Introduction To Nuclear Magnetic Resonance Spectroscopy

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Introduction toNuclear Magnetic ResonanceSpectroscopyDr. Dean L. Olson, NMR Lab DirectorSchool of Chemical SciencesUniversity of IllinoisCalled figures, equations, and tables are from“Principles of Instrumental Analysis, 6th Ed.”Skoog, Holler, and Crouch, 2007;Thompson Corp.http://www.cis.rit.edu/htbooks/nmr/

NMR basic layout & uctingMagnetNMR Probe (the transceiver antennaplaced inside magnet bore;only seen from below)

NMR basic layout & componentsA variety of configurations; UIUC has all Agilent/Varian equipmentNMR console:Latest Agilent/Varian StyleNMR Workstation Computerand Superconductive Magnet

Nuclear Magnetic Resonance NMR is based on the behavior of a sample placedin an electromagnet and irradiated withradiofrequency waves: 60 – 900 MHz (l 0.5 m) The magnet is typically large, strong, , anddelivers a stable, uniform field – required for thebest NMR data A transceiver antenna, called the NMR probe, isinserted into the center bore of the magnet, andthe sample is placed inside the probe Sample can be in a narrow tube, or Sample can flow in via an autosamplerQualitative or Quantitative; liquid or solidUniversal proton (others) detector; non-destructive

NMR, continued NMR is a chemical analysis techniqueMRI magnetic resonance imaging; usually animaging technique, but is also becoming achemical method called functional MRI (fMRI) MRI is also non-destructive Prof. Paul Lauterbur, UIUC, Nobel Laureate forMedicine or Physiology, 2003, with Sir PeterMansfield, U. Nottingham MRI is really NMRI; the MRIindustry cleverly omitted the “nuclear”from their product for easier marketingto the public

A plaque just outsideChemical Life SciencesLaboratory AcommemoratingPaul Lauterbur,Professor of Chemistry,U of Illinois. Nobel Prize,2003 for MRIAnother plaque, outsideNoyes Lab (SE corner),honors Herb GutowskyProfessor of Chemistry,U of Illinois.He was the first to “apply thenuclear magnetic resonancemethod to chemicalresearch. His experimentaland theoretical work on thechemical shift effect and itsrelation to /Herbert S. Gutowsky

NMR componentsMagnet(inside a Dewar)WorkstationcomputerNMR ConsoleOverheadperspective;solenoid inside(creates andreceives pulses)NMR Probe: really atransceiver antenna)(inside magnet)Photos from www.jeol.com

NMR componentsMagnet(inside a Dewar)Overheadperspective;solenoid insideNMR Probe(inside magnet)NMR ProbePneumatic Legs(to stabilize vibrations)

U. Bristol, United Kingdom14.1 Tesla magnetTermed a“600 MHz” magnetBo Static Magnetic FieldVarian is now Agilentas of late 2010600 MHz is the frequencyat which the proton (1H) nucleusspin resonates –in a magnet of this strength(14.1 Tesla)1000 MHz is equivalent to 23.5 Tesla

U. Bristol, United Kingdom14.1 Tesla magnetTermed a“600 MHz” magnet600 MHz is the frequencyat which the proton (1H)nucleus spin resonates –in a magnet of this strength.Bo Static Magnetic FieldThe magnet is superconducting,always charged, but not powered,and surrounded by liquid helium(4.2 K) and the He is surroundedby liquid nitrogen (77 K).The current is “coasting”, that is,persistent, uniform & stable.The big whitetanks outsideNoyes and RALhold liquid N2for NMR andother cold stuff.No high pressuresare involved; vented.

NMR magnet cut-awayBoreLiquid Helium sleeveLiquid Nitrogen sleeveVacuum sleeveSolenoid (cut-away)Superconducting coilBoIn the Atrium ofChemical Life Sciences Lab ABo

NMR sample handling optionsSpinning tube NMRSamplesyringeSamplevialAutomated flow NMRA typical NMR sample tube:8 inches long; 5 mmouter diameter.Inserted into the NMRprobe from aboveeither manually orusing automation.Pumps and solventsAutosampler

How does NMR work?Probe Coils create the Transverse (B1) Fieldfrom a current pulse of time tBoMagnet HousingMagnet HousingBoSolenoid CoilBo Static Magnetic FieldHelmholtz Coilfrom the big supercon ot.com/2008/03/probe-coil-geometry.html

2 Helmholtz Coils:1 inside the otherfor tube NMR. Onecoil for protons, theother for carbon. Theinner coil is themost sensitive.SolenoidalMicrocoilfor flow NMR;one coildoes it ourse/COURSE 2010/Lab 1.pdf

NMR depends on the spin of the nucleusunder study – the most common is 1H Nuclear spin in an appliedmagnetic field Fig. 19-2 A magnetic dipole, m, is producedThe spin precessesThe spin is quantized1H has a spin quantum number ofeither ½ (low E) or – ½ (high E)Many nuclei have suitable spinquantum numbers for NMR: 13C (only 1.1% abundance) 19F 31P 14NMany nuclei are not NMR active: 12C (sadly) & 16O (also sadly)

NMR depends on the spin of the nucleusunder study: the magnetogyric ratiom p magnetogyr ic ratiom dipole momentp angular momentumMagnetogyric ratio gyromagnetic ratio:It’s different for eachtype of nucleus.The bigger the better.Eqn. 19-1, slightly modified to be a ratio

