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An Introduction to NMR Spectroscopy

Shelby Feinberg and Steven Zumdahl

Nuclear magnetic resonance spectroscopy (NMR) has risen to the same level of

importance as electronic and vibrational spectroscopy as a tool for studying molecular properties,

particularly structural properties. Although the following discussion of NMR specifically deals

with hydrogen atom nuclei in organic molecules, the principles described here apply to other

types of molecules as well. Many other types of nuclei (1!, 1"#, 1$, etc.) have nuclear spins and

thus can be studied using NMR techni%ues.

!ertain nuclei, such as the hydrogen nucleus (but not carbon&1' or oygen&1), have a

nuclear spin. *he spinning nucleus generates a small magnetic field termed . +hen placed in a

strong eternal magnetic field, called o, the nucleus can eist in two distinct spin states- a low

energy state A, in which is aligned with the eternal magnetic field, o, and a high energy state

, in which is opposed to the eternal magnetic field, o(figure 1). Alignment of with ois

the more stable and lower energy state.

Figure 1

/n NMR, transitions from the more stable alignment, A, (with the field) to the less stable

alignment, , (against the field) occur when the nucleus absorbs electromagnetic energy that is

Ho

Ho

Spin + 1/2Parallel

ALow Energy

Spin 1/2Anti-parallel

BHigh Energy

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eactly e%ual to the energy separation between the states (0). *his amount of energy is usually

found in the radiofre%uency range. *he condition for absorption of energy is called the condition

of resonance. /t can be calculated as the following-

h,2'h304 ==

h 5 $lanc67s constant

5 the strength of the applied magnetic field, o, at the nucleus

5 the gyromagnetic ratio (a constant that is characteristic of a particular nucleus)

5 the fre%uency of the electromagnetic energy absorbed that causes the change in

spin states

*here are three features of NMR spectra that we will focus on- the number and si8e of

signals, the chemical shift, and spin&spin coupling.

Number and Size of Signals

9et7s consider how the NMR spectrometer can distinguish between hydrogen nuclei and

produce multiple signals. Magnetically e%uivalent hydrogen nuclei produce one signal. *hese

hydrogen nuclei eperience the same local environment. #or eample, in a molecule such as

diethyl ether (#igure '), there are two sets of magnetically e%uivalent hydrogens. *he hydrogens

labeled aare si magnetically e%uivalent methyl hydrogens, while the hydrogens labeled bare

four magnetically e%uivalent methylene hydrogens. Notice that the methyl ( a) hydrogens are all

located ad:acent to a carbon containing two hydrogen atoms. Additionally, the methylene (b)

hydrogens are all located ad:acent to an oygen atom and a carbon atom containing three

hydrogen atoms.

Figure 2

CH3 CH2 O CH2 CH3

a b b a

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ecause of rapid rotations about sigma bonds and molecular symmetry, the si methyl hydrogens

(a) and the four methylene hydrogens (b) comprise two individual magnetically e%uivalent

groups of hydrogens. *he methyl hydrogens (a) eperience a different total magnetic field than

the methylene hydrogens (b), because of different local magnetic fields. As a result, the

resonance energy, 0, corresponding to the fre%uency of absorption, , will be different for these

two groups of hydrogen nuclei. *hus, the NMR spectrometer can distinguish between groups of

hydrogen nuclei which eperience different local magnetic fields. ;ifferent fre%uencies are

re%uired for the two different groups of hydrogens, thereby producing two distinct signals.

*he areas of the signals are directly proportional to the number of hydrogens.

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eternal magnetic field and the magnetic field produced by neighboring electrons. A mathe&

matical representation of the value of this resonance energy is the following-

1h),,(2'

h304 eo =+=

/f we vary the fre%uency () of the electromagnetic energy until h5 0, absorption will cause a

transition in spin states, and a signal will be recorded by the NMR spectrometer.

*he position of an NMR signal is recorded relative to the position of the signal of an

internal standard. *his standard is commonly tetramethyl silane (*M

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ydrogen nuclei which absorb at large are said to be deshielded from the eternal

magnetic field. *hese signals appear downfield (towards 1 ppm) from *M< because the

fre%uency at which they absorb differs greatly from the fre%uency of the *M< hydrogens. As a

result, the value of sis large. ;eshielding is caused by ad:acent atoms which are strongly

electronegative (e.g. oygen, nitrogen, halogen) or groups of atoms which possess &electron

clouds (e.g. !5B, !5!, aromatics).

9et7s now loo6 at the NMR spectrum of ben8yl acetate (#igure =).

Figure 4

AAA AA

c

b

a

*M