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.
-Suppose that a small sample of the cis isomer precipitate was heated in an oven at 180 °C for 30 minutes, cooled, and then analyzed by far-IR spectroscopy. If the spectrum of the thermally treated sample was similar to the spectrum of the cis isomer but displayed no absorbance band at 459 or 281 cm⁻¹, the sample most likely was:

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.
-Suppose that a small sample of the cis isomer precipitate was heated in an oven at 180 °C for 30 minutes, cooled, and then analyzed by far-IR spectroscopy. If the spectrum of the thermally treated sample was similar to the spectrum of the cis isomer but displayed no absorbance band at 459 or 281 cm⁻¹, the sample most likely was:

Quizplus · Accepted Answer

11ecba2d_120f_9ba8_a3bf_abee8faa37fb_MD0008_00 Infrared (IR) spectroscopy can be used to detect structural changes within molecules during chemical conversions by monitoring sample spectra for the emergence or disappearance of peaks that are unique to a given structural feature of a molecule. For example, if a reactant has a structural feature that produces a distinct IR absorbance peak that the product of a reaction does not have, then the formation of the product is indicated in successive IR spectra taken during the reaction as the disappearance of the peak associated with the reactant. Although the far-IR spectrum of the treated sample discussed in this question is not shown, its key features are described and can be compared to the spectra of each isomer given in the passage. Comparing the far-IR spectra of the isomers in the passage shows that the peaks at 459 and 281 cm⁻¹ are unique to the cis isomer and are absent for the trans isomer. Because the far-IR spectrum of the thermally treated sample is said to be similar to the spectrum of the cis isomer but lacks peaks at 459 and 281 cm⁻¹, it is most likely that the cis sample was converted to the trans isomer upon heating. This conclusion is further supported by the observation that the cis isomer converted to the trans isomer when held at an elevated temperature in solution. Therefore, a similar (faster) conversion is plausible for a sample in solid state held at an even higher temperature. (Choice A) If some of the cis isomer were still present in the sample, absorbance peaks at 459 and 281 cm⁻¹ would still be seen in the far-IR spectrum of the sample. (Choice B) The disappearance of the absorbance peaks at 459 and 281 cm⁻¹ indicates that the sample has changed in some way due to heating the sample. (Choice C) If the thermally treated sample had decomposed, its spectrum would be unlikely to resemble the spectra of the isomers as reported. Educational objective: Infrared spectroscopy can be used to detect structural changes within molecules during chemical conversions by monitoring successive sample spectra for the emergence or disappearance of peaks that are unique to a given structural feature of a molecule.

Passage Amino Acids and Copper Both Serve Vital Roles in Many