  Infrared (IR) absorption spectra of individual polycyclic aromatic
hydrocarbons (PAHs) containing methyl (-CH3), methylene (>CH2), or diamond-like
*CH groups and IR spectra of mixtures of methylated and hydrogenated PAHs
prepared by gas phase condensation were measured at room temperature (as grains
in pellets) and at low temperature (isolated in Ne matrices). In addition, the
PAH blends were subjected to an in-depth molecular structure analysis by means
of high-performance liquid chromatography, nuclear magnetic resonance
spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight
mass spectrometry. Supported by calculations at the density functional theory
level, the laboratory results were applied to analyze in detail the aliphatic
absorption complex of the diffuse interstellar medium at 3.4 mu-m and to
determine the abundances of hydrocarbon functional groups. Assuming that the
PAHs are mainly locked in grains, aliphatic CHx groups (x = 1,2,3) would
contribute approximately in equal quantities to the 3.4 mu-m feature (N_{CHx} /
N_{H} approx 10^{-5} - 2 * 10^{-5}). The abundances, however, may be two to
four times lower if a major contribution to the 3.4 mu-m feature comes from
molecules in the gas phase. Aromatic =CH groups seem to be almost absent from
some lines of sight, but can be nearly as abundant as each of the aliphatic
components in other directions (N_{=CH} / N_{H} < 2 * 10^{-5}; upper value for
grains). Due to comparatively low binding energies, astronomical IR emission
sources do not display such heavy excess hydrogenation. At best, especially in
proto-planetary nebulae, >CH2 groups bound to aromatic molecules, i.e., excess
hydrogens on the molecular periphery only, can survive the presence of a nearby
star.
