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Chapter 14 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY 14.1 The position of absorption or chemical shift is given by: δ = observed position of peak (Hz) X 10⁶ operating frequency of instrument (Hz) a) 7.4 δ b) 1480 Hz on a 200 MHz and 2960 Hz on a 400 MHz instrument c) 7.4 δ 14.2 The number of absorption signals in the NMR spectrum is equal to the number of different types of hydrogens (chemically nonequivalent) in a molecule. a) The CH₃ groups are chemically nonequivalent. The hydrogens of the CH₂ group are enantiotopic, so they have the same chemical shift. Thus there are three absorptions. b) The methyl groups are different. There are two different types of H's on the aromatic ring, the two H's ortho to the carbonyl group and the two H's meta to the carbonyl group. Therefore there are four absorptions. c) The two CH₃ groups are different. At first glance, the H's of the CH₂ group appear to be the same. Careful analysis shows that they are diastereotopic, and, thus, different. The situation is similar to the CH₂ group of 2-bromobutane described in the text. (In general, H's of CH₂ groups will be diastereotopic if there is a chirality center elsewhere in the molecule.) In addition, there are the CH and the OH groups. Therefore there is a total of six absorptions. d) The two CH₃ groups are different, as are the two vinylic hydrogens. Therefore there are four absorptions. e) The two OH groups are identical and the two vinylic hydrogens are identical, so there are only two absorptions. 14.3 Approximate chemical shifts for different types of hydrogens can be obtained using Table 14.1. Remember that most values in this table are for CH₃ groups. The signal for a CH₂ group appears about 0.3 δ downfield and that for a CH group appears about 0.7 δ downfield from these values. The presence of an additional functional group causes an additional downfield shift. This shift is relatively large if the additional functional group is attached to the same carbon as the hydrogen under 212