Organic Chemistry

562 CHAPTER 14 NMR Spectroscopy

a b

∆ /J = 20

CH 3 CH 2 X

∆ = 100 Hz

Jab = 5.0 Hz

∆ = 25 Hz ∆ /J = 5

∆ = 15 Hz ∆ /J = 3

∆ = 10 Hz ∆ /J = 2

∆ = 5 Hz ∆ /J = 1

Figure 14.31N
The splitting pattern of an ethyl
group as a function of the
ratio.

¢n>J

The difference in the chemical shifts of the and protons of 2-sec-butylphenol

is 0.8 ppm, which corresponds to 48 Hz in a 60-MHz spectrometer and 240 Hz in a

300-MHz spectrometer (Section 14.5). Coupling constants are independent of the op-

erating frequency, so is 7 Hz, whether the spectrum is taken on a 60-MHz or a

300-MHz instrument. Only in the case of the 300-MHz spectrometer is the difference

in chemical shift more than 10 times the value of the coupling constant, so only in the

300-MHz spectrum do the signals show clean splitting patterns.

in a 300-MHz spectrometer in a 60-MHz spectrometer

14.18 NMR Spectroscopy

The number of signals in a NMR spectrum tells you how many different kinds of

carbons a compound has—just as the number of signals in an NMR spectrum tells

you how many different kinds of hydrogens a compound has. The principles behind

1 H

13 C

13 C

¢n

J

=

48

7

=6.9

¢n

J

=

240

7

= 34

Jac

Ha Hc