Ch6 Molecular Ultraviolet and Visible Complete Test Bank

Problem 6.3: We have stated that the typical range of a UV-vis spectrophotometer is 195 nm to 900 nm. What is this range in Hertz?

Problem 6.4: Using Figure 6.3 as a guide, construct an M.O. diagram for ethylene (ethene). How is the M.O. diagram similar to the M.O. diagram of formaldehyde? How is the M.O. diagram different from the M.O. diagram of formaldehyde? Using your M.O. diagram, predict the UV-vis absorption properties of ethylene.

Problem 6.5: Benzophenone has a structure that is very similar to di-2-pyridyl ketone (dpk). Using our case study of dpk as a guide, discuss in general terms, how the UV-vis spectrum of benzophenone will be similar to that dpk and how will it be different. Then sketch your prediction of the UV-vis spectra of benzophenone.

Problem 6.6: There are three peaks in the UV-vis spectrum of DPK (Figure 6.7) with _max values of 238 nm, 270 nm and 354 nm. Use Equation 6.2 and

1. calculate the energy gap (in Joules) associated with each of these transitions
2. calculate the frequency (in Hz) of the photon absorbed for each of these transitions

Problem 6.7: Refer back to Figure 6.4. Determine the total spin multiplicity of the ground and excited states for panels A & B. State if the transition is spin allowed or spin forbidden.

Problem 6.8: Discuss the two transitions seen in Figure 6.4 in terms of the LaPorte selection rule. Make sure you discuss how the LaPorte selection rule would affect peak intensity.

Problem 6.9: Draw an MO diagram of pyridine and predict the relative energies and relative intensities of the two lowest energy transitions.

Problem 6.10: Using Figure 6.12, determine ₂₀₅ for [Pt(dione)Cl₄] (see Example 6.2 for additional data).

Problem 6.11: What would the concentration of the solution in Figure 6.12 be if you obtained an absorbance of 0.05 at _max= 290.5 nm?

Problem 6.12: Explain why it is considered a “best practice” to keep the total absorbance below 1 when conducting quantitative work.

Problem 6.13: Explain why it is important that the ionic strength of your blank be similar to the ionic strength of your samples, even if the salt does not absorb light at the frequency of interest.

Problem 6.14: What are the two most common sources used in a UV-vis spectrophotometer?

Problem 6.15: Why is it a common design feature to see two different sources used in a UV-vis spectrophotometer?

Problem 6.16: Why is iodine sometimes added to tungsten lamps?

Problem 6.17 For each of the types of noise described so far discuss how the presence of noise affects the output data, give a physical explanation for what caused the noise, include equations (if applicable). Also discuss methods or ways to minimize the noise.

Exercise 6.1: What is the wavelength range of a typical UV-vis spectrophotometer? Briefly discuss the physical constraints that limit a typical UV-vis spectrometer to this wavelength range.

Exercise 6.2: To what does the term n-electrons refer? What is the spectroscopic significance of n-electrons?

Exercise 6.3: Why are the peaks in molecular UV-vis spectroscopy broad relative to atomic UV-vis spectroscopy?

Exercise 6.4: Discuss the significance of _max when reporting molecular UV-vis absorption data. Why do spectroscopist report _max for a transition and not simply ?

Exercise 6.5: What is the percent transmittance if the absorbance is

Exercise 6.6: What is the absorbance if the percent transmittance is

Exercise 6.7: If the molar absorptivity for a compound is 3500 M^-1cm^-1, calculate the concentration of each solution in Exercise 6.5.

Exercise 6.8: If the dark current from a spectrophotometer’s PMT was 0.03 mA and a blank solution produced a current at the PMT of 4.45 mA, what is the absorbance of a sample if the reading at the PMT was 3.75 mA?

Exercise 6.9: Research the photoelectric effect and describe its importance in modern spectrophotometer design. In your discussion explain why PMT detectors are not used for infrared spectroscopy.

Exercise 6.10: The 3s 🡪3p transition in sodiuim is observe at about 5893 Å. What is this wavelength in nanometers? What is the frequency of the absorbed photon? What is the energy of the absorbed photon?

Exercise 6.11: The _max for the complex [Fe(phen)₃]²⁺ is 510 nm. If the molar absorptivity is 1.89 X 10⁴ M^-1cm^-1, what is the concentration of a [Fe(phen)₃]²⁺ solution if it produced an absorbance of 0.03 in a 1 cm cell?

Exercise 6.12: Compare and contrast the optical components of a single-beam and a dual-beam spectrophotometer.

Exercise 6.14: Compare and contrast the optical components of a single-beam and a diode array spectrophotometer.

Exercise 6.14: What were the market forces that have allowed array spectrophotometers to dominate the UV-vis spectrometer market?

Exercise 6.15: Rank each of the following molecules from highest to lowest _max. for the  🡪* transition.

Exercise 6.17: Why is the regulation of the radiant source less critical in a dual-beam spectrophotometer than it is in a single-beam spectrophotometer?

Exercise 6.18: We discussed two basic classes of monochromators: prism and grating. Discuss the advantages and disadvantages of choosing one type of monochromator over the other.

Exercise 6.19: It has been stated that it is considered a best practice to take absorption readings at _max. What were the two reasons given to justify this best practice?

Exercise 6.21: Diamond has a refractive index of 2.42. The frequency of light does not change when it passes between from a material of one refractive index (η) to one with a different η, but both wavelength and velocity do change. Given that (v is the velocity of the light in the medium of higher refractive index) what is the velocity and wavelength of a photon with a fundamental wavelength (λ₀) of 622 nm as it passes through the diamond?

Exercise 6.22: The 340 nm peak in Benzophenone is seen to shift to longer wavelengths when placed in a more polar solvent. What predictions can you make about the type of transition responsible for this peak?

Exercise 6.23: What are the physical characteristics of a grating monochromator responsible for determining the spectral resolution.

Exercise 6.24: What slit width is required to obtain a spectral resolution of 4 nm if the monochromator contains an 1,100 blazes per millimeter diffraction grating with a 50 mm focal length? Assume the angle of incidence on the grating is 0 degrees and the wavelength is 400 nm (m = 1).

Exercise 6.25: Given a diffraction grating with 1,500 blazes per millimeter, what is the linear reciprocal dispersion of the monochromator if the path length between the grating and the exit slit is 0.3 meters and the slit with is 0.50 mm? What is the effective bandwidth of the monochromator?

Exercise 6.26: What wavelength of light is diffracted at 35 degrees and at 40 degrees if light from a tungsten–halogen lamp is incident at 0 degrees on a 1,500 blaze per millimeter diffraction grating?

Exercise 6.27: The for DPK in acetonitrile. What would be the predicted absorption if a 1-ml aliquot of 2 M DPK were diluted to 1 liter?

Exercise 6.28: What is the physical basis of the Nyquist theory ( Johnson noise)

Exercise 6.29: Imagine you have been hired as an outside consultant to a local paint and dye company wishing to purchase a UV-vis spectrophotometer. The spectrophotometer will be used in the quality control laboratory to test colors between different batches of dyes from the supplier. Outline the pros and cons of the various sources and detectors one might choose from, and defend your recommendation.

EXERCISE 6.30: The UV-vis spectrum shown here is for 1,10-phenanthroline-5,6-dione.

Use the data in the table to assign the peaks as nox 🡪 π*, π 🡪 π* (carbonyl), nN 🡪 π*, or π 🡪 π*(aromatic).