In-class Questions: Ultraviolet/Visible Absorption Spectroscopy

Given the background we have already developed on spectroscopy, I give them this set of in-class questions without providing any further background on UV/VIS absorption spectroscopy.

General aspects of UV/VIS absorption spectra

1. Compare and contrast the absorption of ultraviolet (UV) and visible (VIS) radiation by an atomic substance (something like helium) with that of a molecular substance (something like ethylene).
2. Do you expect different absorption peaks or bands from an atomic or molecular substance to have different intensities? If so, what does this say about the transitions?

The background for answering these two questions has usually come up earlier in the course when the students were asked to draw a spectrum because some students think of atomic spectra and others think of molecular spectra. Also, the discussion of molar absorptivity introduced the concept of the probability of transitions explaining the difference in intensities.

Why do atomic spectra consist of discrete lines whereas molecular spectra are broadened and continuous in nature? Are there processes that can occur in molecules that cannot occur in atoms?

With these two prompts, the groups can usually figure out that atoms only undergo electronic excitation whereas molecules can also be vibrationally and rotationally excited. They also know that vibrations are excited in the IR and, from our discussion very early in the class, that rotations are excited in the microwave region. In addition they know that the IR and microwave region of the spectrum is lower in energy than the UV/Vis region of the spectrum. This allows us to draw a diagram similar to Figure 2.4 from the accompanying text showing how the molecule will have many more closely spaced transitions than an atom that lead to the continuous nature of the absorbance spectrum.

3. Compare a molecular absorption spectrum of a dilute species dissolved in a solvent at room temperature versus the same sample at 10K.

What would happen to the sample at a temperature of 10K?

The students quickly realize that it will freeze to a solid. It will also help to point out that the freezing of many solvents will produce a transparent glassy material that UV/VIS radiation readily passes through so that it is possible to obtain an absorption spectrum on a frozen sample. I briefly describe the use of molecular beam apparatus as another way to obtain species at ultracold temperatures.

What takes place in a liquid sample that does not occur in a solid sample?

The groups come up with the realization that the molecules in the liquid sample are moving around and colliding with each other.

What happens when molecules collide with each other?

It may help to remind them that molecules consist of nuclei surrounded by electron “clouds”. They can also reason out that the electron clouds become distorted because of the collisions.

What effect would the distortion of the electron clouds have on the energy of the electrons? Do different molecules collide with different degrees of force?

With these prompts, they come to realize that the collisions of molecules leads to a broadening of the spectrum because the energies of a specific transition is slightly different in different molecules. That allows them to understand the rationale for the spectra observed in Figure 2.5.

4. Are there any other general processes that contribute to broadening in an absorption spectrum?

They are usually stumped by this question.

Have you ever heard of the Doppler effect and, if so, what do you know about it?

Some of them have heard of the Doppler Effect in physics or astronomy courses, and we discuss its significance to broadening in spectroscopic measurements.

5. Compare the UV absorption spectrum of 1-butene to 1,3-butadiene.

Consider the molecular orbitals involved in 1-butene and to draw an energy level diagram that shows the relative energies of the orbitals.

From organic chemistry, they know that \(\sigma\)- and \(\pi\)-orbitals are involved, and they usually know the relative order in which to put them. This enables a brief discussion of the HOMO and LUMO and the lowest energy transition (\(\pi\)-\(\pi\)*) for the molecule, which is the important one to consider in answering the question. They also know from organic chemistry that 1,3-butadiene is a conjugated compound. At this point, I direct them to the next question.

6. Rank these from high to low energy.

They have some experience with this from organic chemistry and are able to draw pictures like those in Figures 2.8, 2.12 and 2.13. They also can usually rank them from low to high energy, and determine which ones are filled with electrons and which ones are empty.

Compare the energy of the HOMO to LUMO.

• They can usually see that it will be lower in 1,3-butadiene than in 1-butene (Figures 2.9 versus 2.14). I then show them the spectra in Figures 2.10 and 2.15.

What would happen to the HOMO to LUMO transition in the absorbance spectra for the series of fused ring polycyclic aromatic compounds benzene, naphthalene, anthracene and pentacene?

They reason that the energy of the HOMO to LUMO transition ought to move toward the red as more rings are added to the compound. I then show them the spectra in Figure 2.16.

7. Compare the UV absorption spectrum of benzene and pyridine.

What distinguishes pyridine from benzene?

They quickly realize that the nitrogen atom in pyridine has a lone pair of electrons, and I introduce the idea that from a spectroscopic and energy level standpoint, we think of these as non-bonding electrons.

