NMR Analysis of Substituted Benzophenones Analysis Guide

last updated:  03/09/2014

Each student is expected to use all of the 2D NMR spectra to confirm the structural assignment.  We will be looking for descriptions of key cross peaks that you believe are important in your characterization. Some of you will need to analyze each NMR spectrum for key information while others may be able to assign the hydrogens and carbons without them. 

1.  Draw out the structure and number the carbons and hydrogens in your molecule.  Be consistent with the H & C labels throughout your NMR analysis. Clearly label the drawn structure with ppm values for each C and H (and F) and give the coupling constants if you know these.  Include this with Report 1 either in the text or as an appendix.

2.     ¹H and ¹³C NMR Spectra.   Sample spectra for toluene:  HNMR    CNMR

• Count the number of hydrogens and carbons from the respective 1D experiments and note the relative ppm for each. Relative integration values for protons are given at the bottom of the HNMR plot.
   

• Signals near electron donating groups are generally more upfield while signals near electron withdrawing groups are generally more downfield.
   

• Fluorine has a 1/2 nuclear spin and therefore will split both hydrogen and carbon.  ¹J C-F coupling is greater than 200 Hz; J_ortho H-F is 6-10 Hz and J_meta H-F is also 4-8 Hz. (recall proton to proton is J_ortho 6-9 and J_meta is 1-3 Hz)

3.  COSY (Homonuclear Correlation Spectroscopy). 
 

• ¹H NMR ppm are given as labels on top and on left side of plot. Ignore the diagonal peaks.
   
• Use this experiment to identify which protons are coupled to each other from the cross peaks.
   
• The signals are symmetrical across the diagonal so information is redundant on the other side of the diagonal.

4.  HSQC (Heteronuclear Single Quantum Correlation Spectroscopy).  
     Sample spectra for  toluene:           HSQC

• ¹H NMR ppm are used as labels on the top and ¹³C NMR ppm on left side.
   
• Use this experiment to identify the "protonated carbons"    ¹J H to ¹³C couplings only.
   
• Use in conjunction with ¹³C NMR spectra to assign all carbons bearing hydrogen.
   
• Only quaternary carbons remain unknown at this point.

5.  HMBC (Heteronuclear Multiple Bond Correlation) . Sample for toluene: HMBC

• H NMR on top and ¹³C NMR on left side of plot.
   
• Use this experiment to identify ¹J, ²J, and ³J  for H to ¹³C couplings.
   
• The ¹J (H-¹³C) couplings are normally suppressed by the HMBC pulse program but not always completely eliminated.  For example, very sharp methyl signals may show as two peaks at the proton ¹³C satellite frequencies.
   
• The longer range ²J/³J couplings for H to ¹³C show up as single cross peaks.
   

• Begin the analysis with cross peaks from the carbonyl carbon to the protons in both aryl rings and  these are very important.
   

• Analyze all couplings to substituents (i.e. methyl group, propyl group, methoxy group, etc.) into the ring (s).

• Analyze all couplings within each ring and out to the substituents.

6.  Coupling Constants

Determine any major coupling constants you have data for.  If you subtract ppm values to find J values you also have to multiply by the ν_r for the instrument to convert to Hz (i.e., 400 MHz).

7.  Calculated Shifts.  If necessary, compare chemical shifts by calculating empirical values using tables such as those in the Mohrig lab text  to calculate an empirical ppm for H and C.

8.  Compare to reported literature values if found for your compound. 

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