NMR

Question 1

Chemical shift =

Answer 1

v(sample)-v(TMS)/operating frequency x 10^6

Question 2

B effective =

Answer 2

B applied - B local

Question 3

Two protons are magnetically equivalent if…

Answer 3

They are chemically equivalent and they have the same geometric relationship with all other protons in the spin system

Question 4

A coupling constant is…

Answer 4

The separation in hertz between two peaks of a split NMR signal

Question 5

Which is bigger J(trans) or J(cis)

Answer 5

J(trans)

Question 6

Heterotopic and diastereotopic protons will…

Answer 6

Have different chemical shifts and couplings to the neighbouring protons

Question 7

Enantiotopic and homotopic protons will…

Answer 7

Have identical chemical shifts. They may or may not have identical coupling to other nuclei

Question 8

Proton equivalence test: Homotopic protons

Answer 8

Exchange with X does not form a new compound

Question 9

Proton equivalence test: Enantiotopic protons

Answer 9

Exchange with X creates a chiral centre which leads to the formation of enantiomers

Question 10

Proton equivalence test: Diastereotopic protons

Answer 10

Exchange with X creates a second chiral centre leading to the formation of diastereomers

Question 11

13C chemical shifts =

Answer 11

Approximately 20 x 1H chemical shifts

Question 12

DEPT 45…

Answer 12

CH, CH2 and CH3

Question 13

DEPT 90…

Answer 13

Only CH carbons are detected

Question 14

DEPT 135…

Answer 14

CH and CH3 are phased up, CH2 phased down

Question 15

How can we simplify complex spectra? (5 answers)

Answer 15

Increase the magnetic field
Decoupling
Chemical exchange or conversion
Change of solvent
Shift reagents

Question 16

Why does increasing the magnetic field simplify spectra?

Answer 16

The larger the magnetic field, the larger the difference in energy between the low spin and high spin orientations. This causes the difference in frequency between signals to increase.

Question 17

What is the ideal signal frequency difference to coupling constant ratio?

Answer 17

dv/J = 10

Question 18

D2O shake can be used on…

Answer 18

Carboxylic acids, alcohols and amines

Question 19

Give another example of chemical exchange

Answer 19

Trifluoroacetic acid can be used on amine groups, the proton closest to the CF3 group is deshielded

Question 20

Type 1 nuclei I =

Answer 20

0

Question 21

Type 2 nuclei, I =

Answer 21

1/2, 1, 3/2,…

Question 22

dE =

Answer 22

h (y/2pi) B0

Question 23

Downfield =

Answer 23

High ppm

Question 24

Upfield =

Answer 24

Low ppm

Question 25

DBE =

Answer 25

C - (H/2) - (X/2) + (N/2) + 1