Alkanes vibrational spectra

The chemistry of all of these molecules is fascinating but, concentrating on the origins of life, a detailed look at the organic species is appropriate to see what molecules are present and how they might have been formed. The only alkane detected directly in the ISM is methane but this is due to the problem of detection. All alkanes are non-polar and so do not have a pure rotation spectrum. However, there is one allowed vibration of methane that is infrared active and with the low moment of inertia of methane the vibration-rotation spectrum can be observed and a rotational progression identifies the molecule with confidence. 

A band of moderate intensity has been observed at 1080 cm-1 in the Raman spectrum of once-folded solution-crystallized n-alkane C168H338.72 This spectral region is associated with C—C stretching vibrations of methylene sequences containing gauche bonds. Extended-chain shorter alkanes (/v-CixHox, n-C72H146) did not show this band. The 1080 cm-1 band is also observed in semicrystalline polyethylene.73 Low-temperature spectra were not reported. 

In the spectra of the short alkanes a clear gap opens below about 100 cm. This lies between the intemal-like TAMs and the extemal-like spectrum. In pentane, hexane, heptane, octane and dodecane this gap appears beyond 80 cm". Generally, however, the gap is not so distinct, as in nonane, decane and the longer alkanes, where it is quite absent. (There is also no identifiable gap in polyethylene.) The external vibrations of the alkane crystalline lattice are expected below the gap. 

In the spectra of those alkanes for which a clear gap is observed it is possible to regard 80 cm" as fflh,ax and the spectrum below as that of the lattice vibrations. A complicating feature of the gap region is the presence of lattice vibration overtones, or multiphonon contribution. However, since the lattice fundamentals are not a sequence of individual transitions their overtone intensity appears as a broad contribution. It builds up rapidly from the very lowest frequency to its strongest underneath the TAMs and provides a decreasing background for the LAM region, it has died away completely by about 600 cm". Beyond dodecane the lack of a clear gap makes the upper firequency bound of the external vibrations unidentifiable and we simply assume that the earlier cut-off (fiw = 80 cm ) remains representative. 

The UV spectrum of cyclopropane is quite different from those of the alkanes we have examined so far. Whereas acyclic alkanes and cyclic alkanes other than cyclopropane and cyclobutane have only Rydberg bands in the lower frequency (and photo chemically important) part of the spectrum, for cyclopropane the lowest Rydberg bands intermingle with bands due to valence-shell transitions. Two structured bands at 63000 and 78000 cm (159 and 128 nm) have been assigned to the 3e a,3p) and (3e o,4p) transitions on the grounds of the similarity of their vibrational fine structure with that of the respective photoelectron bands and their term values. Two other bands, near 70000 cm (143 nm) and 83000 cm (120 nm)... 

Shpol skii spectrofluorimetry has been used for the determination of PAHs in crude enviromnental sample extracts with minimum sample cleanup. This technique gives a vibrationally resolved fluorescence spectrum of samples dissolved in a suitable solvent (usually an -alkane) at cryogenic temperatures, e.g., 26 K. It combines the selectivity of an infrared spectrum with the sensitivity of fluorimetry, though the sensitivity suffers considerably from the presence of large amounts of interfering substances such as fatty components in crude extracts since these give a poor-quality matrix with a high sample absorbance. 

---