Chapter 13 - Nuclear Magnetic Resonance Spectroscopy

How can we know how many signals to expect in a compound's 1H-NMR spectrum given its structural formula? * The solution is that equivalent hydrogen produce the same 1H-NMR signal, but nonequivalent hydrogens produce various 1H-NMR signals. * The chemical environment of equivalent hydrogens is the same. * If one of the following two criteria exists, H atoms are comparable (in the same chemical environment): They are bound to the same sp3 hybridized carbon atom, which may freely spin at ambient temperature.
Because of the quick bond rotation, the H atoms linked to the same carbon atom experience the same chemical environment and are hence equal. * For example, ^^on a freely spinning —CH3 group, all three H atoms are equal, and both H.^^ * ^^They are linked by symmetry, specifically a molecule's plane or point of symmetry.^^
In the 2-chloropropane molecule, for example, all six methyl group H atoms are equal. * The methyl groups rotate freely and are connected by a plane of symmetry, as depicted (as shown in the image attached). * The replacement test is a simple approach to assess whether two hydrogen atoms are comparable. * Replace each of the hydrogen atoms in question with a test atom in your thoughts (e.g., chlorine).
The image attached shows that If the substitution produces the same molecule in each case, then the initial hydrogen atoms are equal.

Structure of 2-chloropropane showing the plane of symmetry responsible for making the two methyl groups (and therefore the six methyl group H atoms) equivalent.
There are four sets of comparable hydrogens in 2-methyl butane. * This molecule has nine main hydrogens: three sets of three and six sets of six.
^^To demonstrate the existence of two sets, consider that replacing any hydrogen in the set of three with chlorine yields 1-chloro-3-methylbutane.^^ * Any hydrogen in the set of six can be replaced with chlorine to produce 1-chloro-2-methylbutane. * Furthermore, the molecule has two comparable secondary hydrogens and one tertiary hydrogen.
You should be able to understand right away that counting the amount of signals in a compound's 1H-NMR spectrum can provide vital information about its molecular structure.
Consider the two constitutional isomers of the chemical formula C2H4Cl2. * In its 1H-NMR spectrum, the molecule 1,2-dichloroethane exhibits one set of equivalent hydrogens and one signal.
%%In its 1H-NMR spectrum, its constitutional isomer 1,1-dichloroethane contains two sets of equivalent hydrogens and two signals.%% * As a result, merely counting signals allows you to differentiate between these two molecules.
The chemical shift of a certain kind of hydrogen is mostly determined by the amount of shielding it receives. Shielding is determined by three factors: * (1) the electronegativity of surrounding atoms, * (2) hybridization of adjacent atoms, and * (3) magnetic induction inside an adjacent p bond. Let us take each of these things one at a time.
%%An electronegative substituent's impact fades fast with distance.%%
The impact of an electronegative substituent two atoms distant is only around 10% of that of an electronegative substituent on the nearby atom. * An electronegative substituent three atoms distant has essentially little impact.
Electronegativity and chemical shift are connected in the following way: the presence of an electronegative atom or group decreases electron density on atoms bound to it, resulting in less shielding. * This action deflects neighboring nuclei, causing them to resonate further downfield (i.e., with a larger chemical shift). * Ring current: refers to An applied magnetic field causes the p electrons of an aromatic ring to circulate giving rise to the so-called ring current and an associated magnetic field that opposes the applied field in the middle of the ring but reinforces the applied field on the outside of the ring.
The vicinal atom Hb, whose nuclear spin may be aligned with or against an applied magnetic field in a 1H-NMR spectrometer, influences nuclear spin and hence the chemical shift of the atom labeled Ha in the image attached above.
Because of spin-spin coupling, aligning the Hb nuclear spin with the applied magnetic field results in a slightly different chemical shift of the signal for Ha than aligning the Hb nuclear spin against the applied magnetic field. * A comparable amount of molecules in a population of molecules in a sample will have each spin alignment for Hb. * Any single molecule produces a single Ha signal, but the spectrum of the overall sample contains both.
The image attached shows a coupling that arises when Hb is split by two different nonequivalent H atoms Ha and Hc%%. * This analysis assumes that there is no other coupling in the molecule and that Jab ? Jbc .%%

So far, we've focused on spin-spin coupling with only one additional set of non-equivalent H atoms. * ^^In molecules that do not have quick bond rotation, however, more complicated circumstances frequently emerge in which the nuclei of a set of H atoms are connected to the nuclei of more than one set of nonequivalent H atoms.^^ * %%In these cases, the connection between nearby non-equivalent sets of H atom nuclei combines to produce more complicated signal splitting patterns.%%
In many circumstances, using a tree diagram might assist you to comprehend splitting. * The various couplings are applied successively in a tree diagram.