Question

how could proton nmr be used to quickly differentiate between a mono versus a di-nitration product

Answer 1

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Chemical shift values, symbolized by δ, are not precise, but typical - they are to be therefore regarded mainly as a reference. The exact value of chemical shift depends on molecular structure and the solvent, temperature, magnetic field in which the spectrum is being recorded and other neighboring functional groups. Hydrogen nuclei are sensitive to the hybridization of the atom to which the hydrogen atom is attached and to electronic effects. Nuclei tend to be deshielded by groups which withdraw electron density. Deshielded nuclei resonate at higher δ values, whereas shielded nuclei resonate at lower δ values.

Some electron withdrawing substituents are –OH, -OR, -NO2. These cause a downfield shift of approximately 2–4 ppm for H atoms on C_α and of less than 1–2 ppm for H atoms on C_β. C_α is an aliphatic C atom directly bonded to the substituent in question, and C_β is an aliphatic C atom bonded to C_α

The labile protons (-OH, -NH₂, -SH) have no characteristic chemical shift. However such resonances can be identified by the disappearance of a peak when reacted with D₂O, as deuterium will replace a protium atom. This method is called a D₂O shake

Another method for identifying protons that are not attached to carbons is theheteronuclear single quantum coherence (HSQC) experiment, which correlates protons and carbons that are one bond away from each other. A hydrogen that is not attached to a carbon can be identified because it does not have acrosspeak in the HSQC spectrum.

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