  The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and
11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH)
molecules, have been recently ascribed to coal- or kerogen-like organic
nanoparticles with a mixed aromatic-aliphatic structure. However, we show in
this Letter that this hypothesis is inconsistent with observations. We estimate
the aliphatic fraction of the UIE carriers based on the observed intensities of
the 3.4 and 6.85 micrometer emission features by attributing them exclusively
to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes,
respectively. We derive the fraction of carbon atoms in aliphatic form to be
<15%. We conclude that the UIE emitters are predominantly aromatic with
aliphatic material at most a minor part of the UIE carriers. The PAH model is
consistent with astronomical observations and PAHs dominate the strong UIE
bands.
