H1 NMR Interpretation

1. H1 NMR Interpretation Lecture for Chem 2140 Spring 2012

2. Main Concepts Symmetry Chemical Shift Integration Splitting Patterns

3. How Symmetry Effects Interpretation A molecule’s symmetry determines how many signals you see Need to determine how many unique proton signals a molecule has A molecule that is completely symmetric will have only one signal In a molecule with no symmetry, every set of protons (on a carbon) will give a signal In molecule with some symmetry, some protons are equivalent and will give one signal

4. Methane (CH4)

5. Propane (CH3-CH2-CH3)

6. Benzene

7. Phenol

8. Hydroquinone

9. Chemical Shift Refers to a shift from the standard tetramethylsilane (TMS) set at zero Downfield: Away from zero/standard Occurs for compounds that are more “deshielded” (i.e. near electronegative groups) Upfield: Toward zero/standard Occurs for compounds that are more “shielded”

10. Typical Chemical Shifts

11. Chemical Shifts to Scale

12. Integration The area “under the curve” of a signal (just like calculus) Each signal’s area is directly proportional to number of protons for that signal One signal is set as the standard All other signals are relative to the standard The standard can be set to any number of protons Automatically set to “1” if not changed (i.e. if CH2 peak is set to 1, then all other signals are 2x the number given)

13. How Integration Relates to Symmetry A molecule’s symmetry determines how many unique proton signals are in the spectrum Thus, if there are equivalent protons (on different carbons), those protons give only one peak Similarly, integration accounts for ALL protons in a given peak

14. Splitting Patterns Different signals have different splitting patterns These patterns come from protons coupling with neighboring protons Each signal spits the neighboring proton (i.e. a methyl group has 3 equivalent protons all giving the same signal, thus neighboring protons split by 3 signals) Only non-equivalent protons will couple (and thus) split each other Equivalent protons are equal and give off the same signal

15. N+1 Rule The # of adjacent neighbors (N) of a given proton signal + 1 = splitting 0 neighbors = singlet (0+1 = 1) 1 = doublet (1+1 = 2) 2 = triplet (2+1 = 3) 3 = quartet (3+1 = 4) 4+= multiplet (4++1 =5+)

16. Pascal’s Triangle and Splitting

17. Ethyl Acetate

18. Pentanoic Acid

19. 3-Chloro 1-Iodopropane

20. 2-Chloropropane

21. Exchangeable Protons Protons that are attached to N, O, or S are often exchangeable with water These protons may or may not appear in a spectra Normally don’t integrate perfectly Normally singlets and do not effect splitting