CH3Br bromomethane low high resolution 1H proton nmr spectrum of analysis interpretation of chemical shifts ppm spin spin line splitting H-1 methyl bromide 1-H nmr explaining spin-spin coupling for line splitting doc brown's advanced organic chemistry revision notes

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Advanced Organic Chemistry: 1H NMR spectrum of bromomethane

The H-1 hydrogen-1 (proton) NMR spectrum of bromomethane (methyl bromide)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing H-1 NMR spectra of bromomethane

CH3Br low and high resolution 1H proton nmr spectrum of bromomethane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for methyl bromide explaining spin-spin coupling for line splitting doc brown's advanced organic chemistry revision notes

TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹H NMR spectroscopy and all other proton resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - bromomethane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of bromomethane represent the peaks of the intensity of the chemical shifts of (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the bromomethane molecule.

Bromomethane, CH₃Br (methyl bromide)

Interpreting the H-1 NMR spectrum of bromomethane

There is only one 1H chemical shift observed, because there is only one protonated carbon atom attached to three equivalent hydrogen atoms - which cannot split each other, so all you see in the 1H NMR spectrum of bromomethane is a single singlet resonance.

Comparing the 1H NMR chemical shift of bromomethane with other halogen compounds

(1) Comparing monosubstituted halogen derivatives of methane

Comparing the effect of a mono-substituent in the methane molecule

Compound	fluoromethane	chloromethane	bromomethane	iodomethane	methane
Formula	CH₃F	CH₃Cl	CH₃Br	CH₃I	CH₄
1H chemical shift/ppm	4.1	3.05	2.68	2.16	0.23
Pauling electronegativity	F 4.0	Cl 3.0	Br 2.8	I 2.5	H 2.1

With the increase in electronegativity of the halogen, the 1H chemical shift for these molecules steadily increases.

(2) Comparing the effects of increasing chlorine substitution of methane

Comparing the effect of polysubstitution of methane with chlorine.

Compound	methane	chloromethane	dichloromethane	trichloromethane	tetrachloromethane
Formula	CH₄	CH₃Cl	CH₂Cl₂	CHCl₃	CCl₄
1H chemical shift/ppm	0.23	3.05	5.30	7.27	not applicable

With increase in chlorine substitution, the 1H NMR chemical shift is increased.

Increasing the number of atoms more electronegative than carbon or hydrogen, increases the 1H NMR chemical shift.

You would expect a similar trend for the equivalent bromine compounds.

Number of directly adjacent protons ¹H causing splitting	Splitting pattern produced from the n+1 rule on spin-spin coupling and the theoretical ratio of line intensities
0 means no splitting							1
1 creates a doublet						1		1
2 creates a triplet					1		2		1
3 creates a quartet				1		3		3		1
4 creates a quintet			1		4		6		4		1
5 creates a sextet		1		5		10		10		5		1
6 creates a septet	1		6		15		20		15		6		1

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