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Unformatted text preview: Chapter 13: Nuclear Magnetic Resonance (NMR) Spectroscopy direct observation of the H's and C's of a molecules Nuclei are positively charged and spin on an axis; they create a tiny magnetic field
+ + Not all nuclei are suitable for NMR. 1H and 13C are the most important NMR active nuclei in organic chemistry Natural Abundance 1H 99.9% 13C 1.1% 12C 98.9% (not NMR active) (a) Normally the nuclear magnetic fields are randomly oriented (b) When placed in an external magnetic field (Bo), the nuclear magnetic field can either be aligned with the external magnetic or oppose the external magnetic field 1 The energy difference between aligned and opposed to the external magnetic field (Bo) is generally small and is dependant upon Bo Applied EM radiation (radio waves) causes the spin to flip and the nuclei are said to be in resonance with Bo E = h E= Note that h 2 Bo = external magnetic field strength = gyromagnetic ratio 1H= 26,752 13C= 6.7 is a constant and is sometimes denoted as h Bo h 2 NMR Active Nuclei: nuclear spin quantum number (I) atomic mass and atomic number Number of spin states = 2I + 1 (number of possible energy levels) Even mass nuclei that have even number of neutron have I = 0 (NMR inactive) Even mass nuclei that have odd number of neutrons have an integer spin quantum number (I = 1, 2, 3, etc) Odd mass nuclei have half-integer spin quantum number (I = 1/2, 3/2, 5/2, etc) I= 1/2: 1H, 13C, 19F, 31P I= 1: 2H, 14N I= 3/2: 15N I= 0: 12C, 16O 2 Continuous wave (CW) NMR Pulsed (FT) NMR Different nuclei absorb EM radiation at different wavelength (energy required to bring about resonance) Nuclei of a given type, will resonate at different energies depending on their chemical and electronic environment. The position (chemical shift, ) and pattern (splitting or multiplicity) of the NMR signals gives important information about the chemical environment of the nuclei
H O H H H H H H C C O C C H 3 Chemical shift: the exact field strength (in ppm) of a nuclei comes into resonance relative to a reference standard (TMS Electron clouds "shield" nuclei from the external magnetic field causing then to absorb at slightly higher energy Shielding: influence of neighboring functional groups on the electronic structure around a nuclei and consequently the chemical shift of their resonance. CH3 Tetramethylsilane (TMS);
O H3C C OCH2CH3
H3C Si CH3 CH3 Reference standard = 0 for 1H NMR Vertical scale= intensity of the signal Horizontal scale= chemical shift (), dependent upon the field strength of the external magnetic field; for 1H, is usually from 1-10 ppm - TMS chemical shift in Hz = = operating frequency in MHz 14,100 gauss: 60 MHz for 1H (60 million hertz) ppm= 60 Hz 15 MHz for 13C 140,000 gauss: 600 MHz for 1H ppm = 600 Hz 150 MHz for 13C 4 Equivalence: chemically and magnetically equivalent nuclei resonate at the same energy and give a single signal or pattern H3C C C H3C H CH3 Test of Equivalence: 1. Do a mental substitution of the nuclei you are testing with an arbitrary label (your book uses X) 2. Ask what is the relationship of the compounds with the arbitrary label 3. If the labeled compounds are identical (or enantiomers), then the original nuclei are chemically equivalent and will normally give rise to a single resonance in the NMR spectra If the labeled compounds are not identical (and not enantiomers), then the original nuclei are not chemically equivalent and can give rise to different resonances in the NMR spectra
H H H C CH3 C C H3C CH3 H H H C CH3 C C H3C CH3 H H H C CH3 C C H3C CH3 H H H C CH3 C C H3C CH3 Identical, so the protons are equivalent
