Physical Chemistry , 1st ed.

splits into a total of 6 1 7 individual absorptions (a heptet). The relative

intensities of the triplet/heptet absorptions should be in the ratio of 62 (or

3 1), which is the ratio of the number of environmentally equivalent hydro-

gens in the molecule. Figure 16.22 shows NMR spectra of the three com-

pounds.

Just like IR spectra for vibrational motions of molecules, the chemical shifts

seen in NMR spectra are generally characteristic of the type of groups in a

molecule. That is, methyl groups typically show up in an NMR spectrum

shifted about 1 ppm away from TMS, hydrogens on aromatic rings typically

show up about 7–8 ppm away from TMS, and so on. General ranges of posi-

tions due to specific groups are gathered in correlation charts (just like IR spec-

troscopy). An example of an NMR correlation chart for proton NMR is shown

in Figure 16.23.

Modern NMR spectrometers have the capability of measuring the reso-

nance absorption of multiple nuclei, like^1 H,^13 C,^31 P, and other elements.

Modern instruments don’t typically use a straight absorption-of-electromag-

netic-radiation mode of measuring a spectrum. Instead, the spectrometer ex-

poses the nuclei to a static magnetic field,B 0 , which aligns the magnetic mo-

ments of the nuclei. Then, a second magnetic field B 1 that is perpendicular to

B 0 is applied to the sample for a short time (1–10 microseconds), as a pulse.

580 CHAPTER 16 Introduction to Magnetic Spectroscopy

8

ppm

6420

8

ppm

6420

8

ppm

6420

Figure 16.22 High-resolution NMR spectra of methane, ethane, and propane show the split-

tings of the proton absorbances, as expected by the number of neighboring hydrogen atoms on

each carbon.