CHEM 420 Exam 1 Study Guide
CHEM 420 Exam 1 Study Guide CHEM 420
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This 6 page Study Guide was uploaded by Sarah Rees on Wednesday February 3, 2016. The Study Guide belongs to CHEM 420 at Ball State University taught by Dr. Zubkov in Spring 2016. Since its upload, it has received 45 views. For similar materials see Chemical Instrumentation in Chemistry at Ball State University.
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Date Created: 02/03/16
NMR Spectroscopy - Chapter Objectives Chapter 19 – Nuclear Magnetic Resonance Spectroscopy. 1. Give the definition of NMR Spectroscopy. Understand what information it can provide. 2. From the number of protons and neutrons in a nucleus, predict if this nucleus is going to be magnetic, and if yes, whether the nuclear spin I is integer or half-integer value. 3. From the value of nuclear spin I, predict (1) how many possible orientations are allowed for such nucleus in an external magnetic field and (2) what the value of the nuclear magnetic quantum number m is iI each case. Understand which of these orientations has the lowest potential energy. 4. Know how the energy gap between different orientations of the nuclear magnetic moment depends on the strength of the applied magnetic field. ▯ B 0 5. Know the formula ▯ 0 ▯ for the resonant frequency of a photon corresponding to the 2▯ transitions between different orientations of the nuclear magnetic moment. Know the meaning of the other terms of this equation. From the strength of the external magnetic field and a magnetogyric ratio for the nucleus, be able to calculate the resonant frequency of nuclear transitions. 6. For a nucleus with two possible orientations in the magnetic field, calculate the numerical ratio of nuclei in the excited state to those in the ground state. Be able to do it from the strength of the external magnetic field and a magnetogyric ratio for the nucleus. Know the formula N ▯ ▯E j ▯ e kT N 0 7. Understand that the nuclear magnetic moment is tilted in the external magnetic field and it precesses around the magnetic field direction at a particular frequency (Larmor frequency). Know the equation for the Larmor frequency. 8. For a given nucleus with a particular spin I, draw the precession cones for each of the nuclear orientations. 9. Understand how a radio frequency photon flips the orientation of the proton magnetic moment and why a particular polarization of the photon magnetic field is important. 10. Understand why relaxation processes are important in NMR, what main mechanisms are, and what optimal half-lifetime for an excited state is. 11. Know what T and T 1re and h2w they affected by the physical state of the sample. 12. Understand why the resonant frequency of a nucleus shifts depending on chemical environment. Know what compound is chosen as a reference for H frequency. 1 ▯ ▯x ref 13. Know the formula for chemical shift ▯ ▯ . Be able to calculate chemical shifts (in ppm) ▯ ref from resonant frequencies given. 14. Be able to interpret spectra. Recognize selected characteristic chemical shifts for protons: saturated carbon sites, saturated carbon sites next to C=C or electronegative groups, alkene protons, aromatic protons, acetylenic protons, alcohols, aldehydes, carboxylic acids. 15. Know what determines the area under an NMR peak. 16. Understand how diamagnetic anisotropy affects the proton NMR signals in aromatics, alkenes, alkynes. 17. Understand what spin-spin coupling is, when it occurs, in what units it is measured, and whether it depends on the external magnetic field strength. 18. For n equivalent neighboring protons, know the “n + 1” multiplicity rule. For nonequivalent neighboring protons, know the “(n +1)(n +1)A multipCicity rule. 19. Predict the relative intensities of peaks within multiplets using Pascal triangle. 20. Know what numeric criterion is used to distinguish between first-order and second-order NMR spectra and what the difference in their appearance is. Know how the spectral appearance of multiplets changes from AX to AB to A type spectra. 2 21. Understand how a fast proton exchange between molecules affects the NMR spectra. 22. For 13C-NMR spectra, understand the reason for the low intensity of signals, for the absence of 13C- C spin-spin splitting, and for the necessity of 13C decoupling from H. 1 23. Understand how a pulse NMR (Fourier Transform NMR) experiment is carried. Understand how individual spins behave during the pulse and during relaxation and how it affects the magnetization of the sample. 24. Know what FID is, how it is obtained, and how it can be converted to an NMR spectrum. 25. Know the location and purpose of the main elements of an NMR spectrometer: superconductive coil, shim stack, sample probe with a transmitter/receiver coil and a locking coil. 26. Understand how the magnetic field inhomogeneities and drift are minimized by shimming, sample spinning, and by the field locking onto the solvent. Reading th 6 Edition: Chapter 19 – NMR Spectroscopy. Read sections A-E Be able to interpret spectra as assigned. I recommend also looking at several useful Shockwave visualizations on NMR at the following website: http://faculty.ccc.edu/cabrams/projects/nmrtutor/. In particular, take a look at NMR01, NMR02, NMR03, INTRO, ALKYNE, BENZENE. Also, see other NMR links posted on Blackboard.
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