06_lec_6_7_2006 - 6 7 Massachusetts Institute of Technology...

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Unformatted text preview: 6 7 Massachusetts Institute of Technology Organic Chemistry 5.13 September 18 and 20, 2006 Prof. Timothy F. Jamison Notes for Lectures #6 and #7 1 H NMR Spectroscopy – Spin-Spin Coupling and Connectivity Signature “Splitting” Patterns in 1H NMR Spectra Prof. Timothy F . Jamison Notes for Lectures #6 and #7 ' H N MR Spectroscopy - Spin-Spin Coupling and Connectivity Signature "Splitting" Patterns in ' H NMR Spectra Singlet, 3H Methyl group CH3-Z Z= non H-bearing atom; typically O, N, C of Ar, etc. - Singlet, 9H at ca. 1ppm t-Butyl group (CH3)3C- Singlet, 3H Methy[ group Singlet, 9H at ca. Ipprn CH3-Z 1-Butyl group {CbhC- Z non H-bearing atom; typically 0. C of Ar, etc. N, Septet (1H) and Doublet (6H) Isopropyl group (CH3)2CH-Z Z = non H-bearing atom,typically O, N, C of Ar, etc. Septet (1H) and Doublet (6H) lsopropyl group (CH3)2CHd Z = non H-bearing atom, typically 0, C o Ar, etc. N, f Two Triplets (each 2H) Adjacent methylene groups 1 2 Z -CH2CH2 -Z 12 Z , Z = very different non H-bearing atoms Two 1 riplets (each 2H) Adjacent rnethylene grw ps Z '-CH~CH~Z* Z' ' = ver)r different ,2 non H-baring atoms Quartet (2H) and Triplet (3H) Ethyl group CH3CH2-Z Z= non H-bearing atom, typically O, N, C of Ar, etc. Quadet (2H) and Triplet (3H) Ethyl group CH3CH2-Z Z = non H-bearing atom, typically 0, C of Ar, etc. N, Doublet of Doublets (1H) Proton coupled to two non-equivalent protons which are attached to the same or different atoms Doublet of Daublets (1H) Proton coupled to two non-equivalent protons which are attached to the same or dmerent atoms Two Triplets (each 2H) and Quintet (2H) Three adjacent 1 2 methylene groups Z -CH2CH2CH2-Z 12 Z , Z = very different non H-bearing atoms Two Triplets (each 2H) and Quintet (2H) f hree adjacent methylene groups Z ' - C H ~ C H ~ CigureZby MIT OCW. FH~- ~ z'. z2= very different non H-bearing atoms 1H NMR Coupling Constants (Expanded) H geminal 12-15 Hz acyclic can be 0-25 Hz in cyclic system H H 6-8 Hz vicinal (averaged by free rotation) H In rigid systems, vicinal coupling can range from 0 to 15 Hz. For example: H H ax-ax H H 6-14 Hz H ax-eq 0-5 Hz H H H eq-eq 0-5 Hz Spin-spin coupling in alkenes: H geminal 0-3-Hz H H H trans 12-18 Hz cis H 6-12 Hz allylic 0-3 Hz H Spin- spin coupling in arenes: Ha Hb Hd 6-10 Hz Jac (meta) Hc Jab (ortho) 1-3 Hz Jad (para) 0-1 Hz Note: Structures shown above represent generic coupling situations and not the specific molecules depicted (in which the labeled protons would be chemically equivalent and would not couple). Figure by MIT OCW. Spin-Spin Coupling for "Vicinal" Protons Ha C The spin of proton Ha influences the energy of the two spin states of proton Hb. This "coupling" is transmitted by the electrons in the bonds linking Ha and Hb. The lowest energy state for proton Hb occurs when the spins of the two protons are antiparallel. Note that this effect does not require that the molecule be in an external magnetic field. C Hb Figure by MIT OCW. Scenario II Ha present Scenario I no Ha Ho Direction of applied magnetic field Ε ca. 50% Ha has spin: ca. 50% Ha has spin: ∆Ε ∆Ε Hb ∆Ε Hb Ε Hb Key Features of Spin-Spin Splitting 1. No coupling occurs among chemically (and magnetically) equivalent atoms. 2. A nucleus coupled to n equivalent nuclei with spin l is split into 2nl + 1 lines. 3. Nuclei coupled to each other have the same coupling constant ("J"). 4. The magnitude of the coupling constant J depends on the dihedral angle and type of intervening bonds, but is not affected by the strength of the applied field. 5. "First-order spectra" are obtained only if (va - vb)/Jab > ca. 7 Hz. 6. The splitting pattern for nuclei coupled to two or more nonequivalent atoms can be predicted using "tree diagrams". ...
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This note was uploaded on 11/27/2011 for the course CHEMICAL E 20.410j taught by Professor Rogerd.kamm during the Spring '03 term at MIT.

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