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:

• Carbon Monoxide (CO)
• Hydrogen Cyanide (HCN)
• Nitrobenzene (C₆H₅NO₂)

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

1. Gutow, J. Molecular Orbitals/Quantum Calculation Experiment 2. 2009, 1-3.
2. Atkins, P.; Paula, J. Physical Chemistry, Eighth Edition, W.H. Freeman and Company, 2006, 363.
3. NIST. Computational Chemistry Comparison and Benchmark DataBase. http://cccbdb.nist.gov/ (accessed March 10, 2009).
4. NIST. NIST Chemistry WebBook. http://webbook.nist.gov/chemistry/ (accessed March 25, 2009).