Proton NMR. Four Questions

Transcription

1 Proton NMR Four Questions How many signals? Equivalence Where on spectrum? Chemical Shift How big? Integration Shape? Splitting (coupling) 1

2 Proton NMR Shifts Basic Correlation Chart How many 1 H signals? Proton NMR Chemical Shift (ppm) Shift? alkane CH alkane CH alkane CH 1.7 X=C C H (Ph C H or 2 3 O=C C H) O C H 3 4 X C H (sp 2 ) C=C H 5 6 Ph H 7 8 O=C H 9 10 O=C O H O H, N H 1 5 2

3 Common exceptions Phenyl protons give one signal even if nonequivalent Common exceptions Exchangeable protons are broad and small Washed away with D 2 O 3

4 Four Questions How many signals? Equivalence Where on spectrum? Chemical Shift How big? Integration Shape? Splitting (coupling) Integration Area under signal is proportional to the number of protons in the set RELATIVE AREA is calculated by computer 4

5 Integration Which proton set is this? Trace of spectrum is cut into sections above each signal to give integration values Integration You need to set the values Here is a more realistic outcome This is the NMR for a C 5 H 10 O compound. What are the integration values?

6 Product Distribution by Integration In a mixture, areas are also proportional to number of protons But they can by fractions 57% B A C B D F E G F 43% C B F G D E A Four Questions How many signals? Equivalence Where on spectrum? Chemical Shift How big? Integration Shape? Splitting (coupling) 6

7 Splitting Splitting Shielding is also affected by magnetic fields of nearby nuclei 7

8 N + 1 Rule The signal for protons b is a quartet Signal b is coupled to the three protons labeled a Four possible affects on signal b Summarized by this simplification: n+1, where n = number of adjacent, nonequivalent protons Splitting 8

9 Exchangeable protons If protons exchange, they are not coupled If protons are not coupled, they will no show splitting in signal Typical for alcohols and acids Label splitting for each proton set O O OH OCH 3 Cl OCH 3 O Br Br OH 9

10 N+1 is a Simplification We learn it first because it applies to many molecules Assumes that all adjacent protons couple to an identical, measureable degree One exception: Aldehyde We will learn a more robust treatment after we master first principles H O H H (often doublet) (often singlet) Very small coupling between these protons, even though they are adjacent Problems: n+1 Spectra 10

11 Predicting Spectra based on Structure Step 1: Identify number of signals A B O O C D Predicting Spectra Step 2: Table of data A O B C O D Signal Shift (ppm) Integration Splitting A singlet B triplet C triplet D singlet 11

12 Predicting Spectra Step 3: Draw spectrum Practice Make problems for yourself: ChemDraw Don t do chiral compounds 12

13 6H, d 5H, multiplet 2H, doublet 1H, multiplet Reverse Process Interpreting Spectra Make table Assign possible pieces Predict structure C 6 H 10 Cl 2 O

14 Make a table Peak Shift (ppm) Int Split A H T, n=2 B H T, n=2 C H Qt, n=3 D H Qt, n=3 E H T, n=2 C 6 H 10 Cl 2 O Assign pieces Peak Shift(ppm) Int Split Piece Adjacent A H T, n=2 CH Electronegative, CH 2 B H T, n=2 CH 2 Oxygen? CH 2 C H Qt, n=3 CH 2 CH 3 or CH, CH 2 D H Qt, n=3 CH 2 CH 3 or CH, CH 2 E H T, n=2 CH 3 CH 2 Cl, Cl, C=O C 6 H 10 Cl 2 O

15 Predict structure Peak Shift(ppm) Int Split Piece Adjacent A H T, n=2 CH Electronegative, CH 2 B H T, n=2 CH 2 Oxygen? CH 2 C H Qt, n=3 CH 2 CH 3 or CH, CH 2 D H Qt, n=3 CH 2 CH 3 or CH, CH 2 E H T, n=2 CH 3 CH 2 Cl, Cl, C=O C 6 H 10 Cl 2 O Coupling Constants Begin Quiz 6 material 15

16 Review of Splitting Caused by shift due to magnetic fields of adjacent protons We say that these protons are coupled Protons may be coupled to different degrees Coupling constant Typically 7 Hz for adjacent sp 3 carbons Tree diagram 16

17 Basis of n+1 Rule Shortcut: N+1 if all protons coupled with same constant Look at tree diagram Coupling constant is Hz Tree diagram Draw the tree diagram that shows why signal A is a triplet What is the coupling constant for signal A 17

18 Coupling constants are not all 7Hz In this class, we will need to know other J values Coupling Constants Typical Constants Use the table to predict typical coupling constants H a C H 3 C Tricky OCH 3 D H b A H 3 C CH 3 D B H H C J ab = J ac = J ad = J bc = J bd = J cd = J ab = J ac = J ad = J bc = J bd = J cd = 18

19 Example: Cinnamic Acid Can the trans and cis isomers be differentiated using proton NMR? Yes with coupling constants H O OH MHz NMR: Doublet at 6.310ppm and 6.355ppm H H HO 5.98 H O 400 MHz NMR: Doublet at 5.925ppm and 5.950ppm Calculate the coupling constants for these doublets 19

20 Spectra that are not N+1 Consider the allylic methyl group Coupled to two protons but not with the same coupling constant Not N+1 Split into a doublet by H a That doublet is split into doublet by H b It is doublet of doublets J ac = 1.7Hz (typical 0 Hz) J bc = 6.9 Hz (typical 4 10 Hz) If J ac = J bc = 6.9Hz, what would we observe? Why not N+1? 20

21 Predict the Splitting What signals would be observed for proton A? Proton A is coupled to one proton B (doublet) Proton A is coupled to three proton C (quartet) Doublet of quartets with J = 15.6 Hz and J = 1.7Hz 21

22 Predict the Splitting Do the same for proton B How is signal same/different than proton A signal? Also a doublet of quartets But coupling constants are 15.6 and 6.9Hz Proton B 22

23 Exercise Explain these two observed signals How would you describe them? Which proton(s) do they belong to? Peak multiplicity J (Hz) A m B sx 7.2 C dd 7.2, 14 D dd 7.2, 14 E d 7.2 Diastereotopic Protons A B C D E 23

24 Types of problems Know typical coupling constants Describe expected signal (dd, dt, etc) Draw expected proton NMR Interpret proton NMR given coupling constants C 4 H 8 O ether IR: 1650 cm 1 Predict Structure 24