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Proton or 1H NMR can look complicated but with practice; there are techniques that can make the peak assignment a little easier.
1. Make sure you count the proton environments correctly, if the molecule displays a form of symmetry you may count an identical environment!
2. If there is a multiplet at 7.2 ppm - this is always indicative of a benzene like ring.
3. Electronegative groups (N, O, Cl, F) cause de-shielding, moving protons in these environments further down the spectrum, use this as an indicator to find which hydrogens are those near functional groups. In contrast, branching alkane groups CH2 and chain end CH3 will mostly have quite low chemical shift.
4. The (n+1) rule can be used if you have proteins in a similar environment. The peak for a proton environment will split if a carbon adjacent to the one the proton you're observing is attached to protons in a different environment - This will cause the peak to split into (n+1) little peaks, where n is the number of protons on neighbouring carbon.
5. Often NMR questions will have other analysis such as MS in the same question. If your molecule you propose based on your NMR isnt compatible with the MS, try again - always look for differences in 14 m/z values for alkane chains etc.
These questions are worth a lot of marks so take your time.see more
A stationary point simply means a point in a curve where the gradient is equal to 0.
For example, in the June 2015 C3 Paper it is asked:
Find the exact values of the coordinates of the stationary points of the curve.
The curve function is f(x) = 6lnx + x^2 - 8x + 3
To calculate the gradient, we need to differentiate, as the gradient can also be represented as the change in y in respect to the change in x, or in other words dy/dx.
dy/dx = 6/x + 2x - 8
Where dy/dx = 0 is where the stationary point will be, 6/x + 2x - 8 = 0; multiplying all by x will give a quadratic: 6 + 2x^2 - 8x = 0, which can then be factorised: (2x-2)(x-3) = 0
Solving this x = 1 or x= 3. Calculating y from the original function gives y = -4 and y = 6ln3 - 12, giving the co-ordinates (1,-4) and (3,6ln3 - 12)
These questions are often worth a substantial amount of marks.see more