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Unformatted text preview: CHEM / BCMB 4190/6190/8189 Introductory NMR Lecture 10 1 Introduction to Complex Pulse Sequences Beyond simple 1D spectra: Simple 1D 1H and 13C spectra are not always sufficient for assigning even small organic compounds. The main problems are: 1) Assignment of the 1D spectra 2) Low S/N in spectra of insensitive nuclei with low natural abundance (e.g. 13C, 15N) Example: Neuraminic Acid derivative 1 O Ac =
1D 13C NMR Spectrum CH3 C 2 We would also like to use the following information: 1) 13C1H correlations 2) The number of protons attach to one carbon 3) 1H1H correlations 4) 13C13C correlations etc. SOLUTION: Complex pulse sequences Use multiple pulses, delays and decoupling schemes • Various pulses: hard pulses: 90˚x, 90˚y, 180˚x, 180˚y, etc. selective pulses: 90˚x, 90˚y, 180˚x, 180˚y, etc. pulse field gradients fixed or variable delays for selective or broadband decoupling • Various delays: • Decoupling: 3 To analyze the effect of complex pulse sequences we use: A) Vector Diagrams: • They are EXTREMELY useful, but it is important to know that they have certain limitations i.e. difficult to explain 2nd order spectra, population transfer, zero or multiple quantum coherence, etc. • For ease of representation, usually in the rotating frame (x', y', and z) instead of the laboratory frame (x, y, and z) . Very important to know what is the frequency (ν) of the rotating frame. B0 z y' x'
B) Energy Diagrams: • EXTREMELY useful for understanding energy transfer in certain experiments (e.g.: SPT, INEPT, HSQC) For 1H at equilibrium: β E (m = 1/2) Eβ = +1/2γhBo Nβ = (Nα + Nβ)/2  δ = N Single quantum transition(∆m = 1) (m = +1/2) Eα = 1/2γhBo Nα = (Nα + Nβ)/2 + δ = N + ∆H α 4 Effect of a pulse on the longitudinal magnetization (Mz): • At equilibrium (Mz): ♦ Bulk magnetization along z caused by Bo ♦ Excess population in the α state • After 90˚, 270˚ pulses: Vector diagrams: B1 field brings Mz to the x'y' plane
90˚x' z
Mz 90˚x' z
Mz 270˚x' z Mz y' x'
x' y'
x' y' 90˚y' z
Mz 90˚y' z Mz y' y' x'
x' Energy diagram: The populations of α and β are now equal
For 1H:
N E β
90˚, 270˚ N + ∆H/2 β N + ∆H α
5 N + ∆H/2 α • After 180˚ pulses: Vector diagrams: B1 field inverts Mz 180˚x' z 180˚x' z Mz Mz y' x' 180˚y' x' 180˚y' y' z Mz y' x' x' z Mz y' Energy diagram: The populations of α and β are inverted For 1H: N E β
180˚ N + ∆H β N + ∆H α
6 N α Effect of a pulse on the transverse magnetization (Mx', My'): • The transverse magnetization (Mx', My') is not at equilibrium ♦ Bulk magnetization in the x'y' plane ♦ Equal populations in the α and β states • Effect of 90˚ and 180˚ x and y pulses on transverse magnetization with My' component only Vector diagrams: 90˚x' z
My' 90˚x' z 180˚x' z
My' y'
x'
90˚y' My' y'
x'
180˚y' y' x' 90˚y' z
My' z
My' z
My' y' x'
x' y'
x' y' Energy diagrams: It all depends where the final magnetization ends up (See above). 7 • Effect of 90˚ and 180˚ x and y pulses on transverse magnetization with Mx' component only Vector diagrams: 90˚x' z 90˚x' z 180˚x' z y' x' 90˚y' Mx' x' Mx' y' x' Mx' y' z 90˚y' z 180˚y' z y' x' Mx' Mx' x' y' x' Mx' y' Energy diagrams: It all depends where the final magnetization ends up (See above). 8 • Transverse magnetization with Mx' and My' components: Where does it come from ? Lets consider a simple pulse sequence: 90˚x 1 H Delay Vector diagrams: A) Effect of Chemical Shift Evolution: example: νH = νrf + 200 Hz
z Mz 90˚x' y' My' x' x' x' y' z Delay y' B) Effect of J coupling: example: νH = νrf
z Mz 90˚x' y' My' x' x' x'
9 z Delay y' ν = νH  1/2J
y' ν = νH +1/2J • Effect of 90˚x pulse on transverse magnetization with Mx' and My' components Vector diagrams: Initial Magnetization: z My' component z Mx' component z y' x' =
x' y' +
x' y' After 90˚x' pulse: z z z y' x' =
x' y' +
x' y' After 90˚y' pulse: z z z y' x' =
x' y' +
x' y'  10  • Effect of 180˚ x and y pulses on transverse magnetization with Mx' and My' components Vector diagrams: 180˚x' y' y' 180˚y' y' x' 180˚x' y' x' 180˚y' y' x' y' x' x' 180˚y' y' x' 180˚x' y' y' x' x' x'  11  ...
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This note was uploaded on 11/13/2011 for the course CHEM 4190 taught by Professor Staff during the Fall '08 term at University of Georgia Athens.
 Fall '08
 Staff

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