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Factors Affecting 13C NMR Spectra: Understanding Dynamics and Mechanisms

Explore the impact of pulse delay, decoupler settings, and dynamic processes on NMR spectra. Learn to analyze chemical shifts, areas, and multiplicities for molecular insights.

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Factors Affecting 13C NMR Spectra: Understanding Dynamics and Mechanisms

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  1. NMR Lecture 5

  2. Factors affecting 13C NMR spectra How do you know 3 microsec = 22° Proton decoupler always on, no pulse delay

  3. Pulse delay 400 s; gated decoupler-on only when acquiring signal

  4. Effects of the pulse delay between pulses, 400 s Proton decoupler always on (NOE)

  5. Information obtained by examining nmr spectra 1. Chemical shift (identifies nature of nucleus 2. Area (identifies relative number of nuclei) 3. Multiplicity ( (NOE) 2D NMR basically provides information about connectivity and proximity

  6. Applications of 1H and 13C NMR

  7. Molecular formula C9H10O area: 5: 2: 3 IR: 1720 cm-1

  8. 3:2:3:6 1H NMR Molecular formula C7H14O2 IR1720 cm-1

  9. F NMR of F3

  10. CF24 CF25H =CF3-

  11. Returning to Dynamic Nuclear Magnetic Resonance For a simple exchange process coalescence  /21/2 Most chemical shift differences are of the order of a few hundred Hz or less. Rate constants are of the order of few hundred sec-1. Large in comparison to k  10-4 – 10-5 sec-1, measured by conventional kinetics but small relative to many dynamic processes occurring in molecular systems such as rotations about bonds.

  12. Assign the spectrum and explain the coupling observed Let’s do a thought experiment

  13. What would the NMR spectrum of the methyl resonances of 4-dimethyl-3-hydroxycyclobutenone look like if all the methine hydrogens of this molecule had the same spin?

  14. J13 J15 are of opposite sign J13, J15 are the same sign

  15. J13, J15 are of opposite sign J13, J15 are the same sign

  16. What are some mechanisms by which the two methyl groups can become identical?

  17. H2O

  18. H2O

  19. 1,3-hydrogen shift

  20. 1,3-hydrogen shift

  21. J is of opposite sign J is the same sign

  22. 1,3-hydrogen shift; JAX and JAM of the same sign

  23. 1,3-hydrogen shift; JAX and JAM of the opposite signs

  24. Problem Set DNMR By line shape analysis, determine the activation energy for rotation about a carbon nitrogen bond in an amide. Go to my gateway and down load the Excel file. A plot of ln k vs 1/T should result in a straight line and the slope of the line should be - Ea/R. Your problem will be to fit the lineshapes as best you can and evaluate k for a series of temperatures. Remember that the most accurate k values will be when the line shapes change the most. k = rate constant

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