Chem 481L
Fall 2009
Lab Module #6
Energy potential and vibrational levels of NH
3
inversion
Due Date:
Noon, Tuesday, October 13
Submit via Blackboard
Goal:
The purpose of this lab is to apply the skills you developed with H
2
+ to the problem of the
inversion bend in NH
3
.
First, using Spartan, you will build an energy profile of the umbrella
inversion bending vibrational mode of ammonia:
HHHN dihedrals versus energy.
You will then
use Mathcad to fit the data to a polynomial.
Next, you will use the variational theorem with
particleinabox (PIB) wavefunctions as a basis set to calculate the first several wavefunctions
(and energies) for NH
3
.
Given that this lab is analogous to Lab Modules #4 & 5, you can write your own research
questions.
Readings:
See Blackboard.
Lab procedure
A.
Potential energy curve (energy as a function of dihedral) in Spartan (as in LM#10)
Build NH
3
in Spartan.
Using the HF 631g* basis set, calculate the equilibrium geometry, and
measure the equilibrium dihedral for the HHHN atoms.
The definition of this dihedral is the
angle between the plane formed by the first three atoms (HHH) and the plane formed by the last
three atoms (HHN).
Be sure to define the dihedral using this order of atoms; to do otherwise
would compromise the reproducibility of the measurement.
Confirm that the molecule is symmetrical, i.e., that all three dihedrals (H1H2H3N, H2H3H1N,
H3H1H2N) have the same value.
The inversion bend, then, is a symmetrical stretch in which
these dihedrals go from equilibrium value (approx 40
o
) to 0
o
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 Spring '07
 PIELAK
 Atom, Energy, Kinetic Energy, Potential Energy

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