SPECTROSCOPY: NMR Flashcards
In NMR when can nuclei be probed
As long as they don’t have a nuclear spin of 0
What do the main characteristics of NMR show
Chemical shift: info about environment
Multiplicity/splitting: info about neighbouring atoms
Coupling constant: identification of neighbouring atoms
Integration: relative ratio of signals
Where do more deshielded atoms lie on an NMR spectrum
Deshielded = less electron density = high frequency
= higher chemical shift
What does ‘X{Y} decoupled’ denote
The NMR spectrum of X decoupled from Y
What are the rules for predicting nuclear spin quantum number from protons and neutrons
Protons and neutrons both even: I = 0
Protons and neutrons both odd: I = integer
Other combination: I = 1/2 integer
What are some limitations to using NMR
Relaxation times are important for many nuclei
Very large chemical shift ranges for many nuclei make observation difficult
Problems with low natural abundance
Stability of compounds in solution
What is the equation for calculating multiplicity
M = 2n(I) + 1
n = number of atoms being considered coupled
I = nuclear spin quantum number of nucleus doing the splitting, not the atom being probed
What is a quadrapolar nucleus
A nucleus where I is greater than 1/2
Explain the relationship between pascals triangle and splitting
Probability of pairing a +1/2 spin with a -1/2 spin. Could have ++, – or +- & -+ which gives intensities 1:2:1.
More complex higher than I = 1/2 so just draw all the peaks the same intensity
What does a coupling constant refer to
The size of the splitting within the signals.
Size of coupling constant decreases as the nuclei become further away from each other.
Explain the notation of a coupling constant
XJ(YZ)
X (superscript) = number of bonds between atoms
Y and Z = atom types where Y is what is being observed in the NMR
What is the effect of cis and trans on coupling constants
Trans has larger coupling than cis therefore likely more stronger than in cis.
Thus, cis and trans are non- equivalent
What happens when we have an NMR spectrum with less than 100% abundant nuclei
One isotope may be I = 0 and the other may be I = 1/2.
When, I = 0, there will be no coupling and a singlet will occur
When, I = 1/2 there will be coupling and will split accordingly.
The product will be these superimposed on each other with relative integration of the abundance of each isotope.
The smaller peaks are called satellite peaks
