These normally cover the range between δ
= 0 and 10 ppm but the range is
increased to δ
= 15 ppm when acidic protons are present in the molecule.
H-NMR spectrum of a molecule gives three key pieces of information
about the structure of a molecule:
- Chemical shift (δ): the peak positions indicate the chemical (magnetic)
environment of the protons, i.e. different protons in the molecule have
different chemical shifts.
- Integration: the relative size of peak area indicates how many protons
have the δ value shown.
- Coupling: the fine structure on each peak (coupling) indicates the
number of protons on adjacent atoms.
These three features make 1
H-NMR a powerful tool in structure
determination and there are two extreme approaches to it:
- Prediction of the spectrum of the expected compound from theoretical
knowledge and then comparison with the spectrum obtained.
You should recognize 'patterns' (e.g. triplet and quartet for an ethyl
group; a singlet of peak area six for two identical methyl groups),
which were present in the starting materials, but the δH values may
have changed in the 'new' molecule. There are computer programs,
such as g-NMR®, which will simulate the NMR spectrum from a
- Interpretation of the spectrum from correlation tables, but this is very
difficult for the inexperienced.
In practice a combination of the two approaches is used with crossreferencing
and checking the proposed structure with tabulated δH
reference spectra until a satisfactory answer is found.
Always make sure that your predicted structure is
consistent with the spectrum.
Factors affecting chemical shift (δH)
values of protons can be predicted to a general approximation from
knowledge of the effects which produce variations in chemical shift.
- The hybridization of the carbon atom to which the hydrogen atom is
- Sp3 hybridized carbon: peaks occur between δ = 0.9 and 1.5 ppm in
simple hydrocarbon systems. The peaks move. downfield with change
of structure from CH3 to CH2 to CH.
- sp hybridized carbon: peaks occur at about δ = 1.5−3.5 ppm in
- Sp2 hybridized carbon: in alkenes the resonances occur around δ =4−8 ppm and the C-H peaks of aromatic rings are found
between b = 6 and 9 ppm. The large downfield shifts of these
Csp2-H nuclei result from deshielding of the protons by fields set up
by circulation of the n-electrons in the magnetic field. The proton of
the aldehyde group (CHO) is particularly deshielded by this effect
and is found at δ = 9−10 ppm.
- Electron attraction or electron release by substituent atoms attached to the
carbon atom. Electron attracting atoms, such as N, O, Hal attached to the
carbon, attract electron density from the C-H bonds and thus deshield the
proton. This results in movement of the chemical shift to higher δ values
(Table 29.1). Conversely, electron-releasing groups produce additional
shielding of the C-H bonds resulting in upfield shifts of δ values.
- All the protons in benzene are identical and occur at δ = 7.27 ppm. In
substituted aromatic compounds, the overall electron-attracting or releasing
effect of the substituent(s) alters the δ values of the remaining ring
protons making them non-equivalent. The ortho protons are affected most.
- For protons attached to atoms other than carbon: the chemical shifts of
protons attached to oxygen increase with increasing acidity of the O-H
group; thus δ = 1-6 ppm for alcohols, 4-12 ppm for phenols and 10-14
ppm for carboxylic acids. Hydrogens bound to nitrogen (l° and 2° amines)
are found at δ = 3-8 ppm. The approximate chemical shift regions are
shown in Fig. 29.5.
|Table 29. 1 Chemical shifts of methyl protons
Integration of peak areas
|Fig.29.5 Approximate chemical shift positions in the
The area of each peak gives the relative number of protons and is produced
directly on the spectrum (Fig. 29.6). On CW-NMR spectrometers the height
of the peak area integration line must be measured using a ruler, whereas on
FT-NMR machines the area is calculated and displayed as a number. You
must remember that:
Coupling (spin-spin splitting)
- The areas are ratios, not absolute values, and you must find a peak
attributable to a specific group to obtain a reference area, e.g. a single
peak at δ = 1.0 ppm is likely to be a CH3 group and thus the area
displayed or measured is equal to three protons.
- You must ensure that you include integrations from all the fine-structure
(coupling) peaks in the peak area.
- Do not expect the peak area integrations to be exact whole numbers, e.g.
an area of 2.8 is probably three protons (CH3), 5.1 is probably five
protons (e.g. a C6H5 group), but 1.5 is probably a CH3 and all the peak
area integrations must be doubled.