In a magnetic field, the spin has twoquantized energy states called high and low mhE Bo2 m spin quantum numberm - ½ for high energy; opposedm ½ for low energy; alignedE 1/ 2 h Bo4 High E; opposedE 1/ 2 h Bo4 Low E; alignedBo in Tesla (T)and E in Joules (J)Bo is the static field. h E Bo2 E high - low

In a magnetic field, the spin has twoquantized energy states called high and lowm spin quantum numberm - ½ for high energy; opposedm ½ for low energy; alignedE 1/ 2 h Bo4 Low E; alignedFig. 19-2

In a magnetic field, the spin has twoquantized energy states called high and lowHigh E;opposedHigh E;opposedLow E;alignedLow E;alignedFig. 19-1

E depends on the applied BoE 1/ 2 h Bo4 Slope E 1/ 2 h Bo4 Slope h4 h4 The strongerthe magnet,the larger the TxtJml/Spectrpy/nmr/nmr1.htm

So, where does the NMR signal come from?Transverse pulsetransmittedby the probeFig. 19-3Fast : msecSlow : secLow E;alignedRelaxationenergyreceivedby the probeHigh E;opposedThe NMR probe coil both transmits and receives: it’s a transceiver.The spin is pulsed by the NMR probe, then the spin relaxation produces the signal.

At equilibrium, the low spin state is slightlyelsefavored – otherwise, no NMR signal Everythingcancels.N Hi eN Lo h Bo 2 k T Boltzmann Distribution Equationfor quantum spin statesin a magnetic fieldIn Example 19-2 (p. 501), for 1,000,000 atoms of hydrogen, 1H,in the high energy state: Bo 4.69 Tesla T 20 C 2.6752 x 108 T-1 sec-1 NHi / NLo 0.999967 For NHi 1,000,000 then NLo 1,000,033 N 33 or just 33 ppm of all the spins present are available for NMRbecause all the rest of the spins are in a dynamic equilibrium This is why NMR is a relatively insensitive technique unfortunate.Thus, big magnets.

Signal area proportionalto amount of protonSpin Relaxation SignalWhat does NMR data look like?This is the acquired signal from the spin relaxation.Time (a few sec of relaxation for 1 pulse)A signal is seen foreach type of proton andeach has its own frequencydepending on its ownelectronic environmentFourierTransform referenceThis is what you look at and analyze:An NMR spectrumx (1 x 106 ) shift in ppm, zeroSame normalized scalefor all magnet strengths

Understanding NMR SpectraDeshieldedprotons absorbmore energy*Si is notelectronwithdrawing*The e- are pulledaway from H anddo a poor job ofblocking themagnetic fieldOxygen roset by TMS(tetramethylsilane)

Understanding NMR h/VirtTxtJml/Spectrpy/nmr/nmr1.htm

Understanding NMR SpectraSmall magnetLarge sch/VirtTxtJml/Spectrpy/nmr/nmr1.htm

Understanding NMR SpectrappmThese ppm are for ALL h/VirtTxtJml/Spectrpy/nmr/nmr1.htm

NMR Spectral NomenclatureRight side of spectrumLeft side of spectrum DeshieldedHigh frequencyDownfieldLow field ShieldedLow frequencyUpfieldHigh field*The e- are pulledaway and do apoor job ofblocking themagnetic ch/VirtTxtJml/Spectrpy/nmr/nmr1.htm

But, the spins couple - they interactFor 2 protons: Each proton has its own spin The spin can be ½ or –½ We can draw all the combinations:Degenerate:both cases havethe same energySkoog,Page 515High E;opposedRelative spin population1Low E;aligned21

But, the spins couple - they interactFor 3 protons: Each proton has its own spin The spin can be ½ or –½ We can draw all the combinations:Degenerate:all 3 cases havethe same energyPage 517Low E;alignedHigh E;opposedRelative spinpopulation1331

The principle of multiplicity: the n 1 ruleand peak splittingn is the number of adjacent(neighboring) protons thatare in a differentchemical environmentMultiplicity, m n 1Pattern follows Pascal’s triangle

The principle of multiplicity: a signal getssplit based on what it’s next tomHn 0H12n 13n 24n 3The splitting is called J couplingProximityisimportant

Do they split – or not?This will yield a spectrumwith one NMR singlet.Protons are not splitby identical neighbors.http://cobalt.rocky.edu/ barbaroj/equivalent hydrogens.pdf

Do they split – or not?aPropane:baSee next panelfor spectrumof propanehttp://cobalt.rocky.edu/ barbaroj/equivalent hydrogens.pdf

1H-NMRSpectrum of PropaneCH3 – CH2 – CH3abaa (triplet)b (septet)Area ratios?

NMR Data Interpretation – Example 1Relative total ative areas1:2:1Splittingrelative areas1:3:3:190-MHzMagnet

NMR Data Interpretation – Example 2Mostshieldedprotons?See if you canwork out thespectral detailsyourself !(areas in green)90-MHzMagnet

NMR Chemical Shifts – helps interpret data

NMR data interpretation – watch the ct-desktop/

Other Things NMR Can Mean

14.1 Tesla magnet Termed a “600 MHz” magnet 600 MHz is the frequency at which the proton (1H) nucleus spin resonates – in a magnet of this strength. The magnet is superconducting, always charged, but not powered, 2 and surrounded by liquid helium (4.2 K) and the He is surrounded by liquid nitrogen (77 K). The current is “coasting .

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