenerate transitions arising from both the phenyl rings. Our quest for development of novel method- ologies to analyze such complex spectra prompted us to synthesize additional molecules of the series, where the position of fluorine in one of the phenyl rings is systematically varied. The 1 H NMR spectra of this series of molecules are not only complex but the complexity drastically differs with the position of substitution of 1090-7807/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2008.10.018 * Corresponding author. Fax: +91 80 2360 1550. E-mail address: nsp@sif.iisc.ernet.in (N. Suryaprakash). Journal of Magnetic Resonance 196 (2009) 119–126 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr