C8953 NMR structural analysis - seminar Vector model of NMR experiments + 13C APT Martin Novák 323460@mail.muni.cz March 16, 2016 Sketch the estimate of 13 C spectrum of attached hypothetical molecule. 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) T1 = τ/2 and b) T1 = 5τ. 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. First ignore 180 pulse in hydrogen channel. 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 APT - Attached Proton Test based on heteronuclear spin echo t1 = 1/1JCH 13 C signals are differentiated according to the number of directly bound 1 H Cq, CH2 positive CH, CH3 negative Evolution of signal governed by the value of 1 JCH =⇒ reflected by the intensity of APT signal 90x 180y 180 H-1 C-13 acq dec t1 -1 -0.5 0 0.5 1 0 0.5 1 1.5 2 Relativeintensity t1 (1/1 JCH) Cq CH CH2 CH3 13 C APT Cinnamic acid 13 C APT of Nicotine 13 C APT 4 Cq Cq CH2 Next topic 2D spectroscopy