Shapes and Structures 1 - Proton NMR

Question 1

What do most protons resonate between?

Answer 1

0-14 ppm

Question 2

What does it mean by integrated?

Answer 2

The area under the peaks of a signal is proportional to the number of protons

Question 3

What is vicinal coupling?

Answer 3

Coupling over three bonds

Question 4

What’s a typical vicinal coupling constant?

Answer 4

7 Hz

Question 5

What is geminal coupling?

Answer 5

Coupling over two bonds

Question 6

For an alkene, what are the coupling constants over two bonds?

Answer 6

0-3 Hz

Question 7

For an alkene, what are the coupling constants over three bonds (cis)?

Answer 7

0-12 Hz

typically 8 Hz

Question 8

For an alkene, what are the coupling constants over three bonds (trans)?

Answer 8

12-18 Hz

typically 15 Hz

Question 9

For a tetrahedral C attached to 2 x H, R1 and R2 (where H’s are differents), what are the coupling constants between the H’s?

Answer 9

8-18 Hz

typically 13 Hz

Question 10

What is roofing?

Answer 10

When two signals couple, the intensity of outermost lines decreases and innermost increases

Question 11

What does the strength of roofing depend on?

Answer 11

The difference between the two signals coupling

Question 12

H in cyclic propyl shift

Answer 12

0.5 ppm

Question 13

H3C-R shift

Answer 13

1.0 ppm

Question 14

R’-CH2-R’’

Answer 14

1.5 ppm

Question 15

H-C=C shift

H-C≡C shift

Answer 15

2.0 ppm

Question 16

H-C-C=O

Answer 16

2.5 ppm

Question 17

H-C-NH2

Answer 17

2.5 ppm

Question 18

H-C-Ph

Answer 18

2.5 ppm

Question 19

H-C-N shift

Answer 19

3.0 ppm

Question 20

H-C-O shift

Answer 20

4.0 ppm

Question 21

H-C-Cl shift

Answer 21

4.0 ppm

Question 22

H-C=C shift

Answer 22

5.0 ppm

Question 23

H-Ph

Answer 23

7.0 ppm

Question 24

H-C-C=O shift

Answer 24

8.0 ppm

Question 25

R-COH shift

Answer 25

10.0 ppm

Question 26

How does the shift of H in a methyl group compare to those in alkyl chains?

Answer 26

Generally 0.5 ppm lower

Question 27

How does shift of H on a C connected to O compare to those connected to N? And what if this is conjugated with a carbonyl?

Answer 27

Usually a shift of 1 ppm greater

If conjugated with carbonyl, shift increases further

Question 28

Why are shifts of H on aromatic rings greater than shifts for alkene H?

Answer 28

Due to local magnetic field set up by electrons in the π system
Aromatic ring H feel increased mag field so resonate at greater shift

Question 29

H-OH shift

Answer 29

1.5 ppm

Question 30

H-O-D shift

Answer 30

4.5 ppm

Question 31

H-O-R shift

Answer 31

2.5 ppm

Question 32

H-O-Ar (phenol)

Answer 32

7 ppm

Question 33

R-COO-H shift

Answer 33

10.5 ppm

Question 34

R-NH2 shift

Answer 34

2.5 ppm

Question 35

Ar-NH2 shift

Answer 35

4.5 ppm

Question 36

R-CO-NH2 shift

Answer 36

8.5 ppm

Question 37

How can shifts from H bonded to O or N be identified?

Answer 37

Very often have hydrogen bonding in these molecules
So very broad signals
OR
“D2O shake” test as deuterates N-H/O-H to N-D/O-D and resonate at a different frequency so signals disaappear

Question 38

Why are N-H/O-H peaks sometimes not seen?

Answer 38

No coupling due to hydrogen bonding

Exchangeable protons are rapidly swapping and equally spin up or down so an average of 0 spin is seen

Question 39

What are the necessary conditions to increase the chance of seeing coupling through an O or N?

Answer 39

Very dilute solution in an extremely dry solvent