Spin-Spin Splitting (7)

Question 1

What is spin-spin splitting?

Answer 1

two nuclei can be sufficiently close in space that they exert a magnetic effect on one another, manifested through the observed splitting of the NMR resonance caused by the presence of the second nuclei

Question 2

Other names for spin-spin splitting

Answer 2

through-bond, scalar, indirect, J

Question 3

What is J - the coupling constant?

Answer 3

the space between splittings

Question 4

What are the units of J?

Answer 4

Hz

Question 5

How is J written in terms of the interacting atoms?

Answer 5

superscript describing number of bonds separating the coupled nuclei, subscript describing the two atoms involved

Question 6

What is the relationship of J to B0?

Answer 6

independent of the magnitude of B0

Question 7

How do two nuclei exert magnetic field effects on one another?

Answer 7

sufficiently close in space

Question 8

What is the fermi contact term?

Answer 8

direct interaction between magnetic moments of nuclei and those of bonding electrons in s states

Question 9

Is the interaction between two nuclei through space or through chemical bonds?

Answer 9

bond

Question 10

How are these nuclei interaction magnetic effects observed in the NMR spectrum?

Answer 10

by splitting

Question 11

How does the observed splitting depend on the s character of the bonds between the interacting nuclei?

Answer 11

more s character, higher J Hz

Question 12

What are the possible combinations of spin states for the 2 CH2 protons?

Answer 12

1 combination of spins paired and aligned with field
1 combination of spins paired and aligned against field
2 combinations of opposed spins

Question 13

What is the effect of the magnetic difference of the 2 CH2 protons on the 3 CH3 protons?

Answer 13

CH3 senses the magnetic effect, splits into three peaks

Question 14

Into how many peaks does the CH2 resonance split?

Answer 14

three

Question 15

What are the areas of the resulting peaks when CH2 resonance splits?

Answer 15

middle peak twice the area of the side peaks

Question 16

What is the practical definition of a first-order spectrum?

Answer 16

∆v/J > 10

Question 17

What are the rules governing the interpretation of first order spectra?

Answer 17

1. Equivalent nuclei do not interact with one another to give multiple absorption peaks
2. Coupling constants decrease with separation of groups
3. Multiplicity of a band is determined by the number of magnetically equivalent protons on a neighboring atom

Question 18

What is multiplicity?

Answer 18

possible orientations of the total spin

Question 19

What is the n+1 rule?

Answer 19

multiplicity is determined by the number of protons on adjacent groups n, +1

Question 20

How can multiplicity be calculated for adjacent non-equivalent nuclei?

Answer 20

(nA + 1) * (nB + 1)

Question 21

How is the coupling constant related to B0?

Answer 21

independent

Question 22

How can multiplets be distinguished from closely spaced chemical shift peaks?

Answer 22

by changing B0

Question 23

What are some common spin-spin splitting patterns?

Answer 23

triplet, doublet, quartet

Question 24

What is an A-X spectrum?

Answer 24

first order, two spin

Question 25

What is an example of an A-X system?

Answer 25

1-chloro-4-nitrobenzene

Question 26

What is an A2-X spectrum?

Answer 26

first order, three spin

Question 27

What is an example of an A2-X system?

Answer 27

1,1-2 trichloroethane

Question 28

What is an example of an A2-X3 system?

Answer 28

diethyl ether

Question 29

How is pascal’s triangle related to the relative intensities of the multiplet components in an NMR signal split by spin-spin coupling?

Answer 29

the relative areas of the split NMR peaks in the multiplet are symmetric around the theoretical midpoint of the peak, and are proportional to the coefficients