Draw representative energy level diagrams for benzene and pyridine, show which orbitals are filled, and to compare the energy of the HOMO to LUMO transition for the two compounds. Most are able to reason that the non-bonding electrons ought to go between the \(\pi\) and \(\pi\)* orbitals and that the lowest energy transition of pyridine ought to be red-shifted relative to that of benzene. I then show them the two spectra in Figures 2.18 and 2.20 to confirm their conclusion.

8. The peaks in the 320-380 nm portion of the UV absorption spectrum of pyridine shifts noticeably toward the blue (high energy) portion of the spectrum on changing the solvent from hexane (C₆H₁₄) to methanol (CH₃OH). Account for this change.

What would occur with pyridine that is different in the two solvents?

Groups have the background to know that methanol will form a hydrogen bond with the nitrogen atom of the pyridine.

What would this do to the energy of the non-bonding electrons?

Usually they are able to determine that it would stabilize them and lower their energy.

Would potential hydrogen bonding by methanol have any effect on electrons in the \(\pi\) or \(\pi\)* orbitals of the pyridine (pointing out that when a molecule of pyridine absorbs radiation, it promotes an electron to the \(\pi\)* orbital)?

They often seem to think that association of the slightly positive hydrogen atom of methanol would have more of a stabilizing effect on the \(\pi\)-electrons rather than the \(\pi\)* electrons.

Draw the locations of these orbitals in the molecule and which is more exterior and exposed to the solvent?

With this prompt, they realize that an electron in the \(\pi\)*-orbital extends out further and shows more stabilization than the \(\pi\)-electrons. They also realize that the stabilization of the non-bonding electrons will be much greater than the other orbitals, thereby allowing them to explain the blue shift.

9. The peaks in the UV spectrum of benzene shift slightly toward the red (low energy) portion of the spectrum on changing the solvent from hexane (C₆H₁₄) to methanol (CH₃OH). Account for this change.

Based on the information developed in answering the previous question, it is relatively straightforward for the students to explain the observation in this situation.

UV/VIS spectroscopy as a qualitative and quantitative tool

I first remind them what we mean by a qualitative and quantitative tool before giving them the following question.

1. Is UV/VIS spectroscopy useful as a qualitative tool?

Consider the general features of a UV/VIS absorption spectrum for organic compounds. Is this sufficient to identify an unknown compound?

We also explore whether a match in the UV/VIS spectrum might be used to confirm the identity of an unknown if the person has an idea what its identity might be. We also explore that many transition metal species have more distinct absorption spectra that can be used to confirm the identity of a species.

2. Is UV/VIS spectroscopy useful as a quantitative tool?

Consider things like the power of UV/VIS sources, the sensitivity of detectors, the magnitude of extinction coefficients, and given a certain extinction coefficient (e.g., 5,000) with a certain minimal reading for absorbance (e.g., 0.01), what would be the minimal concentration that could be measured?

With that, they see that UV/VIS spectroscopy is a useful quantitative tool.

3. If you were using UV spectroscopy for quantitative analysis, what criteria would you use in selecting a wavelength for the analysis?

This was covered earlier but serves as a review that \(\lambda\)_max is preferable provided there are no interferences in the sample.

4. What variables influence the recording of UV/VIS absorption spectra and need to be accounted for when performing qualitative and quantitative analyses?

Some of these have been covered as well during the discussion of solvent effects. The students can usually think of things like solvent effects and pH as possible variables may alter the value of \(\lambda\)_max.

Are there inorganic species that might be in the sample? If so, what effect would they have?

These questions are sufficient to get them to think about the potential impact of metal ions in a sample.

5. Provided the UV/VIS absorption spectra of HA and A^– differ from each other, describe a method that you could use to measure the pKa of the acid.

It will be necessary to set this up by writing the appropriate reaction and discussing the time scale of the reaction relative to the time scale of absorption of a photon. With the understanding that the reaction is slow relative to the process of absorption, students can appreciate that a species is either in the HA or A^– form during absorption. It also helps to examine the possible resonance forms for a carboxylate ion (RCO₂^-) versus its corresponding carboxylic acid (RCO₂H) to have the students appreciate that the absorption spectra of the two species are likely to be different. At this point, there are usually some students in the class who remember the Henderson-Hasselbalch equation. If not, I ask them:

Do you remember an equation in acid-base chemistry that had an HA and A^- species in it?

With the equation in hand, they sometimes can figure out that they would need to examine a solution that was strongly acidic and another that was strongly basic to determine the extinction coefficients for HA and A^- respectively. If not, I ask them:

How could you prepare a solution with essentially all HA and then one with essentially all A^-? By this point they usually recognize that buffered solutions at intermediate pH will give some of both and with the known pH and measured concentrations, they can use the Henderson-Hasselbalch equation to calculate the pKa.

I save a discussion of the relative value of \(\lambda\)max for the HA and A^– form for our unit on fluorescence, but it could be done at this time as well.