(Useful constants: h=6.6261034J s; c=2.998108ms1;82=78.957; mass of H atom =1.6731027kg.) 4.1 (a) What is meant by the term polarizability? (b) State the selection rules for Raman scattering. (c) What technological advances have enabled the routine use of Raman spectroscopy? 4.2 With which type of spectroscopy would one observe the pure rotation spectrum of H2 ? If the bond length of H2 is 0.07417nm, what would be the spacing of the lines in the spectrum? 4.3 The spin of the hydrogen nucleus is 21; does this make any difference to your answer to Prob. 4.2? 4.4 Which type of vibrational spectroscopy (IR or Raman) would you use to measure the vibrational frequency of the following bonds: The stretching frequency of 14N15N The CC stretch in ethyne, HCCH The C=O stretch in acetone, CH3.CO2CH3 The ReRe stretch of the inorganic cluster compound, (CO)5ReRe(CO)5 4.5 The ReRe vibration of (CO)5ReRe(CO)5 is observed at 122cm1, while that of the Re2Cl82 ion occurs at 275cm1. Without calculating separate force constants, calculate the ratio of the ReRe bond force constants of the two molecules. Use your result to comment on the bond orders in the two species. (Note: Here, as frequently, it is a sufficiently good approximation to treat the vibration of the heavy Re atoms as being independent of the rest of the molecule.) 4.6 A molecule A2B2 has infra-red absorptions and Raman spectral lines as in the following table: Deduce what you can about the structure of the molecule and assign the observed vibrations to particular molecular modes as far as possible. Hint: Use data from Table 3.4 to help with your answer. 4.7 A molecule AB2 has the following infra-red and Raman spectra: The rotational fine structure of the infra-red bands is complex and does not show simple PR or PQR characteristics. Comment on the molecular structure, and assign the observed lines to specific molecular vibrations as far as possible. 4.8 Both N2O and NO2 exhibit three different fundamental vibrational frequencies, and for the two molecules some modes are observed in both the infra-red and the Raman. The bands in N2O show only simple PR structure (no Q branches) while those in NO2 show complex rotational structure. What can be deduced about the structure of each molecule