PowerPoint PresentationDE = hnWhat Kind of States?The two nuclei that are most useful to organic chemists are:Nuclear SpinThe distribution of nuclear spins is random in the absence of an external magnetic field.An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field.There is a slight excess of nuclear magnetic moments aligned parallel to the applied field.Energy Differences Between Nuclear Spin StatesSome important relationships in NMRSlide 11Slide 12Slide 13Nuclear Shielding and 1H Chemical ShiftsShieldingSlide 16Slide 17Chemical ShiftSlide 19Slide 20Effects of Molecular Structure on 1H Chemical ShiftsElectronegative substituents decrease the shielding of methyl groupsEffect is cumulativeProtons attached to sp2 hybridized carbon are less shielded than those attached to sp3 hybridized carbonInformation contained in an NMR spectrum includes:Slide 261H and 13C NMR compared:Slide 28Slide 2913C Chemical shifts are most affected by:Slide 31Slide 32Principles of Molecular Principles of Molecular Spectroscopy:Spectroscopy:Electromagnetic Radiation and Electromagnetic Radiation and Molecular structureMolecular structureNuclear Magnetic Resonance (NMR)Nuclear Magnetic Resonance (NMR)Electromagnetic radiation is absorbed when theElectromagnetic radiation is absorbed when theenergy of photon corresponds to difference in energy of photon corresponds to difference in energy between two states.energy between two states.E = hE = helectronicelectronicvibrationalvibrationalrotationalrotationalnuclear spinnuclear spinUV-VisUV-VisinfraredinfraredmicrowavemicrowaveradiofrequencyradiofrequencyWhat Kind of States?What Kind of States?NMR is concerned with change in the direction of NMR is concerned with change in the direction of spin orientation as the result of the absorption of spin orientation as the result of the absorption of radiofrequency radiation.radiofrequency radiation.11H and H and 1313CCboth have spin = ±1/2both have spin = ±1/211H is 99% at natural abundanceH is 99% at natural abundance1313C is 1.1% at natural abundanceC is 1.1% at natural abundanceThe two nuclei that are most useful toThe two nuclei that are most useful toorganic chemists are:organic chemists are:Nuclear SpinNuclear SpinA spinning charge, such as the nucleus of A spinning charge, such as the nucleus of 11H H or or 1313C, generates a magnetic field. The C, generates a magnetic field. The magnetic field generated by a nucleus of spin magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that +1/2 is opposite in direction from that generated by a nucleus of spin –1/2.generated by a nucleus of spin –1/2. +++++++ The distribution of The distribution of nuclear spins is nuclear spins is random in the random in the absence of an absence of an external magnetic external magnetic field.field.+++++An external magnetic An external magnetic field causes nuclear field causes nuclear magnetic moments to magnetic moments to align parallel and align parallel and antiparallel to applied antiparallel to applied field.field.HH00++++ +There is a slight There is a slight excess of nuclear excess of nuclear magnetic moments magnetic moments aligned parallel to aligned parallel to the applied field.the applied field.HH00no energy difference in absence of magnetic fieldno energy difference in absence of magnetic fieldproportional to strength of external magnetic field proportional to strength of external magnetic field Energy Differences Between Nuclear Spin StatesEnergy Differences Between Nuclear Spin States ++ EEE E ''increasing field strength, Hincreasing field strength, HZZSome important relationships in NMRSome important relationships in NMRThe frequency of absorbedThe frequency of absorbedelectromagnetic radiationelectromagnetic radiationis proportional tois proportional tothe energy difference betweenthe energy difference betweentwo nuclear spin statestwo nuclear spin stateswhich is proportional towhich is proportional tothe applied magnetic fieldthe applied magnetic fieldSome important relationships in NMRSome important relationships in NMRThe frequency (The frequency () of absorbed) of absorbedelectromagnetic radiationelectromagnetic radiationis proportional tois proportional tothe energy difference (the energy difference (E) betweenE) betweentwo nuclear spin statestwo nuclear spin stateswhich is proportional towhich is proportional tothe applied magnetic field (Hthe applied magnetic field (H00))UnitsUnitsHz (sHz (s-1-1))kJ/molkJ/mol(kcal/mol)(kcal/mol)tesla (T)tesla (T)Some important relationships in NMRSome important relationships in NMRThe frequency of absorbed electromagneticThe frequency of absorbed electromagneticradiation is different for different elements, radiation is different for different elements, and for different isotopes of the same element.and for different isotopes of the same element.For a field strength of HFor a field strength of H00 = 4.7 T: = 4.7 T:11H absorbs radiation having a frequencyH absorbs radiation having a frequencyof 200 MHz (200 x 10of 200 MHz (200 x 1066 s s-1-1))1313C absorbs radiation having a frequencyC absorbs radiation having a frequencyof 50.4 MHz (50.4 x 10of 50.4 MHz (50.4 x 1066 s s-1-1))Compare to 10Compare to 101515 s s-1-1 for electrons; 10 for electrons; 101313 s s-1-1 for vibrations for vibrationsSome important relationships in NMRSome important relationships in NMRThe frequency of absorbed electromagneticThe frequency of absorbed electromagneticradiation for a particular nucleus (such as radiation for a particular nucleus (such as 11H or H or 1313C) C) depends on the depends on the molecularmolecular environment of the environment of the nucleus (the electronic environment). nucleus (the electronic environment). This is why NMR is such a useful toolThis is why NMR is such a useful toolfor structure determination. The signals of different for structure determination. The signals of different protons and carbon atoms in a molecule show protons and carbon atoms in a molecule show different signals, just like different functional groups different signals, just like different functional groups show different signals in the IR.show different signals in the IR.Nuclear Shieldingand1H Chemical ShiftsWhat do we mean by "shielding?"What do we mean by "shielding?"What do we mean by "chemical shift?"What do we mean by "chemical shift?"ShieldingShieldingAn