H3C C C H3C CH3 H3C CH3 H3C C C CH3 H3C CH3 H3C C C CH3 H3C CH3 H3C C C CH3 H3C CH3 H3C C C CH3 CH3 Identical, so the methyl groups are equivalent 5 H3C C C H3C H3C C C H3C CH3 H H3C H CH3 H C C CH3 H3C C C H3C H3C H CH3 H C C CH3 H3C H3C H3C C C H3C H CH3 H3C C C H3C H CH3 These are geometric isomers (not identical and not enantiomers). The three methyl groups are therefore not chemically equivalent and can give rise to different resonances H3C C C H3C H CH3 H H H H CH2CH3 H H H CH3 H H CH3 H H CH3 H H H H H H CH2CH3 H H H H CH2CH3 H H H CH2CH3 H H H H H H H H H H CH2CH3 H H H CH2CH3 H H H H H H H H CH2CH3 H H H CH2CH3 H H H CH2CH3 H H H CH2CH3 H H 6 H Cl H3C C C CH3 H H H Cl H3C C C CH3 H H H Cl H3C C C CH3 H H H Cl H3C C C CH3 H H H H H3C CH3 H Cl H H H3C CH3 H Cl H H CH3 H H CH3 H H CH3 Homotopic: equivalent Enantiotopic: equivalent Diastereotopic: non-equivalent Cyclohexane: two different types of protons, axial and equitorial
H H H H H H H H H H H H H H H H H H H H H H H H The chair-chair interconversion interchanges the axial and equatorial protons and is a fast process at room temperature. NMR is like a camera with a slow shutter speed and a blurred image of fast processes is observed. At room temperature the cyclohexane protons are observed as a time-average and appear as a single resonance. At -90 C the chair-chair interconversion is sufficiently slow that axial and equatorial are observed as two separate resonances. 7 Typical 1H NMR chemical shifts ranges also see Table 13.2 and 13.3 (pages 495-6)
12 11 10 9 8 7 6 5 H
1 4 3 2 1 H R2N CR2 R2NH X H 0 H NMR Shift Ranges RO O RCO CR2 H CR2 Ar H CR2 H HO CR2 RC CR2 X=O, CR2 O PhOH N H R2N-H RC-H H X CR2 X= F, Cl, Br, I O C C H 2 ROH sat. alkanes R-H RCHO RCO2H aromatics vinyl 12 11 10 9 8 7 6 5 4 3 1 0 (PPM) The influence of neighboring groups (deshielding) on 1H chemical shifts is additive (to an extent) Shoolery's additivity rules for predicting the chemical shift of protons of the type: Y - or X C H for protons on X C H sp3 carbons only (ppm) = 0.233 + i
Funtional Group (X,Y) -Cl -Br -I -OH -OR
O O R i (ppm) 2.53 2.33 1.82 2.56 3.23 3.13 1.64 1.57 0.47 2.53 Funtional Group (X,Y) i (ppm) 1.85 R O R O OR O NR2 1.44 1.70 1.55 1.59 1.14 1.70 -SR -NR2 -CH3
R R R -CF3 -CN 8 = 2.0 = 1.2 = 4.1 H O H H H H H O H H H C C O C C H H H -or- H C C C O C H H H = 0.233 + 3.13 + 0.47 = 3.833 = 0.233 + 1.70 = 1.933 = 0.233 + 1.70 + 0.47 = 2.43 = 0.233 + 3.13 = 3.363 Integration of 1H NMR resonances The area under an NMR resonance is proportional to the number of nuclei that give rise to that resonance. = 1.2 = 3.6 6 integral 2 The relative area under the resonances at = 3.6 and 1.2 is 1:3 The integral is superimposed over the spectrum as a "stair-step" line. The height of each "step" is proportional to the area under the resonance. 9 = 1.6 = 3.4 4.5 3 H O H H H H H H C C O C C H = 2.0 3H = 1.2 3H = 4.1 2H Spin-Spin Coupling (splitting) protons on adjacent carbons will interact and "split" each others resonances into multiple peaks (multiplets) n + 1 rule: equivalent protons that have n equivalent protons on the adjacent carbon will be "split" into n + 1 peaks.
H O H H H H H H C C O C C H = 2.0 3H = 1.2 3H = 4.1 2H Resonances always split each other. The resonance at = 4.1 splits the resonance at = 1.2, therefore the resonance at = 1.2 must split the resonance at = 4.2. 10 The multiplicity is defined by the number of peaks and the pattern
H H H C C Br H H = 1.7 -CH2- -CH3- 1:3:3:1 1:2:1 One proton on an adjacent carbon will split a proton into a doublet (d), two peaks of 1:1 relative intensity Two proton on an adjacent carbon will split a proton into a triplet (t), three peaks of 1:2:1 relative intensity Three proton on an adjacent carbon will split a proton into a quartet (q), four peaks of 1:3:3:1 relative intensity Equivalent protons do not show spin-spin coupling
H H H H H H C Br H Br C C Br Singlet (s) R2 R1 11 The resonance of a proton with n equivalent protons on the adjacent carbon will be "split" into n + 1 peaks with a coupling constant J. Coupling constant: distance between peaks of a split pattern; expressed in Hz. Protons coupled to each other have the same coupling constant J.
