How To Interpret An NMR Spectrum

How To Interpret An NMR Spectrum - How To Interpret An NMR...

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How To Interpret An NMR Spectrum 1.0 The NMR spectrum . 1.1 Because different amounts of electron density are around different non-equivalent nuclei, the different non-equivalent nuclei in a molecule are experiencing slightly different net magnetic fields in an NMR experiment (Review Section 5.2A of the theory handout) . Recall also that the difference in energy between the two allowed spin states (+1/2 and -1/2 spin states) of a spin 1/2 nucleus (like in 1H and 13C nuclei) depends on the exact magnetic field felt by the nucleus (Review Section 2.3C in the theory handout) . Recall further that in the NMR experiment, when and only when nuclei are irradiated with electromagnetic radiation of energy that exactly corresponds to the energy difference between the +1/2 and -1/2 spin states, the nuclei absorb the energy and the NMR spectrometer measures this absorbance (Review section 3.1 of the theory handout) . The absorbance of energy to convert a nucleus from a +1/2 to a -1/2 spin state is referred to as "resonance" of that nucleus. 1.1A The key conclusion is that nuclei with different electron densities have +1/2 and -1/2 spin states that differ in energy by differing amounts, so these nuclei will absorb electromagnetic radiation of different frequencies in the NMR experiment. 1.1B Nuclei surrounded by greater amounts of electron density will be more shielded from the external magnetic field, so they will absorb electromagnetic radiation of lower energy, that is, lower frequency (energy is proportional to frequency). You may want to review Section 5.2A of the theory handout again . 1.1C The converse is also true, namely that nuclei surrounded by lesser amounts of electron density will be less shielded (referred to as being "deshielded") from the external magnetic field, so they will absorb electromagnetic radiation of higher energy, that is, higher frequency (energy is proportional to frequency). 1.1D The three most important factors influencing the electron density around a hydrogen nucleus are: (i) adjacent electronegative atoms remove electron density; (ii) hybridization of the attached carbon atom, increasing shielding is observed in the order sp1, sp2, sp3; (iii) adjacent pi bonds are deshielding. 1.2 An NMR spectrum is a plot of absorbance versus frequency. 1.2A To make different spectra directly comparable, a standard is used for all NMR spectra. For 1H NMR spectra, the standard is called tetramethylsilane (TMS ) and a small amount of TMS is usually added to any 1H NMR sample. 1.2B Magnets of different strengths lead to absorbance of electromagnetic radiation at different frequencies for the same nucleus, meaning that if simple frequency were plotted in an NMR spectra, you could not compare spectra taken of the same sample on machines with different magnet strengths. To solve this problem, the frequency of absorption plotted on NMR spectra are
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corrected for the magnet strength. In addition, frequency is correlated to the reference compound
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This note was uploaded on 09/21/2010 for the course CH 53200 taught by Professor Bocknack during the Spring '10 term at University of Texas at Austin.

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How To Interpret An NMR Spectrum - How To Interpret An NMR...

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