Knowledge Bank

In a recent study of vibrational properties of deuterium isolated CH bonds in $n-alkanes,1$ it was found that when the proton can occupy more than one position (e.g., propane $CHD2CD2CD3$, and $cyclohexane-d11$), the relative Raman intensity ratios for the CH stretching bands varied significantly from values predicted on the basis of widely held assumptions regarding the intensity behavior and relative conformer abundance. To resolve this difficulty, we have measured the absolute Raman trace scattering cross sections of selected isotopomers of various alkanes. The observed cross sections for a set of isotopomers were fit in terms of isotopically invariant intensity parameters and, for isotopomers where more than one conformer can coexist, the relative conformer abundance. In an initial study of ethane, we verified that the over-all fit of the integrated cross sections was not significantly affected by neglecting the anharmonic contributions to the vibrational force field and the intensity. In subsequent studies of propane and cyclohexane, we have found that the reason for the anomalous intensity ratios is the unexpectedly large variation ($\sim 15\%$) in the intensity parameter $\partial a¯/\partial rCH$ for the in-plane and out-of-plane methyl CH bonds in propane and for the axial and equatorial CH bonds in cyclohexane. The relative conformer abundance result is consistent with a random distribution of the proton among available sites, to within the accuracy of our experiment. We also find that there is a significant variation in the parameter $\partial \alpha ¯/\partial rCC$ for the three alkanes studied. These results call into serious question the assumption of the transferability of such parameters when estimating Raman trace scattering cross sections.