Erica Vander Mause and Rob Hodgson
March 11, 2014
This experiment uses three different ab initio basis sets, 6-21G, 6-31G, and DZV, to predict the molecular properties is Br2, NHF2
and nitrobenzene. Comparing the calculated characteristics to
experimental data, it was found that 6-31G(a smaller basis set than DZV)
more accurately predicted the properties of BR2. As
expected the largest basis set, DZV, most accurately predicted the
properties of the other two molecules nitrobenzene and NHF2.
The goal of this experiment is to successfully
describe the overall electronic structure of the molecule at study.
Three different molecules will be examined: Br2, NHF2, and Nitrobenzene.
Accurately describing the most probable locations of electrons and
their energies available, will predict useful properties of the molecule
such as dipole moment, polarizability, vibrational
frequencies,absorption of visible light. Software was used to aid in the
calculation intensive process of describing electrons. Avogadro
was used to build the molecules, wxMacMolplt to optimize geometry,
GamessQ ran all of the calculations, and then Jmol analyzed the
characteristics of interest in the calculations made in GamessQ.
The calculations set up in wxMacMolplt consisted of
different levels of theory. The levels of theory (basis sets) that we
will be using for our calculations are 6-21G (3-21G for Br2 only),
6-31G, and DZV (which is in order of increasing size of calculations
taken). By using these different levels of theory we will look into the
intuitive nature of electronic structure within a molecule.
Each theory of calculations used a different number
of Gaussian functions to describe the electron wave function. The
first number in each theory level signifies the number of Gaussian’s
added together to describe inner electrons. The number following
the dash describes the number of Gaussian’s used to describe the valance
electrons in each atom of the molecule. More Gaussian’s were used
to more accurately model the behavior of the valance electrons because
those are the most important for bonding. For example, in the
6-21G 6 Gaussian’s were added together to describe the inner
electrons. The DZV or double zeta valance basis set is the biggest
basis set used in this procedure.
Each of these basis sets was evaluated using
Harte-Frock Self Consistent Field calculations. A single electron
in the molecule was chosen. All of the other electrons in the
molecule were treated as single electronic field acting on the selected
electron. Based off of the coulomb interactions with the all
electron encompassing electronic field and the nuclei, the lowest energy
position for electron and nuclei were found. Then another
electron was chosen and the process was repeated until the energy of the
molecule reached a minimum.
Use the following links to navigate to the calculations of each molecule.
Computational results are useful when there is
little or no reliable experimental data on a molecule.
Computations may be useful when making predicitons about reactions that
use expensive materials. Unsuccessful reactions and reagent waste
could be avoided if computional models were used to find reactions that
are likely to take place. Computational results could also be
useful if the molecule is dangerous to handle or synthesize.
Computational results also provide a visual of the molecule that cannot
be seen experimentally. This can lead to a greater understanding
of what is going on at the molecular level. As discussed in this
report, the accuracy of the calculated results vary, and a bigger basis
set does not always improve accuracy. Experimental results are
necessary to back up the computational results.