H H H C C Br H H 12 More complex spin-spin coupling: non equivalent protons will couple independently. H2 splits H3 into a doublet with coupling constant J2-3 H2 splits H1 into a doublet with coupling constant J1-2 H1 splits H2 into a doublet; H3 splits H2 into a doublet (doublet of doublets) with coupling constants J1-2 and J2-3. 13 Summary of 1H-1H spin-spin coupling chemically equivalent protons do not exhibit spin-spin coupling to each other. the resonance of a proton that has n equivalent protons on the adjacent carbon is split into n+1 peaks (multiplicity) with a coupling constant J. protons that are coupled to each other have the same coupling constant non-equivalent protons will split a common proton independently. complex coupling. Spin-spin coupling is normally observed between nuclei that are one, two and three bonds away. Four-bond coupling can be observed in certain situations but is not common. Summary of 1H-NMR Spectroscopy the number of proton resonances equals the number of non-equivalent protons the chemical shift (, ppm) of a proton is diagnostic of the chemical environment (shielding and deshilding) Integration: number of equivalent protons giving rise to a resonance spin-spin coupling is dependent upon the number of equivalent protons on the adjacent carbon 14 13C NMR Spectroscopy: Natural Abundance 1H 99.9% (I= 1/2) O 12C 13C 98.9% (I= 0) 1.1% (I= 1/2) H3C C OCH3 1.1 % 1.1 % 1.1 % E= Bo h 2 Bo = external magnetic field strength = gyromagnetic ratio 1H= 26,752 13C= 6.7 13C is a much less sensitive nuclei than 1H for NMR spectroscopy New techniques (hardware and software) has made 13C NMR routine Pulsed NMR techniques (FT or time domain NMR) Signal averaging (improved signal to noise) Pulsed NMR Techniques:
Z EM pulse "tips" the magnetization 90 into the XY-plane
X The magnetization is detected in the X-axis
Y Y Z The magnetization precesses in the XY-plane at the Larmor frequency of the nuclei, which is directly related to the chemical shift () of the nuclei relaxation (recovery)
X Y The magnetization will relax (recover) back to the Z-axis. As the magnetization precesses in XY-plane, it "spirals" back to the Z-axis. Animation: http://www.nmr.ethz.ch/education/PCV/anim/puls_evol.html http://www.nmr.ucdavis.edu/BCM230_F2001/Flash/PresentationSecondWeek.swf 15 Free Induction Decay (FID)- time domain NMR time In pulse (FT) NMR, all nuclei are tipped at the same time and the FID's are superimposed. Fourier Transform (FT) deconvolutes all of the FID's and gives an NMR spectra. Signal averaging: pulsed NMR allows for many FID's (NMR spectra) to be accumulated over time. These FID's are added together and averaged. Signals (resonances) build up while the "noise" is random and cancels out during the averaging. Enhanced signal to noise ratio and allows for NMR spectra to be collected on insensitive nuclei such as 13C and small samples. Chemical shifts give an idea of the chemical and electronic environment of the 13C nuclei due to shielding and deshielding effects range: 0 - 220 ppm from TMS
13C NMR spectra will give a map of the carbon framework. The number of resonances equales the number of non-equivalent carbons. 16 Signal-Averaging 13C-spectra of CH CH CH CH CH OH 3 2 2 2 2
after one scan after 200 scans Chemical Shift Range of 13C
220 200 180 160 140 120 100 80 60 40 20 R3C-Br C NMR Shift Ranges R3C-F R3C-Cl R2N-CR3 nitriles aromatics vinyl R3C OH RO CR3 O RC CR3 Ar-CR3 R3C-I 0 -20 13 carbonyls ketones & esters, amides aldehydes & acids 220 200 180 160 alkyne saturated alkanes 140 120 100 80 60 40 20 0 -20 (PPM) Note the carbonyl range 17 1H-13C spin-spin coupling: spin-spin coupling tells how many protons are attached to the 13C nuclei. (i.e., primary, secondary tertiary or quaternary carbon) 13C spectra are usually collected with the 1H-13C coupling "turned off" (broad band decoupled). In this mode all 13C resonances appear as singlets. DEPT spectra (Distortionless Enhancement by Polarization Transfer) a modern 13C NMR spectra that allows you to determine the number of attached hydrogens. Run: broad-band decoupled spectra DEPT-90: only CH's show up DEPT-135: CH's and CH3's give positive resonances CH2's give negative resonances 18 8 7 6 5 4 3 OH Broad-band decoupled 2 1 6 5 2 4 3 8 7 1 DEPT-90 CH CH DEPT-135 CH3 CH3 CH3 CH2 CH2 Solving Combined Spectra Problems: Mass Spectra: Molecular Formula Nitrogen Rule # of nitrogen atoms in the molecule M+1 peak # of carbons Degrees of Unsaturation: # of rings and/or -bonds Infrared Spectra: Functional Groups C=O C=C CC 1H NMR: Chemical Shift () chemical environment of the H's Integration # of H's giving rise to the resonance Spin-Spin Coupling (multiplicity) # of non-equivalent H's on the adjacent carbons (vicinal coupling). Shoolery's Rules: finial check on the structure assignment by 1H NMR 13C NMR: # of resonances symmetry of carbon framework Type of Carbonyl Each piece of evidence gives a fragment (puzzle piece) of the structure. Piece the puzzle together to give a proposed structure. The proposed structure should be consistent with all the evidence. 19 Magnetic Resonance Imaging (MRI): uses the principles of nuclear magnetic resonance to image tissue MRI normally used the magnetic resonance of protons on water and very sophisticated computer methods to obtain images. Other nuclei within the tissue can also be used (31P) or a imaging (contrast) agent can be administered Normal 25 years old Normal 86 years old Alzheimer's Disease 78 years old 20 ...
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- Fall '08