Question

MCAT Tutorial: Proton NMR Spectroscopy

Answer 1

\({\bf{Basic~Mechanism:}}\) molecules are irradiated with ~radio frequency radiation which induces a magnetic field within the nuclei of the involved atoms. Nuclei must have nuclear spin. Induced mag field can be aligned with the external field (parallel) or against it (antiparallel).

>The amount of energy absorbed/reflected is detected and plotted in an NMR spectrum.
> TMS (tetramethylsilane) is added as a reference; the absorption signal of TMS is assigned to 0 ppm

\({\bf{Definitions:}}\)
- Antiparallel: higher energy
- Spin-flipping: absorbing enough energy to become antiparallel
- Chemical shift: the absorption peak frequency (compared to 0)/ frequency of the spectrometer
- Upfield: the low frequency region near the right end of a typical NMR spectrum
- Downfield: the high frequency region near the left end of a typical NMR spectrum

Answer 2

\({\bf{Interpreting~an~NMR~Spectrum:}}\)
> the number of different peaks = the number of distinct proton environments

let's start with a simple example: ethanoic acid



let's start with the carbon on the left, all of these three hydrogen atoms are considered to be in the same proton environment because they are bonded to the same atom, and are adjacent to the same kind of functional group

the second carbon contains a hydrogen that is bonded to a distinct carbon, and is closer to the double bonded O, and thus is a separate proton environment

> the position of the peaks indicates the relative shielding/deshielding

shielding: when a proton is "protected" against the effects of the magnetic field by nearby electron centers, such as electronegative atoms
when a proton environment is shielded it will appear further upfield

> the splitting pattern indicates the presence of neighboring proton environments

n + 1 rule: consider a proton environment with an adjacent proton environment. Count the number of adjacent protons and add 1. The original proton peak gets split into that many smaller peaks. The heights of the peaks can be proportioned out using pascal's triangle.

Answer 3

let's use ethyl acetate to demonstrate the n + 1 rule:

Answer 4

first group, has 3 hydrogens (you are free to try sketching out the hydrogens to make this simpler) with no adjacent proton environments so this appears as only 1 peak

Answer 5

second group has 2 hydrogens with an adjacent proton group with 3, therefore the second group gets split into 3 + 1 = 4 peaks (note: be careful to use the adjacent group when determining the # of peaks)

third group has 3 hydrogens as stated before, adjacent to the second group with 2 hydrogens, therefore getting split into 3 peaks

Answer 6

now let's take a look at the relative shielding. the first group is the closest to the very EN atom oxygen, so it will be shielded (upfield)

followed in order by the second and third proton environments.

giving us:

Answer 7

> finally the area under the the peaks indicates the relative # of hydrogen atoms in that enviornment. on an NMR graph you will often be given an integral line that indicates the relative area under the curve compared to the other peaks.

note: you cannot just look at the highest peak, you have to consider the height of every split peak within an environment combined.

Answer 8

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT