Magnetic Resonance 3: Dynamics and Chemical Exchange

Vocabulary flashcards covering the study of dynamics via NMR, chemical exchange regimes, historical Nobel milestones, and the fundamental principles of MRI.

Slow processes

Processes slower than seconds that can be studied by measuring the build-up or decay of NMR signals with time, with the fast limit set by the acquisition time (e.g., a few minutes for ${}^{1} ext{H}$ NMR).

Fast processes ($T_1$ timescale)

Processes with motional frequencies around the NMR frequency, occurring on a nanosecond ($ext{ns}$) timescale, which affect $T_1$ relaxation times.

T1 Relaxation Efficiency

Efficiency is highest (and $T_1$ is shortest) when the rate of motion ($k$) is comparable with the NMR frequency.

Activation barrier ($E_a$)

A value that can be obtained for internal motion by measuring the temperature dependence of $T_1$ relaxation.

Chemical exchange

A process where a nucleus swaps between sites with different NMR frequencies, causing line broadening and coalescence as the exchange rate increases relative to the frequency difference.

Slow swapping

A regime where component frequencies of swapping sites are still observed but are affected by "lifetime broadening."

Slow exchange limit (k ext{ << } riangle u)

A regime showing separate peaks for each site that broaden as the exchange rate $k$ increases, where the additional linewidth is riangle ext{exchange} = rac{k}{ ext{\pi}}.

Fast exchange limit (k ext{ >> } riangle u)

A regime showing a single line at the mean shift, which broadens as the exchange rate $k$ decreases.

Coalescence point

The point in an NMR spectrum where separate peaks merge into one, giving an estimate of the exchange rate $k$ calculated as k = rac{ ext{\pi} riangle u}{ ext{\sqrt{2}}}.

Labile protons ($ext{H}$)

Protons, such as those in $ext{OH}$ groups, whose resonances are often broadened or invisible due to exchange in protic or wet solvents.

Anomer exchange requirement

The exchange rate between separate anomers must be slow (slower than $100 ext{s Hz}$) for sharp peaks of separate anomers to be visible in the spectrum.

Mean chemical shift ($ext{\delta}$)

In the fast exchange limit, the observed resonance is the mean of individual chemical shifts weighted by their mole fractions, calculated as $ext{\delta} = p_A ext{\delta}_A + p_B ext{\delta}_B + ext{\dots}$.

Cornelius Gorter

Researcher who made an unsuccessful attempt to measure NMR in 1936 using a solid sample at low temperature, where $T_1$ was too long for magnetization to build up.

Ed Purcell and Felix Bloch

Independently observed NMR in 1945/6 using paraffin wax and water respectively, for which they shared the 1952 Physics Nobel Prize.

Richard Ernst

Winner of the 1991 Chemistry Nobel Prize for the development of high-resolution Fourier Transform (FT) NMR.

Kurt Wüthrich

Winner of the 2002 Chemistry Nobel Prize for the structure solution of biomolecules by NMR.

Paul Lauterbur and (Sir) Peter Mansfield

Winners of the 2003 Medicine Nobel Prize for the development of NMR imaging (MRI).

Martin Karplus

Winner of the 2013 Chemistry Nobel Prize for computational methods, including those used for structure solution by NMR.

NMR Imaging (MRI) frequency

In MRI, a magnetic field gradient is applied so that the resonance frequency depends on position, defined as ext{\nu}_{ ext{NMR}} = rac{ ext{\gamma}B(x)}{2 ext{\pi}}.