I. Terms and short answer (Note: some questions contained errors which are corrected) 1. For each brief description below, describe the relevant spectroscopy as microwave, infrared, Raman, UV-visible, photoelectron, fluorescence a) A photon of l =280 nm is absorbed by a molecule, which then emits a photon of l =630 nm. fluorescence ______b) The absorption band is at 2130 cm-1. ___IR______c) A photon is emitted after intersystem crossing from a singlet state to a triplet state. _phosphorescence_____ d) A molecule absorbs a photon at 65 cm-1, making a transition from J=4 to J=5 __microwave______e) Describe the change in molecular motion for the transition of (d) _higher energy rotational state f) A molecule absorbs a photon at 2175 cm-1, producing a v=0 to v=1 transition in the symmetric stretch normal mode of CO2; then the photon is re-emitted with no change in the dipole moment. __Raman______
2. What are the names of the 3 normal modes of vibration of CO2? _symmetric stretch__, __asymmetric stretch_, ____bending mode___
Which mode(s) is/are IR active? __asymmetric stretch_, ____bending mode__
Sketch the nuclear displacement(s) in this(these) mode(s):
Figs. 11.24b, 11.25 in the text
Which mode(s) is/are Raman active? ____symmetric stretch______
Sketch the nuclear displacement(s) in this(these) mode(s):
Identify the parallel and perpendicular modes. __asymmetric stretch - parallel__
___bending mode - perpendicular_
3. How many normal modes of vibration do each of the following have?
i) H2O______3______ii) CH4______9______iii) benzene______30______4. A photon is absorbed by a vibrational band at 3610 cm-1. What is the photon
i) frequency (Hz)__1.08 x 10^14 iii) wavelength (nm)___2770____
ii) energy (J)__7.18 x10^-20___
This photon excites a (rotational, vibrational, electronic) transition. (circle one)
5. A molecule absorbs a photon at 16,310 cm-1, then emits a photon at 14,190 cm-1. i) what kind of spectroscopy is this? ___Raman______
ii) describe in words how the energy of the incident photon is conserved. Where does that energy go? ___The energy difference appears as vibrational energy______
6. A molecule absorbs a photon at l =180 cm-1. The molecule is ionized and an electron is emitted with velocity of 4.1x105 m-s-1. what kind of spectroscopy is this? _____photoelectron spectroscopy_
what is the binding energy of the photoelectron? __1.028 x10^-18 J___
7. What is the difference between a singlet and a triplet state? They are electronic states which: contain no unpaired electrons (singlet state) contain two electrons with the same spin (triplet state)
8. What is the difference between fluorescence and phosphorescence? fluorescence - emission from an excited electronic singlet state (is relatively rapid, about 1 ns) phosphorescence - emission from an excited electronic triplet state (is relatively slow, about 1 ms) 9. In fluorescence spectroscopy, a photon is absorbed and a different photon is emitted. i) Is the emitted photon (lower, equal, higher) in frequency than the absorbed photon?
ii) Is the absorbed photon in the (X-ray, UV-vis, IR, microwave, radio) region?
10. Which kind of spectroscopy is often used to "fingerprint" molecules? ____IR______11. Which kind of spectroscopy could be used to measure the binding energy of an electron in the
1pu M.O. of O2? _____photoelectron spectroscopy___
12. A photon in what spectral region (X-ray, UV-vis, IR, microwave, etc.) induces a p-to-p* transition of a C=O double bond? _____UV-visible______
13. Calculate the moment of inertia of CO if the bond length is 1.128 Å. effective mass = 1.138 x10^-26 kg
moment of inertia = 1.45 x10^-46 kg-m2
14. H-Cl absorbs a photon of energy 5.94x10-20 J. What spectral region (X-ray, UV-vis, IR, microwave) is this in? _____IR______If the quantum number of the initial state of the transition is v=1, what is the quantum number of the final state of the transition? ___v=2______
15. Calculate the force constant of H-Cl. kappa = 516 N/m (see section 11.6)