Candidate carriers and synthetic spectra of the 21- and 30-mu
  protoplanetary nebular bands
  Computational chemistry is used here to determine the vibrational line
spectrum of several candidate molecules. It is shown that the thiourea
functional group, associated with various carbonaceous structures (mainly
compact and linear aromatic clusters), is able to mimic the 21-$\mu$m band
emitted by a number of proto-planetary nebulae. The combination of nitrogen and
sulphur in thiourea is the essential source of emission in this model: the band
disappears if these species are replaced by carbon.
  The astronomical 21-$\mu$m feature extends redward to merge with another
prominent band peaking between 25 and 30 $\mu$m, also known as the 30-$\mu$m
band. It is found that the latter can be modelled by the combined spectra of
aliphatic chains, made of CH$_{2}$ groups, oxygen bridges and OH groups, which
provide the 30-$\mu$m emission. The absence of oxygen all but extinguishes the
30-$\mu$m emission. The emission between the 21- and 30-$\mu$m bands is
provided mainly by thiourea attached to linear aromatic clusters.
  The chemical software reveals that the essential role of the heteroatoms N, S
and O stems from their large electronic charge. It also allows to determine the
type of atomic vibration responsible for the different lines of each structure,
which helps selecting the most relevant structures.
  A total of 22 structures have been selected here, but their list is far from
being exhaustive; they are only intended as examples of 3 generic classes. When
background dust emission is added, model spectra are obtained, which are able
to satisfactorily reproduce recent observations of proto-planetary nebulae.
  The relative numbers of atomic species used in this model are typically
H:C:O:N:S=53:36:8:2:1.
