1H Nuclear Magnetic Resonance
1HNuclear Magnetic Resonance
Gas Chromatograph of Molecular Hydrogen at –100 604020101601652170175Time180185190
Whats going on?Ortho HydrogenO/P 1:1 at 80 KPara Hydrogen
The magnetic moment of a spinning nucleus is colinear withthe axis of spinNSFor a nucleus with a spin of 1/2, only two orientations in amagnetic field are allowed. Regardless of the strength of themagnetic fields currently available, only a very slight difference inpopulation exist between the two orientations
Processional frequency, ν, is proportional to the magnetic field strength,Hν γ H1 Hz 1 cycle/secondFor 1HFor 13CAt 14,000 Gauss (1.4Tesla)ν 60 MHzν 14 MHzAt 70,500 Gauss (7.05 Tesla)ν 300 MHzν 70 MHzAt 141000 Gauss (14.1 Tesla)ν 600 MHzν 140 MHzc λν;λ 3*1010 cm sec-1/600*106 sec-1 50 cm
EEnergydifferencebetweenthe twostates0H Magnetic field strengthIf a nucleus experiences a different magnetic field it willprecess at a different frequency and absorb at a differentfrequency (E hν)
Consider a set of identical excess nuclei with the followingorientationSNBefore the rf pulse
detector coilSNrfgeneratorJust after the 90 rf pulse
Resultant momentdetector coilSNrf coilWhen the rf is turned off
yReceiver coilHxNSz
Receiver coil output with time2receiver output10-1-20246810time12141618
2210 HH10-1-1-2020406080100time-20No magnetic inhomogeneity20406080100120timeWith magnetic inhomogeneityNet magnetic moment detected by the detector coil. Thissignal is converted by the electronics and software tolook like an absorption signal via a Fourier transform
IntensityA typical 1H NMR spectrum in frequency reported in parts per million0109876543210ppmSince the frequency (cycles/sec) depends on the strength of the magnetic field, chemical shiftsare reported relative to a standard, frequently tetramethylsilane (0 ppm). If the spectrum is runat a magnetic field strength that generates precessional frequencies of 300 MHz (300 millioncycles per sec, 7.05 Tesla), 1 ppm is equal to 300 Hz (300 cycles per second ). At 600MHz, 1ppm 600 Hz. Chemical shifts reported in ppm are independent of the field strength (orfrequency)
Typical NMR Chemical ShiftsTetramethylsilane 0 ppm (standard)
What causes the chemical shift?The protons are immersed in a sea of electrons which are charged and have both anelectric field and a magnetic field associated with their charge.
Information obtained by a NMR spectrum1. Chemical shift: indicates the type of proton2. Relative areas: provide a ratio of the number of each type of H nucleus3. Number of nearest neighbors (first order spectra)
The nmr of 2,2,-dimethylpropanolCH3CH3CCH2OHCH3219Figure NMR-2. The NMR spectrum of 2,2-dimethylpropanol
2 3Why the multiplicity?3
OThe hydrogens are three bonds removedCH3HCH3CH3CCH3The hydrogens are four or more bonds removedCH2OHexcept for the OH
Consider the following:HaHbHaHbCCCCJabνaIf Ha has a different chemical shift from Hb, roughly 50% of the nuclei of Hb willhave one orientation of their magnetic moment and the other will have just theoppositeThe total field felt by Ha will be due to the external field, the local field due toelectronic environment and the field due to the proton(s) three bonds removed. Thefield due to the local proton will either add or subtract to the total field experience byHa. Roughly 50% will have a neighbor with a and 50 % with a neighbor resultingin two lines observed for HaJab is called the coupling constantJabWith two identical neighbors:HaHbHaHbHaHbHaHbCCCCCCCCHbHbHbHbνa
