Lecture 6 NMR Chem 237 February 24 2015 NMR Spectroscopy In the presence of a magnetic field Nuclear Magnetic Resonance NMR nuclear spin of certain nuclei when NMR exploits the property of magnetic field they are placed in a Spin up aligned with magnetic field energetically favored Spin flipped absorption of energy nucleus of I 0 nucleus of I 0 nucleus of I 0 radiation of energy B0 external magnetic field Spin down aligned against magnetic field energetically disfavored Any nucleus with a nuclear spin I 0 odd number of protons and or neutrons can be studied by NMR The most commonly studied 1H 13C nuclei are and NMR Sensitivity issues E h is quite small in the radio wave rf portion of the electromagnetic spectrum 0 4 J mol rf MHz or Ms 1 A magnetic field is required to see an energy difference The greater the magnetic field the greater the energy difference Measure the resonance frequencies relative to a reference The units on the x axis of the spectrum are ppm Hz MHz Units of ppm are independent of magnetic field strength 2 Ho 2 1H 42 57 MHz T 13C 10 71 MHz T 15N 4 32 MHz T NMR Instrumentation Superconducting Magnet 300 MHz spectrometer 750 MHz spectrometer Nuclear Magnetic Resonance Nuclear phenomenon Nucleus must have magnetic moment Nuclear spin quantum number I 0 Nuclei with odd mass number or even mass number and odd atomic number have a magnetic moment I 0 Common I nuclei 1H 13C 15N 19F Natural Abundance Sample or Scans Needed 99 985 1 108 0 370 100 000 very little much much very little Effective Magnetic Field Changes in the distribution of electrons around a nucleus changes effective magnetic field Bo applied field Blocal nucleus electrons Beffective Bo Blocal More electrons proton will be shielded from the magnetic field Summary of Information from NMR Spectra The number of signals indicates the number of sets of equivalent hydrogen The chemical shift of the signals indicates the type of hydrogen in each set The integration of the signal tells the relative number of protons in each set The multiplicity of the signal tells the number of protons bonded to adjacent carbons coupling constant identifies which protons are The coupled to one another Chemical Shift Equivalency Example 1 Match the 1H NMR spectra to the structures The scale is parts per million the scale Summary of Information from NMR Spectra The number of signals indicates the number of sets of equivalent hydrogen The chemical shift of the signals indicates the type of hydrogen in each set The integration of the signal tells the relative number of protons in each set The multiplicity of the signal tells the number of protons bonded to adjacent carbons coupling constant identifies which protons are The coupled to one another 1H NMR SPECTRA 7 26 Cl H CH3 H3C Cl Cl H3C 7 0 Si CH3 0 0 ppm Delta scale What Determines 1 Nearby Electron Density CH3 X Electronegativity of X CH3 F 4 26 3 98 CH3 Cl 3 05 3 16 CH3 Br 2 68 2 96 CH3 I 2 16 2 66 CH3 OH 3 41 3 44 O CH3 O 3 66 Me More electronegative groups pull electron density away from CH3 resulting in deshielding What Determines 2 Magnetic Anisotropy of Adjacent Groups deshielded 7 8 5 6 R O H shielded 2 3 What Determines 3 Substitution Y CH3Y RCH2Y R2CHY OH 3 39 3 5 3 9 CO2H 2 08 2 3 2 6 More substitution more deshielded carbon can bear more partial positive charge Protons attached to certain functional groups will absorb RF radiation at characteristic s TMS 10 9 deshielded downfield 8 7 6 5 4 3 2 1 0 ppm shielded upfield Summary of Information from NMR Spectra The number of signals indicates the number of sets of equivalent hydrogen The chemical shift of the signals indicates the type of hydrogen in each set The integration of the signal tells the relative number of protons in each set The multiplicity of the signal tells the number of protons bonded to adjacent carbons coupling constant identifies which protons are The coupled to one another What determines signal intensity Integral Relationships area under the curve is relative number of nuclei proportional to the causing that signal 12 00 2 00 3 5 3 0 2 5 2 0 1 5 8 94 1 0 0 5 0 0 0 5 As you look at molecules you should be able to predict how many different kinds of H s Structure of diff H s Rel ratio 3 3 2 3 2 2 2 1 1 1 NA 2 4 4 1 1 1 NA 3 2 4 2 1 2 1 Summary of Information from NMR Spectra The number of signals indicates the number of sets of equivalent hydrogen The chemical shift of the signals indicates the type of hydrogen in each set The integration of the signal tells the relative number of protons in each set The multiplicity of the signal tells the number of protons bonded to adjacent carbons coupling constant identifies which protons are The coupled to one another Spin Spin Splitting Discovered in Noyes Lab by H S Gutowsky C P Slichter D W McCall An Extremely Important Discovery 1 00 15 10 2 04 5 Peaks are split 0 5 these peaks are 6 Hz apart these peaks are 6 Hz apart 1 00 6 0 2 04 5 5 5 0 4 5 4 0 What s going on Consider the CH2 group J coupling constant Hz spins aligned w B0 spins aligned against B0 deshielded 4 10 shielded 4 05 4 00 3 95 3 90 3 85 3 80 What s going on Consider the CH group Bo spins cancel no net effect spins aligned w B0 spins aligned against B0 deshielded shielded Spin Spin Coupling Splitting or Multiplicity split Neighboring hydrogens will the peak of a unique set of hydrogens due to coupling n equivalent hydrogens will be split into a A peak coupled to n 1 multiplet with peaks n n 1 relative intensities multiplet name 0 1 1 singlet s 1 2 1 1 doublet d 2 3 1 2 1 triplet t 3 4 1 3 3 1 quartet q 4 5 1 4 6 4 1 quintet 5 6 1 5 10 10 5 1 sextet 6 7 septet 7 8 1 6 15 20 15 6 1 1 7 21 35 35 21 7 1 8 9 1 8 28 56 70 56 28 7 1 nonet octet The space between the peaks of a multiplet is called the coupling constant J and is reported in Hz