Nuclear Magnetic Resonance (NMR) Spectroscopy Lecture Notes

Vocabulary-style flashcards covering the principles of NMR, Carbon-13 and Proton NMR, chemical shifts, splitting patterns, and applications as described in the lecture transcript.

Nuclear magnetic resonance spectroscopy (NMR)

A technique used particularly in organic chemistry that applies a magnetic field and radio waves to a sample to generate an energy change in the nuclei of atoms, allowing structures of complex molecules to be determined.

Spin

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

Parallel and Anti-parallel States

The two orientations nuclei with spin can take in an external magnetic field; parallel is the lower energy state, while anti-parallel is the higher energy state.

Resonance

The process where nuclei flip between parallel and anti-parallel positions when supplied with electromagnetic energy (in the radio region) exactly equal to the energy difference ($\Delta E$) between the two states.

Resonant frequency

The specific frequency of radio waves required to cause flipping of a particular atomic nucleus within a given magnetic field.

Fourier transformation

A computer technique used by modern NMR instruments to analyze the response from pulses of radio waves of a range of frequencies.

Carbon-13 ($^{13}C$) NMR

A form of NMR spectroscopy that specifically targets the 1% of carbon atoms possessing nuclear spin to obtain details about carbon environments in organic compounds.

Shielding

The effect where electrons surrounding a nucleus protect it from the full external magnetic field; nuclei with higher electron density feel a smaller magnetic field and resonate at lower frequencies.

Chemical shift ($\delta$)

A quantity related to the resonant frequency measured in units of parts per million ($ppm$) from a defined zero reference point.

Tetramethylsilane (TMS)

The standard reference compound $Si(CH_3)_4$ used to calibrate NMR spectra; it is assigned a chemical shift value of exactly $0$ by definition.

Deshielded

An environment where electronegative atoms (like oxygen) 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.

Proton ($^1H$) NMR

A form of NMR spectroscopy examining hydrogen nuclei, which typically produces values of chemical shift between $0$ and $10\,ppm$.

Integration trace

A line on a proton NMR spectrum where the relative heights of the steps are proportional to the relative number of each type of hydrogen atom in the molecule.

Spin-spin coupling

Also called spin-spin splitting, this phenomenon occurs when the magnetic field felt by a hydrogen atom is affected by the magnetic field of hydrogen atoms on neighboring carbons, causing peaks to split into patterns.

$n + 1$ rule

A rule stating that $n$ hydrogen atoms on an adjacent carbon atom will split the NMR signal of a particular hydrogen into $n + 1$ smaller peaks.

Multiplet Split Ratios

The height ratios for split peaks following the $n + 1$ rule: a doublet ($1:1$), a triplet ($1:2:1$), and a quartet ($1:3:3:1$).

Deuterium ($D$)

An isotope of hydrogen that does not produce an NMR signal in the same range as $^1H$, used in solvents like $CD C l_3$ and $D_2O$ to avoid swamping the sample's signal.

Magnetic resonance imaging (MRI)

A medical application of NMR that uses protons in water molecules within the body to create three-dimensional images for diagnosis.