Chapter 3 Multiple Choice Questions

. The diamagnetic contribution to shielding is

dominant for light atoms and is generally insensitive to changes in the chemical environment, resulting in a narrow range of chemical shifts for light atoms.

dominant for heavy atoms and is generally insensitive to changes in the chemical environment, resulting in a narrow range of chemical shifts for heavy atoms.

dominant for heavy atoms and is generally sensitive to changes in the chemical environment, resulting in a narrow range of chemical shifts for heavy atoms.

dominant for light atoms and is generally sensitive to changes in the chemical environment, resulting in a broad range of chemical shifts for light atoms.

. The paramagnetic contribution to shielding is

dominant for light atoms and provides important information about the electronic structure and bonding in molecules and results in a significantly broader range of chemical shifts.

dominant for heavy atoms and provides important information about the electronic structure and bonding in molecules and results in a significantly narrower range of chemical shifts.

dominant for heavy atoms and provides important information about the electronic structure and bonding in molecules and typically results in a significantly broader range of chemical shifts.

dominant for light atoms and provides important information about the electronic structure and bonding in molecules and typically results in a significantly narrower range of chemical shifts.

. The symmetry in complexes is an important factor affecting chemical shifts. For fac-Mo(CO)₃(PPh₃)₃, how many signals due to CO will appear in ¹³C{¹H} NMR spectrum?

Two signals due to CO as only two carbonyls from three are equivalent.

Three signals due to CO as all the carbonyls are equivalent by being placed trans to PPh₃.

Three signals due to CO as all the carbonyls are non-equivalent.

One signal due to CO as all the carbonyls are symmetry equivalent by being placed trans to PPh₃.

. The oxidation state of the element is known to affect its chemical shift. For P(III) versus P(V)

chemical shifts of ³¹P for P(III) compounds are only known as P(V) is paramagnetic.

chemical shifts of ³¹P for P(III) compounds are deshielded when compared to P(V).

chemical shifts of ³¹P for P(III) compounds are nearly identical to those of P(V).

chemical shifts of ³¹P for P(III) compounds are shielded when compared to P(V).

. For the silicon(IV) tetrahalides, replacement of the halogen in the order Cl to Br to I, will result in

an increase in deshielding and consequently lower/less positive chemical shifts for ²⁹Si on going from Cl to Br to I.

an increase in shielding and consequently lower/less positive chemical shifts for ²⁹Si on going from Cl to Br to I.

an increase in deshielding and consequently higher/more positive chemical shifts for ²⁹Si on going from Cl to Br to I.

an increase in shielding and consequently higher/more positive chemical shifts for ²⁹Si on going from Cl to Br to I.

. Adding small amounts of Cr(III) compounds to the sample during acquisition of ¹³C{¹H} spectra

may result in significantly slower relaxation of ¹³C spins, particularly affecting quaternary carbon centres due to the contribution of the paramagnetic relaxation pathway.

may result in significantly faster relaxation of ¹³C spins due to the contribution of the quadrupolar relaxation pathway, particularly affecting protonated carbon centres.

may result in significantly faster relaxation of ¹³C spins due to the contribution of the paramagnetic relaxation pathway particularly affecting the detection of quaternary carbon centres.

has no effect on ¹³C{¹H} acquisition.

. Chiral lanthanide shift reagents can be used to

separate the signals of individual enantiomers of target compounds in solution through the formation of diastereomers exhibiting different chemical shifts.

separate the signals of isomers of target compounds in solution through the formation of enantiomers exhibiting different chemical shifts.

separate the signals of rotamers of target compounds in solution through the formation of enantiomers exhibiting different chemical shifts.

separate the signals of individual enantiomers of target compounds in solution through the formation of diastereomers exhibiting similar chemical shifts.

. A reduced coupling constant is introduced to

remove the influence of the natural abundance of spins when comparing the coupling constants for any selected pair of nuclei.

remove the influence of the gyromagnetic ratios when comparing the coupling constants for any selected pair of nuclei.

remove the influence of the coupling constant when estimating the gyromagnetic ratios of a pair of nuclei.

remove the influence of the gyromagnetic ratios when comparing the chemical shifts of pair of nuclei.

. Consider ¹J(PtP) in cis-[PtCl₂(PBu₃)₂] and cis-[PtCl₂(P(OBu)₃)₂]

the coupling constant will be larger in cis-[PtCl₂(P(OBu)₃)₂] since the electronegativity of the substituents on the phosphine ligand has increased.

the coupling constant will be larger in cis-[PtCl₂(PBu₃)₂] since the electronegativity of the substituents on the phosphine ligand has increased.

the coupling constant will be larger in cis-[PtCl₂(PBu₃)₂] since the electronegativity of the substituents on the phosphine ligand has decreased.

the coupling constant will be larger in cis-[PtCl₂(P(OBu)₃)₂] since the electronegativity of the substituents on the phosphine ligand has decreased.

. The size of the coupling constant can be used to judge the mutual placement of ligands at metal centre and the interbond angle

the coupling constant between similar groups will be larger if the coupling nuclei are mutually cis, and smaller for groups that are mutually trans.

coupling between similar groups that are mutually trans is generally too small to be visible.

the coupling constant between similar groups will be larger if the coupling nuclei are mutually trans, and smaller for groups that are mutually cis.

coupling constants are generally similar for both trans and cis coupled ligands and are mostly dependent of the metal centre used.

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