Physical Chemistry , 1st ed.

Solution

a.The benzene molecule is highly symmetric, and all its hydrogens are envi-

ronmentally equivalent. Therefore, only a single absorption is expected.

b.The addition of a methyl group to the benzene ring changes the environ-

ments of the hydrogens on the ring. Two hydrogens are on the carbon near-

est to the methyl group, two are on the carbons next removed, and one hy-

drogen is directly opposite it. Also, three equivalent hydrogens are on the

methyl group itself, assuming that the methyl group is free to rotate. Therefore,

toluene has four chemically different hydrogens, and one would expect four

absorptions in the NMR spectrum.

A conceptual diagram of an absorption-type NMR spectrometer is shown

in Figure 16.15. The magnetic field strength is usually between 1 and 10 teslas,

depending on the model of spectrometer. Experimentally, it is more conve-

nient to fix the radio frequency and change the magnetic field strength until

resonance conditions are established, and monitor the absorption of the radio

waves versus field strength. A typical low-resolution NMR is shown in Figure

16.16. NMR spectra that detect absorption by hydrogen nuclei are called pro-

ton NMR or^1 H NMR spectra. It is typical to measure an NMR spectrum us-

ing some standard reference material as an internal calibration compound, and

plot the values ofBtotfor the environmentally different nuclei with respect to

the internal standard. For^1 H NMR, tetramethylsilane (TMS), Si(CH 3 ) 4 , is used

as an internal standard. This compound has a very high shielding constant;

most compounds have lower shielding constants than TMS. The difference be-

tween the shielding constant of TMS and another compound is called the

chemical shift,labeled . Since shielding constants have values in the 10^5 to

10 ^6 gauss range, it is common to multiply the difference in shielding con-

stants by 10^6 to yield manageable numbers. In such cases, the chemical shift is

said to have units ofparts per million(ppm):

(in ppm) (TMSnucleus)  106 (16.28)

Figure 16.17 shows a low-resolution NMR spectrum in terms of.

576 CHAPTER 16 Introduction to Magnetic Spectroscopy

Sample

To

detector

From radio

frequency

generator

Sweep

coil

Sample holder

Electromagnet pole Electromagnet pole

Sweep

coil

Figure 16.15 A diagram of a standard NMR spectrometer. The sweep coils vary the magnetic

field in small amounts while radio waves bombard the sample. Any absorption of radio waves

will be detected by the detector circuit. The samples are usually spun to minimize inhomo-

geneities.