> See also: > - [[Spectroscopy]] > - [[Nuclear Spin]] # Nuclear Magnetic Resonance (NMR) Spectroscopy > [!summary] Key Points > - Every signal peak in an NMR spectrum is a **chemically distinct** group of protons > - Signals that occur further **downfield** have been **deshielded** by nearby electronegative atoms Nuclear magnetic resonance (NMR) spectroscopy provides information about how the individual carbon and hydrogen atoms in a molecule are connected to each other Without being subjected to a strong magnetic field, a sample does not absorb radio frequency (RF) radiation and therefore cannot produce an NMR spectrum. The two ### Chemical Equivalence > See also: [[Isomers]] **Homotopic** and **enantiotopic** protons are considered identical and will emit a single signal while **diastereoptopic** protons are not chemically equivalent as they cannot be interchanged symmetrically (**heterotopic**). ### Solvent Effects The molecules being analyzed through NMR are almost always combined with a solvent. Most traditional solvents contain hydrogen atoms, which would interfere with the reading. To solve this issue, chemists created **deuterated solvents** where the hydrogen atoms are replaced with deuterium (hydrogen+1 isotope) atoms ## Reading Signals The number of signals present within a There are three main factors to consider when reading 1H NMR Spectra: 1. Chemical Shift 2. Induction 3. Multiplicity (Shows ) ### Chemical Shift **Chemical Environment** - Hydrogen atoms bonded to heteroatoms (non-carbon elements) typically do not have splitting patterns and are always equivalent to each other if attached to the same atom. > [!tldr]- Common Chemical Shifts > ![[Pasted image 20230131194703.png|500]] > --- > ![[Pasted image 20230131194831.png|500]] **Inductive Effects** - More electronegative atoms can "pull" away electron density from protons (and other atoms) within the molecule - When the electron density is pulled away from an atom due to inductive effects, it has been *deshielded*. **Magnetic Anisotropy** - Aromatic compounds such as benzene will produce a local magnetic field (donut-like internal flow) that, unlike other molecules, will be flowing in the same direction (in-line) as the external magnetic field. - This increases the chemical shifts - This effect actually affects all $\pi$ bonds (double bonds) and greatly increases the chemical shift of vinyllic protons/atoms despite lacking the "donut-shape" found in aromatic compounds - Magnetic anisotropy can also produce inductive effects that deshield the electron density of connected atoms - [ ] TODO: get pictures from power point ### Integration The area below the signals of an NMR spectrum represents the proportion of protons [[Integration]] can be used to quantify the area below the signal and determine the proportion of protons that signal contributes to the overall molecule. - The value will plateau and produce an **integration value** for a given peak ### Multiplicity Protons are charged particles and as a result undergo [[Nuclear Spin]] which produced a local magnetic field. When protons are close enough to each other within a a molecule these magnetic fields can interact with each other ![[Pasted image 20230125073918.png]] ![[Pasted image 20230125073937.png]] If there is an indeterminable splitting pattern, it is referred to as a **multiplet**. Coupling does not occur between protons bonded to heteroatoms (non-carbons), especially oxygen and nitrogen. This is due to the hydrogen bonding **The N + 1 Rule** - The When there is a high amount of symmetry within the molecule that causes chemically equivalent groups at separate locations, you do not add together the protons connected to each group. - The amount of neighboring protons should be identical if the groups are truly chemically equivalent #### Complex Splitting Patterns When using the N + 1 rule, you only count the protons of chemically equivalent groups. When there are different groups in coupling range you must use splitting trees: ![[Pasted image 20230125090537.png|150]] ![[Pasted image 20230125090550.png|300]] - NOTE: ### Coupling Constants and Signal Resolution ## 13C NMR Spectroscopy While the more common carbon-12 isotope has a nuclear spin of 0, the carbon-13 isotope has an odd number of protons leading to a magnetic moment within the nucleus. - The radio frequency (RF) of 13C NMR is significantly lower than 1H NMR to ensure that no hydrogen signals appear in the readings - The relative abundance of carbon-13 is extremely low (~1.1%), meaning that around 98.9% of carbon atoms within the solution are inactive. - Leads to much longer run time for the NMR machine, with many samples being collected and averaged together to reduce the signal noise caused by this. - While 1H NMR can prevent solvents from producing signals by replacing hydrogen with the deuterium isotope, there is no similar solution for 13C NMR. As a result, a solvent peak will always be present in 13C NMR (along with the TMS standard peak)