| We report a series of computational and experimental studies of the high vibrational overtone of alkanes. Experiments used an argon laser-pumped dye laser to measure the overtone absorption with photoacoustic spectroscopy. The computational results used the Gaussian '03 package on the Wake Forest Linux cluster. The observed spectra are explained by the Local Mode Model of loosely-coupled carbon-hydrogen (C-H) oscillators. We have used computational models that include electron correlation to describe the potential energy surfaces over a range of C-H bond lengths, including those near dissociation. Such methods scale poorly, and calculations for alkanes larger than C 5 are very expensive. For larger molecules, we have used the ONIOM method as implemented in Gaussian '03. Results are shown where the high-level ONIOM calculation was used in the local mode region of interest and a lower level of theory was used for the remainder of the molecule. The ONIOM results compared favorably with those of conventional computational methods. |
