Quantum Calculations of
Diatomic Fluorine, Dioxygen Difluoride, and Styrene
Cody Beck and Robert Robinson
Abstract In
this experiment, quantum calculations were performed to determine several
molecular properties of diatomic fluorine, dioxygen difluoride, and styrene.
Properties determined included: molecular geometries, molecular orbitals,
dipole moments, partial atomic charges, vibrational frequencies, electrostatic
potential, as well as electronic transitions and energy vs. bond length for
select molecules. Basis sets used to evaluate these properties included AM1, PM3,
6-21G, 6-31G, and DZV. In comparison to literature values, DZV theory usually
gave the best results. However, even the best basis set had significant amount
of error (>10%) in most cases, suggesting that even higher levels of theory
are not always reliable.
Introduction Quantum calculations play an important role in the field of
physical chemistry, allowing chemists to learn many properties
of a molecule that cannot normally be observed. These properties
are determined by the electronic structure of a molecule and play an
important role in many physical processes. Wavefunctions are
mathematical models that give scientists needed information about the a
given molecule. Unfortunately, only atoms with a single atom can be
solved exactly for the corresponding eigenvalues and eigenfunctions.
Therefore, to make estimates about multi-electron molecules, computer
software is used to make predictions using "trial" wavefunctions in
attempt to estimate the exact values. While mathematically, this could
be solved on paper, it would be extremely cumbersome so calculations are
normally performed using computers.
The variational principle expresses the approximate wavefunction as a
linear combination of trial wavefunctions. The constants (ci)
in front of each psi represent the contribution to each psi listed in
the sum. Using this information, the software then tries to find the set
of coefficients that will give the overall wavefunction the lowest
energy. The expectation value of the energy is computed beacause the
trial wavefunction is not a "true" solution to the Schrodinger equation.
As the level of theory increases, the more calculations the computer
will make in an attempt to minimize the energy of the wavefunction. In
theory, higher basis sets should give more accurate results as they
perform more calculations, and therefore a lower overall energy of the
trial wavefunction. The objective of this experiment was to use various
levels of theory to perform quantum calculations on diatomic fluorine,
dioxygen difluoride, and styrene, and compare these results with
literature values to asses the validity of each basis set
Experimental To carry out the quantum calculations for each of the following molecules, the following software packages were used.
-Avogadro
-wxMacMolPlt
-GamessQ
-Jmol (also used in development of website along with SeaMonkey)
Instructions for using these software packages were found in Chem371 Lab Manual Fall 20191. Discussion Results and discussion of results for each of the following molecules can be found by clicking the hyperlinks below. Diatomic Fluorine Diatomic Fluorine Vibration Dioxygen Difluoride Dioxygen Difluoride Vibrations Dioxygen Difluoride Partial Atomic Charges Styrene
Conclusion Using
various computational software, we were able to calculate several molecular
properties of diatomic fluorine, dioxygen difluoride, and styrene. Given the
relative error for each of the basis sets, in general, the higher level of theory
produced more accurate predictions. However, these results prove that it is
important to obtain data from multiple basis sets. Additionally, the practicing
scientist should be skeptical of the predictions made and should use his/her
best judgement when interpreting results.
References 1.Gutow, J. Chemistry 371 Lab Manual Fall 2019,2019 pg. 17-28
2.Lide, D.R.; Handbook of Chemistry and Physics
73rd Edition; CRC, 1993
3.National Institute of Standards and Technology, NIST
Chemistry Webbook, https://webbook.nist.gov/chemistry/, accessed 2019
4.Oberhammer, H. Gas Phase Structures of Peroxides:
Experiments and Computational Problems, ChemPhysChem2015, 16,
282-290
5. Loos, K.R., et. al.; Dioxygen Difluoride: Infrared Spectrum, Vibrational Potential Function and Bonding, J. Chem. Phys. 1970, 52, 4418