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Return to NMR Spectroscopy in Inorganic Chemistry 2e Student resources
Chapter 4 Multiple Choice Questions
Experimental methods: pulses, the vector model and relaxation
*
not completed
.
Fourier-transform NMR experiments when compared to continuous wave experiments
are slower and do not allow for repetition of the measurement to improve the signal to noise ratio.
correct
incorrect
are slower and allow for repetition of the measurement to improve the signal to noise ratio.
correct
incorrect
are faster and allow for repetition of the measurement to improve the signal to noise ratio.
correct
incorrect
are slower and allow for repetition of the measurement to improve the resolution in the spectrum.
correct
incorrect
*
not completed
.
If signal averaging is not being used the intensity of the NMR signal detected in the x,y plane is maximized when
a 90° excitation pulse along the x or y axis is applied to the initial vector of magnetisation.
correct
incorrect
a 180° excitation pulse along the x or y axis is applied to the initial vector of magnetisation.
correct
incorrect
a 90° excitation pulse along the z axis is applied to the initial vector of magnetisation.
correct
incorrect
a 30° excitation pulse along the x or y axis is applied to the initial vector of magnetisation.
correct
incorrect
*
not completed
.
For small molecules two types of relaxation processes are important, which of the following statements is correct?
Spin-lattice relaxation is characterized by the time constant T2 while spin-spin relaxation is characterized by the time constant T1. T2 relaxation is faster than T1 relaxation; one should wait for 1 T1 to allow sufficient relaxation to occur between acquisitions.
correct
incorrect
Spin-lattice relaxation is characterized by the time constant T1 while spin-spin relaxation is characterized by the time constant T2. T1 and T2 are equal; one should wait for 1 T1 to allow sufficient relaxation to occur between acquisitions.
correct
incorrect
Spin-spin relaxation is characterized by the time constant T2 while spin-lattice relaxation is characterized by the time constant T1. T2 relaxation is slower than T1 relaxation; one should wait for 5-10 T2 to allow sufficient relaxation to occur between acquisitions.
correct
incorrect
Spin-spin relaxation is characterized by the time constant T2 while spin-lattice relaxation is characterized by the time constant T1. T2 relaxation is faster than T1 relaxation; one should wait for 5-10 T1 to allow sufficient relaxation to occur between acquisitions.
correct
incorrect
*
not completed
.
Among relaxation mechanisms
dipole-dipole relaxation is the most efficient for the nuclei with large gyromagnetic ratios, the relaxation rate is temperature dependent.
correct
incorrect
dipole-dipole relaxation is most efficient for the nuclei with small gyromagnetic ratio, the relaxation rate is temperature dependent.
correct
incorrect
dipole-dipole relaxation is most efficient for the nuclei with large gyromagnetic ratio, the relaxation rate is temperature independent.
correct
incorrect
dipole-dipole relaxation is most efficient for the nuclei with low gyromagnetic ratio, the relaxation rate is temperature independent.
correct
incorrect
*
not completed
.
Too short a relaxation delay between subsequent NMR measurements is of particular significance for
13
C spins and may result in
a complete disappearance of primary carbons as these relax more quickly than others.
correct
incorrect
a complete disappearance of tertiary carbons as these relax more slowly than others.
correct
incorrect
a complete disappearance of quaternary carbons as these relax faster than the others.
correct
incorrect
a complete disappearance of quaternary carbons as these relax more slowly than the others.
correct
incorrect
*
not completed
.
Heteronuclear decoupling is a powerful procedure used during acquisition of many insensitive spins, e.g.,
13
C as
it increases the intensity of the
13
C signals for carbons bearing hydrogens (i.e., primary, secondary and tertiary) due to polarization transfer between neighbouring
13
C and
1
H spins.
correct
incorrect
it increases the intensity of the
13
C signals for carbons bearing hydrogens (i.e., primary, secondary and tertiary) due to the NOE effect between neighbouring
13
C and
1
H spins.
correct
incorrect
it increases the intensity of the
13
C signals for quaternary carbons due to the NOE effect between neighbouring
13
C and
1
H spins.
correct
incorrect
it increases the intensity of the
13
C signals of all carbons due to efficient spin-spin relaxation.
correct
incorrect
*
not completed
.
In an inversion recovery experiment to measure T
1
fitting the measured intensity of the signals as a function of the relaxation delay using an exponential function is required. Alternatively, T1 can be calculated from the zero-crossing point of the plot when T1=τ/ln2 where τ is the zero-crossing time.
correct
incorrect
fitting the data for the measured intensity of the signals as a function of the relaxation delay using a linear function is required. T1 can be calculated from the zero-crossing point of the plot when T1=τ/2 where τ is the zero-crossing time.
correct
incorrect
fitting the data for the measured intensity of the signals as a function of the relaxation delay using a logarithmic function is required. T1 time can be calculated from the zero-crossing point of the plot when T1=τ/ln2 where τ is the zero-crossing time.
correct
incorrect
fitting the data for the measured intensity of the signals as a function of the relaxation delay using an exponential function is required. Alternatively, T1 can be calculated from the zero-crossing point of the plot using T1=ln2/τ where τ is the zero-crossing time.
correct
incorrect
*
not completed
.
In an experiment to measure T2 a 90° x pulse was used. T2 can be determined from
the rate of decay of the FID or by using periodic refocussing (a spin echo) of the magnetisation along the x axis followed by fitting the loss of intensity of signals to the echo time.
correct
incorrect
the rate of decay of the FID or by using periodic refocussing (a spin echo) of the magnetisation along the y axis followed by fitting the loss of intensity of signals to the echo time.
correct
incorrect
the rate of decay of the FID or by using periodic refocussing (a spin echo) of the magnetisation along the z axis followed by fitting the loss of intensity of signals to the echo time.
correct
incorrect
the rate of decay of FID or by using periodic refocussing of the magnetisation along the x axis followed by fitting the loss of intensity of signals in a decoupling experiment.
correct
incorrect
*
not completed
.
Spectral editing using differences in spin-lattice relaxation times, T1, for different sites
can be used to resolve overlapping resonances efficiently if the species concerned relax at similar rates.
correct
incorrect
can be used to improve sensitivity of measurements efficiently if the species concerned relax at different rates.
correct
incorrect
can be used to resolve overlapping resonances efficiently if the species concerned relax at different rates.
correct
incorrect
decrease the resolution of overlapping resonances if the species concerned relax at different rates.
correct
incorrect
*
not completed
.
Magnetic resonance imaging relies on the application of
diamagnetic contrast agents to increase the relaxation rate of the water molecules contained within tissue to differentiate it from the surrounding water molecules. Most commonly used contrast agents are based on diamagnetic Ru(II) complexes of macrocyclic ligands.
correct
incorrect
paramagnetic contrast agents to increase the relaxation rate of water molecules surrounding live tissue. Most commonly used contrast agents are based on paramagnetic Gd(III) complexes with macrocyclic ligands.
correct
incorrect
paramagnetic contrast agents to increase the relaxation rate of water in live tissue. Most commonly used contrast agents are based on paramagnetic Fe(III) complexes with macrocyclic ligands.
correct
incorrect
paramagnetic contrast agents to increase the relaxation rate of water molecules contained within the tissue to differentiate it from the surrounding water molecules. Most commonly used contrast agents are based on paramagnetic Gd(III) complexes with macrocyclic ligands.
correct
incorrect
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