Chapter 29: Chromotography and Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy uses a combination of strong magnetic field and radio frequency radiation. The nuclei of some atoms absorb and release the radiation repeatedly in a process called nuclear magnetic resonance

Tetramethylsilane (TMS) (CH3)4Si

chemical shift depends on the chemical environment of atoms and is greatly influenced by the presence of nearby electronegative atoms or pi-bonds

Deuterium is the 2H isotope of hydrogen, and therefore D2O (heavy water) doesn’t produce either H+ or C-13 peaks

number of carbon environments (number of peaks) types of carbon environments present (chemical shift)

have the same chemical environment and therefore the same chemical shift

have different environments and therefore different chemical shifts

• are equivalent and have the same chemical environment - have the same chemical shift and contribute to the same peak

• numer of proton environments (number of peaks) • types of proton environment (chemical shift) • relative numbers of each type of molecule (relative peak areas) • number of adjacent non-equivalent protons (spin-spin splitting pattern)

n + 1 i.e. for n protons (H+) on an adjacent carbon atom, the peak will be split into n + 1 -OH and -NH are always singlets

the NMR peaks are: often broad (from hydrogen bonding) and do not have any splitting of variable chemical shift

this is for identifying O-H and N-H peaks • a H+ NMR spectrum is ran as normal • a few drops of D2O are added, the mixture is shaken, and a second spectrum is run • deuterium exchanges with any OH or NH protons, removing their peaks from the second NMR so that they can be identified

peak number chemical shift environment splitting no. of adjacent hydrogens relative peak areas no. of atoms in environment conclusion