  Based on the results of chemical analysis and simulation of kerogens and
immature coals, a large number of chemical structures carrying the 3.4-mum
feature were studied by means of computer simulation codes. Further selection
criteria were the integrated strength of the absorption lines in the aliphatic
stretchings wavelength band, weak IR activity in the aromatic stretching band
and absence of notable activity outside the astronomical UIBs (Unidentified
Infrared Bands). Most of the structures that were retained can be classed as
branched, short and oxygen-bridged CH_{2} chains, and naphtenic chains.
Combinations of their absorption spectra deliver spectra comparable to those
observed in the sky. Absorption spectra were derived from Normal Mode Analysis.
Emission spectra of the same structures were computed by monitoring their
overall dipole moment as they vibrate freely in vacuum after excitation. These
spectra were then combined in suitable proportions, together with those of
aromatic structures, so as to simulate various typical near IR emission spectra
observed in the sky.
