Molecular
Orbital Calculations for Fluorine Gas, Water, and
Phenylacetylene
Baldomero, Miguel; Gilly, Shannon
Abstract
This study determines bond lengths, bond angles, dipole
moments vibrational frequencies, and molecular orbitals using
geometrically optimized molecule models of Flourine gas,
Water, and Phenylacetylene. Different basis set sizes were
used in optimizing geometries and were compared and
contrasted. Results show that using the larger basis sets does
not necessarily mean better results. Large basis sets results
gave bond length, bond angles and vibrational frequencies that
are close to the literature values. However, a smaller basis
set is better in determining dipole moments.
Introduction
Determining the structure, geometry, and molecular orbitals of
polyatomic molecules are vital entities in understand the
behavior, reactivity, dipole moment, vibrational frequencies,
polarizability, and other properties of molecules. Quantum
mechanical principles can be used to describe molecular
orbitals and structure of a molecule. Specifically, behavior
of molecular orbitals can be condensed into wavefunctions.
However, due to complexity of polyatomic molecules,
approximations must be made to make an appropriate
wavefunction. These approximations can be done by applying the
variation principle which allows approximations of
wavefunctions by using linear combinations of a trial
wavefunction as shown in eq 1.
(1)
The coefficients, c, are
then solved to produce the lowest energy for the electron.
Expectation value is then calculated and the wavefunction
is normalized. Each of the basis set wavefunctions used to
form the trial wavefunction is normalized. The larger the
basis set, the more accurate the energy prediction would
then be. In a geometry optimization calculation, one
searches for the best geometry to minimize the energy of
the system
In this study, geometry optimization are applied to
fluorine gas, water, and phenyacetelyne molecule to
determine bond-length, bond angles, dipole moment and many
other properties. The main basis set used were 6-21G,
6-31G, and DZV, where DZV have the largest basis set,
smallest for 6-21G.
Fluorine
Gas
Water
Phenyacetylene
Conclusion
Results show that using quantum mechanics to model
structure and behavior of molecules is an effective way to
determine physical quantities such as bond length, bond
angles, dipole moments, vibrational frequencies,
transition frequencies. However, certain basis sets are
more effective in calculating a specific property or
behavior. DZV, a large basis set, was effective in
calculating bond lengths, bond angles, vibrational
frequencies. Small basis sets, with diffused functions are
more effective in determining dipole moments,
electronegativity mapping, and partial charges.