Chemical shift is determined by the structural electronical environment of the nuclei producing that signal. The position of a signal along the x-axis of an NMR spectra is called chemical shift, or δ, of the signal. Let’s explain how that works and what information can be obtained. 6.6a), the x-axis units of NMR spectrum are in ppm (not in Hz as we would expect for frequency), and the two signals stand at different position along the x-axis. Four signals total in 1H NMR spectrum.Īs seen in the 1H NMR spectrum of methyl acetate ( Fig. Therefore, the four aromatic protons can be divided to three sets. 1,3-dimethylbenzene: H b is situated between two methyl groups, the two H c protons are one carbon away from a methyl group, and H d is two carbons away from a methyl group.Both methyl groups are in the same bonding and symmetric to each other, they are equivalent. So the four aromatic protons are divided to two sets. 1,2-dimethylbenzene: both H a protons are adjacent to a methyl substituent, while both H c protons are two carbons away.The two methyl groups are equivalent to each other as well. 1,4-dimethylbenzene: all four aromatic protons in are chemically equivalent because of the symmetry.Acetaldehyde: The three H a protons in the methyl group are chemical equivalent, and they all bonded to an sp 3-hybridized carbon but they are different to the H b proton that is bonded to an sp 2–hybridized carbonyl carbon.The molecules in the next figure contains more sets of chemically equivalent protons. The protons that are symmetric to each other by a certain plane of symmetry are chemical equivalent. Notes: As you probably already realized, chemical equivalence or non-equivalence in NMR is closely related to symmetry. Acetone: both methyl groups (two CH 3) bonded with C=O bond, so they are in the same chemical environment, and as a result all the six protons are chemical equivalent that show only one signal.Benzene: all six protons are chemical equivalent (have the same bonding and in the same chemical environment) to each other and have the same resonance frequency in an 1H NMR experiment, therefore show only one signal.To do that, we need to count how many distinct proton sets are included in the molecule.įor each of the following molecule, the chemically equivalent protons are labelled in the same color to facilitate the understanding. Here we will go through several examples for the first situation, that is to predict the number of signals in 1H NMR spectrum with the structure of a compound given. On the other side, if the 1H NMR spectrum is available for an unknown compound, counting the number of signals in the spectrum tells us the number of different sets of protons in the molecule, and that is the very important information to determine the structure of the compound. For the compound with structure given, we should be able to predict how many signals are there in 1H NMR spectrum. The ability to recognize chemical equivalent and non-equivalent protons in a molecule is very important in understanding NMR spectrum. That is why there are total two signals for compound methyl acetate. The three H b protons in the methyl group bonded with O atom are chemical equivalent as well and show the other signal. All chemical equivalent hydrogens have the same resonance frequency with applied to an external magnetic field, so show only one signal in 1H NMR spectrum. The total six hydrogens can be divided to two groups, the three H a protons in the methyl group that bonded with C=O are all in the same chemical environment, therefore they are chemical equivalent. Why only two signals for a compound containing total six hydrogens? So the compound methyl acetate shows two signals in 1H NMR spectrum.
The peak at the far right is for the standard reference compound tetramethylsilane (TMS, more discussions in chemical shift section 6.6.2), not for the compound.
6.6a), we can see that there are three signals. In the above 1H NMR spectrum of methyl acetate ( Fig. \)Ħ.6.1 Chemical Equivalent and Non-Equivalent Protons
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