What does the n+1 rule represent in proton NMR? a) The number of neighbouring hydrogens b) Splitting pattern c) Number of signals on the spectrum d) Integration number
The Correct Answer and Explanation is:
Correct answer: a) The number of neighbouring hydrogens
Explanation:
In proton nuclear magnetic resonance (¹H NMR) spectroscopy, the n+1 rule is a basic and widely used principle that helps predict the splitting pattern of a proton signal based on the number of neighboring hydrogen atoms. According to this rule, a hydrogen atom (proton) will split into n+1 peaks, where n is the number of equivalent hydrogen atoms on adjacent (neighboring) carbon atoms.
For example, if a proton has two neighboring hydrogens (n = 2), its signal will appear as a triplet (n + 1 = 3). If it has three neighboring hydrogens, the signal will be a quartet (n + 1 = 4), and so on. This pattern results from spin-spin coupling, which arises from the interaction of magnetic fields between neighboring protons.
This rule applies primarily to first-order spectra, where coupling constants are small compared to the difference in chemical shifts. In more complex molecules or where coupling constants are similar to chemical shift differences, the patterns may deviate from the simple n+1 rule and become more complicated.
It is important to note that the n+1 rule tells you about the number of neighboring hydrogens, not the total number of hydrogens in the molecule or the number of signals in the spectrum. The number of signals reflects the number of chemically distinct types of hydrogen environments. The integration of each peak or set of peaks gives the relative number of protons contributing to each signal.
In summary, the n+1 rule specifically refers to the relationship between a proton’s signal splitting and the number of hydrogens on adjacent atoms, helping chemists deduce molecular structure from the NMR spectrum.
