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Consider the following molecules: a) H2C=CH2 ethene (ethylene) b) HC=CH ethyne (acetylene) c) CH3HC=CH2 propene (propylene) d) CH3C=CH propyne (methyl acetylene) Which have no C-to-C multiple bond infrared stretching frequencies (bands)? 1) (a) and( c) 2) (b) and (d) 3) (a) and (b) 4) (c) and (d) 5) all have multiple bond infrared stretching frequencies ANS: (3) Ethene and ethyne (unlike propene and propyne) have no CC-to-C multiple bond stretching frequencies because they are symmetrical and do not undergo a change in dipole moment when excited. ALTERNATIVE QUESTION: How would you account for the fact that ethane and ethyne exhibit no C-to_C multiple bond infrared stretching frequencies (bands) in the infrared whereas propene and propyne do? 2. Consider the symmetric stretching modes of the following molecules: a) CO2 b) COS c) O3 d) (NC)2 Indicate which one(s) are infrared INACTIVE. only (a) 2) (a) and (c) (a), (c), and (d) only (d) (a) and (d) ANS: (5) (a) and (d) would not result in a change in dipole moment and are therefore inactive. If (c) were linear and not bent it would be inactive… but it is bent… and is active. And because O and S are different, (b) is active. 3. Select the best answer(s) as indicated. As a general rule, bond strengths (increase, decrease) with increasing multiplicity as you move from C-to-C single, to double, then to triple bonds. And as the masses of the halogen (X) atoms in monohalomethanes (CH3-X) increases from monofluoro-, to monochloro-, then to monobromomethane, the stretching frequencies (increase, decrease). 1) increase, decrease 2) decrease, increase 3) decrease, decrease 4) increase, increase 5) Bond strength and atom masses have nothing to do with frequencies. ANS: (1) The increasing order of frequencies goes something like single (1200 cm-1), double (1650 cm-1), and triple (2150 cm-1)… according to force constant and “stiffness.” For C-F (1200 cm-1), C-Cl (800cm-1), and C-Br (600cm-1), as mass increases, the frequency (wave number) decreases. That is, frequency goes inversely with mass (according to Hooke’s law) and is consistent with the observed decrease in bond strength for the homonuclear diatomic halogens in descending order, through the family from F2 to Cl2 to Br2. 4. Compare the C-H and C-D stretching frequencies in chloroform (CHCl3) and deuterochloroform (CDCl3) and briefly explain the differences. 5. Assuming gasoline to be pure iso-octane (C8H18), estimate the CO2 produced in kg per day from the 10-million cars driving around greater Los Angeles on the major freeways (the 405, the 5, the 10, and the 101 each day, assuming each burns an average of 2-gallons per day. 6. In 1967, the United Nations rightly predicted an increase due to anthropogenic industrial activity of thermal waste on the order of 5%. Thermal waste is another way of saying that not all the solar input is eventually reradiated away from the earth-atmosphere system. That is, over 33 years (to the year 2000) the heart content of the earth-atmosphere system increased by 5%. Estimate the rise in the ambient earth-atmosphere temperature that would accompany continuation of such a heat rejection over the next 33 years, assuming the present earth-atmosphere temperature to be 257K. Why is this a problem/not a problem? Argue both sides…. briefly. 7. The methane molecule has a dipole moment of zero, yet it is a greenhouse gas that contributes significantly to global warming. Briefly explain! 8. Would you expect to find degenerate modes (of equal energy) in the spectrum of SO2 as is the case for CO2? Why (or why not)? 9. The effective radiative blackbody temperature of the mean earth-atmosphere system  lightbulb earth. That is, l№max is centered at about 11 microns. Calculate the wavelength in nanometers and the wavenumber in reciprocal centimeters corresponding to l№max. 10. Compare the two fundamental vibrational modes  stretching and bending  for molecules. How are they essentially different? In what essential ways are they the same? ANS: DIFFERENCES: Extension and compression of the “coiled spring” (model) with weights for atoms at each end occurs with characteristic frequency that is a function of the stiffness of the spring (bond) and the masses (weights) of the atoms at the ends. The assigned bond length is the average length resulting from these vibrations. Bending (or deformation) is different. A three-atom sequence is required, bond lengths remain constant but the bond angles change. Again, the assigned bond angle is the average of the opening and closing of the angle. 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