Protons in Nuclear Magnetic Resonance (NMR) Spectroscopy: Properties and Applications, Lab Reports of Organic Chemistry

Download Protons in Nuclear Magnetic Resonance (NMR) Spectroscopy: Properties and Applications and more Lab Reports Organic Chemistry in PDF only on Docsity! 1 Protons (nucleus of a hydrogen atom) also have a net spin and can be observed in the NMR. •Hydrogen atoms are more than 99% 1H. Other isotopes have special names 2H = deuterium 3H = Tritium (radioactive) •Remember 13C is only 1% of carbon; 1H NMR is much more sensitive. –Practically speaking for a typical lab molecule, you can obtain a 1H-NMR wit 1-2 mg of compound in a matter of minutes but the same sample may require a few hours to obtain a 13C spectrum. 1H NMR En er gy No Field Magnetic Field Stronger Magnetic Field hv En er gy } ΔΕ = hv In a strong magnetic field, the energy level difference corresponds to the energy of radio waves 2 FYI: Q&A How strong is a magnetic field in a typical NMR? 300 MHz NMR = 7.0459 tesla 600 MHz NMR = 14.0918 tesla What other nuclei are commonly observed by NMR? 15N, 19F, 31P What type of NMR is used in MRI? MRI methods typically look at 1H-NMR of water in the different environments of the body. 0123456789101112 TMS O H H C C H C H X (X = O, Hal) O C H C H Down Field Up Field Typical locations of 1H-NMR resonances. C H Protons resonate at a different frequency than 13Cʼs. Typical compounds lie within a smaller range of frequencies than 13C. PROTONS ON TYPICAL ORGNIC COMPOUNDS 1-12 ppm 5 1H-NMR peaks can be integrated!! { 5 : 3 Tells you the ratio of the number of protons that compose each resonance. This is particularly helpful when you have symmetry. 5 3 Protons can couple (cause splitting) of C-13 peaks but we donʼt see splitting of proton peaks by carbon!! C-H splitting is very small because only 1% of Carbon has a spin!. C-13 satellites You can see carbon-proton coupling if you look very closely! 6 Protons can split other protons by through bond J-coupling. C C H H 1 2 3 Are coupled over three bonds. H C H1 2 Splitting over two bonds is rarely observed. Geminal coupling is only observed when the two protons are in different “chemical environments”, this happens when the protons are diastereotopic. Protons on adjacent carbons are said to be Vicinal protons Coupling can be observed between two protons. The coupling originates because in a magnetic field, the nuclear spin will perturb the local distribution of electrons. Protons on the same carbons are said to be geminal protons Old NMR with Small wimpy magnet! Proton NMR signals are also split following the N+1 rule but for proton NMR N typically represents the number of adjacent protons. 7 O H3C O H2 C CH3 N+1 RULE STILL APPLIES (but it can get more complicated) PAY ATTENTION!: 13C-NMR 1H-NMR C C H H H C H H Splitting pattern reflects how many protons are directly attached to the carbon atom A Triplett Splitting pattern reflects how many protons are connected to adjacent carbon atom(s) A doublet A Triplett 10 C3H6O2 O O Same compound run on a 60 MHz and a 300 MHz spectrum Coupling constant is measured by taking the difference between peaks (in ppm)within a multiplet and dividing by the field strength in Hz. 60 MHz NMR 1 ppm = 60 Hz 300 MHz NMR 1 ppm = 300 Hz 8 hz 8 Hz 11 Common Splitting Patterns 0123 PPM CH3CH2Cl 0123 PPM Br Ethyl Isorpopyl 01234567 PPM Cl O aromatic protons have (almost) the same chemical shift 12 01234567 PPM Br a b c a b b/c a C4H10O 15 C10H12O from: structural database for organic compounds (sept) Problem: Protons in different chemical structures have different amounts of splitting or “coupling constants”. •Bad News: Life gets more complicated •Good News: Splitting tells us more about the chemical structure. 16 Coupling Constants Depends on Structure and Geometry Approximate Coupling Constants. C C H H 7 Hz (free rotation) C C HH C C H H 10 Hz 15 Hz C C H H 2 Hz H H H H C C C H H H 8 Hz 2 Hz 6 Hz C C CH H C C C H H 0-3 Hz depends on geometry C H H 1 Hz Special Couplings over more than 3 bonds H H J = 8-10 HzH H J = 2-3 Hz Karplus Equation: 17 N H O H H3C H H H H A B C D E (i.e. Yuck!) 20 Hc Ha Hb O O How can coupling constats help you assign the spectrum of vinyl acetate? a b c Assign the aromatic protons: H H 8 Hz H H 2 HzHint: 21 Assembling Structures Using 1H and 13C NMR. 1. Determine and Assign IHD (Mol formula/C13) 2. Identify symmetry (C13) 3. Identify Functional Groups (C13 or other techniques). 4. Explore around functional groups Identify or “map” groups in H-NMR and infer adjacent groups 5. Assemble fragments. C-13 71.5 t 68.7 d 36.9 d 26.6 t (2) 23.4 t (2) 23.0 q (2) H-1 3.33 d 2H 3.19 sept 1H 1.83 m 1H 1.47 m 4H 1.51 m 4H 1.16 d 6H C9H18O O H H H H H Should be 1.47 d,d,d,d = multiplet Should be 1.83 t,t,t = multiplet Why Multiplets? O 22 O O 26.6 23.4 23.4 26.6 36.9 71.5 68.7 23.0 23.0 O 1.47 1.51 1.51 1.47 1.83 3.33 3.19 1.16 1.16 O H H H H H Should be 1.47 d,d,d,d = multiplet Should be 1.83 t,t,t = multiplet Why Multiplets? O 13.3 13C- NMR: 173.6, s 51.3, q 42.0, t 34.9, d 33.1, t (2) 26.2, t (2) 26.1, t 1H- NMR: 3.67, s, 3H 2.19, d, J = 6.4 Hz, 2H 1.70, m, 6H 0.9-1.3, m, 5H C9H16O2 O O 25 C C H HR H BIG Big small C8H9Br Br 26 020406080100120140160180200220 PPM d d d t t t qC7H12O Where is the double bond? Is the double bond cis or trans? H O H O H O H O H O H O 020406080100120140160180200220 PPM d d d t t t qC7H12O 1H-NMR 9.72 t J = 8.1 Hz, 1H 5.48 dq, J = 15.9, 6.2 Hz, 1H 5.20, dt, J = 15.9, 5.9, 1H 2.40 dt, J = 8.1, 7.4 Hz, 2H 1.96, td, J = 7.9, 5.9Hz, 2H 1.66, quint, J = 8.0 Hz, 2H 1.71, d, J = 6.2 Hz, 3H H O H O H O H O H O H O 27 Question: Would you be able to solve the structure of this compound by 1H and 13C NMR? O O