AQA Chemistry A Level: 32 Structure Determination (NMR Spectroscopy)

A set of vocabulary flashcards based on the lecture notes regarding the theory, implementation, and interpretation of Carbon-13 and Proton NMR spectroscopy.

Nuclear magnetic resonance (NMR) spectroscopy

A technique in organic chemistry used to determine the structures of even complex molecules by detecting the energy changes in the nuclei of atoms within a magnetic field and radio wave source.

Spin

A property of nuclei with odd mass numbers, such as $^{1}H$, $^{13}C$, $^{15}N$, $^{19}F$, and $^{31}P$, which gives them a magnetic field similar to that of a bar magnet.

Resonance

The flipping of nuclei between parallel and anti-parallel energy states when supplied with electromagnetic energy in the radio region that is exactly equal to the energy difference $\Delta E$ between those states.

Resonant frequency

The specific frequency of radio waves required to cause a particular atomic nucleus to flip between energy states within a specific magnetic field.

Fourier transformation

A computer technique used by modern NMR instruments to analyze the response from pulses containing a range of radio wave frequencies all at once.

Chemical shift ($\delta$)

A quantity related to the resonant frequency, measured in units of $\text{ppm}$ from a defined zero, which indicates the specific chemical environment of a nucleus.

Tetramethylsilane (TMS)

The standard calibration compound $Si(CH_{3})_{4}$ used in NMR spectroscopy; its hydrogen atoms are all in the same environment and are assigned a chemical shift value of exactly $0\,\text{ppm}$.

Shielding

The effect where electrons surrounding a nucleus reduce the strength of the external magnetic field felt by that nucleus, resulting in lower resonant frequencies.

Deshielding

The process where electronegative atoms draw electrons away from a carbon or hydrogen atom, causing its nucleus to feel a greater magnetic field and resonate at a higher $\delta$ value.

Chemical environment

The specific surrounding of an atom in a molecule; atoms in different environments experience different magnetic shielding and thus yield different chemical shift values.

Integration trace

A line on a proton NMR spectrum where the relative heights of the steps represent the relative ratio of hydrogen atoms in each environment.

Spin-spin coupling

Also known as spin-spin splitting, this phenomenon occurs when the magnetic field of a hydrogen atom is affected by the magnetic fields of hydrogens on neighboring carbon atoms, creating specific peak patterns.

$n + 1$ rule

A guideline stating that $n$ hydrogen atoms on an adjacent carbon atom will split a proton NMR signal into $n + 1$ smaller peaks.

Deuterium ($D$)

An isotope of hydrogen used in NMR solvents like $CDCl_{3}$ or $D_{2}O$ because it does not produce a signal in the same range as $^{1}H$, preventing the solvent from swamping the sample signal.

Magnetic resonance imaging (MRI)

A medical application of NMR that uses the signals from water molecules in human tissue to build three-dimensional images for diagnosing medical conditions.

Parts per million ($\text{ppm}$)

The standard units used to measure the chemical shift $\delta$ in an NMR spectrum relative to the TMS peak.