NMR Spectroscopy

Question 1

2 kinds of NMR focused on

Answer 1

1H (proton) and 13C

Question 2

NMR experiment

Answer 2

• apply external magnetic field to make all spins either parallel or anti-parallel to magnetic field; more spins in lower energy state
• high energy light pulse -> excites low energy electrons -> photon emits as e- falls back -> detected

Question 3

what happens to spins as magnetic field increased

Answer 3

larger difference between parallel and anti-parallel spins

Question 4

standard reference for NMR

Answer 4

TMS; at 0 ppm

Question 5

what do the number of signals tell you

Answer 5

the number of different environments in the molecule

Question 6

what does the chemical shift tell you

Answer 6

type of environment H atom is in; downfield shift as deshielding occurs

Question 7

downfield shift

Answer 7

increase ppm; C bonded to electroneg. atom which takes electron density = deshielding
*less electron density around carbon bonded to H

Question 8

upfield shift

Answer 8

decrease in ppm value; impact of electron shielding
* more electron density around carbon bonded to H

Question 9

what do the integration values tell you

Answer 9

number of H in the environment
1 = -CH
2 = -CH2
3 = -CH3

Question 10

what do coupling patterns tell you

Answer 10

how many hydrogens in close proximity (in diff environments)

Question 11

N+1 rule

Answer 11

predicts number of peaks for any given proton (coupling)

Question 12

what does coupling constant show you

Answer 12

stereochem/ isomers

Question 13

what is n in the n+1 rule

Answer 13

n= number of protons on adjacent carbon (s)

Question 14

how are ratios for coupling determined

Answer 14

pascals triangel
ex.
triplet: 1:2:1
quartet: 1:3:3:1

Question 15

Curphy Morrison parameters

Answer 15

used to calculate chemical shifts based on sub groups;
standard shift + (sum of alpha shifts) + (sum of beta shifts)

Question 16

how does symmetry in molecules impact peaks

Answer 16

since environments are identical, there are less peaks but they are more intense

Question 17

steps for predicting structure of molecule

Answer 17

1) look at formula and calculate unsaturated units
2) think about possible functional groups (use IR)
3) think about connectivity (use NMR)
4) confirm with mass spec (draw out fragmentation)

Question 18

how to calculate saturation unit

Answer 18

U= [ 2C +2 + N - H - X]/2

Question 19

H-NMR coupling with long carbon chains

Answer 19

*N+1 rule does not work; as chain extents growth of peaks representing each environment
*second order effects increase

Question 20

diastereotopic protons

Answer 20

2 protons very close in chemical shift AND coupled with one another
*N+1 rule does not apply; second order effects
*diastereomers

Question 21

how is 13C-NMR different from proton NMR

Answer 21

-no coupling; because only 1% of all C is 13 C
- more sensitive to small changes in molecular environment

Question 22

how does the solvent CDCl3 show up on C-NMR

Answer 22

1:1:1 triplet between 8 and 75 ppm

Question 23

impact of H- decoupling

Answer 23

enhance total intensity of signal

Question 24

primary, secondary, etc carbons ppm

Answer 24

highest ppm -> lowest ppm =
4 > 3> 2> 1

Question 25

impact of planes of symmetry on NMR spectra

Answer 25

simplify the spectra; less signals overall BUT signal is intensified if equivalent environment on other side of plane of symmetry

Question 26

Vinyl group

Answer 26

alkene function group with one of the carbons having 2H and the other having 1 H and 1 R group

Question 27

alkene magnetic field

Answer 27

protons of C=C group experience enhanced magnetic field -> higher ppm
*5.8 plus or minus 1.5 ppm

Question 28

Vinyl group coupling

Answer 28

trans H, cis H, geminal H (on same carbon)

Question 29

vinyl group coupling intensity

Answer 29

strongest -> weakest coupling
trans > cis > gem

Question 30

coupling trees

Answer 30

diagrams to interpret splitting pattern shown in NMR spectra peaks; strongest coupling pattern (trans> cis > gem) splits first -> other coupling exhibited

Question 31

Curphy Morrison Parameters

Answer 31

use Curphy Morrison Parameters for H- NMR constants;
standard value for group (-CH3, vinyl group, etc) + value for coupled functional group (cis/trans/gem function group)

Question 32

H-NMR aromatic ppm

Answer 32

7.5 plus or minus 1 downward ppm shift; conj pi system creates magnetic feild

Question 33

benzene ring coupling

Answer 33

strongest -> weakest
ortho> meta> para

ortho = sub groups on neighboring carbons; A and B
meta = sub groups on non neighboring carbons; A and A’

Question 34

C-NMR: what does the height of carbon peak indicate

Answer 34

number of H attached to it; taller the peak, more saturated the carbon

Question 35

ipso carbon

Answer 35

carbon with no hydrogens attached > small signal