Spectroscopy Topic 5 Flashcards
What represents letter φ?
Dihedral angle between protons
Wherew there is confirmational flexibility in regions of an organic molecule, what is the value of 3JHH seen in the 1H NMR spectrum?
Approximately 7 Hz
What is Karplus curve?
A plot of 3JHH against φ
What is the shape of Karplus curve?
It iis approximately cosinusoidal in shape, with maxima at φ=0 and 180 degrees, and a minumum at 90 degrees.
Where do we see a fixed values of φ?
In rings and alkenes as bonds are not free to move.
Why value of J is higher for π bond compared to σ bond?
Because π electrons assist in transmitting the spin-spin information from one nucleus to the other.
Why we get the largest value of 3JHH with antiperiplanar arrangements (φ=180 degrees)?
On the left hand side you have the bonding orbital between C-H and on the right hand side we have antibonding C-H bond. And when you have an antiperiplanar arrangement between the two hydrogens on adjacent carbons that results in maximal orbital overlap between σ and σ*.
Why 3JHH=0 when φ=90 degrees?
Because orbitals at 90 degrees cannot overlap.
What is value of J for φ=0 degrees Z ankene?
J=8-12 Hz
What is value of J for φ=180 degrees E ankene?
J=12-18 Hz
What is value of J for φ=180 degrees 2 axia bonds in ring?
J= 10-12 Hz
What is value of J for φ=60 degrees 1 axial 1 equatorial or 2 equatorial bonds in ring?
J=2-5 Hz
In what molecules coupling is measured further than 3 bonds?
- with π-bonding
- with special arrangements of bonds
Why π-bonding shows coupling over 3 bonds?
The additional electrons and orbitals from π-bonding assist in the transmission of spin-spin coupling between protons. Notice that longer range coupling leads to a much smaller value of J, making it easily distinguishable from the usual 3 bond coupling.
Why special arrangements of bonds show coupling over 3 bonds?
Special arrangements of protons, such as is seen in so-called W-coupling can give surprisngly large values of 4J in saturated system, but these are very unusual, and rarely encountered. The arrangement of C-H σ and C-C σ* orbitals leads to efficient coupling over 4 bonds.
What is this product called?
Diels-Alder product
How doublet is formed?
By having one proton that couples with one proton. Proton B splits the signal of proton A into a doublet with coupling constant JAB
What coupling is formed when Ha with the same coupling constant?
First we get spliting caused by proton b, we get doublet. Superimposing onto that the splitting due to proton c. So proton C splits the signal again into doublets. And because coupling constants are the same there is so overlap in the pattern. And signal in the middle is twice as big as signals on the side.
What coupling is formed when three protons coupling to Ha with the same coupling constant?
Proton b splits the signal a into doublet. And proton c splits each of those signals into a doublet with a central signals overlaping. and proton d splits those signals into a doublet with overlaping. We get intensities 1:3:3:1 as quartet
What coupling is formes when two protons couple to Ha with a different coupling constant?
Proton b splits signal of proton a into a doublet. And proton c splitts each signal into a doublet. Because coupling constants are different we do not get overlapping and we get 4 peaks called double doublet.
What coupling is formed when three protons couple to Ha one with a different coupling constant and two with the same coupling constant?
Proton b splits signal of proton a into doublet. Proton c splits both signals into a doublets. As coupling constants are different there is no overlaping in intensities. The 4 signals are split into doublets by proton d and as proton c and d has the same coupling constants there is overlaping and 6 signals are formed with intensities 1:2:1:1:2:1. This is called double triplet.
What coupling is formed when three protons couple to Ha one with a small coupling constant and two with the large coupling constant?
Proton b splits the signal into doublet. Each signal is split by proton c into doublets but because coupling constants are the same there is overlaping of the signal and just 3 signals formed. Each signal is split into doublet by proton d with smalled proton constant so no overlaping happening and we get 6 peaks with intensities 1;1:2:2:1:1. This is called triple doublet.
