  The so-called unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7,
8.6, and 11.3 micrometer are ubiquitously seen in a wide variety of
astrophysical regions. The UIE features are characteristic of the stretching
and bending vibrations of aromatic hydrocarbon materials, e.g., polycyclic
aromatic hydrocarbon (PAH) molecules. The 3.3 micrometer aromatic C--H
stretching feature is often accompanied by a weaker feature at 3.4 micrometer.
The latter is often thought to result from the C--H stretch of aliphatic groups
attached to the aromatic systems. The ratio of the observed intensity of the
3.3 micrometer aromatic C--H feature to that of the 3.4 micrometer aliphatic
C--H feature allows one to estimate the aliphatic fraction of the UIE carriers,
provided that the intrinsic oscillator strengths of the 3.3 micrometer aromatic
C--H stretch (A3.3) and the 3.4 micrometer aliphatic C--H stretch (A3.4) are
known. While previous studies on the aliphatic fraction of the UIE carriers
were mostly based on the A3.4/A3.3 ratios derived from the mono-methyl
derivatives of small PAH molecules, in this work we employ density functional
theory to compute the infrared vibrational spectra of several PAH molecules
with a wide range of sidegroups including ethyl, propyl, butyl, and several
unsaturated alkyl chains, as well as all the isomers of dimethyl-substituted
pyrene. We find that, except PAHs with unsaturated alkyl chains, the
corresponding A3.4/A3.3 ratios are close to that of mono-methyl PAHs. This
confirms the predominantly-aromatic nature of the UIE carriers previously
inferred from the A3.4/A3.3 ratio derived from mono-methyl PAHs.
