C8953 NMR structural analysis - seminar Basic concepts & Vector model Jan Novotný novotnyjan@mail.muni.cz March 14, 2018 Assign correct value of chemical shift to labelled NMR active atoms1 : 1 http://www.chem.wisc.edu/areas/reich/chem605/ Diastereotopicity1 1 http://www.chem.wisc.edu/areas/reich/chem605/ Determination of regioisomers 300MHz 1 H NMR spectrainCDCl3 OH CH3 CH3 2,4-methylphenol OH CH3 CH3 2,5-dimethylphenol OH CH3CH3 2,6-dimethylphenol OH CH3 CH3 2,3-methylphenol OH CH3 3,4-dimethylphenol OH CH3CH3 3,5-dimethylphenol CH3 H3 H4 H6 H3 H4 H5 H2H6 H5 H2H6 H4 H6 H5 H3 H4 H5 H6 8 7 6 5 4 3 2 1 0 5X expansion 8 7 6 5 4 3 2 1 0 5X expansion This work by Dr. James S. Nowick, Professor of Chemistry, University of California, Irvine, is licensed under a Creative Commons Attribution 4.0 International License. Spectra are from Sigma-Aldrich (www.sigmaaldrich.com) under fair use. 8 7 6 5 4 3 2 1 0 5X expansion 8 7 6 5 4 3 2 1 0 5X expansion 5X expansion 8 7 6 5 4 3 2 1 0 Draw the estimate of 13 C NMR spectrum (with and without 1 H decoupling) and APT experiment: Analysis of simple pulse sequences using vector model � simple model based on rotation of the vector of bulk magnetization in the plane perpendicular to the vector of magnetic field, direction is determined by the "right-hand rule" � NMR signal is detectable only as coherent magnetization oscillating in xy plane � the free precession ω (due to the B0) of magnetization vector is eliminated by introducing rotating frame ω0 ⇒ magnetic field of excitation pulses (B1) is motionless and the individual resonance frequencies differs in so called offset Ωi = ωi − ω0 � applicability of vector model is rather limited to simple single-quantum experiments without transfer of polarisation x y z B0 Bpulse T1 relaxation Apply following sequence (inversion recovery) to isolated spin characterized by a) τ = 2 ∗ T1 and b) τ = 0.2 ∗ T1. Draw semi-quantitatively resulting spectrum. τ 180x 90y- 1-1 sequence Draw the evolution of macroscopic magnetization through the sequence: 90(y) - τ - 90(y) - aq Consider the evolution of an isolated spin due to the chemical shift. 1. How does the result differ for the following offsets: Ωτ = 0, π/2, π. 2. Draw lineshapes of resulting signal assuming the a) y+ b) x+ corresponds to zero phase of receiver. τ 90y 90y Heteronuclear spin echo By using vector diagrams determine the result of attached pulse sequence. 1. Ignore 180 pulse in hydrogen channel for isolated spin systems a) 13C-1H and b) 13C-1H2. Explain the role of CPD block. 2. Lets consider the complete sequence and isolated spin systems a) 13C-1H and b) 13C-1H2. τ=1/2J 90y 180x 13 C τ=1/2J 180x 1 H CPD Next topic 2D NMR - homonuclear experiments