Lecture 6 (NMR)!!Chem 237 February 24, 2015NMR SpectroscopyIn the presence of a magnetic fieldNuclear Magnetic Resonance (NMR)NMR exploits the property of ___________ of certain nuclei when they are placed in a ______________.nuclear spinmagnetic fieldAny nucleus with a nuclear spin (I) > 0 (odd number of protons and/or neutrons) can be studied by NMR. The most commonly studied nuclei are ____ and ____.1H13Cnucleusof I > 0absorptionof energyradiationof energy“Spin-flipped”B0 external magnetic fieldSpin “up” (aligned with magnetic field) energetically favoredSpin “down” (aligned against magnetic field) energetically disfavorednucleusof I > 0nucleusof I > 0NMR - Sensitivity issuesΔE = hν 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 strengthν =γHo2π γ2π 1H = 42.57 MHz / T 13C = 10.71 MHz / T 15N = -4.32 MHz / TNMR Instrumentation - Superconducting Magnet300 MHz spectrometer 750 MHz spectrometerCommon I = ½ nuclei 1H 99.985% very little 13C 1.108% much 15N 0.370% much 19F 100.000% very little•Nuclei with odd mass number or even mass number and odd atomic number have a magnetic moment, I > 0)Nuclear Magnetic ResonanceNatural AbundanceSample or Scans Needed•Nuclear phenomenon •Nucleus must have magnetic moment •Nuclear spin quantum number, I > 0Effective Magnetic Field Changes in the distribution of electrons around a nucleus changes effective magnetic field. applied field Bo nucleus electrons Blocal Beffective = Bo - Blocal More electrons – proton will be shielded from the magnetic fieldSummary of Information from NMR Spectra • The _______________ indicates the number of sets of equivalent hydrogen. • The ____________ of the signals indicates the type of hydrogen in each set. • The _________ of the signal tells the relative number of protons in each set. • The _________ of the signal tells the number of protons bonded to adjacent carbons. • The _______________ identifies which protons are coupled to one another. number of signals chemical shift integration multiplicity coupling constantChemical Shift Equivalency - Example #1Match the 1H NMR spectra to the structuresThe scale is parts per million - the δ scaleSummary of Information from NMR Spectra • The _______________ indicates the number of sets of equivalent hydrogen. • The ____________ of the signals indicates the type of hydrogen in each set. • The _________ of the signal tells the relative number of protons in each set. • The _________ of the signal tells the number of protons bonded to adjacent carbons. • The _______________ identifies which protons are coupled to one another. number of signals chemical shift integration multiplicity coupling constant1H NMR SPECTRA HClClClH3CSiCH3CH3H3C0.0 ppm7.0 7.26 Delta scale (δ)What Determines δ? 1. Nearby Electron Density CH3–X δ 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 3.66 More electronegative groups pull electron density away from CH3, resulting in deshielding CH3O MeOWhat Determines δ? 2. Magnetic Anisotropy of Adjacent Groups deshielded shielded δ 7 – 8 δ 5 – 6 δ 2 – 3 HROWhat Determines δ? 3. Substitution Y δCH3Y δRCH2Y δR2CHY -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 δ’s0123456789 δ (ppm)10TMSdeshieldedshieldeddownfieldupfieldSummary of Information from NMR Spectra • The _______________ indicates the number of sets of equivalent hydrogen. • The ____________ of the signals indicates the type of hydrogen in each set. • The _________ of the signal tells the relative number of protons in each set. • The _________ of the signal tells the number of protons bonded to adjacent carbons. • The _______________ identifies which protons are coupled to one another. number of signals chemical shift integration multiplicity coupling constantWhat determines signal intensity?Integral Relationships – area under the curve is proportional to the _____________________ _______________3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.512.008.942.00relative number of nucleicausing that signalAs you look at molecules, you should be able to predict how many different kinds of H’sStructure# of diff. H’sRel. ratio3212133:2:32:2 (1:1)NA4:4 (1:1)NA2:4:2 (1:2:1)Summary of Information from NMR Spectra • The _______________ indicates the number of sets of equivalent hydrogen. • The ____________ of the signals indicates the type of hydrogen in each set. • The _________ of the signal tells the relative number of protons in each set. • The _________ of the signal tells the number of protons bonded to adjacent carbons. • The _______________ identifies which protons are coupled to one another. number of signals chemical shift integration multiplicity coupling constant151050-52.041.00Spin-Spin Splitting6.05.55.04.54.02.041.00Peaks are splitDiscovered in Noyes Lab by H.S. Gutowsky, C.P. Slichter, & D.W. McCall An Extremely Important Discovery!these peaks are 6 Hz apartthese peaks are 6 Hz apartWhat’s going on?Consider the CH2 group4.104.054.003.953.903.853.80δJ = coupling constant (Hz)spins aligned w/ B0deshieldedspins aligned against B0shieldedWhat’s going on?BoConsider the CH groupδspins aligned w/ B0deshieldedspins aligned against B0shieldedspins cancel no net effectSpin-Spin Coupling (Splitting or Multiplicity)Neighboring hydrogens will ________ the peak of a unique set of hydrogens due to coupling. A peak coupled to ____ equivalent hydrogens will be split into a multiplet with ________ peaks. The space between the peaks of a multiplet is called the ________________ (J) and is reported in Hz.splitnn + 1coupling constantnn + 1relative intensitiesmultiplet name011singlet (s)121:1doublet (d)231:2:1triplet (t)341:3:3:1quartet