H-NMR signals do not consist of a single line but are usually
associated with several lines (splitting patterns). Protons giving multiline
signals are said to be coupled
. This coupling arises from the magnetic
influence of protons on one atom with those on an adjacent atom(s). Thus
information about the nature of adjacent protons can be determined and fed
into the structural elucidation problem. To a simple first approximation the
following three general points are useful in the interpretation of coupling
- Aliphatic systems: if adjacent carbon atoms have different types of
protons (a and b), then the protons will couple. If a proton is
coupled to n (n = 1, 2, 3, 4, 5, etc.) other protons on an adjacent
carbon atom, the number of lines observed is n + 1, as shown in the
||Protons a are coupled to two protons b: n = 2;
therefore the peak for protons a is split into three
lines (a triplet).
Protons b are coupled to three protons a: n = 3;
therefore the peak for protons b is split into four
lines (a quartet).
Protons c have no adjacent protons and therefore are
not coupled and give a single line (singlet) (Fig. 29.6).
|Fig. 29.6 1H-NMR spectrum of methoxyethane.
||Protons a are coupled to one proton b: n = 1;
therefore the peak for protons a is split into two lines
Protons b are coupled to three protons a and two
protons c: n = 5; therefore the peak for protons b is
split into six lines (sextet).
Protons c are coupled to one proton b: n = 1;
therefore the peak for protons c is split into two lines
Protons a and protons c are not adjacent and do not
couple (Fig. 29.7).
|Fig.29.7 1H-NMR spectrum of 1,2-dibromopropane.
The intensity of each peak in the resulting singlet, doublet, triplet,
quartet, etc., is calculated from Pascal's triangle (Fig. 29.8).
The separation between the coupled lines is called the coupling
constant, J, and, for aliphatic protons CH, CH2 and CH3, it is usually
|Fig. 29.8 Intensities of coupled peaks from
*Note: The (n + 1) rule only applies in systems where the
coupling constant (J) between the protons is the same. Fortunately this
is common in aliphatic systems.
- Alkene hydrogens: hydrogen atoms on double bonds have different
coupling constants depending upon the stereochemistry of the alkene.
Alkene hydrogens in the Z (cis) configuration have J = 5-14Hz,
whereas those in the E (trans) configuration have J = 11-19 Hz (Figs
29.9a and b ).
|Fig.29.9 1H-NMR spectra of: (a) (Z)-3- bromopropanonitrile; (b) (E)-3-bromopropanonitrile.
- Aromatic hydrogens: coupling of hydrogens, which are non-adjacent, is
readily observed in aromatic compounds. Different protons ortho to each
other couple with J = 7-10 Hz, while those in a meta relationship have J = 2- 3 Hz. Para coupling (J = 0-1 Hz) is not usually seen on the
spectrum. The types of aromatic compound you are likely to meet most
- Monosubstituted aromatic compounds, in which three basic patterns
are found in the aromatic region of the spectrum. If the substituent
exerts a weak electronic effect on the ring, the δ values of the ring
protons are similar and the protons appear as a single peak of area
five (Fig. 29. l0a). If the group is strongly electron releasing (OH,
NH2, OCH3, etc.), the protons appear as complex multiplets (ortho and meta coupled), below δ = 7.27 ppm of relative areas two to
three (Fig. 29.10b). If the group is electron attracting (e.g. NO2,
COOH, etc.), then the complex multiplets have δ > 7.27 ppm (Fig.
- para disubstituted aromatic compounds, which are of two types. If
the substituents are the same, then all the ring protons are identical
and a singlet, of relative area four, is seen (Fig. 29.lOd). If the
substituents are different, then the pairs of hydrogens ortho to each
substituent are different and ortho-couple to give what appears to be
pair of doublets, each of relative area two (Fig. 29.l0e).
- Increasing numbers of substituents, which decrease the number of
aromatic hydrogens and the spectrum becomes simpler. Thus the
common 1,2,4-trisubstituted pattern (Fig. 29.10f) is recognized easily
as an ortho-coupled doublet, a meta-coupled doublet and a doublet
of doublets (coupled ortho and meta).
The chemical shifts of aromatic protons can be calculated from
detailed correlation tables.
|Fig.29.10 1H-NMR spectra of (a) methyl benzene; (b) methoxybenzene; (c) nitrobenzene; (d) l,4-dimethylbenzene; (e) 4- methoxynitrobenzene; (f) 4-amino-3-bromonitrobenzene (NH2 protons not shown).