Chapter 6 Multiple Choice Questions

. When studying dynamic processes

the NMR timescale is more suited to study slow exchange, while IR is able to distinguish very fast exchanging sites.

the IR timescale is more suited to study slow exchange, while NMR is able to distinguish very fast exchanging sites.

the NMR timescale is more suited to study fast exchange, while IR can only distinguish very slowly exchanging sites.

the IR and NMR timescales are comparable and can be used to study slow and fast motions.

. For dynamic systems studied by the NMR, the following temperature dependence is observed:

At higher temperature, two sets of signals are observed for a dynamic system, upon cooling these coalesce into one set and sharpen as the slow exchange limit is reached.

At lower temperature, one set of signals is observed for a dynamic system; this splits into two upon warming as the fast exchange limit is reached.

At lower temperature, two sets of signals are observed for the dynamic system. These coalesce into one set and sharpen upon warming as the fast exchange limit is reached.

At high and low temperature, two sets of signals are observed.

. In the slow exchange NMR regime

separate resonances, one for each site, are observed; the lines become narrower as the exchange rate increases.

separate resonances, one for each site, are observed; the lines become broader as the exchange rate increases.

one averaged resonance is observed; the line becomes broader as the exchange rate increases.

one averaged resonance is observed; the line becomes narrower as the exchange rate decreases.

. In the fast exchange NMR regime

a single line is observed at the weighted average resonance position; the line becomes narrower as the exchange rate increases.

two separate resonances, one for each site, are observed; the lines become broader as the exchange rate increases.

a single line is observed at the weighted average resonance position; the line becomes broader as the exchange rate increases.

separate resonances, one for each site, are observed; the lines become narrower as the exchange rate increases.

. In the fast exchange limit involving two sites

the system behaves as two equally populated sites and shows two similar sets of resonances at all times.

the exchanging sites can be distinguished using their distinct chemical shifts.

the system behaves as two unequally populated sites and shows two differing sets of resonances at all times.

the distinction between the exchanging sites is completely lost and the system behaves as if a single site existed.

. The free energy of activation ΔG^‡ for a two-site dynamic system can be calculated

from the coalescence temperature of the dynamic system if both sites are equally populated.

from the coalescence temperature of the dynamic system if the sites are not equally populated.

from the linewidth of the resonances if both sites are equally populated.

from the linewidth of the coalesced signal if both sites are equally populated.

. The phenomenon of differential broadening refers to

unequal broadening of the resonances of two unequally populated sites involved in a dynamic process; the sites with the lower population to broaden first.

equal broadening of the resonances of two equally populated sites involved in a dynamic process.

unequal broadening of the resonances of two sites involved in a dynamic process; the sites with the higher population broadening first.

unequal broadening of resonances of two sites involved in a dynamic process, which site broadens first cannot be predicted.

. A saturation transfer experiment can be used to

follow the movement of the excitation around an exchanging system. It can be used to determine the rates of intermolecular processes only.

follow the movement of the excitation around an exchanging system and can be used to identify the exchanging sites. It can be used to determine the rates of intramolecular processes only.

cannot be used for the identification of exchanging sites.

follow the movement of the excitation around an exchanging system and can be used for the identification of exchanging sites and the determination of the rates of intra- and inter-molecular processes.

. During intra-molecular exchange

the ligand/migrating group dissociates from the rest of the molecule during the dynamic process; the observed coupling constants correspond to the weighted average of the individual couplings and the coupling pattern is as expected.

the ligand/migrating group dissociates from the rest of the molecule during the dynamic process; the observed coupling constants correspond to the individual couplings for each coupling partner.

the ligand/migrating group remains bound to the rest of the molecule at all times; the observed coupling constants correspond to the weighted average of the individual couplings and the coupling pattern is as expected for equal coupling to all sites visited.

the ligand/migrating group remains bound to the rest of the molecule at all times; the observed coupling constants correspond to the weighted average of the individual couplings and a different coupling pattern is expected for each site visited.

. An advantage of EXSY spectroscopy over 1D saturation transfer experiments is

that it generates a 2D map containing diagonal peaks, which can be helpful for identification of some exchange sites.

that it generates a 2D map of all the exchanging sites. Exchange pathways can be identified by analysis of the cross-peaks.

that it generates a 1D map of all exchanging sites. Exchange pathways can be identified by analysis of the spectrum.

that it generates a 2D map of some exchange sites that can be identified by analysis of the coupling constants for each cross-peak.

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