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NMR Spectroscopy and Stereo Chemistry

The following illustration gives some idea about chemical shift.

Using a 60 MHz instrument one part per million would be 60 Hz or 1 ppm that is 1 δ . If a signal appears at 360 Hz from TMS, it speak position in δ = 360-0 /60 = 6 (Note the denominator 60 is in MHz, )If it is in Hz, the entire term is multiplied by 106 in order to rationalise the value.

Integral (peak area)

• The peak area depends on the number of protons it represents. The ratio of the peak areas will be the ratio of the protons they represent. BrCH2CH2CH2Br has two types of protons their relative ratios is 4:2 which is 2:1. With the knowledge of the molecular formula, the actual number of protons each peak represents can be assessed.
• The machine integrates the peak areas and represents it as a step function on each peak, the height of the step function will represent the peak area. So the relative ratio of the heights would be the relative ratio of the protons they represent.
Multiplicity (Splitting pattern)
Consider the diagram1 in the above representation.
There are two hydrogens Ha and Hb and they are non equivalent and have different chemical shift.
• Consider Ha : It feels the external magnetic field and the induced magnetic field of the neigbouring Hb which is a tiny magnet by itself. There are two possible spin orientations (+1/2 and -1/2) for Hb, the induced magnetic filed due to one orientation will add on to the external magnetic field felt by Ha and the other will correspondingly reduce. Thus the absorption due to Ha will instead of appearing at one specific position will appear at two different positions closely spaced, and the signal is called a doublet.
• In the second diagram 2 Ha has two Hb atoms as neighbours (hydrogen atoms attached to the next carbon) the possible spin states of these atoms are shown. Influenced by these, the signal due to Ha splits into three closely spaced peaks called a triplet. And as can be seen from the spin population the intensities within the triplet will be 1:2:1
• In a similar manner when Ha has three neighbours (diagram 3) its signal will be a quartet and so on.
Coupling constant

For a doublet it is the distance between two peaks in Hz. For a triplet it is the distance between the central and the terminal peak and so on.. The coupling constant gives valuable stereo chemical information.The value of coupling constant indicates the intensity of coupling, for instance