11.3 Nuclear Magnetic Resonance Spectroscopy

those with odd mass numbers, odd atomic numbers, or both (aka odd protons or odd neutrons)

based on the fact that certain atomic nuclei have magnetic moments that are oriented at random when such nuclei are placed in a magnetic field, their magnetic moments tend to align with or against the direction of the applied field

the lower energy state of nuclei with magnetic moments that are aligned with the field

the higher energy state of nuclei once irradiated with uses that match the energy gap between the 2 states

a noninvasive diagnostic tool that uses proton NMR multiple cross sectional scans of the patients body are taken, and the various chemical shifts of absorbing protons are translated into specific shades of grey produces a picture that shows the relative density of specific types of protons

frequency vs absorption of energy

chemical shift plotted on x-axis

parts per million (ppm) of spectrophotometer frequency

NMR used to study H nuclei (protons) the only type of NMR tested by the MCAT

when the magnetic field of the protons align against the external magnetic field applied lower energy than beta spin state

when the magnetic field of the protons align against the external magnetic field applied

measures the alignment of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus itself used for determining the structure/connectivity of a compound, including functional groups

using tetramethylsilane (TMS), which as a chemical shift of 0 ppm TMS is used because it has one of the “most upfield” positions due to the electron-DONATING properties of the S atom; making it VERY shielded

downfield

upfield

has its own peak

the area under the curves/peaks proportional to the number of protons contained under the peak

when electron-withdrawing groups pull electron density away from the nucleus, allowing it to be more easily affected by magnetic field moves a peak further downfield

Ha is connected to one other hydrogen that is 3 bonds away so it will have 1+1 = 2 peaks

Hb is connected to two other hydrogens that are 3 bonds away so it will have 2+1 = 3 peaks

if a proton has n protons that are 3 bonds away, it will be split into n+1 peaks (BUT do NOT include protons attached to N or O)

occurs when 2 protons close to one another have an effect on the other’s magnetic environment results in the splitting of peaks into doublets, triplets, and multiplets, depending on the environment

between 9 and 10 ppm

between 10.5 and 12 ppm