Chapter 4 Multiple Choice Questions

. 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.

are slower and allow for repetition of the measurement to improve the signal to noise ratio.

are faster and allow for repetition of the measurement to improve the signal to noise ratio.

are slower and allow for repetition of the measurement to improve the resolution in the spectrum.

. 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.

a 180° excitation pulse along the x or y axis is applied to the initial vector of magnetisation.

a 90° excitation pulse along the z axis is applied to the initial vector of magnetisation.

a 30° excitation pulse along the x or y axis is applied to the initial vector of magnetisation.

. 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.

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.

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.

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.

. Among relaxation mechanisms

dipole-dipole relaxation is the most efficient for the nuclei with large gyromagnetic ratios, the relaxation rate is temperature dependent.

dipole-dipole relaxation is most efficient for the nuclei with small gyromagnetic ratio, the relaxation rate is temperature dependent.

dipole-dipole relaxation is most efficient for the nuclei with large gyromagnetic ratio, the relaxation rate is temperature independent.

dipole-dipole relaxation is most efficient for the nuclei with low gyromagnetic ratio, the relaxation rate is temperature independent.

. Too short a relaxation delay between subsequent NMR measurements is of particular significance for ¹³C spins and may result in

a complete disappearance of primary carbons as these relax more quickly than others.

a complete disappearance of tertiary carbons as these relax more slowly than others.

a complete disappearance of quaternary carbons as these relax faster than the others.

a complete disappearance of quaternary carbons as these relax more slowly than the others.

. Heteronuclear decoupling is a powerful procedure used during acquisition of many insensitive spins, e.g., ¹³C as

it increases the intensity of the ¹³C signals for carbons bearing hydrogens (i.e., primary, secondary and tertiary) due to polarization transfer between neighbouring ¹³C and ¹H spins.

it increases the intensity of the ¹³C signals for carbons bearing hydrogens (i.e., primary, secondary and tertiary) due to the NOE effect between neighbouring ¹³C and ¹H spins.

it increases the intensity of the ¹³C signals for quaternary carbons due to the NOE effect between neighbouring ¹³C and ¹H spins.

it increases the intensity of the ¹³C signals of all carbons due to efficient spin-spin relaxation.

. In an inversion recovery experiment to measure T¹

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.

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.

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.

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.

. 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.

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.

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.

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.

. 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.

can be used to improve sensitivity of measurements efficiently if the species concerned relax at different rates.

can be used to resolve overlapping resonances efficiently if the species concerned relax at different rates.

decrease the resolution of overlapping resonances if the species concerned relax at different rates.

. 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.

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.

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.

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.

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