Applications of NMR

Question 1

Where would a solvent peak of CDCl₃ show up in ¹H NMR and what causes it

Answer 1

δ_H = 7.27

Due to residual amounts of CHCl₃

Question 2

How is the δ_H value calculated?

Answer 2

Taking the average of the peaks to two decimal places

Question 3

How is the J value calculated?

Answer 3

Taking the difference between the peaks and multiplying by the frequency of the spectrometer

Question 4

Generally is coupling seen in a C¹³ spectum?

Answer 4

Generally the spectra are proton decoupled but could show coupling for other NMR active nuclei

Question 5

What are the typical chemical shifts for carbon NMR?

Answer 5

δ_C 220-150 C=O e.g ketones, aldehydes (δ_C 210-190 esters and amides)

δ_C 150-100 - C=C and aromatic

δ_C 100-50 - R₃COH, R₂CHOH, RCH₂OH

δ_C 60-40 - R₃CN, R₂CHN, RCH₂N

δ_C 50-0 - other C, CH, CH₂ and CH₃S

Question 6

How would CDCL₃ show up in a ¹³C NMR and why?

Answer 6

Show up as a 1:1:1 triplet

Due to coupling from deuterium - Deuterium has spin of 1 and so three lines show up due to (2I+1) rule

Question 7

Describe the chemical shift scale

Answer 7

For ¹H NMR the scale generally goes from 0-10 ppm

High ppm value is downfield and deshielded

Question 8

What does integration tell us

Answer 8

Tells us how many protons the signal is worth

Question 9

What does multiplicity tell you and what doe sit come from?

Answer 9

Due to coupling

Tells you about the environment

3 bond coupling is the most common type of coupling although 2 and 4 bond coupling is possible

Coupling is a through bond effect - equivalent protons do not couple and protons coupledto each other have the same J values

Question 10

What is distortionless Enhancement by Polarisation Transfer (D.E.P.T)

Answer 10

Heteronuclear pulse sequence which involves the simultaneous application of a programmed sequance of pulses to the ¹H and ¹³C nuclei

D.E.P.T provides an enhancement of intensity by changing the pulse angle so it is possible to change the phase of a signal.

When θ = 135°:

CH and CH₃ point up

CH₂ point down

C (quaternary) do not appear

Question 11

What happens to a proton when it couples to two other that do not have the same J value?

Answer 11

First J coupling constant splits δ_H peak into 2 then the second J coupling constant splits each of these loines into two again

In the spectrum you would see 4 peaks of equal intensity (not a quartet which shows 1:3:3:1)

Peaks 1-2 gives smaller J value

Peaks 1-3 gives largerJ value

Peak 1-4 give sum of J values

This is called a doublet doublet

Question 12

What happens to a proton if it is coupled to two protons in different environments with the same J value by chance?

Answer 12

The signal shows up as an apparent triplet (has a 1:2:1 ratio) but is different to a proton coupled top 2 protons in the same environment

Should be reported as a doible doublet with two dentical J values

The same effect is seen when a proton couples to 3 protons in different environments with the same J values by chance - shows 1:3:3:1 as an apparent quartet but is reported as a double double doublet

Question 13

What splitting pattern is shown for a proton coupled to 3 others in different environments with J values 12, 4 and 2Hz

Answer 13

Appearance: 8 lines - double double doublet

reported as ddd J = 12,4,2 Hz

Lines 1-2 shows 2Hz

Lines 1-3 shows 4 Hz

Lines 1-5 shows 12 Hz

Question 14

What is the splitting pattern for a proton that couples to 3 others with J values 12Hz, 10Hz and 4Hz

Answer 14

Appearance: 8 lines

Shows crossover between the lines: when the sum of the second two J values is greater than the first cross over occurs

Lines 1-2 gives 4 Hz

Lines 1-3 gives 10 Hz

Lines 1-4 gives 12 Hz

Question 15

What is the splitting pattern for a proton couled to 3 others in different environments with J vaues 12, 10 and 2 Hz

