Passage
Amino acids and copper both serve vital roles in many biological processes of living organisms. Beyond their separate biological roles, however, chemists have also discovered that amino acids are able to function as chelates with Cu²⁺ ions to form blue complexes with potential medicinal applications. One such complex, copper(II) glycinate, has shown to be effective against some oral diseases. The reaction to form copper(II) glycinate is shown in Figure 1.
Figure 1 Synthesis of two isomers of copper(II) glycinateTwo different isomers can form depending on the reaction conditions. Mixing hot aqueous glycine (70 °C) with a hot solution of copper(II) acetate dissolved in 50% aqueous ethanol, followed by cooling the mixture on ice, yields a precipitate of only the cis isomer. If a suspension of the cis isomer is held at boiling for an hour or longer in a solution of glycine and copper(II) acetate, the trans isomer is obtained.Because of the geometric differences between the two isomers, the stretching vibrations of the central Cu-N and Cu-O bonds in each isomer are different. Symmetric and asymmetric bond stretching give four possible vibrational modes for each isomer, as shown in Figure 2.
Figure 2 Vibrational modes of the coordinate bonds in copper(II) glycinateIf the stretching of the bonds during vibration causes a transient, unequal distribution of electron density between the atoms within the bond, an oscillating dipole will form. Bonds with dipoles that are not cancelled out by another dipole in the molecule will absorb infrared light at particular frequencies (often expressed in wave numbers) and are designated as IR-active. As a result, the vibrational differences between the two isomers can be analyzed by far-infrared (far-IR) spectroscopy, as shown in Figure 3.
Figure 3 Far-IR spectra of the cis and trans isomers of copper(II) glycinateBoth isomers show a skeletal absorbance (unrelated to coordinate bond vibration) near 385 cm⁻¹. Additional absorbance bands are seen that specifically relate to the vibrational modes of the bonds in each isomer.
-How many vibrational modes of the coordinate bonds from each isomer of copper(II) glycinate are observable by far-IR spectroscopic analysis? (Note: Exclude skeletal vibrational modes unrelated to the coordinate bonds.)

Question

Passage
Amino acids and copper both serve vital roles in many biological processes of living organisms. Beyond their separate biological roles, however, chemists have also discovered that amino acids are able to function as chelates with Cu²⁺ ions to form blue complexes with potential medicinal applications. One such complex, copper(II) glycinate, has shown to be effective against some oral diseases. The reaction to form copper(II) glycinate is shown in Figure 1.

Figure 1 Synthesis of two isomers of copper(II) glycinateTwo different isomers can form depending on the reaction conditions. Mixing hot aqueous glycine (70 °C) with a hot solution of copper(II) acetate dissolved in 50% aqueous ethanol, followed by cooling the mixture on ice, yields a precipitate of only the cis isomer. If a suspension of the cis isomer is held at boiling for an hour or longer in a solution of glycine and copper(II) acetate, the trans isomer is obtained.Because of the geometric differences between the two isomers, the stretching vibrations of the central Cu-N and Cu-O bonds in each isomer are different. Symmetric and asymmetric bond stretching give four possible vibrational modes for each isomer, as shown in Figure 2.

Figure 2 Vibrational modes of the coordinate bonds in copper(II) glycinateIf the stretching of the bonds during vibration causes a transient, unequal distribution of electron density between the atoms within the bond, an oscillating dipole will form. Bonds with dipoles that are not cancelled out by another dipole in the molecule will absorb infrared light at particular frequencies (often expressed in wave numbers) and are designated as IR-active. As a result, the vibrational differences between the two isomers can be analyzed by far-infrared (far-IR) spectroscopy, as shown in Figure 3.

Figure 3 Far-IR spectra of the cis and trans isomers of copper(II) glycinateBoth isomers show a skeletal absorbance (unrelated to coordinate bond vibration) near 385 cm⁻¹. Additional absorbance bands are seen that specifically relate to the vibrational modes of the bonds in each isomer.
-How many vibrational modes of the coordinate bonds from each isomer of copper(II) glycinate are observable by far-IR spectroscopic analysis? (Note: Exclude skeletal vibrational modes unrelated to the coordinate bonds.)

Quizplus · Accepted Answer

11ecba2d_1210_5ef9_a3bf_191ef7f038bf_MD0008_00 Atoms within a molecule exhibit continual vibrational motion that can occur in a variety of bond stretching and bending modes. If the vibrational motion of a bond causes a transient, unequal distribution of electron density between the atoms within the bond, an oscillating dipole will form. If a net change in a dipole (not cancelled out by another dipole in the molecule) is produced by a particular vibrational mode, the vibration is IR-active and will absorb IR light at particular frequencies proportional to the frequency of the bond vibration. Because of the geometric differences between the two isomers of copper(II) glycinate, the stretching vibrations of the central Cu-N and Cu-O bonds in each isomer are different. Symmetric and asymmetric bond stretching give four possible vibrational modes for these bonds in each isomer. If all four modes were IR-active in each isomer, the far-IR spectrum of both isomers would show four absorbance peaks attributed to these vibrational modes. Comparing the number of vibration modes for each isomer to their respective far-IR spectra, the cis isomer is seen to display four peaks (in addition to the skeletal vibration peak at 385 cm⁻¹); however, the trans isomer displays only two peaks along with the skeletal vibration peak. This indicates that two of the four vibrational modes of the coordinate bonds in the trans isomer are not IR-active. Therefore, there are four vibrational modes from the coordinate bonds in the cis isomer and two vibrational modes from the coordinate bonds in the trans isomer that are observable by far-IR spectroscopy. (Choices A and C) In the far-IR spectra of the isomers, the trans isomer displays two peaks (not four) and the cis isomer displays four peaks (not two) in addition to the skeletal absorbance peak. (Choice D) In the far-IR spectra of the isomers, the trans isomer displays a total of three peaks and the cis isomer displays a total of five peaks, but the skeletal absorbance peak at 385 cm⁻¹ is unrelated to the coordinate bond stretching modes and must be excluded. Educational objective: Vibrational modes that do not produce a net change in a dipole are not IR-active and will not produce an absorbance peak in an IR spectrum.

Passage Amino Acids and Copper Both Serve Vital Roles in Many