Advanced Molecular Spectroscopy and Applications — DU B.Sc (hons) Chemistry Previous Year Question Papers

DSE: Advanced Molecular Spectroscopy and 3. Attempt FOUR questions in all. a. Name any two factors that affect the width of spectral lines. Discuss how the linewidth depends on temperature and molecular mass, and explain. why lighter molecules exhibit broader lines? b. c. d. A 2.5 x 10-3 M solution in a cell of path length 1 cm has an absorbance of o.75 at 560 nm. What is the molar absorptivity coefficient of the substance? Calculate the absorbance and transmittance if the concentration of the solution is 7.5 x Show that if the ground and excited vibrational states of a molecule have identical potential energy curves, then the (0,0) transition is the most intense. Which principle dictates this observation? List two main points of difference between charge transfer transitions and cJ-d tranitions. Explain the origin of the intense purple colour of potassium permanganate in terms of electronic transitions. e. How does conjugation effect 7[ i 7r*transitions in a molecule? Anthracene is colpurless, but tetracene is bright orange. Justify this observation. f. With suitable reasons, explain the increasing order of chemical shift in complexes of Fe, Fe2+ and Fe3+ ions, in M6ssbauer spectroscopy. 9. i. Why does fluorescein fluoresce but a similar molecule phenophthalein does not? ii. n i 7r*transitions are a/zfe-sfrzJ/ed in the presence of H-bonding solvents. 2. a. Derive the following expression for the probability of transition from state / to state in when a molecule interacts with a time-dependent electromagnetic field. b. c. Derive the selection rules for the n-7[*, 7[-7[*, n-o* and a-7c* transitions of (5) formaldehyde using the symmetry principle. Character table of C2v is given as How many signals are observed in the following complexes and explain it (5) briefly with their spectral transition diagram. 3. a. Write the time-dependent Schr6dinger wave equation. Show that the total (5) wavefunction for a system that varies with time js given by the expression -iEt ny(q't)--¢(a)eH b. What do you understand by the term "fluorescence quenching". Derive Stem- (5) Volmer equation and explain the significanc.e of this plot. c. For each of the following electronic transitions, detemine whether the transition is crJ/owec7 or/orbz.ddeJz based on the selection rules: Also, the transition between the 02 states 3Zg and ]Ag violates three selection rules. Name them. 4. a. b. c. Derive the relationship between Einstein coefficients Ami and Bmi, and showthat 877hi/3 The rotational lines crowd closely together with increasing z„ on the R-branch side, while the P-branch lines become widely spaced as in increases, where ur is (" = Bi-Bo). Using relevant equations, explain this. Write the expression for the wavenumbers of the P and R branch lines in. terns of rotational constants B I and Bo, for vibrationa] state v= 1 and v=0, respectively. The observed spectroscopic lines in the spectrmi of CO is given by fspec£ = 2143.28 + 3.813 in -0.0175 m2cm-I