Answer 15

Appearance 7 lines: middle lines coincident

LInes 1-2 show 2 Hz

Line 1-3 show 10 Hz

Lines 1-4/5 show 12 Hz

Question 16

What is the splitting pattern for a proton coupled to 3 other in different environments with J values 12, 5 and 5 Hz

Answer 16

Appearance 6 lines - looks like a doublet of triplets

Lines 1-2 shows 5Hz

Lines 1-4 shows 12 Hz

Question 17

What is the splitting patter of a proton coupled to 3 others in different environments with J values 12, 12 and 5 Hz?

Answer 17

Appearance 6 lines - looks like triplet of doublets

Lines 1-2 gives 5 Hz

Lines 1-3 gives 12 Hz

Question 18

What is roofing?

Answer 18

The intensity of doublet peaks is not always equal so we sometimes see a roofing effect where one of the peaks is smaller than another - usually see between two separate doublets

If separation between peaks of different environemnts is good the roofing effect is reduced

If separation is poor the two doublets can look like a quartet

Question 19

What is ²J coupling?

Answer 19

Coupling through only 2 bonds when the protons are different

The magnitude of the J constant is often large

Average J value- 12 Hz

Question 20

What are the factros affecting ²J coupling?

Answer 20

Adjacent electronegative atoms reduce ²J value ~ 10.5 Hz

Adjacent pi systems increase ²J value ~ 16 Hz

terminal alkenes have small ²J values ~ 0-3 Hz

Question 21

Where can ²J coupling be observed?

Answer 21

When 2 protons are bonded to the same carbon atom but are in different environments

These can be found on terminal alkenes (when an alkene has 2 hydrogens on one end) or when a molecule has 1 or more chiral centres

Question 22

What is the typical range of J values for 3 bond coupling and the average value?

Answer 22

Range of values: 0-12 H_z

Average value: 7H_z

Question 23

How do J vlaues depend on dihedral angle?

Answer 23

Karplus equation: ³J = 4.22 - 0.5cosΘ + 4.5cos2Θ

When Θ = 0° ³J = 9-12 Hz

When Θ = 90° ³J = 0 Hz

When Θ = 180° ³J = 9-12 Hz

Question 24

What can J values tell you about cyclic systems?

Answer 24

In cyclo ring the conformation is often fixed (dihedral angle fixed)

The magnitude of ³J can tell you about the orientation of the protons

There are two types of protons: axial and equatorial

Question 25

What is the ³J value for axial axial coupling?

Answer 25

Θ = 180°

³J ax-ax = 9-12 Hz

Question 26

What is the ³J value for axial equatorial coupling?

Answer 26

Θ = 60°

³J = 2-6 Hz

Question 27

What is the ³J value for equatorial equatorial coupling?

Answer 27

Θ = 60°

³J = 2-6 Hz

Question 28

What ²J coupling will you see in cyclo compounds?

Answer 28

²J coupling can be seen between axial and equatorial protons on the same carbon

Typical value ²J = 12Hz

²J does not depend on the dihedral angle

Question 29

What are the typical values for the ³J coupling constants for trans, cic and cyclic alknes?

Answer 29

Trans: ³J = 14-16 Hz

Cis: ³J = 8-10 Hz

cyclohexene ³J = 10 Hz

cyclopentene ³J = 6 Hz

cyclobutene ³J = 3 Hz

cyclopropene ³J = 1 Hz

Question 30

What is 4 bond coupling and what are the typical values?

Answer 30

⁴J coupling is coupling across 4 bonds (long range)

Requires a pi system or a W shape

Aromatic W coupling: ⁴J = 0-3 Hz

Allylic coupling: ⁴J = 0-3 Hz

Cyclohexene W coupling: ⁴J = 0-3 Hz

Question 31

What are diastereoisomers?

