Principles of MolecularPrinciples of MolecularSpectroscopy:Spectroscopy:Electromagnetic Radiation andElectromagnetic Radiation andMolecular 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 = E = hh""electronicelectronicvibrationalvibrationalrotationalrotationalnuclear spinnuclear spinUV-VisUV-VisinfraredinfraredmicrowavemicrowaveradiofrequencyradiofrequencyWhat Kind of States?What Kind of States?NMR is concerned with change in the direction ofNMR is concerned with change in the direction ofspin orientation as the result of the absorption ofspin orientation as the result of the absorption ofradiofrequency 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 11HHor or 1313C, generates a magnetic field. TheC, generates a magnetic field. Themagnetic field generated by a nucleus of spinmagnetic field generated by a nucleus of spin+1/2 is opposite in direction from that+1/2 is opposite in direction from thatgenerated by a nucleus of spin generated by a nucleus of spin ––1/2.1/2. +++++++ The distribution ofThe distribution ofnuclear spins isnuclear spins israndom in therandom in theabsence of anabsence of anexternal magneticexternal magneticfield.field.+++++An external magneticAn external magneticfield causes nuclearfield causes nuclearmagnetic moments tomagnetic moments toalign parallel andalign parallel andantiparallel antiparallel to appliedto appliedfield.field.HH00++++ +There is a slightThere is a slightexcess of nuclearexcess of nuclearmagnetic momentsmagnetic momentsaligned parallel toaligned parallel tothe 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 tesla (T)(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; 10for 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 orH or1313C) depends on the C) depends on the molecularmolecular environment of the environment of thenucleus (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 differentfor structure determination. The signals of differentprotons and carbon atoms in a molecule showprotons and carbon atoms in a molecule showdifferent signals, just like different functional groupsdifferent signals, just like different functional groupsshowshow different signals in the IR.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 external magnetic fieldAn external magnetic fieldaffects the motion of theaffects the motion of theelectrons in a molecule,electrons in a molecule,inducing a magnetic fieldinducing a magnetic fieldwithin the molecule.within the molecule.CCHHHH 00ShieldingShieldingAn external magnetic fieldAn external magnetic fieldaffects the motion of theaffects the motion of theelectrons in a molecule,electrons in a molecule,inducing a magnetic fieldinducing a magnetic fieldwithin the molecule.within the molecule.The direction of theThe direction of theinduced magnetic field isinduced magnetic field isopposite to that of theopposite to that of theapplied field.applied field.CCHH HH 00ShieldingShieldingThe induced field shieldsThe induced field shieldsthe nuclei (in this case,the nuclei (in this case,1313C and C and 11H) from theH) from theapplied field.applied field.A stronger external field isA stronger external field isneeded in order for energyneeded in order for energydifference between spindifference between spinstates to match energy ofstates to match energy ofrf rf radiation.radiation.CCHH