NMR Spectroscopy

Question 1

Which coulping will be the greatest between two protons?

Ax-Ax, Ax-Eq, Eq-Eq?

Answer 1

Ax-Ax, the other two are rather small.

Question 2

How can you tell if protons are chemically equivalent

Answer 2

Can be related using a symmetry operation.

Make sure to take bond rotations into consideration.

Question 3

Why is it possible to measure the relaxation of the spin of a nuclei to find the frequency of resonances?

Answer 3

Fourier transform takes the units and transforms it into its reciprocal.

i.e. time -> frequency

The frequencies will be the ones needed to make up the relaxation time.

Question 4

How do you know if two protons have magnetic equivalency?

Answer 4

Protons must be chemically equivalent first before considring this

Of two chemically equivalent protons couple to another proton differently such as cis/trans or ortho/para/meta then they will be magnetically inequivalent

Question 5

What happens if protons are magnetically inequivalent

Answer 5

More complex NMR spectra

Question 6

For APT, what do +ve and -ve lines denote?

Answer 6

+ve, C/CH₂

-ve. CH, CH₃

Question 7

For DEPT 45, 90 and 135 what do signals denote?

Hence how can you tell if a carbon is C, CH, CH₂ or CH₃?

Answer 7

45 - All carbons with protons attached

90 - CH only

135 - CH + CH₃ +ve

CH₂ -ve

C - Appears in DEPT 45 but not in any of the others

CH - Appears on DEPT 90

CH₂ - Appears as -ve pek in DEPT 135

CH₃ - Left over peaks in DEPT 135 after accounting for CH₂

Question 8

What does COSY NMR detect?

Answer 8

Direct J coupling

Question 9

What does TOCSY NMR detect?

Answer 9

J coupling which continues until it cannot couple anymore

May be relevant for proof of two systems such as in a disacharide

Question 10

What does NOESY NMR detect?

Answer 10

Correlates spins close in space