Magnetic Resonance and Chemical Shifts Flashcards

A set of vocabulary-style flashcards covering the fundamental concepts of chemical shifts, shielding, and NMR referencing based on lecture notes.

Chemical shift ($\delta$)

A measure of the deshielding of a nucleus relative to a reference compound, which is independent of the external magnetic field ($B_0$) and expressed in parts per million ($\text{ppm}$).

Shielding constant ($\sigma$)

A value that represents the opposing magnetic field induced by circulating electrons, which increases with electron density and atomic number.

$$B_{\text{effective}}$$

The actual magnetic field experienced by a nucleus, calculated as being less than the external field ($B_0$) due to the shielding effect of surrounding electron density.

Tetramethylsilane (TMS)

The classic reference compound ($(CH_3)_4Si$) used for $^{13}C$, $^{1}H$, and $^{29}Si$ NMR; it is inert, provides a strong clean signal, and is defined as $\delta = 0\,\text{ppm}$.

Deshielding

The process where a decrease in electron density increases the effective magnetic field experienced by a nucleus, leading to a higher NMR frequency and a higher $\delta$ value.

Ring current effect

An effect in aromatic molecules where external magnetic fields cause delocalized $\pi$ electrons to circulate, generating an opposing field that causes external protons to experience a higher field ($\delta \approx 7\,\text{ppm}$).

Anisotropic shielding

A shielding effect that is orientation-dependent, typically occurring in functional groups such as $C=O$ and $C=C$ where the induced field is not necessarily aligned with the external field direction.

Empirical

An approach to understanding or identifying chemical shifts based on observation or experience (such as correlation charts) rather than pure logic or theory.

Hydrogen bonding (NMR effect)

A local contribution that reduces the electron density around a proton, thereby increasing its chemical shift ($\delta$).

$^{129}Xe$ NMR

A specific type of NMR used to illustrate sensitivity to chemical environment; although the nucleus is inert, its shift is extremely sensitive to its surrounding context.

Low field / High field

Anachronistic terminology referring to the use of electromagnets; modern nomenclature prefers 'shielded/deshielded' or 'high/low frequency'.