1 13 C-NMR, 2D-NMR, and MRI Lecture Supplement: Take one handout from the stage

2 Midterm Exam 2 Date: Monday May 21 Time: 5:00-6:50 PM Topics: All of spectroscopy (mass spectrometry  today) Location: last name A-La in Haines 39 last name Le-Z in Moore 100 Calculators not allowed Question and Answer Session Lecture time, Monday May 21 Submit questions to Label as “Question for Q&A” Deadline for possible inclusion: noon Sunday May 20 Extra Office Hours Saturday 3-5 PM, Young Hall 3077F (Steve Joiner) Sunday ???

3 13 C-NMR Is NMR limited to 1 H? Any nucleus with I  0 can be observed I  0 when nucleus has odd number of protons or odd number of neutrons Includes 1 H, 2 H, 13 C, 19 F, 29 Si, 31 P, 127 I, etc. Examples 19 F: 9 protons, 10 neutrons; 100% natural abundance 31 P: 15 protons, 16 neutrons; 100% natural abundance } Easily observed by NMR Limited value for organic structure analysis 13 C-NMR Carbon is backbone of organic molecules so 13 C-NMR has high potential, but... Low natural abundance: 13 C = 1.1% ( 12 C = 98.9% but has 6 protons and 6 neutrons) Low probability that photon absorption causes spin flip: 1.6% compared to 1 H Result: 13 C spin flip much harder to observe than 1 H spin flip Modern NMR spectrometers have overcome these problems; 13 C-NMR now routine

4 13 C-NMR What can we deduce about molecular structure from 13 C-NMR spectrum? NMR fundamentals are the same regardless of nucleus Information from carbon NMR spectrum Number of signals: equivalent carbons and molecular symmetry Chemical shift: presence of high EN atoms or pi electron clouds Integration: ratios of equivalent carbons Coupling: number of neighbors

5 13 C-NMR: Number of Signals Number of 13 C-NMR signals reveals equivalent carbons One signal per unique carbon type Reveals molecular symmetry Examples CH 3 CH 2 CH 2 CH 2 OHCH 3 CH 2 OCH 2 CH 3 Two 13 C-NMR signals 2 x CH 3 equivalent 2 x CH 2 equivalent No equivalent carbons Four 13 C-NMR signals Symmetry exists when # of 13 C-NMR signals < # of carbons in formula

6 13 C-NMR: Position of Signals Position of signal relative to reference = chemical shift 13 C-NMR reference = TMS = 0.00 ppm 13 C-NMR chemical shift range = ppm Downfield shifts caused by electronegative atoms and pi electron clouds OH does not have carbon  no 13 C-NMR OH signal Example: HOCH 2 CH 2 CH 2 CH 3

7 13 C-NMR: Position of Signals It is not necessary to memorize this table. It will be given on an exam if necessary. Trends RCH 3 < R 2 CH 2 < R 3 CH EN atoms cause downfield shift Pi bonds cause downfield shift C=O ppm

8 13 C-NMR: Integration 1 H-NMR : Integration reveals relative number of hydrogens per signal 13 C-NMR : Integration reveals relative number of carbons per signal Rarely useful due to slow relaxation time for 13 C Relaxation time important phenomenon for MRI time for nucleus to relax from excited spin state to ground state

9 13 C-NMR: Spin-Spin Coupling Spin-spin coupling of nuclei causes splitting of NMR signal Only nuclei with I  0 can couple Examples: 1 H with 1 H, 1 H with 13 C, 13 C with 13 C 1 H NMR: splitting reveals number of H neighbors 13 C-NMR: limited to nuclei separated by just one sigma bond; no pi bond “free spacers” Conclusions Carbon signal split by attached hydrogens (one bond coupling) No other coupling important 1H1H 13 C 12 C Coupling observed Coupling occurs but signal very weak: low probability for two adjacent 13 C 1.1% x 1.1% = 0.012% No coupling: too far apart No coupling: 12 C has I = 0

10 13 C-NMR: Spin-Spin Coupling Carbon signal split by attached hydrogens N+1 splitting rule obeyed Quartet TripletDoublet Singlet Example 1 H- 13 C Splitting Patterns How can we simply this?

11 Proton decoupled 13 C-NMR: Spin-Spin Coupling Broadband decoupling: all C-H coupling is suppressed All split signals become singlets Signal intensity increases; less time required to obtain spectrum Simplification of Complex Splitting Patterns

12 13 C-NMR: Spin-Spin Coupling Distortionless Enhancement by Polarization Transfer (DEPT ) Example All carbons Assigns each 13 C-NMR signal as CH 3, CH 2, CH, or C CH 3 onlyCH 2 onlyCH only

13 Two-Dimensional NMR (2D-NMR) Basis: interaction of nuclear spins ( 1 H with 1 H, 1 H with 13 C, etc.) plotted in two dimensions Applications: Simplifies analysis of more complex or ambiguous cases such as proteins Obtain structural information not accessible by one-dimensional NMR methods Techniques include: Correlation Spectroscopy (COSY) Heteronuclear Correlation Spectroscopy (HETCOR) Heteronuclear Multiple-Quantum Coherence (HMQC) Nuclear Overhauser Effect Spectroscopy (NOESY) Incredible Natural Abundance Double Quantum Transfer Experiment (INADEQUATE) Many others

14 2D-NMR COSY: Correlation of 1 H- 1 H coupling Dots = correlations Ignore dots on diagonal Sucrose 1 H-NMR Examples H 6 and H 5 are coupled Identify H 8 by its coupling with H 9 H8H8

15 2D-NMR HMQC: Correlation of spin-spin coupling between 1 H and nuclei other than 1 H such as 13 C Sucrose 13 C-NMR Sucrose 1 H-NMR No diagonal Example Which carbon bears H 6 ? 92 ppm

16 Magnetic Resonance Imaging (MRI) Basis: Spin-excited nuclei relax at a rate dependent on their environment Environmental factors = bonding to other atoms, solvent viscosity, etc. Photons released upon excitation are detected 1 H relaxation times varies with tissue type (brain, bone, etc.) Therefore tissues may be differentiated by NMR Timeline 1971: First MRI publication: “Tumor Detection by Nuclear Magnetic Resonance” Science, 1971, 171, : 22,000 MRI instruments in use; 6 x 10 7 MRI exams performed 2003: Nobel Prize in Physiology or Medicine: to Paul Lauterbur and Peter Mansfield for “their discoveries concerning magnetic resonance imaging”

17 Magnetic Resonance Imaging (MRI) NMR and MRI Use Similar Instruments Powerful magnets An NMR spectrometerAn MRI instrument

18 Magnetic Resonance Imaging (MRI) MRI Images: Quite Different from NMR Spectra! MRI image: a foot MRI image: a head