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Up: Chemistry 3730 Fall 1998 Previous: 2 Answer any two

3 Answer any two questions in this section.

  Value of this section: 20 marks
  1. We often use Gaussian variational wavefunctions, i.e. wavefunctions whose radial part is of the form

    displaymath243

    in molecular problems. Suppose that we wanted to approximate the ground-state wavefunction of a hydrogen atom by the above Gaussian function. What would the percent error in the energy be? [10 marks]

    Maple hints: The exponential function tex2html_wrap_inline219 is exp(u) in Maple and tex2html_wrap_inline195 is Pi. Maple will need to know that c>0. Use subs() to substitute your best value of c into your variational energy, then use simplify() to get an initial simplification. You will need to complete the simplification process by hand.

  2. The lowest-energy tex2html_wrap_inline253 electronic transition of 1,3-butadiene

    displaymath255

    occurs at 217nm. Calculate the value of the resonance integral tex2html_wrap_inline257 from this information. Note that tex2html_wrap_inline257 is negative.

    Maple hints: See question 7 of section 1.

  3. The effective potential for bound states of a diatomic molecule can be approximately represented by a Morse function:

    displaymath261

    where R is the internuclear distance and D, tex2html_wrap_inline257 and tex2html_wrap_inline269 are parameters specific to a particular molecule.

    1. Show that D is the asymptote of the potential, i.e. the limit of V as tex2html_wrap_inline275 .

      Maple hint: If you use Maple for this problem, Maple will need to know that tex2html_wrap_inline277 .

    2. Show that tex2html_wrap_inline269 is the equilibrium bond length.
    3. Calculate k as a function of the parameters D and tex2html_wrap_inline257 .

      Maple hints: Use subs() to substitute the equilibrium bond length into the expression and simplify() to clean up the answer.

    4. Give an expression for the dissociation energy.
  4. Recall that in the harmonic oscillator model, tex2html_wrap_inline287 is the displacement of the bond length from its equilibrium. Suppose that for a certain molecule, the electronic ground state is such that tex2html_wrap_inline289 and tex2html_wrap_inline291 while for an excited electronic state, tex2html_wrap_inline293 and tex2html_wrap_inline295 . Are electronic transitions in which the molecule is in the ground vibrational state both before and after the transition likely to occur?

    Maple hint: tex2html_wrap_inline195 is Pi in Maple.

Useful information

tabular88

tex2html_wrap_inline309

Hückel molecular orbitals

Hückel matrix: Element (i,j) is -1 if atoms i and j participate in the tex2html_wrap_inline195 electron network and are bonded to each other, 0 otherwise.

The orbital energies are scaled as follows: tex2html_wrap_inline321 .

Harmonic oscillator

tex2html_wrap_inline323 , tex2html_wrap_inline325 , tex2html_wrap_inline327

Ground-state wavefunction: tex2html_wrap_inline329

tex2html_wrap_inline331 where tex2html_wrap_inline333 and tex2html_wrap_inline335 .

Reduced mass: tex2html_wrap_inline337

Spherical polar coordinates

Volume element: tex2html_wrap_inline339

Angular ranges: tex2html_wrap_inline341 , tex2html_wrap_inline343

tex2html_wrap_inline345

Hydrogen atom

tex2html_wrap_inline347

1s wavefunction: tex2html_wrap_inline349


next up previous
Up: Chemistry 3730 Fall 1998 Previous: 2 Answer any two

Marc Roussel
Wed Dec 16 12:51:39 MST 1998