all JabWith three identical HbHbHbHaHbCCHb CHbCHb2 down, 1 upHbHbHaHbHbHaHbCCHb2 up, 1 downHbn 1 rule number of lines observed equals the number of nearest Hb neighbors 1
11211341 neighbor12 identical neighbors13641 5 10 1013 identical neighbors16 15 20 154 identical neighbors1515 identical neighbors6 16 identical neighborsneighbors are identical in the sense that the coupling constants are the same or very similar
HCH3COHCH34.0116
523
23321
Simulation of AB2 (10-09-2013, 09:57:45)One Criterium for First OrderSpectraThe chemical shift differencebetween the hydrogens that areJab 3.01Vab 109.00Vcentr 150.00Va 95.50Vb 204.50Wa 1.10Right-Hz 30.64WdthHz 280.72Freq in Hz (cps)coupled to each other is greaterthan 10 times the magnitudeof the coupling constant.1.00.80.6ppm0.40.2Simulation of AB2 (10-09-2013, 09:59:21)X-CH2CHY2Jab 19.01Vab 109.00Vcentr 150.00Va 95.50Vb 204.50Wa 1.10Right-Hz 30.64WdthHz 280.721.00.80.6ppm0.40.2
13C NMR: natural abundance of 1%, spin of ½ (like hydrogen)The probability of finding 2 13 C atoms next to each other is 0.012 or 10-4Hydrogen and carbon do couple to each other. However because the amount of 13 Cis so small, spectra are generally decoupled during aquisition. This is done byirradiating all the hydrogens and causing them to rapidly change their spinorientation. It is somewhat analogous to the decoupling observed in alcohols wherethe hydrogens three bonds removed are decoupled to an OH hydrogen due to rapidchemical exchangeInformation from 13C spectra1. Number of chemically different carbons2. Chemical shift information: the chemical environment of the carbon
13C NMR Chemical Shiftscarbon environment chemical shift (ppm)C O (in ketones)205 - 220C O (in aldehydes)190 - 200C O (in acids and esters)170 - 185C in aromatic rings125 - 150C C (in alkenes)115 - 140RCH2OH50 - 6540 - 45RCH2ClRCH2NH237 - 4525 - 35R3CHCH3CO20 - 30R2CH216 - 25RCH31Information from 13 C NMRCarbon countChemical Shift
CH2 CH-CO-CH3OCH2CH3COCH3CHCH3
Which of the following alcohols produced the 13 C NMR spectrum below?CH3CH2CH2CH2OHCH3CH2OHCH3CH3COHCHCH3CH3CH3CH CH2OHCH3
Common NMR solvent: CDCl3; CCl4, CD3SOCD3; C6D6; CD3COCD3; ; D2ODeuterium 2H also has a spin; it has three orientation in a magnetic fieldCDCl3SolventChloroform-dFormulaH-NMR shift* C-NMR shiftCDCl37.24 (singlet)multiplet77.0tripletCarbon tetrachloride CCl496.7tripletDimethylsulfoxide-d6 CD3SOCD3 2.49 (pentet)39.7septetBenzene-d6C6D67.2 (singlet)128.0tripletAcetone-d6CD3COCD32.04 (pentet)29.8septetWater-d2D2O4.65 (singlet)*The H-NMR peak is due to a residual amount of 1H in the deuterated solvent and is small
13C NMRCDCl3CDCl3
1HAcetone13C NMRNMR
13 C NMR: natural abundance of 1%, spin of ½ (like hydrogen) The probability of finding 2 13-C atoms next to each other is 0.012 or 10 4 Hydrogen and carbon do couple to each other. However because the amount of 13 C is so
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Magnetic Resonance / excitation 52 Magnetic Resonance / excitation In an absolute referential 53 Magnetic Resonance / relaxation T1 M M B dt dM z z z Return to equilibrium / B0: time constant T1 Spin dephasing: Time constant T2 2,,, T M M B dt dM x y x y x y 54 Magnetic Resonance / relaxation T1 (ms) TISSUE 0.5 T 1.5 T T2(ms) Muscle 550 .
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