Table 1- Typical 2D NMR experiments used for molecular structure determination (*) Experiment Typical quantity (mg) Exp. Time* COSY 5 mg 5 to 30 min Long-range 5 mg 5 to 30 COSY min COSY-45 5 mg 5 to 30 min DQ-COSY 5 mg 10 to 60 min Relayed-COSY 5 mg 5 to 30 or RELAY min TOCSY 5 mg 5 to 30 min NOESY 10 mg 1 to 2 hours ROESY 10 mg 1 to 2 hours HETCOR 20 mg 1 to 2 (1-bond) hours HMQC/HSQC 10 mg 0.5 to 2 (1 bond) hours HETCOR 20 mg 4 to 12 (n-bond) hours HMBC 10 mg 2 to 12 (n-bond) hours HMQC-TOCSY 10 mg 0.5 to 2 hours INADEQUATE 100 mg 24-72 hours What kind of information is obtained? Establishes correlation between spins with scalar coupling (J) but does not determine coupling constants. Emphasize correlation with small Js. Decreases the intensity of the diagonal peaks with respect to the correlation peaks, thus identifying correlation between strongly coupled spins. Establishes correlation between spins with scalar coupling (J); can be used to measure Js. Singlets are removed (CH3 signals, for example) and solvent (H20) by means of filtration of the correlation signals with a double quantum filter. Magnetization transfer to 1 or 2 chemical bonds beyond those of the COSY transfer to determine coupled spin systems (only via J). Show correlation among all the spins that have a common coupling partner, e.g., A -> B -> C, where Jab, Jbc *■ 0, BUT JAC = 0. Stereochemical information via dipolar coupling using cross-relaxation (longitudinal); determination of chemical exchange processes. Stereochemical information via dipolar coupling using cross-relaxation (transversal); adequate for molecules with average MW in the range of 1000-3000 and/or when coxc ~ 1 -12 (where co is the spectrometer frequency and xc the correlation time). Heteronuclear assignment Heteronuclear assignment using inverse detection, i.e., using 1Hs to detect heteronuclear frequencies (more often) or using a nucleus with larger (to detect a low-( nucleus (e.g. use of 19F to detect 13C frequencies). H-Xlong range heteronuclear assignment (via 2Jxh and 3Jxh ). H-Xlong range heteronuclear assignment (via 2Jxh and 3Jxh ) using inverse detection. H-X long range heteronuclear assignment (via 2Jxh e 3Jxh ) using inverse detection and protonated Xs Establishes 13C-13C connectivities. For structural elucidation of organic molecules, it is the most powerful experiment, but with the lowest sensitivity. (*) The acquisition times and quantities mentioned above are for phase-cycled 2D experiments, i.e., experiments that require a minimum number of transients in F2 to eliminate axial peaks and make quadrature detection. The experiments with pulsed field gradients tend to be faster than the phase-cycled ones. (a) Explain the small outer peaks of the multiplet. (b) Analyze the multiplet. Give approximate coupling constants, report them in the form nJXY =_Hz. Assign the couplings. (c) Why do the small outer peaks become broad at higher field strength? This is not a fluxional molecule, and there is no ligand exchange under these conditions. 15 Problem R-11Q (CnH19N08). This problem requires you to determine the stereochemistry of two isomers of sialic acid (A and B). Below is shown a portion of the 126 MHz 13C NMR spectrum (D20 solvent) of a 10:1 mixture of two isomers (Hori, H.; Nakajima, T.; Nishida, Y.; Ohrui, H.; Meguro, H. Tetrahedron Lett. 1988, 29, 6317). Spectrum 1 is the fully proton decoupled. Spectrum 2 has the decoupler turned off. Spectrum 2 Spectrum 1 178.0 T) C02H T ho ' O OH A n ho ' 40 30 20 10 0 Hz 177.0 176.0 175.0 ppm (a) Which carbons of the sialic acid are being shown here? Mark the shifts on the structures. (b) Interpret the multipicity of the signal at 177.7 ppm in the coupled spectrum (2). Estimate coupling constants, and assign them. (c) Which is the major isomer (A or B)?. Give your reasoning below. Be specific and brief. 10 Problem R-11S (C16H22Fe202P2). Below are the 60 MHz 1H NMR spectra of two stereoisomers (E and Z) of the iron Cp complexes shown (J. Am. Chem. Soc 1963, 85, 3120). CH3 £ Z 1 £ Z J Spectrum 1 i i \ \ L i i i i 1 i i i i 1 i i i i 1 i i i i 1 i i i i 1 i i i i 1 i i i i 1 i i i i 1 i i i i 1 i i i i i i 1 J 4 3 2 ppm 3 Spectrum 2 1 k 1 ............................. Hz D 20 10 0 0 i i i i i i i i i I i i i i i i i i i I i i i i i i i i i I i i i i i i i i i I i i i i i i i i i I i 4 3 2 1 0 ppm v ) (a) Which isomer corresponds to Spectrum 1_, and which to Spectrum 2 _? Explain (b) Explain the appearance of the multiplet at 5 1.6 (i.e. why does it look like this). (c) Would you expect the spectrum to look significantly different at 300 MHz (instead of the 60 MHz of the spectra shown)? 15 Problem R-12M. You are asked to interpret the coupled 13C NMR spectrum of an oxazoline. (a) Which carbon are we looking at? _i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_ 58.0 57.8 57.6 57.4 57.2 57.0 56.8 56.6 56.4 56.2 56.0 55.8 55.6 55.4 55.2 ppm (b) Analyze the spectrum, report all coupling constants in the standard format (nJX-Y = 000 Hz). (c) The spectrum below is of the same compound with one H replaced by D. Where is the deuterium? Place it on the structure, and explain briefly. 3 _i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_ 58.0 57.8 57.6 57.4 57.2 57.0 56.8 56.6 56.4 56.2 56.0 55.8 55.6 55.4 55.2 ppm (d) What is the proton NMR frequency of the spectrometer they were using?_ Problem R-11T (C19H25f02). Below are part of the 60 MHz 1H NMR spectra of two stereoisomers (A and B) of the fluorinated steroids shown. To aid in your analysis, a conformational drawing is also provided (J. Am. Chem. Soc. 1963, 85, 3038). q m q Spectrum 2 _i_I_I_I_I_I_I_I_I_i_ 6.0 5.5 5.0 4.5 (a) Which protons are being shown here? Analyze the coupling, and report them in the standard format (give 5 and identify any couplings you found). Spectrum 1: Spectrum 2: (b) Which isomer corresponds to Spectrum 1 which to Spectrum 2. Explain briefly. K IL-JUi J_I jUL j_i