Introduction
This
webpage provides access to quantum calculations for carbon monoxide,
hydrogen cyanide, and nitrobenzene. The programs used for these
calculations were GAMESS, MOPAC, and Molecular Mechanics via WebMO.
MacMolPlt was used to analyze the bond lengths, bond angles, 3D
HOMO, and vibrations of each molecule. UV-Vis transitions were
calculated for nitrobenzene, and potential energy versus bond length
was calculated for carbon monoxide.
The calculations for bond
lengths, bond angles, 3D HOMOs, and vibrations all include interactive
displays of the molecules using the program Jmol. The vibration
calculations also have a link to an IR spectrum, courtesy of the NIST
website, to compare the frequencies to. The UV-Vis transitions are
accompanied by a link to nitrobenzene's UV-Vis spectrum, and the
potential energy versus bond length data for carbon monoxide was
plotted using IGOR.
All of the calculations carried out on GAMESS were done using the best
ab initio
results (PM3 and 6-311G basis set) from previous work. These results were
recalculated using basis sets 3-21G, 6-31G, and 6-311G to compare the
calculations between basis set sizes. More about how the optimized
geometry of each molecule was chosen can be found on their respective
webpages.
Data
To view the quantum calculations on separate pages, click on the link to the desired molecule below:
Conclusion
For those interested in using the data presented:
These
quantum calculations are extremely useful as preliminary data for
comparison and as estimates for true values, but cannot be relied on
for very accurate work. As can be seen from the comparisons to
experimental data on the molecule webpages, the computational results
are generally greater in value. The vibrational data in particular had
large errors in value. The bond lengths and bond angles were the
exception, with values that resembled the experimental data more
closely. Quantum calculations work very well for "big picture"
questions, such as "Will this reaction work?". In this way, these
calculations can prevent the wasted time and materials spent on
reactions that don't work kinetically. Careful experimental data may
still be the best way to acquire very accurate numbers because no
matter how large the basis set or how high the level of theory, the
computational method is still an approximation.
References
- Gutow, J. Molecular Orbitals/Quantum Calculation Experiment 2. 2009, 1-3.
- Atkins, P.; Paula, J. Physical Chemistry, Eighth Edition, W.H. Freeman and Company, 2006, 363.
- NIST. Computational Chemistry Comparison and Benchmark DataBase. http://cccbdb.nist.gov/ (accessed March 10, 2009).
- NIST. NIST Chemistry WebBook. http://webbook.nist.gov/chemistry/ (accessed March 25, 2009).