Shapes and Structures 1 - C-13 NMR Flashcards
A nucleus with a spin I has how many different energy levels?
2I + 1 energy levels
What is the spin of nuclei with odd masses?
half-integral spin
What is the spin of nuclei with odd numbers of protons and neutrons?
Integral spin
What is the spin of nuclei with even numbers of protons and neutrons?
Zero spin
Why does a C-13 resonance frequency increase as the atoms attached become more electronegative?
Electronegative elements withdraw electrons, deshielding the C atom
Electrons cause the nucleus to experience a weaker magnetic field overall, so lower electron density means a larger energy difference between spin states and larger frequency absorbed
Describe the peak given by the solvent CDCl3
Triplet at 75-80 ppm
How many peaks will be shown for a molecule in C-13?
Number of carbon environments
What should initially be looked for when predicting the spectrum of a molecule?
Planes/axes of symmetry
How is the chemical shift in ppm worked out?
10^6 x (frequency of resonance - frequency of reference)/(frequency of reference)
Outline the reference compound and its properties
Tetramethylsilane - TMS - Si(CH3)4
The chemical shift of 0 ppm by definition
In which range of ppm are sp3 carbons?
Give two examples of sp3 carbons
0 - 50 ppm
C-C, C-N
In which range of ppm are sp3 carbons with very electron-withdrawing groups?
Give examples
50 - 100 ppm
C-O, C-F
C≡C also in this range
In which range of ppm are sp2 carbons?
Give examples
100 - 150 ppm
C=C, benzene
C≡N also in this range
In which range of ppm are sp2 carbons with very electron-withdrawing groups?
Give examples
150 - 200 ppm
C=O
With respect to the carbonyl region, how can aldehydes and ketones be distinguished?
Ketones typically just over 200 ppm
Aldehydes typically just under 200 ppm
At what range do carbonyl carbons for acid derivatives resonate at? What are the groups in this range?
160 - 170 ppm
Carboxylic acid, ester, acyl chloride, amide, acid anhydride
What does the difference between the shift of aldehydes and ketones with that of acid derivatives indicate?
Carbon atoms in acid derivatives are more shielded
Heteroatoms (O, N, Cl) must be donating electron density
Why would a CH3F molecule result in a doublet?
C and F nuclei couple
F has spin 1/2 so two energy states
C - F spin up, increased field, increased shift
C - F spin down, decreases field, smaller shift
Why is C-H coupling not usually seen?
Usually proton decoupled
Why is C-C coupling not usually seen?
A low abundance of C-13 in naturally occurring carbon
If a C atom couples to two nuclei that give different coupling constants, what peak type is shown?
Doublet of doublets
If a C atom couples to two equivalent nuclei, what peak type is shown?
Triplet
What produces a quartet?
A C atom couples to three equivalent nuclei
What produces a triplet of doublets?
A C atom couples to two equivalent nuclei and one different nucleus
What is the term for C-13 spectra that do not show coupling to protons?
Proton decoupled
Explain broadband proton decoupling
While the C-13 NMR spectrum is being recorded
Protons irradiated such that nuclear spins move rapidly between up and down
Fast enough the couplings average to zero and splittings disappear
With broadband proton decoupling, what is the significance of “broadband”?
Irradiating over a range of frequencies so that all protons are rapidly moving between spin states
What are the advantages and disadvantages of proton coupling?
Advantages:
Easy to see whether carbons are CH3, CH2, CH or quaternary since they appear as q, t, d and s respectively
Disadvantages:
Signal strength weaker so signals can be lost in the background
Most compounds multiplets will overlap and sometimes can’t distinguish C peaks
Outline the Attached Proton Test, APT
Carbons with 0 or 2 protons attached point one way
Carbons with 1 or 3 protons attached point the other
You can tell which way by how CDCl3 points as it has 0 protons
Where would a methyl group (CH3) peak show?
10-20 ppm
Where would an alkyl group (CH2) peak show?
20-30 ppm
Where would an alkyne carbon peak show?
70 ppm
Where would an aromatic carbon attached to an electronegative atom peak show?
150 ppm
Where would an aromatic carbon peak show?
130 ppm
With the solvent CDCl3, knowing that D has I=1, and Cl-35 and Cl-37 have I=3/2, why is a triplet shown?
I=1 for D so spin states = 2x1 + 1 = 3
Cl has no observed coupling because spin states relax too quickly
