NMR Spectroscopy and Stereo Chemistry

By

Srinivasa K

Srinivasa K

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 10

^{6 }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. BrCH
_{2}CH_{2}CH_{2}Br 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 H_{a}and H_{b}and they are non equivalent and have different chemical shift.Coupling constant

- Consider H
_{a}: It feels the external magnetic field and the induced magnetic field of the neigbouring H_{b}which is a tiny magnet by itself. There are two possible spin orientations (+1/2 and -1/2) for H_{b}, the induced magnetic filed due to one orientation will add on to the external magnetic field felt by H_{a}and the other will correspondingly reduce. Thus the absorption due to H_{a}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 H
_{a}has two H_{b}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 H_{a}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 H
_{a}has three neighbours (diagram 3) its signal will be a quartet and so on.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

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