Answer 31

A stereoisomerthat is not an enantimor

Diastereoisomers have >1 chiral centre

Question 32

What are diastereotopic protons?

Answer 32

Protons that are chemically different - can be distnguished even by systems that cannot tell right from left - appear at different chemical shifts in NMR

Question 33

How do OH or NH protons show up?

Answer 33

OH signals can be unpredictable

Chemical shift values vary

signals are often broad but can be sharp and sometimes show coupling.

Both chemical shift valiues and coupling depend on hydrogen binding which depends on solvent, concentration and temperature

R-O-H + H⁺ <–> R-O-H + H⁺ - proton exchange faster than NMR timecale - shows broad peak

Question 34

How can Oh or Nh peaks be discovered?

Answer 34

With a D₂O shake:

1) record ¹H NMR spectrum as normal
2) Add two drops of deuterium, shake then record ¹H NMR spectrum again:

D₂O_(excess) + R-OH <—> R-OD + H-O-D

Missing signals can be assigned to OH or NH

The H in H-O-D shows up at 4.8

Question 35

What methods can be used to increase chance of observing coupling to/from OH

Answer 35

Rigorously dry NMR solvent - no H₂O (no H⁺)

Low concentration of compound - exchange is an intermolecular process - lower concentration reduces rate of exchange

Use H bonding solvent (DMSO) - H bonding of OH to DMSO slows rate of exchange

Best situation - dry d⁶-DMSO

Question 36

What is the Nuclear Overhauser Effect?

Answer 36

results inchange of intensity of NMR signals of a nucleus when the resonance of some other nucleus in the molecule is saturates - undergopes irradiation by string radiofrequency.

Arise via dipole dipole interactions between saturated and observed nuclei

It is a through space interaction

Gives 3D solution structure of conformation of a molecule - shows protons close in space

Question 37

How do we measure NOEs?

Answer 37

Measure normal ¹H spectrum and observe peaks

Record spectrum while saturating one group

Take the difference between the two spectra - peaks that show up are close in space to saturated environment.

Question 38

What can we us NOEs to assign?

Answer 38

Trisubstituted alkenes - cannot use J bvalues as they dont exist - look at which groups are close together

Assign stereochemistry in cyclic ring - groups close in space, on same side of the ring will show NOEs

Often used to assign spectra of proteins

Question 39

What is (2D ¹H ¹H COSY) spectroscopy?

Answer 39

Uses a pulse sequence which modulates the resonance of each proton in an NMR with the chemical shift of protons which are coupled to it

Presence or absence of spin spin coupling between each proton is obtained.

Output: 2D contour map:

each axis is the 1D ¹H NMR spectrum, contours represent signal intensity, diagonal is birds eye view

Question 40

In COSY what do cross peaks show?

Answer 40

Cross peaks represent coupled protons

if there are no cross peaks those protons are not coupled#a cross peak indicates coupling

Question 41

What is COSY: 2D ¹H ¹³C HMQC or HSQC?

Answer 41

HMQC = heteronuclear multiple quantum correlation spectroscopy

HSQC = heteronuclear single quantum correlation spectroscopy

identifies 1 bond C-H connectivity

one axis = 1D ¹H spectrum

one axis = 1D ¹³C spectrum

Question 42

What do cross peaks show in HMQC and HSQC?

Answer 42

Cross peaks show which carbon a particular proton is connected to

a diastereotopic CH₂ will show cross peak to from one carbon to 2 protons

a cross peak indicates a C-H bond

Question 43

What is NOESY 2D ¹H ¹H

Answer 43

The presence or absence of NOE interactions between each proton is obtained

Each axis is the 1D ¹H NMR

Contours represent intensity

Diagonal is a birds eye view

Question 44

What do cropss peaks show in NOSEY?

Answer 44

Cross peaks represent protons that show NOE (are close in space)

If there are no cross peaks then those protons are not coupled (not close)

Cross peaks indicates two protons close in space