Instant Notes: Analytical Chemistry

● Note the presence or absence of saturated structures, most of which give

resonances between 0 and 5 d/ppm (trichloromethane at 7.25d/ppm is a

notable exception).

● Note the presence or absence of unsaturated structuresin the region between

about 5 and 9 d/ppm (alkene protonsbetween 5 and 7 d/ppm and aromatic

protonsbetween 7 and 9 d/ppm). (N.B. Alkyne protonsare an exception,

appearing at about 1.5d/ppm.)

● Note any very low field resonances (9 to 16d/ppm), which are associated

with aldehydic and acidic protons, especially those involved in strong

hydrogen bonding.

● Measure the integrals, if recorded, and calculate the numbers of protons in

each resonance signal.

● Check for spin-spin splitting patterns given by adjacent alkyl groups

according to the n+1rule and Pascal’s triangle. (N.B. The position of the

lower field multiplet of the two is very sensitive to the proximity of electro-

negative elements and groups such as O, CO, COO, OH, Cl, Br, NH 2 , etc.)

● Examine the splitting pattern given by aromatic protons, which couple

around the ring and are often complex due to second order effects.

● 1,4- and 1,2-disubstituted rings give complex but symmetrical looking

patterns of peaks, whereas mono-, 1,3- and tri-substituted rings give more

complex asymmetrical patterns.

● Note any broad single resonances, which are evidence of labileprotons from

alcohols, phenols, acids andaminesthat can undergo slow exchangewith

other labile protons. Comparison of the spectrum with another after shaking

the sample with a few drops of D 2 O will confirm the presence of an

exchangeable proton by the disappearance of its resonance signal and the

appearance of another at 4.7d/ppm due to HOD.

Some examples of proton spectra with their resonances assigned are shown in

Figures 3- 8.

The two aromatic protons, Aand X, in cytosine(Fig. 3) are coupled to give an

AXpattern of two doublets. The A proton is deshielded more than the Xproton

due to its closer proximity to nitrogens and the oxygen atom. The intensities of

the doublets are slightly distorted by second order effects. The OHand NH 2

protons have been exchanged with D 2 O, and their resonances replaced with a

HOD peak at 4.7 d/ppm.

E13 – NMRS: interpretation of proton and carbon-13 spectra 263

543

d 1 H (ppm)

2 1

OH CH 2 CH 3

Relative peak areas:

(integrals)

12 3

Fig. 2. Proton (^1 H) spectrum of ethanol.