Omsu由Pue5eene5especLnaacd einethmmHH nene DEPT spectra: A DEPT experiment consists of three spectra 动南动 Spectrum B:CH carbons umAines-SpectrumCli:quatemary c 0ra8naPpehimaRoeHeiePgpotheeroblem 10 Important Concepts 兰 a SHCHcCHa ed by radio-fre eadn eE9g5t08c5pem8R8easnertea aong Deoeet58PPm0eem 10 Important Concepts 10 Important Concepts 5.Chemical Shifts 物shie mical Equ alency-Equv 10.Non on Noighboring Hydrogons 12.DEPT C NMR eo gen 1212 On such pulse sequence is the distortionless enhanced polarization transfer (DEPT) 13C NMR spectrum. DEPT gives information specifying which type of carbon gives rise to a specific signal in the normal 13C spectrum: CH3, CH2, CH, or Cquaternary. A DEPT experiment consists of three spectra: Normal broad-band decoupled spectra; DEPT-90 pulse sequence spectra, which reveals signals only of carbons bound to one hydrogen; DEPT-135 pulse sequence spectra which produces normal CH3 and CH signals, but negative absorptions for CH2 and no peaks for quaternary carbons. Limonene DEPT spectra: Spectrum A: All 10 lines – 6 alkyl C at high field, 4 alkenyl C at low field Spectrum B: CH carbons Spectrum C: CH carbons, positive signal CH3 carbons, negative signal CH2 carbons Spectrum A lines – Spectrum C lines: quaternary carbon We can apply 13C NMR spectroscopy to the problem of the monochlorination of 1-chloropropane. Both 1,1- and 1,2-dichloropropane should exhibit three carbon signals each. The groups of three signals would be spaced differently due to the different arrangement of chlorine substituents. 1,3-dichloropropane should exhibit only two carbon signals. The two deshielded chlorine bearing carbons assignments in 1,2- dichloropropane can be made using DEPT data. The signal at 49.5 ppm appears inverted in a DEPT-135 spectrum (CH2). The signal at 55.8 ppm is the only absorption in a DEPT-90 spectrum 10 Important Concepts 1. NMR – Most important spectroscopic tool for elucidating organic structures. 2. Spectroscopy – Based on lower energy forms of molecules being converted into higher energy forms by the absorption of electromagnetic radiation. 3. NMR – Based on alignment of the nuclei of certain nuclei (i.e.. 1H and 13C) with (α) and against (β) a strong magnetic field. • α to β transition affected by radio-frequency radiation leading to resonance and characteristic absorption spectra. • High magnetic fields lead to higher resonant frequencies. 4. High Resolution NMR – Allows differentiation of 1H and 13C nuclei in different environments. • Spectral positions are measured as the chemical shift, δ, in ppm from an internal standard (TMS). 10 Important Concepts 5. Chemical Shifts – Highly dependent on presence (shielding) or absence (deshielding) of electron density. • Electron donor substituents shield. • Electron withdrawing substituents deshield. • Hydrogen bonding or proton exchange result in broad peaks. 6. Chemical Equivalency – Equivalent hydrogens or carbons have the same chemical shift. 7. Integration of peak area indicates number of contributing hydrogens. 8. Spin-Spin Splitting – • Pattern determined by number of hydrogen neighbors (N+1 Rule). • Equivalent hydrogens show no mutual splitting. 10 Important Concepts 9. Non-First-Order Spectra – Complicated patterns created when chemical-shift difference between coupled hydrogens is comparable to their coupling constant. 10. Non-Equivalent Neighboring Hydrogens – (N + 1) rule is applied sequentially. 11. Carbon NMR – Utilizes low abundance 13C nuclei. C-C coupling is not observed. C-H coupling can be removed by proton decoupling. 12. DEPT 13C NMR – Allows peak assignment to CH3, CH2, CH, and quaternary carbons respectively