Rules of the Day
Click here for a copy of my lecture notes from today's lecture
N. Adjacent nuclei have magnetic fields associated with their spins. The spins of equivalent adjacent nuclei can be either +1/2 or -1/2, and at room temperature they are found in about a 50:50 mixture at any given nucleus (very slight excess of lower energy +1/2). These can add to give n+1 different spin combinations in the proportions predicted by Pascal's triangle. Each different spin combination produces a different magetic field, which leads to n+1 splittings in the peaks of the NMR spectra of the adjacent (no more than three bonds away) nuclei.
3. For alkenes or ring structures such as cyclopropanes the splitting does not simplify (no bond rotation) and you see full multiplicative splitting ("doublet of doublets", etc.) Click here to go to Pictures of the Day for today in which the NMR spectra for an alkene and a cyclic structure are explained.O. THEORY: When there are two sets of adjacent H atoms, the number of peaks multiply. For example, a CH2 group with a CH2 group and a CH3 group on either side should show 3 x 4 = 12 splittings! You can say this group is a "triplet of quartets" (or a "quartet of triplets").
P. WHAT YOU WILL SEE IN REALITY : For alkyl groups complex splittings simplify because coupling constants ("J") are all about the same. In practice, if there are n adjacent H atoms, equivalent or not, you will see n+1 peaks. This is an approximation, but almost always true on spectra taken with all but the most sophisticated NMR spectrometers.
Q. Non-equivalent H atoms on the same C atom can split each other (called geminal coupling), for example on alkenes or small rings. This coupling usually has very small coupling constants, so is difficult to see on some spectra. Note that EQUIVALENT H ATOMS on the same C atom do NOT split each other.
R. Deuterium atoms do not show up in 1H-NMR spectra, so deuerated solvents are used to dissolve NMR samples.
S. The H atoms of relatively acidic functional groups (alcohols, carboxylic acids, amines) exchange rapidly, so they often do not split adjacent protons, and they can be replaced (signal disappears) with deuterium by adding a drop of D2O to the NMR sample.
T. H-bonding changes the location of a signal for H-bonding groups in a concentration dependent manner explaining why -OH and -NH2 group signals can vary so much in location.
U. The splitting of a -CH2- group adjacent to a chiral center will be "messed up", that is split into many peaks. This is useful for identifying chiral centers in molecules.
V. When solving NMR spectra problems 1) Determine number and relative integrations of signals predicted for a given structure 2) Make sure the splitting pattern matches with the spectrum for each signal and 3) If the number and relative integrations as well as splitting patterns match with the spectra, compare expected chemical shifts with those of the signals in spectra.
7. The popular medical diagnostic technique of magnetic resonance imaging (MRI) is based on the same principles as NMR, namely the flipping (i.e. resonance) of nuclear spins of H atoms by radio frequency irradiation when a patient is placed in a strong magnetic field. Magnetic field gradients are used to gain imaging information, and rotation of the gradient around the center of the object gives imaging in an entire plane (i.e. slice inside patient). In an MRI image, you are looking at individual slices that when stacked make up the three-dimensional image of relative amounts of H atoms, especially the H atoms from water and fat, in the different tissues. [Memorize the preceding passage, as it will be worth 14 points on the first page of the Final] Click here for a handout on MRI. IF YOU ARE STRUGGLING TO CATCH THE WAVE IN THIS CLASS, DO NOT GIVE UP, IT IS NOT TOO LATE!! YOU NEED TO DO THE FOLLOWING!!